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FEATURE ARTICLE
Cite this: Chem. Commun., 2014, 50, 9816
Received 1st April 2014, Accepted 21st May 2014 DOI: 10.1039/c4cc02399d
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Well-defined transition metal hydrides in catalytic isomerizations Evgeny Larionov, Houhua Li and Cle ´ment Mazet* This Feature Article intends to provide an overview of a variety of catalytic isomerization reactions that have been performed using well-defined transition metal hydride precatalysts. A particular emphasis is placed on the underlying mechanistic features of the transformations discussed. These have been categorized depending upon the nature of the substrate and in most cases discussed following a
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chronological order.
Introduction Since the turn of the century and the growing appreciation for the need for sustainable chemistry, the ‘twelve principles of Green Chemistry’ have more and more systematically guided the design of novel synthetic methods.1 In this context, catalytic isomerizations are perceived as nearly ideal transformations.2 As catalytic redox-economical processes which do not generate any waste they obey several principles at once (#1 prevention; #2 atom-economy; #3 energy efficiency; #8 reduce derivatizations; #9 catalysis; #10 design for degradation. . .). Furthermore, if properly designed at the outset, an isomerization reaction not Organic Chemistry Department, University of Geneva, 30 quai Ernest Ansermet, Geneva 1204-CH, Switzerland. E-mail: [email protected]; Fax: +41 22 379 32 15; Tel: +41 22 379 62 88
Evgeny Larionov was born in St Petersburg (Russia) in 1985. He studied chemistry at the St Petersburg State University and received his MSc in Chemistry in June 2007. In 2011 he received his PhD from LMU Munich under the supervision of Prof. H. Zipse. He subsequently moved to the University of Geneva for a postdoctoral stay with Prof. E. P. ¨ndig (2011–2012) and in 2012 Ku joined the group of Prof. C. Mazet Evgeny Larionov at the same university. His research interests include application of computational and experimental methods for understanding mechanisms of organic reactions.
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only consists of a simple internal redox process, but additional oxidation state manipulations and a number of protection/ deprotection sequences may also be by-passed, while still generating a substantial increase in molecular complexity. As an illustration of its relevance in a variety of important chemical processes the transition-metal catalyzed isomerization of olefins is undeniably the most documented of all isomerizations.3,4 There are two most commonly accepted mechanisms that account for the isomerization of olefins, even though convincing alternatives have been demonstrated (Fig. 1).4,5 The metal hydride mechanism involves addition of a kinetically long-lived metal-hydride across a carbon–carbon double bond to generate a secondary metal–alkyl intermediate. Subsequent b-hydride elimination yields the isomerized product and regenerates the initial metal-hydride catalyst. The less-documented p-allyl mechanism invokes a non-hydride catalyst which oxidatively adds to an
Houhua Li
Houhua Li studied carbohydrate chemistry under the supervision of Prof. Xinshan Ye during his BSc and MSc in Peking University (China). In 2009 he moved to the National Institute of Biological Sciences (NIBS) and worked with Prof. Xiaoguang Lei on Lycopodium alkaloid synthesis. He joined the University of Geneva (Switzerland) in 2011 to start his graduate studies in the group of Prof. Cle´ment Mazet. His research project focuses on the Ir-catalyzed selective isomerizations of allylic alcohol.
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Fig. 1 Generally accepted (and simplified) mechanisms for the metalcatalyzed isomerization of olefins.
olefin to produce a p-allyl metal-hydride intermediate. Next, hydride transfer to the opposite terminus of the allyl fragment liberates the metal at its initial oxidation state and results in olefin isomerization in an overall formal 1,3-H shift. When a metal hydride precatalyst [M–H] is employed, once activated, it is anticipated to follow the hydride mechanism and to simplify the mechanistic understanding of the isomerization at work. As will be discussed later in this Feature Article, the situation is not necessarily always as simple. Prior to going into a more specific discussion, a number of aspects need to be specified to clearly delineate the scope of this article. There are usually two distinct approaches to develop a catalytic reaction. The in situ approach consists in mixing a transition metal precursor with an array of ligands in varying relative stoichiometries in separate experiments. Whereas this strategy might accelerate the discovery of a lead candidate it does not provide information regarding the nature of the active form of the catalyst. Subsequent mechanistic studies often suffer from limited structural information on the catalytically competent intermediates. If it is often believed that in organic synthesis, the art of mixing and testing is somewhat similar to culinary instinct, the uncertain nature of the results of in situ catalysis might have probably more to do with English food rather than the delicacy of French cuisine. In contrast, the ex situ approach relies on the use of well-defined
Cle´ment Mazet received his PhD from the University of Strasbourg under the supervision of Prof. L. H. Gade (2002). After postdoctoral stays with Prof. A. Pfaltz (University of Basel – 2003–2005) and Prof. E. N. Jacobsen (Harvard University – 2006–2007) he joined the University of Geneva to establish his independent research program. His interests include mechanistic and synthetic chemistry with particular emphasis on ´ment Mazet Cle asymmetric catalysis. He recently received the Zasshikai Lectureship Award from the University of Tokyo (2012) and the Werner Prize from the Swiss Chemical Society (2013).
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transition metal complexes as precatalysts, the structure of which is expected to closely resemble their active form in the catalytic cycle. Often a simple structural change in coordination geometry or ligand dissociation is necessary to funnel the precatalyst into the productive reaction pathway. During optimization studies, the necessity of systematically preparing each and every catalyst might be a rather time consuming task, but mechanistic information may be gleaned more readily by following this approach. Another distinct advantage of ex situ catalysis is that the initial (pre)catalyst loading is probably a more realistic reflection of the active catalyst concentration while with the in situ approach multiple inactive or competing species might be generated concomitantly with the truly active form of the catalyst. Herein, emphasis will be placed exclusively on isomerization reactions which employ a well-defined transition metal hydride precursor. This deliberately excludes a vast body of literature in which transient metal hydrides have been either invoked or identified as being responsible for an isomerization process (note, these species were typically generated by the activation of a non-hydride containing precatalyst).3–6 The isomerization reactions discussed herein have been classified according to the nature of the substrate rather than the nature of the catalyst and their chronological order of appearance in the literature has been followed. When appropriate the underlying mechanistic features of these transformations are presented.
Isomerization of olefins In transition metal-based homogeneous catalysis, olefinic substrates are traditionally divided into two categories. Functionalized olefins are substrates with an adjacent functional group (i.e. usually a heteroatom containing function) that can effectively coordinate to the metal and exert a direct influence on the outcome of the reaction, both in terms of reactivity and selectivity. These olefins are often referred to as 2-point binding substrates. By opposition, unfunctionalized olefins or 1-point binding olefins interact with the catalyst solely by p-coordination of the CQC bond. These are often considered as more challenging candidates as the benefits of directed-catalysis are lost. Between these two extremes there is of course a continuum of situations which depends (i) on the ability of the functional group to form a firm chelate with the metal center which may vary as a function of the distance between the coordinating function and the olefin, and (ii) on the personal appreciation one may have of a functional group.4b,7 Isomerization of unfunctionalized olefins In the early days of the transition metal-catalyzed hydroformylation of olefins (oxo synthesis), it was recognized that olefin isomerization plays a central role in the control of the final linear/branched ratio of aldehydes produced.8 Subsequently, several studies were focused on better understanding the overall reaction mechanism by specifically scrutinizing the isomerization of terminal olefins. The ubiquitous [HCo(CO)4] catalyst became the center of particular attention, and independent seminal contributions by Heck and
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Feature Article
Fig. 2 Typical [M–H] catalysts used in the 60s–70s for the isomerization of terminal olefins and still often employed today.
Breslow and the Orchin group provided important mechanistic information.9,10 The underlying features are that (i) CO inhibits the reaction presumably by competing binding with the incoming olefin, (ii) the dominant reaction pathway involves a 1,2-addition/ elimination of [HCo(CO)4] with preferential Markovnikov selectivity in the absence of a CO atmosphere. Interestingly, on the basis of labelling experiments a conflicting formal 1,3-H shift was postulated in the case of allylbenzene.11 Subsequent investigations by Wilkinson and co-workers showed that the extent of competing isomerization and hydrogenation could be substantially reduced using [HRh(CO)(PPh3)3], an observation which later led to the development of the Union Carbide hydroformylation process.12 In this context, the specific isomerization of cis and trans terminal olefins was carefully investigated and other catalysts such as [HRu(Cl)(PPh3)3] were also surveyed. As these studies were mostly based on kinetic analyses, limited structural information regarding the nature of the active form of the catalyst or the preferred reaction mechanism was collected. Importantly, it was found that isomerization and hydrogenation of terminal olefins occurred at similar rates (Fig. 2).13 Both from historical and mechanistic vantage points, the Pd-catalyzed isomerization of olefins has been the source of intense research and controversy.5a,14 Intriguingly, examples of well-defined palladium-hydride precursors employed in isomerization reactions are scarce (Fig. 3A). This certainly reflects the challenges associated with their preparation, purification, handling and characterization. Portnoy and Milstein reported that the hydride-bridged dinuclear complex 1 catalyzed the internal isomerization of cyclooctene (as evidenced by the in situ generation of the deuteride version of 1).15 Bunel and co-workers showed that the mononuclear complex 2 was
Fig. 3 Pd hydride catalyst for the isomerization of cyclic (A) and linear olefins (B). The catalyst loading was not specified and is given as x mol%.
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Fig. 4 Pd-catalyzed one-carbon isomerization of olefins.
competent in the slow isomerization of 1-pentene into 2-pentene at 80 1C, but unfortunately no information on the cis/trans ratio was provided (Fig. 3B). Added PCy3 (Cy = cyclohexyl) was found to have an inhibitory effect indicating that dissociation of phosphine occurs before the rate-determining step.16 Both Milstein and Bunel proposed an insertion/b-H elimination mechanism to account for these observations. More recently, Skrydstrup and Lindhardt reported the onecarbon isomerization of a wide variety of terminal alkenes into 2-alkenes with often high control of olefin geometry.17,18 Synthetically, this reaction is appealing as it potentially connects allyl- and vinyl-based chemistry, in particular, when the thermodynamically least stable products can be accessed.19 Some representative examples are disclosed in Fig. 4. Although initial experiments were conducted by in situ generation of the catalytically active [Pd–H] intermediate, supporting organometallic experiments and mechanistic investigations revealed that the known complex [HPd(Cl)(PtBu3)2] 3 could also be employed as a precatalyst. Activation by phosphine dissociation and coordination of the olefinic substrate were proposed as initial steps to enter into the productive catalytic manifold. Of note, isomerization of safrole was found to proceed with a catalyst loading as low as 0.01 mol% when the welldefined hydride 3 was used rather than the in situ protocol. Over the course of metathesis reactions employing the wellestablished Grubbs first and second generation ruthenium catalysts (4 and 5 respectively), isomerization of the olefinic substrates or the newly formed double bond in the products was initially considered as an undesired side-reaction (Fig. 5).20,21 Initial efforts were essentially directed towards inhibiting this secondary pathway. The potential of this isomerization in its own right was put forward by Nishida and co-workers, notably for the preparation of indoles.22 By adding 1–10 equiv. of trimethyl(vinyloxy)silane to 5, a discrete [Ru–H] was generated
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Fig. 7 Comparative study between [Ru–H] catalysts for the isomerization of estragole.
Fig. 5
[Ru] metathesis catalysts and hydrides derived thereof.
by degradation of the transient Fischer carbene complex initially formed, as later supported by mechanistic investigations.23 Subsequently, an impressive number of studies exploiting the ability of ruthenium metathesis catalysts to evolve in situ into isomerization catalysts have been developed.24 Among other examples, decomposition of ruthenium metathesis catalysts was accomplished by use of molecular hydrogen, protic solvents or inorganic bases.19,25 In most cases, the exact structure of the active isomerization catalyst was never investigated thoroughly, probably because the ruthenium hydrides 6, 7 and 8 prepared independently from 4, 5 or their methylidene analogues were recurrently invoked as plausible intermediates. In contrast, well-defined [Ru–H] complexes were employed only on rare occasions.13d,f,26 The potential of 7 as a general catalyst for the positional isomerization of 1,3-dienes into more substituted dienes was only established recently by Diver and co-workers (Fig. 6).27 The use of n-butanol was found to substantially increase the chemical yields, the substrate scope was relatively broad and the reaction highly stereoconvergent as the most thermodynamically stable (E,E)-1,3-dienes were systematically obtained. Because the
Fig. 6
[Ru–H]-catalyzed positional isomerization of 1,3-dienes.
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starting 1,3-dienes are themselves products of ene–yne metathesis, the authors developed an efficient one-pot tandem protocol using 5 as a single catalyst and various terminal alkynes and alkenyl alcohols as substrates. NMR investigations and spiking experiments secured that 7 is indeed generated by decomposition of 5 and responsible for the isomerization reaction. Further stoichiometric studies using 7-d1 supported the intermediacy of a [Ru–(p-allyl)] complex in the 1,3-diene isomerization. The use of nBuOH was proposed to irreversibly favor the final p-allyl formation/b-H elimination sequence. Importantly, Fogg and co-workers recently asserted that in 6, 7 and 8, the [Ru–H] typically assumed to account for isomerization of olefins during metathesis are not the truly catalytically competent species (Fig. 7).28 A systematic structure–activity analysis using estragole as a model substrate and several related [Ru–H] complexes suggests that the isomerization-active species may be generated by more extensive decomposition of the metathesis catalysts, potentially involving degradation of the strongly coordinated PCy3 or NHC ligands (NHC = N-heterocyclic carbene).25b,29 The authors also propose that a p-allyl-type mechanism rather than a hydride-type mechanism may be operating. These observations further advocate the notion that proper characterization of reactive intermediates and decomposed catalysts may have important implications in methodology development. In 2006, Goldman, Brookhart and co-workers reported a tandem catalytic system for the formal metathesis of alkanes utilizing a pincer iridium catalyst for the dehydrogenation of alkanes (step 1) and the hydrogenation of olefins (step 3) and a Mo catalyst for olefin metathesis (step 2; Schrock-type) (Fig. 8).30 In this system, parasitic isomerization of the transient terminal alkene into internal analogues is a major problem which affects the product selectivity of C2n2 products from Cn substrates. Using a combined experimental and theoretical approach, the groups of Goldman, Brookhart and Krogh-Jespersen specifically investigated the isomerization of terminal olefins using pincer iridium dihydride complexes 11 and 12.31 Unexpectedly, they clearly established that isomerization occurs via the intermediacy of a [(pincer)Ir(Z3-allyl)(H)] complex rather than by insertion of 11 or 12 across an incoming olefin. In situ observation of the catalyst resting state, labelling experiments, independent synthesis and evaluation of an iridium-Z3-allyl hydride for olefin isomerization were collective evidences in support of this proposal. DFT calculations further substantiated the empirical data and provided additional structural details on the counter-intuitive allyl-mechanism. Formation of the p-allyl hydride intermediate arises from dissociation of an olefin double bond from the
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Isomerization of functionalized olefins
Fig. 8 Mechanism for the Ir-catalyzed olefin isomerization in the context of alkane metathesis.
metal center, followed by allylic C–H activation and final recoordination of the double bond (I - II - III). Almost concomitantly, Chianese and co-workers reported a related study of the isomerization of terminal olefins using two Ir(III) hydride complexes supported by CCC pincer ligands which differ only in their coordination number.32 Whereas experimental evidence was in agreement with a p-allyl mechanism for the sterically less demanding catalyst, the operating mechanism of the bulkier adamantyl catalyst could not be elucidated unambiguously but was proposed to differ significantly. Collectively, the results obtained with the Brookhart–Goldman and Chianese [(pincer)Ir] catalysts clearly indicate that the use of well-defined transition metal hydrides for the isomerization of olefins does not imply the reaction will necessarily proceed via a hydride-type mechanism as one may expect at the outset. This further underscores the importance and necessity to conduct appropriate mechanistic investigations. Most of the catalytic systems described so far rely on the use of late transition metal hydride complexes. Examples of well-defined hydrides based on early transition metals are less abundant in the literature and the catalytic studies with either terminal or internal alkenes were conducted at the NMR scale.33 Nonetheless, there is obviously plenty of room for improvements and more efficient and practical isomerization catalysts built on early transition metal hydrides will certainly appear in the coming years.
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A. Allylic alcohols. As part of their studies on olefin hydroformylation and isomerization with [HCo(CO)4], Goetz and Orchin also evaluated the potential of this hydride complex in the isomerization of allylic alcohols.34 A measurable yield (21% yield) was obtained only for 2-propenol when 50 mol% of the cobalt complex were employed. Isotope labelling using the cobalt deuteride complex showed exclusive incorporation at C(3), which was rationalized to arise from a 1,3H-shift similar to that invoked for allylbenzene. A decade later, Sasson and Rempel showed that the air-sensitive [HRu(Cl)(PPh3)3] 10 was a highly active catalyst for the isomerization of linear and cyclic secondary allylic alcohols as well as homoallylic alcohols using neat substrates at 110 1C (Fig. 9A).35 The corresponding ketones were obtained in high isolated yields. Primary allylic alcohols and secondary propargylic alcohols were not isomerized under these conditions. Although the enantioselectivities were in the marginal range and were assessed by optical rotation measurements, the first asymmetric version of the isomerization of allylic alcohols was reported in 1974 by Botteghi and Giacomelli using [HRh(CO)(PPh3)3] as metal precursor in combination with ()-DIOP, one of the very few chiral ligands available at that time (Fig. 9B).36 Bergens and co-workers reported a rare example of a chiral monohydride ruthenium complex [HRu(R)-(BINAP)(THF)2(CH3CN)]BF4 13 which proved to be catalytically active in the hydrogenation and hydrosilylation of ketones and olefins.37 As shown in Fig. 9C, using 2 mol% of 13, a partial kinetic resolution was also achieved by preferential isomerization of one enantiomer of 3-buten-2-ol into the corresponding enol and ketone products. En route to Superambrox, a highly valuable fragrance chemical with a strong ambery odor, Fehr and Farris attempted hydrogenation of the bicyclic intermediate 14 using Crabtree’s catalyst [(Cy3P)(Py)Ir(cod)]PF6.38 An unexpected highly diastereoselective isomerization leading to lactol 15 was observed instead. Switching to Chaudret’s ruthenium hydride catalyst [HRu(Z2C8H11)2] 16 enabled substantial improvement of the method (0.5 mol%, room temperature) and the target compound was isolated in 76% yield over 2 steps after silane mediated reduction (Fig. 9D). The approach was later demonstrated to provide general access to diverse 1,2-annulated trans-tetrahydrofurans and the related trans-g-butyrolactones. Labelling and cross-over experiments were consistent with an intermolecular addition/ elimination mechanism.39 B. Allylic amines and amides. During the early development of the industrially-relevant isomerization of allylic amines, a cobalt hydride catalyst [HCo(N2)(PPh3)3] was first employed.40 Isomerization of N,N 0 -diethylgeranylamine and N,N 0 -diethylnerylamine into the corresponding enamine was highly efficient with 1 mol% of the catalyst (THF, 80 1C, 15 h) and only traces of the regioisomeric 1,3-dienamine were observed (Fig. 10A). A preliminary screening of chiral ligands was performed by in situ generation of the active cobalt hydride and, despite low yields, enantioselectivities of up to 33% were measured with (+)-DIOP. Further optimizations of this reaction
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Fig. 9
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Well-defined [Ru–H] and [Rh–H] in the isomerization of allylic alcohols.
led to the development of what remains one of the most formidable achievements in homogeneous enantioselective catalysis: the [Rh(BINAP)]-catalyzed isomerization of allylic amines.41 In 2001, Krompiec and co-workers described the highly (E)-selective Ru-catalyzed isomerization of N-allyl-N-arylamides into N-propenyl-N-arylamides (Fig. 10B).42 A hydride mechanism was postulated when [HRu(Cl)(PPh3)3] 10 was employed. Efforts were mostly directed toward the understanding of the selectivitydetermining interactions between the aryl ring and the metal center by combining Hammett correlations with computational studies. C. Allylic ethers. Binding of the oxygen atom of allylic ethers to the metal center during isomerization has been often
invoked but never truly demonstrated. Hence, these are ambiguous candidates and can be regarded equally as 1-point or 2-point binding olefinic substrates. Because of their structural similarity with allylic alcohols and amines, we have arbitrarily decided to discuss their isomerization in this section. Based on the initial studies at the stoichiometric level, a set of several [Pt–H] complexes was used by Clark and Kurosawa for the catalytic isomerization of allylmethylether and allylphenylether (Fig. 11A).43 With the usually very low loadings of catalyst, the reaction was highly cis-selective in benzene. Appearance of the trans isomer with prolonged reaction time and labelling experiments were in support of a reversible addition/elimination process. The authors also proposed coordination of the oxygen
Fig. 10 Isomerization of allylic amines (A) and allylic amides (B).
Fig. 11 Isomerization of allylic ethers (A) and silylated allylic alcohols (B).
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atom to account for the high cis-selectivity observed. The absence of stereospecificity in the isomerization of butene further reinforced this hypothesis. Inspired by the Sasson and Rempel studies on allylic alcohols, Suzuki, Moro-Oka and co-workers disclosed that [(H)2Ru(PPh3)4] performed the isomerization of silylated allylic alcohols under somewhat forcing conditions (150 1C, benzene or o-xylene).44 The desired silyl enol ethers were obtained in appreciable yields but with usually quite modest (E)/(Z) selectivity. Interestingly, both double bonds were isomerized and kept in conjugation for the 2,4-pentadienyl derivative whereas only the allylic double bond migrated for the geranyl and neryl substrates, leaving the remote prenyl moiety intact (Fig. 11B). D. Acetylenic derivatives. Following up on their previous work, Suzuki and Moro-Oka investigated the related isomerization of acetylenic silyl ethers to dienol silyl ethers catalyzed by [HRu(Cl)(PPh3)3(toluene)].45 After 5 h at 150 1C in benzene, the product was obtained quantitatively as a ca. 1/1 mixture of (Z,Z) and (Z,E) isomers. This selectivity was explained by initial formation of an allene intermediate followed by the addition/elimination of the [Ru–H] catalyst across the most accessible double bond and from the sterically least encumbered face of the diene (Fig. 12). A series of others dienol silyl ethers were obtained in appreciable yields using the same method, but the stereospecificity of the reaction was unfortunately not discussed. Terminal acetylenes as well as free hydroxyl group were not tolerated. As part of their studies on transition metal hydride catalysis, the Lu group reported the highly stereoselective isomerization of a,b-ynones to the geometrically pure conjugated (E,E)-dienones.46 Both [H5Ir(PiPr3)2] and [(H)2Ru(PPh3)4] proved competent catalysts for this transformation. With or without added phosphine, the reaction proceeded rapidly under mild conditions and was applied to a significantly diversified set of substrates. Importantly, the monohydride syn,syn-(Z4-dienone)-metal complex 17 was isolated from stoichiometric reactions between [H5Ir(PiPr3)2] and either the substrate 18 or product 19. Under similar reaction conditions, it was found to catalyze the isomerization of 18 to 19 with reactivity
Fig. 12
Ru-catalyzed isomerization of acetylenic silyl ethers.
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Fig. 13
Ir-catalyzed isomerization of ynones to conjugated dienones.
and selectivity comparable to that of the iridium pentahydride, thus lending credence to its potential role as a crucial reactive intermediate (Fig. 13). Shortly after, closely related studies on the Ir-catalyzed isomerization of propargylic secondary alcohols (mono and diols) were reported by the same group using again the iridium pentahydride catalyst.47 E. Deconjugation of a,b-unsaturated esters. During the course of their investigations on the Ru-catalyzed cyclization of enynes, Mori and co-workers observed that – quite remarkably – [HRu(Cl)(CO)(PPh3)3] 9 induced the thermodynamically disfavoured deconjugation of an a,b-unsaturated ester by migrating the carbon double bond away from the ester functionality.48 A specific study in this intriguing direction led them to investigate the scope of this reaction in more details using the same catalyst in refluxing toluene.49 Thus, a,b-unsaturated esters and amides with different terminal functional groups were deconjugated efficiently, independently of the chain length of the linker between both ends of the substrate (Fig. 14). The corresponding products were obtained in practical yields and E/Z selectivities varying from 1.0/2.2 to 4.6/1.0.50
Fig. 14
Ru-catalyzed deconjugation of a,b-unsaturated esters and amides.
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Tandem processes involving isomerization Although seemingly simple at first sight, the combination of two compatible catalysts in a single vessel for sequential operations is faced with numerous challenges and is currently the subject of intense research efforts.51 The use of well-defined [M–H] isomerization catalysts in several tandem processes is discussed in the present section. A. Isomerization–metathesis. As discussed above, the ability of ruthenium metathesis catalysts to decompose into isomerizationactive ruthenium hydride has often been exploited to conduct metathesis and isomerization reactions sequentially in a one-pot protocol. Adopting an opposite strategy, van Otterlo and co-workers, have accessed a series of benzo[1,4]dioxins in good yields using [HRu(Cl)(CO)(PPh3)3] 9 as isomerization catalyst and 5 as ringclosing metathesis catalyst.52 B. Isomerization/C–C bond formation. Sorimachi and Terada have developed a binary catalytic system composed of a [Ru–H] and a Brønsted acid for a three-step sequence resulting in an overall isomerization/C–C bond formation process.53 In the first event, [HRu(Cl)(CO)(PPh3)3] 9 isomerizes a N-protected-Nallylamine into an enamine intermediate, itself being directly isomerized to the corresponding imine in the presence of a modified biaryl phosphoric acid or Tf2NH. The same catalyst is then instrumental in promoting the final C–C bond forming event with the electron-rich arene or heteroarene coupling partner. A selection of the formal Friedel–Crafts products accessible by this method is displayed in Fig. 15. Control experiments showed that both catalysts were required for the reaction to
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proceed. Furthermore, much reduced yields were obtained when isolated enamides were employed, suggesting a synergistic effect between the organometallic and organic catalysts. A related enantioselective Pictet–Spengler cyclization was next developed using the same [Ru–H] isomerization catalyst in combination with chiral binaphthol-derived phosphoric acids serving as Brønsted acid catalysts.54 The corresponding tetrahydroisoquinoline derivatives were obtained in yields ranging from 50 to 87% and a 53% ee value was reported in the best case. A similar isomerization/cyclization strategy was followed by Nielsen and co-workers for the development of very general routes to access tetrahydro-b-carbolines and tetrahydropyrano[3,4]-indoles.55 After extensive screening, the ubiquitous [HRu(Cl)(CO)(PPh3)3] complex (9) was identified as the best isomerization catalyst and phosphoric acid the most efficient acid catalyst. Promising enantioselectivity levels were also obtained when chiral phosphoric acids were evaluated. C. Isomerization–hydroformylation–hydrogenation. Nozaki and co-workers recently described a tertiary catalytic system for the sequential one-pot isomerization–hydroformylation– hydrogenation of internal alkenes to the corresponding homologated n-alcohols (Fig. 16).56 In this system, the measure of the n/i ratio (i.e. linear vs. branched products) provides a direct indication of the efficiency of the isomerization step. Interestingly, their initial study focused on a binary system composed of a non-hydride Rh complex for the isomerization– hydroformylation sequence associated with Shvo’s catalyst 20 for the final hydrogenation. In control experiments, the concomitant use of the Rh and Ru catalysts led to higher n/i ratio in comparison to those obtained with the Rh catalyst alone. This observation indicates participation of the [Ru–H] complex in the isomerization and supports a synergistic effect between the two organometallic species. Similar cooperativity was also found in the hydrogenation step. The tandem process next evolved into a tertiary catalytic system by addition of [Ru3(CO)12] to further improve the efficiency of the isomerization step.
Miscellaneous isomerizations Our group recently reported the [Pd–H]-catalyzed selective isomerization of terminal epoxides into the corresponding
Fig. 15
Relay catalysis by a [Ru–H]/Brønsted acid binary system.
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Fig. 16 Cooperative catalysis in the sequential isomerization/hydroformylation/hydrogenation of internal alkenes to n-alcohols.
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Conclusions
Fig. 17 Pd-catalyzed isomerization of terminal epoxides. Mechanistic proposal and representative examples.
aldehydes (Fig. 17).57 Although deeply rooted in the literature, this rearrangement is still faced with multiple challenges.58 These include the discovery of a truly practical catalytic method with wide functional group tolerance. The subsequent development of an enantioselective version of this process would provide an elegant route to chiral aldehydes with a a-tertiary center. Careful examination of the species formed during screening of various ligands and palladium precursors led to the independent isolation of an unusual dinuclear palladium hydride complex [(dippp)3(Pd–H)2](OTf)2 21 (dippp = 1,3-bis-(di-iso-propylphosphino)propane) which proved particularly effective for the isomerization of a variety of terminal but also trisubstituted epoxides. Functional groups that would not be compatible with the traditional Lewis acid reagents employed for this isomerization were particularly well-tolerated by 21. Experimental and theoretical investigations unveiled an unusual hydride-type mechanism characterized by an epoxide opening/hydride transfer sequence. Consistent with this rationale, preliminary observations support the existence of two distinct enantio-determining steps for the kinetic resolution of terminal epoxides (I and II). An off-cycle racemization pathway involving the [Pd–H] catalyst was also identified (IV).59 Although the selectivity factors obtained were not practical (Smax = 4.4), the delineation of the reaction mechanism and the operational simplicity of the protocol hold promise for future developments.
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For a long time considered as an undesired process in hydrogenation and hydroformylation reactions, the isomerization of olefins has been initially studied with the primary aim of suppressing its detrimental impact on these industrially relevant transformations. This is particularly true for unfunctionalized olefins which usually serve as feedstock chemicals in industry. The situation changed in the late 1970s, and early 1980s with the success story of the Rh-catalyzed enantioselective isomerization of allylic amines – which eventually also led to the development of an industrial application. Nonetheless, the successful implementation of a transformation in industry cannot become a simple measure of its relevance. The knowledge and understanding of the underlying mechanistic aspects of a chemical transformation are also of prime importance. We have already discussed the potential benefits of carrying out ex situ rather than in situ catalysis. In the context of this Feature Article, this justified placing a specific focus on isomerizations performed with well-defined transition metal hydrides – catalysts for which a greater mechanistic predictability is expected a priori (i.e. via the hydride mechanism). The independent contributions of Goldman, Brookhart, Krogh-Jespersen and Chianese demonstrate that the situation is not necessarily that simple. Their clear observation of allyl-type mechanisms despite the use of well-defined metal hydride precatalysts is an interesting case study. From a synthetic point of view, it is tempting to conclude that most of the methods presented herein are not exactly practical and do not provide access to structurally relevant and diversified libraries of products. Though, a fair estimation of the synthetic potential of isomerization reactions in their whole depth and breadth would require including reactions with in situ generation of the active species. Unfortunately, this would go much beyond the scope of this short review. Nonetheless, the work of Skrydstrup and Lindhart shows that there is much to learn mechanistically and to benefit synthetically from a combined in situ/ex situ approach. Finally, the recent tandem processes by Terada and the [Pd–H]-catalyzed rearrangement of epoxides have just unveiled some of the immense synthetic potential that remains to be explored for isomerization reactions.
Acknowledgements The authors thank the University of Geneva and the Swiss National Foundation (Project PP00P2_133482) for financial support.
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FEATURE ARTICLE
Cite this: Chem. Commun., 2014, 50, 9816
Received 1st April 2014, Accepted 21st May 2014 DOI: 10.1039/c4cc02399d
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Well-defined transition metal hydrides in catalytic isomerizations Evgeny Larionov, Houhua Li and Cle ´ment Mazet* This Feature Article intends to provide an overview of a variety of catalytic isomerization reactions that have been performed using well-defined transition metal hydride precatalysts. A particular emphasis is placed on the underlying mechanistic features of the transformations discussed. These have been categorized depending upon the nature of the substrate and in most cases discussed following a
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chronological order.
Introduction Since the turn of the century and the growing appreciation for the need for sustainable chemistry, the ‘twelve principles of Green Chemistry’ have more and more systematically guided the design of novel synthetic methods.1 In this context, catalytic isomerizations are perceived as nearly ideal transformations.2 As catalytic redox-economical processes which do not generate any waste they obey several principles at once (#1 prevention; #2 atom-economy; #3 energy efficiency; #8 reduce derivatizations; #9 catalysis; #10 design for degradation. . .). Furthermore, if properly designed at the outset, an isomerization reaction not Organic Chemistry Department, University of Geneva, 30 quai Ernest Ansermet, Geneva 1204-CH, Switzerland. E-mail: [email protected]; Fax: +41 22 379 32 15; Tel: +41 22 379 62 88
Evgeny Larionov was born in St Petersburg (Russia) in 1985. He studied chemistry at the St Petersburg State University and received his MSc in Chemistry in June 2007. In 2011 he received his PhD from LMU Munich under the supervision of Prof. H. Zipse. He subsequently moved to the University of Geneva for a postdoctoral stay with Prof. E. P. ¨ndig (2011–2012) and in 2012 Ku joined the group of Prof. C. Mazet Evgeny Larionov at the same university. His research interests include application of computational and experimental methods for understanding mechanisms of organic reactions.
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only consists of a simple internal redox process, but additional oxidation state manipulations and a number of protection/ deprotection sequences may also be by-passed, while still generating a substantial increase in molecular complexity. As an illustration of its relevance in a variety of important chemical processes the transition-metal catalyzed isomerization of olefins is undeniably the most documented of all isomerizations.3,4 There are two most commonly accepted mechanisms that account for the isomerization of olefins, even though convincing alternatives have been demonstrated (Fig. 1).4,5 The metal hydride mechanism involves addition of a kinetically long-lived metal-hydride across a carbon–carbon double bond to generate a secondary metal–alkyl intermediate. Subsequent b-hydride elimination yields the isomerized product and regenerates the initial metal-hydride catalyst. The less-documented p-allyl mechanism invokes a non-hydride catalyst which oxidatively adds to an
Houhua Li
Houhua Li studied carbohydrate chemistry under the supervision of Prof. Xinshan Ye during his BSc and MSc in Peking University (China). In 2009 he moved to the National Institute of Biological Sciences (NIBS) and worked with Prof. Xiaoguang Lei on Lycopodium alkaloid synthesis. He joined the University of Geneva (Switzerland) in 2011 to start his graduate studies in the group of Prof. Cle´ment Mazet. His research project focuses on the Ir-catalyzed selective isomerizations of allylic alcohol.
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Fig. 1 Generally accepted (and simplified) mechanisms for the metalcatalyzed isomerization of olefins.
olefin to produce a p-allyl metal-hydride intermediate. Next, hydride transfer to the opposite terminus of the allyl fragment liberates the metal at its initial oxidation state and results in olefin isomerization in an overall formal 1,3-H shift. When a metal hydride precatalyst [M–H] is employed, once activated, it is anticipated to follow the hydride mechanism and to simplify the mechanistic understanding of the isomerization at work. As will be discussed later in this Feature Article, the situation is not necessarily always as simple. Prior to going into a more specific discussion, a number of aspects need to be specified to clearly delineate the scope of this article. There are usually two distinct approaches to develop a catalytic reaction. The in situ approach consists in mixing a transition metal precursor with an array of ligands in varying relative stoichiometries in separate experiments. Whereas this strategy might accelerate the discovery of a lead candidate it does not provide information regarding the nature of the active form of the catalyst. Subsequent mechanistic studies often suffer from limited structural information on the catalytically competent intermediates. If it is often believed that in organic synthesis, the art of mixing and testing is somewhat similar to culinary instinct, the uncertain nature of the results of in situ catalysis might have probably more to do with English food rather than the delicacy of French cuisine. In contrast, the ex situ approach relies on the use of well-defined
Cle´ment Mazet received his PhD from the University of Strasbourg under the supervision of Prof. L. H. Gade (2002). After postdoctoral stays with Prof. A. Pfaltz (University of Basel – 2003–2005) and Prof. E. N. Jacobsen (Harvard University – 2006–2007) he joined the University of Geneva to establish his independent research program. His interests include mechanistic and synthetic chemistry with particular emphasis on ´ment Mazet Cle asymmetric catalysis. He recently received the Zasshikai Lectureship Award from the University of Tokyo (2012) and the Werner Prize from the Swiss Chemical Society (2013).
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ChemComm
transition metal complexes as precatalysts, the structure of which is expected to closely resemble their active form in the catalytic cycle. Often a simple structural change in coordination geometry or ligand dissociation is necessary to funnel the precatalyst into the productive reaction pathway. During optimization studies, the necessity of systematically preparing each and every catalyst might be a rather time consuming task, but mechanistic information may be gleaned more readily by following this approach. Another distinct advantage of ex situ catalysis is that the initial (pre)catalyst loading is probably a more realistic reflection of the active catalyst concentration while with the in situ approach multiple inactive or competing species might be generated concomitantly with the truly active form of the catalyst. Herein, emphasis will be placed exclusively on isomerization reactions which employ a well-defined transition metal hydride precursor. This deliberately excludes a vast body of literature in which transient metal hydrides have been either invoked or identified as being responsible for an isomerization process (note, these species were typically generated by the activation of a non-hydride containing precatalyst).3–6 The isomerization reactions discussed herein have been classified according to the nature of the substrate rather than the nature of the catalyst and their chronological order of appearance in the literature has been followed. When appropriate the underlying mechanistic features of these transformations are presented.
Isomerization of olefins In transition metal-based homogeneous catalysis, olefinic substrates are traditionally divided into two categories. Functionalized olefins are substrates with an adjacent functional group (i.e. usually a heteroatom containing function) that can effectively coordinate to the metal and exert a direct influence on the outcome of the reaction, both in terms of reactivity and selectivity. These olefins are often referred to as 2-point binding substrates. By opposition, unfunctionalized olefins or 1-point binding olefins interact with the catalyst solely by p-coordination of the CQC bond. These are often considered as more challenging candidates as the benefits of directed-catalysis are lost. Between these two extremes there is of course a continuum of situations which depends (i) on the ability of the functional group to form a firm chelate with the metal center which may vary as a function of the distance between the coordinating function and the olefin, and (ii) on the personal appreciation one may have of a functional group.4b,7 Isomerization of unfunctionalized olefins In the early days of the transition metal-catalyzed hydroformylation of olefins (oxo synthesis), it was recognized that olefin isomerization plays a central role in the control of the final linear/branched ratio of aldehydes produced.8 Subsequently, several studies were focused on better understanding the overall reaction mechanism by specifically scrutinizing the isomerization of terminal olefins. The ubiquitous [HCo(CO)4] catalyst became the center of particular attention, and independent seminal contributions by Heck and
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Fig. 2 Typical [M–H] catalysts used in the 60s–70s for the isomerization of terminal olefins and still often employed today.
Breslow and the Orchin group provided important mechanistic information.9,10 The underlying features are that (i) CO inhibits the reaction presumably by competing binding with the incoming olefin, (ii) the dominant reaction pathway involves a 1,2-addition/ elimination of [HCo(CO)4] with preferential Markovnikov selectivity in the absence of a CO atmosphere. Interestingly, on the basis of labelling experiments a conflicting formal 1,3-H shift was postulated in the case of allylbenzene.11 Subsequent investigations by Wilkinson and co-workers showed that the extent of competing isomerization and hydrogenation could be substantially reduced using [HRh(CO)(PPh3)3], an observation which later led to the development of the Union Carbide hydroformylation process.12 In this context, the specific isomerization of cis and trans terminal olefins was carefully investigated and other catalysts such as [HRu(Cl)(PPh3)3] were also surveyed. As these studies were mostly based on kinetic analyses, limited structural information regarding the nature of the active form of the catalyst or the preferred reaction mechanism was collected. Importantly, it was found that isomerization and hydrogenation of terminal olefins occurred at similar rates (Fig. 2).13 Both from historical and mechanistic vantage points, the Pd-catalyzed isomerization of olefins has been the source of intense research and controversy.5a,14 Intriguingly, examples of well-defined palladium-hydride precursors employed in isomerization reactions are scarce (Fig. 3A). This certainly reflects the challenges associated with their preparation, purification, handling and characterization. Portnoy and Milstein reported that the hydride-bridged dinuclear complex 1 catalyzed the internal isomerization of cyclooctene (as evidenced by the in situ generation of the deuteride version of 1).15 Bunel and co-workers showed that the mononuclear complex 2 was
Fig. 3 Pd hydride catalyst for the isomerization of cyclic (A) and linear olefins (B). The catalyst loading was not specified and is given as x mol%.
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Fig. 4 Pd-catalyzed one-carbon isomerization of olefins.
competent in the slow isomerization of 1-pentene into 2-pentene at 80 1C, but unfortunately no information on the cis/trans ratio was provided (Fig. 3B). Added PCy3 (Cy = cyclohexyl) was found to have an inhibitory effect indicating that dissociation of phosphine occurs before the rate-determining step.16 Both Milstein and Bunel proposed an insertion/b-H elimination mechanism to account for these observations. More recently, Skrydstrup and Lindhardt reported the onecarbon isomerization of a wide variety of terminal alkenes into 2-alkenes with often high control of olefin geometry.17,18 Synthetically, this reaction is appealing as it potentially connects allyl- and vinyl-based chemistry, in particular, when the thermodynamically least stable products can be accessed.19 Some representative examples are disclosed in Fig. 4. Although initial experiments were conducted by in situ generation of the catalytically active [Pd–H] intermediate, supporting organometallic experiments and mechanistic investigations revealed that the known complex [HPd(Cl)(PtBu3)2] 3 could also be employed as a precatalyst. Activation by phosphine dissociation and coordination of the olefinic substrate were proposed as initial steps to enter into the productive catalytic manifold. Of note, isomerization of safrole was found to proceed with a catalyst loading as low as 0.01 mol% when the welldefined hydride 3 was used rather than the in situ protocol. Over the course of metathesis reactions employing the wellestablished Grubbs first and second generation ruthenium catalysts (4 and 5 respectively), isomerization of the olefinic substrates or the newly formed double bond in the products was initially considered as an undesired side-reaction (Fig. 5).20,21 Initial efforts were essentially directed towards inhibiting this secondary pathway. The potential of this isomerization in its own right was put forward by Nishida and co-workers, notably for the preparation of indoles.22 By adding 1–10 equiv. of trimethyl(vinyloxy)silane to 5, a discrete [Ru–H] was generated
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Fig. 7 Comparative study between [Ru–H] catalysts for the isomerization of estragole.
Fig. 5
[Ru] metathesis catalysts and hydrides derived thereof.
by degradation of the transient Fischer carbene complex initially formed, as later supported by mechanistic investigations.23 Subsequently, an impressive number of studies exploiting the ability of ruthenium metathesis catalysts to evolve in situ into isomerization catalysts have been developed.24 Among other examples, decomposition of ruthenium metathesis catalysts was accomplished by use of molecular hydrogen, protic solvents or inorganic bases.19,25 In most cases, the exact structure of the active isomerization catalyst was never investigated thoroughly, probably because the ruthenium hydrides 6, 7 and 8 prepared independently from 4, 5 or their methylidene analogues were recurrently invoked as plausible intermediates. In contrast, well-defined [Ru–H] complexes were employed only on rare occasions.13d,f,26 The potential of 7 as a general catalyst for the positional isomerization of 1,3-dienes into more substituted dienes was only established recently by Diver and co-workers (Fig. 6).27 The use of n-butanol was found to substantially increase the chemical yields, the substrate scope was relatively broad and the reaction highly stereoconvergent as the most thermodynamically stable (E,E)-1,3-dienes were systematically obtained. Because the
Fig. 6
[Ru–H]-catalyzed positional isomerization of 1,3-dienes.
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starting 1,3-dienes are themselves products of ene–yne metathesis, the authors developed an efficient one-pot tandem protocol using 5 as a single catalyst and various terminal alkynes and alkenyl alcohols as substrates. NMR investigations and spiking experiments secured that 7 is indeed generated by decomposition of 5 and responsible for the isomerization reaction. Further stoichiometric studies using 7-d1 supported the intermediacy of a [Ru–(p-allyl)] complex in the 1,3-diene isomerization. The use of nBuOH was proposed to irreversibly favor the final p-allyl formation/b-H elimination sequence. Importantly, Fogg and co-workers recently asserted that in 6, 7 and 8, the [Ru–H] typically assumed to account for isomerization of olefins during metathesis are not the truly catalytically competent species (Fig. 7).28 A systematic structure–activity analysis using estragole as a model substrate and several related [Ru–H] complexes suggests that the isomerization-active species may be generated by more extensive decomposition of the metathesis catalysts, potentially involving degradation of the strongly coordinated PCy3 or NHC ligands (NHC = N-heterocyclic carbene).25b,29 The authors also propose that a p-allyl-type mechanism rather than a hydride-type mechanism may be operating. These observations further advocate the notion that proper characterization of reactive intermediates and decomposed catalysts may have important implications in methodology development. In 2006, Goldman, Brookhart and co-workers reported a tandem catalytic system for the formal metathesis of alkanes utilizing a pincer iridium catalyst for the dehydrogenation of alkanes (step 1) and the hydrogenation of olefins (step 3) and a Mo catalyst for olefin metathesis (step 2; Schrock-type) (Fig. 8).30 In this system, parasitic isomerization of the transient terminal alkene into internal analogues is a major problem which affects the product selectivity of C2n2 products from Cn substrates. Using a combined experimental and theoretical approach, the groups of Goldman, Brookhart and Krogh-Jespersen specifically investigated the isomerization of terminal olefins using pincer iridium dihydride complexes 11 and 12.31 Unexpectedly, they clearly established that isomerization occurs via the intermediacy of a [(pincer)Ir(Z3-allyl)(H)] complex rather than by insertion of 11 or 12 across an incoming olefin. In situ observation of the catalyst resting state, labelling experiments, independent synthesis and evaluation of an iridium-Z3-allyl hydride for olefin isomerization were collective evidences in support of this proposal. DFT calculations further substantiated the empirical data and provided additional structural details on the counter-intuitive allyl-mechanism. Formation of the p-allyl hydride intermediate arises from dissociation of an olefin double bond from the
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Isomerization of functionalized olefins
Fig. 8 Mechanism for the Ir-catalyzed olefin isomerization in the context of alkane metathesis.
metal center, followed by allylic C–H activation and final recoordination of the double bond (I - II - III). Almost concomitantly, Chianese and co-workers reported a related study of the isomerization of terminal olefins using two Ir(III) hydride complexes supported by CCC pincer ligands which differ only in their coordination number.32 Whereas experimental evidence was in agreement with a p-allyl mechanism for the sterically less demanding catalyst, the operating mechanism of the bulkier adamantyl catalyst could not be elucidated unambiguously but was proposed to differ significantly. Collectively, the results obtained with the Brookhart–Goldman and Chianese [(pincer)Ir] catalysts clearly indicate that the use of well-defined transition metal hydrides for the isomerization of olefins does not imply the reaction will necessarily proceed via a hydride-type mechanism as one may expect at the outset. This further underscores the importance and necessity to conduct appropriate mechanistic investigations. Most of the catalytic systems described so far rely on the use of late transition metal hydride complexes. Examples of well-defined hydrides based on early transition metals are less abundant in the literature and the catalytic studies with either terminal or internal alkenes were conducted at the NMR scale.33 Nonetheless, there is obviously plenty of room for improvements and more efficient and practical isomerization catalysts built on early transition metal hydrides will certainly appear in the coming years.
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A. Allylic alcohols. As part of their studies on olefin hydroformylation and isomerization with [HCo(CO)4], Goetz and Orchin also evaluated the potential of this hydride complex in the isomerization of allylic alcohols.34 A measurable yield (21% yield) was obtained only for 2-propenol when 50 mol% of the cobalt complex were employed. Isotope labelling using the cobalt deuteride complex showed exclusive incorporation at C(3), which was rationalized to arise from a 1,3H-shift similar to that invoked for allylbenzene. A decade later, Sasson and Rempel showed that the air-sensitive [HRu(Cl)(PPh3)3] 10 was a highly active catalyst for the isomerization of linear and cyclic secondary allylic alcohols as well as homoallylic alcohols using neat substrates at 110 1C (Fig. 9A).35 The corresponding ketones were obtained in high isolated yields. Primary allylic alcohols and secondary propargylic alcohols were not isomerized under these conditions. Although the enantioselectivities were in the marginal range and were assessed by optical rotation measurements, the first asymmetric version of the isomerization of allylic alcohols was reported in 1974 by Botteghi and Giacomelli using [HRh(CO)(PPh3)3] as metal precursor in combination with ()-DIOP, one of the very few chiral ligands available at that time (Fig. 9B).36 Bergens and co-workers reported a rare example of a chiral monohydride ruthenium complex [HRu(R)-(BINAP)(THF)2(CH3CN)]BF4 13 which proved to be catalytically active in the hydrogenation and hydrosilylation of ketones and olefins.37 As shown in Fig. 9C, using 2 mol% of 13, a partial kinetic resolution was also achieved by preferential isomerization of one enantiomer of 3-buten-2-ol into the corresponding enol and ketone products. En route to Superambrox, a highly valuable fragrance chemical with a strong ambery odor, Fehr and Farris attempted hydrogenation of the bicyclic intermediate 14 using Crabtree’s catalyst [(Cy3P)(Py)Ir(cod)]PF6.38 An unexpected highly diastereoselective isomerization leading to lactol 15 was observed instead. Switching to Chaudret’s ruthenium hydride catalyst [HRu(Z2C8H11)2] 16 enabled substantial improvement of the method (0.5 mol%, room temperature) and the target compound was isolated in 76% yield over 2 steps after silane mediated reduction (Fig. 9D). The approach was later demonstrated to provide general access to diverse 1,2-annulated trans-tetrahydrofurans and the related trans-g-butyrolactones. Labelling and cross-over experiments were consistent with an intermolecular addition/ elimination mechanism.39 B. Allylic amines and amides. During the early development of the industrially-relevant isomerization of allylic amines, a cobalt hydride catalyst [HCo(N2)(PPh3)3] was first employed.40 Isomerization of N,N 0 -diethylgeranylamine and N,N 0 -diethylnerylamine into the corresponding enamine was highly efficient with 1 mol% of the catalyst (THF, 80 1C, 15 h) and only traces of the regioisomeric 1,3-dienamine were observed (Fig. 10A). A preliminary screening of chiral ligands was performed by in situ generation of the active cobalt hydride and, despite low yields, enantioselectivities of up to 33% were measured with (+)-DIOP. Further optimizations of this reaction
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Fig. 9
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Well-defined [Ru–H] and [Rh–H] in the isomerization of allylic alcohols.
led to the development of what remains one of the most formidable achievements in homogeneous enantioselective catalysis: the [Rh(BINAP)]-catalyzed isomerization of allylic amines.41 In 2001, Krompiec and co-workers described the highly (E)-selective Ru-catalyzed isomerization of N-allyl-N-arylamides into N-propenyl-N-arylamides (Fig. 10B).42 A hydride mechanism was postulated when [HRu(Cl)(PPh3)3] 10 was employed. Efforts were mostly directed toward the understanding of the selectivitydetermining interactions between the aryl ring and the metal center by combining Hammett correlations with computational studies. C. Allylic ethers. Binding of the oxygen atom of allylic ethers to the metal center during isomerization has been often
invoked but never truly demonstrated. Hence, these are ambiguous candidates and can be regarded equally as 1-point or 2-point binding olefinic substrates. Because of their structural similarity with allylic alcohols and amines, we have arbitrarily decided to discuss their isomerization in this section. Based on the initial studies at the stoichiometric level, a set of several [Pt–H] complexes was used by Clark and Kurosawa for the catalytic isomerization of allylmethylether and allylphenylether (Fig. 11A).43 With the usually very low loadings of catalyst, the reaction was highly cis-selective in benzene. Appearance of the trans isomer with prolonged reaction time and labelling experiments were in support of a reversible addition/elimination process. The authors also proposed coordination of the oxygen
Fig. 10 Isomerization of allylic amines (A) and allylic amides (B).
Fig. 11 Isomerization of allylic ethers (A) and silylated allylic alcohols (B).
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atom to account for the high cis-selectivity observed. The absence of stereospecificity in the isomerization of butene further reinforced this hypothesis. Inspired by the Sasson and Rempel studies on allylic alcohols, Suzuki, Moro-Oka and co-workers disclosed that [(H)2Ru(PPh3)4] performed the isomerization of silylated allylic alcohols under somewhat forcing conditions (150 1C, benzene or o-xylene).44 The desired silyl enol ethers were obtained in appreciable yields but with usually quite modest (E)/(Z) selectivity. Interestingly, both double bonds were isomerized and kept in conjugation for the 2,4-pentadienyl derivative whereas only the allylic double bond migrated for the geranyl and neryl substrates, leaving the remote prenyl moiety intact (Fig. 11B). D. Acetylenic derivatives. Following up on their previous work, Suzuki and Moro-Oka investigated the related isomerization of acetylenic silyl ethers to dienol silyl ethers catalyzed by [HRu(Cl)(PPh3)3(toluene)].45 After 5 h at 150 1C in benzene, the product was obtained quantitatively as a ca. 1/1 mixture of (Z,Z) and (Z,E) isomers. This selectivity was explained by initial formation of an allene intermediate followed by the addition/elimination of the [Ru–H] catalyst across the most accessible double bond and from the sterically least encumbered face of the diene (Fig. 12). A series of others dienol silyl ethers were obtained in appreciable yields using the same method, but the stereospecificity of the reaction was unfortunately not discussed. Terminal acetylenes as well as free hydroxyl group were not tolerated. As part of their studies on transition metal hydride catalysis, the Lu group reported the highly stereoselective isomerization of a,b-ynones to the geometrically pure conjugated (E,E)-dienones.46 Both [H5Ir(PiPr3)2] and [(H)2Ru(PPh3)4] proved competent catalysts for this transformation. With or without added phosphine, the reaction proceeded rapidly under mild conditions and was applied to a significantly diversified set of substrates. Importantly, the monohydride syn,syn-(Z4-dienone)-metal complex 17 was isolated from stoichiometric reactions between [H5Ir(PiPr3)2] and either the substrate 18 or product 19. Under similar reaction conditions, it was found to catalyze the isomerization of 18 to 19 with reactivity
Fig. 12
Ru-catalyzed isomerization of acetylenic silyl ethers.
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Fig. 13
Ir-catalyzed isomerization of ynones to conjugated dienones.
and selectivity comparable to that of the iridium pentahydride, thus lending credence to its potential role as a crucial reactive intermediate (Fig. 13). Shortly after, closely related studies on the Ir-catalyzed isomerization of propargylic secondary alcohols (mono and diols) were reported by the same group using again the iridium pentahydride catalyst.47 E. Deconjugation of a,b-unsaturated esters. During the course of their investigations on the Ru-catalyzed cyclization of enynes, Mori and co-workers observed that – quite remarkably – [HRu(Cl)(CO)(PPh3)3] 9 induced the thermodynamically disfavoured deconjugation of an a,b-unsaturated ester by migrating the carbon double bond away from the ester functionality.48 A specific study in this intriguing direction led them to investigate the scope of this reaction in more details using the same catalyst in refluxing toluene.49 Thus, a,b-unsaturated esters and amides with different terminal functional groups were deconjugated efficiently, independently of the chain length of the linker between both ends of the substrate (Fig. 14). The corresponding products were obtained in practical yields and E/Z selectivities varying from 1.0/2.2 to 4.6/1.0.50
Fig. 14
Ru-catalyzed deconjugation of a,b-unsaturated esters and amides.
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Tandem processes involving isomerization Although seemingly simple at first sight, the combination of two compatible catalysts in a single vessel for sequential operations is faced with numerous challenges and is currently the subject of intense research efforts.51 The use of well-defined [M–H] isomerization catalysts in several tandem processes is discussed in the present section. A. Isomerization–metathesis. As discussed above, the ability of ruthenium metathesis catalysts to decompose into isomerizationactive ruthenium hydride has often been exploited to conduct metathesis and isomerization reactions sequentially in a one-pot protocol. Adopting an opposite strategy, van Otterlo and co-workers, have accessed a series of benzo[1,4]dioxins in good yields using [HRu(Cl)(CO)(PPh3)3] 9 as isomerization catalyst and 5 as ringclosing metathesis catalyst.52 B. Isomerization/C–C bond formation. Sorimachi and Terada have developed a binary catalytic system composed of a [Ru–H] and a Brønsted acid for a three-step sequence resulting in an overall isomerization/C–C bond formation process.53 In the first event, [HRu(Cl)(CO)(PPh3)3] 9 isomerizes a N-protected-Nallylamine into an enamine intermediate, itself being directly isomerized to the corresponding imine in the presence of a modified biaryl phosphoric acid or Tf2NH. The same catalyst is then instrumental in promoting the final C–C bond forming event with the electron-rich arene or heteroarene coupling partner. A selection of the formal Friedel–Crafts products accessible by this method is displayed in Fig. 15. Control experiments showed that both catalysts were required for the reaction to
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proceed. Furthermore, much reduced yields were obtained when isolated enamides were employed, suggesting a synergistic effect between the organometallic and organic catalysts. A related enantioselective Pictet–Spengler cyclization was next developed using the same [Ru–H] isomerization catalyst in combination with chiral binaphthol-derived phosphoric acids serving as Brønsted acid catalysts.54 The corresponding tetrahydroisoquinoline derivatives were obtained in yields ranging from 50 to 87% and a 53% ee value was reported in the best case. A similar isomerization/cyclization strategy was followed by Nielsen and co-workers for the development of very general routes to access tetrahydro-b-carbolines and tetrahydropyrano[3,4]-indoles.55 After extensive screening, the ubiquitous [HRu(Cl)(CO)(PPh3)3] complex (9) was identified as the best isomerization catalyst and phosphoric acid the most efficient acid catalyst. Promising enantioselectivity levels were also obtained when chiral phosphoric acids were evaluated. C. Isomerization–hydroformylation–hydrogenation. Nozaki and co-workers recently described a tertiary catalytic system for the sequential one-pot isomerization–hydroformylation– hydrogenation of internal alkenes to the corresponding homologated n-alcohols (Fig. 16).56 In this system, the measure of the n/i ratio (i.e. linear vs. branched products) provides a direct indication of the efficiency of the isomerization step. Interestingly, their initial study focused on a binary system composed of a non-hydride Rh complex for the isomerization– hydroformylation sequence associated with Shvo’s catalyst 20 for the final hydrogenation. In control experiments, the concomitant use of the Rh and Ru catalysts led to higher n/i ratio in comparison to those obtained with the Rh catalyst alone. This observation indicates participation of the [Ru–H] complex in the isomerization and supports a synergistic effect between the two organometallic species. Similar cooperativity was also found in the hydrogenation step. The tandem process next evolved into a tertiary catalytic system by addition of [Ru3(CO)12] to further improve the efficiency of the isomerization step.
Miscellaneous isomerizations Our group recently reported the [Pd–H]-catalyzed selective isomerization of terminal epoxides into the corresponding
Fig. 15
Relay catalysis by a [Ru–H]/Brønsted acid binary system.
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Fig. 16 Cooperative catalysis in the sequential isomerization/hydroformylation/hydrogenation of internal alkenes to n-alcohols.
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Conclusions
Fig. 17 Pd-catalyzed isomerization of terminal epoxides. Mechanistic proposal and representative examples.
aldehydes (Fig. 17).57 Although deeply rooted in the literature, this rearrangement is still faced with multiple challenges.58 These include the discovery of a truly practical catalytic method with wide functional group tolerance. The subsequent development of an enantioselective version of this process would provide an elegant route to chiral aldehydes with a a-tertiary center. Careful examination of the species formed during screening of various ligands and palladium precursors led to the independent isolation of an unusual dinuclear palladium hydride complex [(dippp)3(Pd–H)2](OTf)2 21 (dippp = 1,3-bis-(di-iso-propylphosphino)propane) which proved particularly effective for the isomerization of a variety of terminal but also trisubstituted epoxides. Functional groups that would not be compatible with the traditional Lewis acid reagents employed for this isomerization were particularly well-tolerated by 21. Experimental and theoretical investigations unveiled an unusual hydride-type mechanism characterized by an epoxide opening/hydride transfer sequence. Consistent with this rationale, preliminary observations support the existence of two distinct enantio-determining steps for the kinetic resolution of terminal epoxides (I and II). An off-cycle racemization pathway involving the [Pd–H] catalyst was also identified (IV).59 Although the selectivity factors obtained were not practical (Smax = 4.4), the delineation of the reaction mechanism and the operational simplicity of the protocol hold promise for future developments.
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For a long time considered as an undesired process in hydrogenation and hydroformylation reactions, the isomerization of olefins has been initially studied with the primary aim of suppressing its detrimental impact on these industrially relevant transformations. This is particularly true for unfunctionalized olefins which usually serve as feedstock chemicals in industry. The situation changed in the late 1970s, and early 1980s with the success story of the Rh-catalyzed enantioselective isomerization of allylic amines – which eventually also led to the development of an industrial application. Nonetheless, the successful implementation of a transformation in industry cannot become a simple measure of its relevance. The knowledge and understanding of the underlying mechanistic aspects of a chemical transformation are also of prime importance. We have already discussed the potential benefits of carrying out ex situ rather than in situ catalysis. In the context of this Feature Article, this justified placing a specific focus on isomerizations performed with well-defined transition metal hydrides – catalysts for which a greater mechanistic predictability is expected a priori (i.e. via the hydride mechanism). The independent contributions of Goldman, Brookhart, Krogh-Jespersen and Chianese demonstrate that the situation is not necessarily that simple. Their clear observation of allyl-type mechanisms despite the use of well-defined metal hydride precatalysts is an interesting case study. From a synthetic point of view, it is tempting to conclude that most of the methods presented herein are not exactly practical and do not provide access to structurally relevant and diversified libraries of products. Though, a fair estimation of the synthetic potential of isomerization reactions in their whole depth and breadth would require including reactions with in situ generation of the active species. Unfortunately, this would go much beyond the scope of this short review. Nonetheless, the work of Skrydstrup and Lindhart shows that there is much to learn mechanistically and to benefit synthetically from a combined in situ/ex situ approach. Finally, the recent tandem processes by Terada and the [Pd–H]-catalyzed rearrangement of epoxides have just unveiled some of the immense synthetic potential that remains to be explored for isomerization reactions.
Acknowledgements The authors thank the University of Geneva and the Swiss National Foundation (Project PP00P2_133482) for financial support.
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