Helicene-like chiral auxiliaries in asymmetric catalysis.

Published on 06 August 2014. Downloaded by State University of New York at Binghamton on 29/09/2014 13:12:30.

Dalton Transactions View Article Online

PERSPECTIVE

Cite this: DOI: 10.1039/c4dt01935k

View Journal

Helicene-like chiral auxiliaries in asymmetric catalysis P. Aillard, A. Voituriez and A. Marinetti* This literature overview demonstrates that helically chiral ligands and organocatalysts have been largely neglected so far. However, a few recent studies on helical pyridine, the corresponding ammonium salts and N-oxides have highlighted the significant potential of these compounds as organocatalysts for

Received 26th June 2014, Accepted 4th August 2014 DOI: 10.1039/c4dt01935k www.rsc.org/dalton

1.

Michael type additions, aldehyde propargylations, epoxide openings, and others. In addition, helicenes displaying a fused phosphole ring at the end of their polyaromatic structures, have been used as ligands in enantioselective gold promoted cycloisomerization reactions, giving both excellent catalytic activity and high enantiomeric excesses. These recent results are expected to stimulate further research on the catalytic applications of helically chiral auxiliaries in the next few years.

Introduction

This review focuses on the applications of chiral helicene-like compounds in both organo- and organometallic catalysis. The term ‘helicene’ refers to ortho-fused polyaromatic compounds which adopt a helical configuration, due to geometrical constraints and overlapping of the terminal rings. In a broader sense, the term ‘helicene’, or more appropriately ‘helicene-like’, refers to helical derivatives which include both aromatic or heteroaromatic and partially saturated rings in their π-conjugated scaffolds. These compounds display extremely interesting optoelectronic properties and applications in a variety of fields, as exhaustively reviewed in recent articles.1–5 With special focus on catalysis, this account will emphasize synthetic methods for obtaining enantiomerically pure chiral auxiliaries, i.e. alcohols, nitrogen and phosphorus derivatives, as well as their uses in catalytic processes.6 The field being rather underdeveloped to date, this review is intended to underline the potential of helical chirality in chiral induction processes so as to stimulate more extensive investigations. We will especially highlight phosphorus containing helicenes by showing methods for the synthesis of racemic or enantioenriched compounds, resolution procedures, transition metal complexes and uses in (enantioselective) organometallic catalysis. To start with, it is worth mentioning that the high efficiency of helical compounds in stereocontrol phenomena related to chiral induction, has been demonstrated by Martin

Institut de Chimie des Substances Naturelles, CNRS UPR 2301 1, av. de la Terrasse, 91198 Gif-sur-Yvette, France. E-mail: [email protected]

This journal is © The Royal Society of Chemistry 2014

et al.7–9 These authors demonstrated for instance that the racemic 2-hydroxy-[7]helicene derived pivalate 1 undergoes addition of Grignard reagents with total diastereoselectivity (Scheme 1a).10 They also showed that the enantioenriched 2-cyano[7]helicene 3 mediates the oxidation of E-stilbene into the corresponding epoxide in >99% ee (Scheme 1b).11 These initial studies on stoichiometric reactions highlighted the undoubtedly strong potential of helical auxiliaries in chiral induction processes. Further studies validated then the concept through catalytic experiments.

Scheme 1

Chiral helicenes as stoichiometric auxiliaries and reagents.

Dalton Trans.

View Article Online

Perspective

Dalton Transactions


2. Non-functionalized helicenes as chiral inducers We would like to open this review by mentioning the rather non-conventional use of [5]- and [6]helicenes reported by Soai et al. in 2001.12,13 These helical hydrocarbons, devoid of any functional group, have been used as chiral inducers in a chiral amplification process related to the addition of diisopropylzinc to the pyrimidine-5-carbaldehyde in Scheme 2. It is known that, in this reaction, the final product itself is able to catalyse the addition process. The final alcohol is therefore responsible for the chiral amplification by which the enantiomeric excess of the final product increases over the time up to over 90% ee, irrespective of the optical purity of the catalyst used. Soai demonstrated that [5]- and [6]helicenes are able to initiate this reaction giving the final alcohol in up to 95% ee. The most relevant point here, in the context of this review, is that these helicenic hydrocarbons, devoid of any heteroatom, may act as efficient chiral inducers via simple coordination with the carbonyl moiety and the pyrimidine ring of the substrate. The charge-transfer complex has been evidenced by 1H NMR studies.

Scheme 3

Scheme 2 Enantioselective organo-zinc addition initiated by chiral helical hydrocarbons.

3. Catalytic uses of helicenic alcohols The first example of catalytic uses of helicenes with hydroxyl functions has been reported by Katz in 2000.14 Inspired by the success of BINOL, VAPOL and analogous biaryl derived diols as chiral auxiliaries, Katz designed and investigated the new chiral diol 7, called [5]HELOL, which displays two [5]helicene units connected through their 2-positions, so as to give a biaryl-type structure (Scheme 3). The [5]helicene units of 7 have been obtained via a Diels– Alder approach in which the silyl enol ether of 2-acetylphenanthrene, 4, reacts with benzoquinone. Resolution of the helicenic alcohol has been performed through separation of the corresponding diastereomeric esters of (1S)-camphanic acid, 6. After removal of the camphanoyl group, the enantioenriched

Dalton Trans.

Synthesis of [5]HELOL.

alcohols (>98% ee) racemize appreciably at room temperature, nevertheless the stereochemical integrity could be retained when the alcohols were engaged immediately, at 0 °C, in the next steps of the reaction sequence in Scheme 3. Thus, both the Ag2O mediated oxidative coupling of (P)-6 and the subsequent Zn dust mediated reduction have been performed at 0 °C, leading to the desired (P,P)-[5]HELOL, 7, with less than 2% of the meso-isomer. The intermediate 1-OH-[5]helicene 6′ (X* = H) displays a moderate, but nonetheless increased configurational stability with respect to that of simple, non-substituted [5]-helicenes.15,16 This is due to the presence of the hydroxyl function in position 1, whose steric hindrance increases the inversion barrier. Steric effects account as well for the higher configurational stability of the dimeric species [5]HELOL: monitoring of [5]HELOL by circular dichroism suggests that this compound does not racemize significantly after six days at room temperature in degassed acetonitrile. The configurational stability of [5]HELOL, 7, being established, the diol has been evaluated as chiral auxiliary in the addition of diethylzinc to aryl aldehydes (Scheme 4). The diolchelated zinc catalyst was formed in situ. At a 50% catalyst loading, it displayed good catalytic activity and afforded the expected alcohols in moderate to good enantiomeric excess, depending on the substrate. The highest, 81% ee was attained starting from benzaldehyde (R = H), while 4-methoxybenzaldehyde and 4-chlorobenzaldehyde gave 46% and 69% ee,


View Article Online

Dalton Transactions

Perspective


4.1

Scheme 4

HELOL promoted diethylzinc addition to aldehydes.

respectively. The enantioselectivity levels were higher than those obtained with BINOL under the same reaction conditions (34% ee in the addition of Et2Zn to benzaldehyde), showing that the helical structure amplifies the stereoselectivity in these reactions, to some extent at least. The (P,P)configured [5]HELOL, 7, afforded (S)-configured alcohols. As far as we know, reactions in Scheme 4 remain the only catalytic application of helical alcohols reported to date.

4. Catalytic uses of helicenes with nitrogen functions Helicenes with nitrogen functions have been used so far as either Lewis bases or H-bonding auxiliaries in organocatalytic processes. The most common catalysts include pyridine, pyridine oxide and pyridinium salts derivatives. Since the field has been reviewed recently,17,18 this account will summarize briefly the main achievements only and will update the more recent developments. Azahelicenes with pyridine subunits have been prepared via either oxidative photocyclization of diaryl olefins or, alternatively, via metal promoted reactions including Stille–Kelly couplings (Scheme 5a)19 and alkyne cyclotrimerizations (Scheme 5b).20 Two of these catalytic methods are typified in Scheme 5. Optically pure azahelicenes are obtained then by fractional crystallization of their dibenzoyltartaric acid salts20 or by preparative chiral HPLC separations.20,21

Scheme 5

Synthesis of azahelicenes.


Amines as Lewis bases

Pyridine-based azahelicenes have been investigated as nucleophilic organocatalysts with the aim of highlighting new chiral DMAP equivalents (DMAP = 4-dimethylaminopyridine). Azahelicenes have been screened mainly in enantioselective acylation reactions. Thus, pioneering studies of the Starý’s group investigated the use of simple, non-functionalized 1-aza- and 2-azahelicenes for the kinetic resolution of racemic phenylethanol (Scheme 6).22 1-Azahelicenes proved inactive, probably due to the high steric hindrance of the nitrogen atom, while the 2-azahelicene (M)-12 was able to promote acylation reactions quite efficiently. In initial experiments a 53.1% ee was obtained for the starting alcohol, at a 46.9% conversion rate. In spite of the moderate selectivity factors (S = 7), these experiments demonstrated that amines with helical chirality might complement DMAP analogues displaying axial,23 central24 or planar chirality,25 as a new design concept. Later, Carbery introduced an improved catalyst design in which the helix entails a 4-aminopyridine unit, closely mimicking the DMAP structure, as typified by (P)-13 in Scheme 6.21,26 The use of (P)-13 in the acylation of 1-phenylethanol resulted in an increased catalytic activity as well as in an improved selectivity factor (S = 17). The azahelicene 13 could be used at a low, 0.5 mol% loading. The scope of the reaction could be extended to other aryl– alkylcarbynols, showing that substrates with ortho-substituted aryl groups, as well as naphthyl and anthracenyl substituted alcohols display enhanced selectivity (Table 1). In the acylation of the 1-naphthyl-substituted alcohol, at a 5 mmol scale, the catalyst loading could be reduced to 0.05 mol%, that is one of the lowest loading ever reported for reactions promoted by chiral DMAP analogues. The reaction proceeded with the same selectivity. Thus, at a 60% conversion rate, the chiral alcohol was isolated in >99% ee (S = 34).

Scheme 6 Enantioselective acylation of 1-phenylethanol under kinetic resolution conditions.

Dalton Trans.

View Article Online

Perspective

Dalton Transactions


Table 1 Selectivity factors for the kinetic resolution of alcohols promoted by the azahelicene (P)-13

Scheme 8

4.2

Pyridinium salts as H-bonding catalysts

The catalytic uses of helical pyridinium salts mainly concern helicenes terminated by 2-aminopyridinium functions, introduced by Takenaka. These helicenes efficiently activate nitroolefins and enable various catalytic processes via dual H-bonding interactions between the NO2 group and the nitrogen functions of the helical auxiliary. On the other hand, the helical scaffold operates a highly effective chiral induction. Two different reactions have been reported, namely the 1,4addition of 4,7-dihydroindoles to the olefin (Scheme 7) and Diels–Alder type cyclizations on nitroalkenes (Scheme 8). In the 1,4-addition of 4,7-dihydroindoles to β-nitrostyrene (R2 = Ph),27 the best catalyst 14a, shown in Scheme 7, displays a 1-adamantylamino substituent. Single H-bond donors, i.e. azahelicenes devoid of the 2-amino group, were non-selective, while the enantioselectivity gradually increased going from NH2 to 2-adamantyl-NH, t-BuNH and 1-adamantyl-NH substituted azahelicenes. The steric hindrance of the 1-adamantyl group is therefore crucial for high enantioselection. The scope of the

Scheme 7

Dalton Trans.

1,4-addition of 4,7-dihydroindoles to nitroolefins.

Enantioselective Diels–Alder reactions on nitroethylene.

reaction has been extended to 16 substrates with different substitution patterns, giving good yields and >80% ee. Helicenes with 2-aminopyridinium functions have been shown to catalyze Diels–Alder type cycloadditions between cyclopentadienes and nitroethylene (Scheme 8).28 These reactions afforded selectively the endo isomer of the final norbornene, the enantioselectivity levels were however only moderate, even with the best catalyst 14b (30–40% ee). Thus, although these results demonstrate the ability of hydrogenbond donors to promote enantioselective Diels–Alder reactions on nitroalkenes, further optimization of the catalyst is needed to attain synthetically useful enantioselectivity levels.

4.3

Helical pyridine N-oxides as Lewis base catalysts

Chiral pyridine N-oxides are known to be privileged catalysts for reactions involving silicon reagents, since they generate pentavalent silicon adducts with strong Lewis acid character.29 As shown by Takenaka and co-workers, this is the case also for azahelicene N-oxides. These compounds, easily obtained by oxidation of the corresponding azahelicenes with m-chloroperbenzoic acid, are effective catalysts for the desymmetrization of meso-epoxides via ring opening with SiCl4 (Scheme 9).19 In these reactions, the benzo-fused azahelicene derivative (P)15 performs better than the analogous, non-substituted [6]azahelicene oxide in terms of enantioselectivity. It gives up to

Scheme 9

Enantioselective ring opening of meso-epoxides by SiCl4.


View Article Online


Dalton Transactions

Perspective

chlorine atoms. This stereochemical model contrasts with previous proposals30 which postulated a different ligand distribution on silicon and a role of π-stacking interactions in the orientation of the substrate. According to calculation results, such π-stacking interactions seem to have indeed a negligible effect. Overall, the recent studies above have afforded the proof-ofconcept for the efficiency of helical pyridinium salts and pyridine N-oxides as chiral H-bonding donors and as Lewis bases for the activation of silicon reagents, respectively. The scope of these catalysts is expected to expand in the future through more extensive screening, combined with the fine modulation of the helical scaffolds and substitution patterns.

Scheme 10

Propargylation of aldehydes with allenyltrichlorosilane.

94% ee in the ring opening/chlorination of stilbene oxide and other aryl-substituted epoxides (5 examples with 87–94% ee). The same catalyst, (P)-15, has been evaluated in the additions of allenyltrichlorosilane to aromatic aldehydes (Scheme 10). The enantiomeric excesses were promising (68% ee), but better results were obtained then by using the new 2-pyridyl-substituted azahelicene (P)-16 as the catalyst.30 Aromatic aldehydes were converted into propargyl alcohols in high yields and 74–96% ee (17 examples). The scope of the reaction seems to be restricted to aromatic substrates. However, the analogous propargylation of benzaldehyde acylhydrazone proceeded with good enantioselectivity also (78% ee). A stereochemical model which correctly predicts the sense of chiral induction in these reactions has been afforded by Wheeler through DFT studies.31 Calculations at the B97-D/TZV (2d,2p) level have been used to define both the preferred arrangement of ligands around the hexacoordinate silicon center and the substrate positioning in the transition state. One of the postulated key intermediate is depicted in Scheme 11. According to the model, the chlorine atoms on silicon adopt a cis axial/equatorial arrangement and the allyl group (Nu) occupies the second axial position, cis to both the N(O) function and the aldehyde. Calculations suggest that the geometry of the transition state should be determined mainly by electrostatic interactions between the carbonyl group and the

Scheme 11 Stereochemical outcome and a key intermediate for the enantioselective propargylation of aldehydes.


5. Catalytic uses of helicenes with phosphorus functions Whilst axial, central and planar chirality have been largely exploited to build chiral phosphorus ligands and organocatalysts, helical chirality has been rather neglected so far in this field. Several carbohelicenes and heterohelicenes with appended phosphorus functions have been prepared in either racemic or enantiomerically pure form, but catalytic screenings are limited mainly to a few benchmark reactions, such as olefin hydrogenations or palladium promoted allylic substitutions. In this chapter we will exhaustively review these catalytic studies, together with the most relevant synthetic strategies to access phosphorus ligands. Finally, we will summarize recent studies on the synthesis and uses of phosphahelicenes including studies from our group in gold catalysis. 5.1

Helicenes with appended phosphorus functions

5.1.1 Synthesis. The largest class of helicenes with appended phosphorus functions is made of helical derivatives displaying P(X)Ph2 substituents (X = O, BH3, lone pair). The most common access to these helicenes involves introduction of the phosphorus function from a suitably functionalized helical precursor, at a late step of the synthetic sequence. Thus, one of the first known methods is the reaction of bromohelicenes with ClPPh2 or ClP(O)Ph2 via halide-lithium exchange, typified in Scheme 12a,b.32,33 This method has been applied to the synthesis of a variety of mono- and diphosphine derivatives displaying carbohelicene (e.g. 17 and 18),34,35 as well as heterohelicene scaffolds.36 Starting from helicenes terminated by thiophene units, such as 19 (Scheme 12c), the direct lithiation of the thiophene rings at their 2-positions allowed introduction of PR2 groups without previous functionalization of the helical scaffold.37,38 By this method, Ph2P, Cy2P, (3,5-Me2-C6H3)2P and P(O)(OEt)2 functions were introduced on the same thiahelicene scaffold. Optically pure phosphines have been obtained by either resolution of the starting thia[7]helicene 19 by HPLC (Chiralpak IA column)37 or by HPLC separation of the corresponding phosphine oxides.38

Dalton Trans.

View Article Online


Perspective

Dalton Transactions

Scheme 14

Scheme 12 Synthetic approaches to helicenes with appended phosphorus functions.

Alternatively, phosphorus functions can be introduced on helical scaffolds by palladium promoted couplings between Ph2PH39 or Ph2P(O)H40 and helicenes with suitable leaving groups, as shown in Scheme 13. Both helical bromides and triflates are suitable substrates for these catalytic reactions.

Scheme 13 Synthesis of helical phosphines and phosphine oxides through palladium promoted couplings.

Dalton Trans.

Synthesis of a phosphole-substituted azahelicene.

Helicenes displaying phosphole substituents, e.g. 24 in Scheme 14, represent a different class of phosphorus containing helicenes. They have been prepared according to an original strategy developed by Crassous and Réau, typified in Scheme 14.41,42 The phosphole unit has been formed from an aza[6]helicene precursor 23 displaying a 1,7-diyne function. The key reaction is the Fagan–Nugent cyclization, that is formation of a zirconocene intermediate via oxidative coupling of the alkyne functions, followed by zirconium/phosphorus exchange. The starting material 23 could be resolved into enantiomers by chiral HPLC on a Chiralcel OD-H column, and converted then into the enantiomerically pure phospholecontaining helicene 24. Compounds of this class have been especially targeted mainly because of their peculiar chiroptical properties that should be easily tunable by coordination of the phosphorus function to transition metals. The reported uses of these helicenes are outside the scope of this review. Finally, a totally different approach to helical phosphines involves building of the helical scaffold from non-helical precursors which bear the phosphorus function. Thus, for instance, the oxidative photocyclization of the dienic phosphine oxide 25 has afforded the 2-diphenylphosphinoyl[7]helicene 26 in 57% yield (Scheme 15).43 The starting material 25 has been prepared easily by two subsequent Heck-type reactions of 3,6-dibromophenanthrene with styrene and ( p-styryl) diphenyl-phosphine oxide respectively. Resolution of the trivalent helical phosphine has been carried out through formation of diastereomeric palladium complexes.

Scheme 15 Oxidative photocyclization of a diphenylphosphinoyl substituted substrate.


View Article Online


Dalton Transactions

From this literature overview it results that only a few structural variations have been made so far on phosphane containing helicenes: (i) beside PPh2 only PCy2 and a few PAr2 functions have been considered for catalytic purposes; (ii) with the only exceptions of compounds shown in Schemes 13a and 14, the phosphorus functions are located at the 2-position of the helical scaffolds; (iii) concerning the helical scaffold itself, the known phosphines are based either on [5]-, [6]- and [7]carbohelicenes, with a small range of functional groups, or on non-functionalized thiahelicene backbones. It is easily understood that more extensive investigations and structural variations are needed to fully establish the potential and usefulness of these compounds in both catalysis and other fields. In addition to phosphane derivatives, the literature reports a few examples of helical phosphites. These compounds are available from helical alcohols44 or diols45,46 and chloro- or dichlorophosphites, under classical conditions, as shown in Scheme 16. The helical phosphites 28 and 31 have been obtained in enantiomerically pure form from enantiopure precursors and evaluated then as chiral ligands in catalysis (see below). Thus, for instance, the bis-[4]helicene-derived diol 27, which displays both helical and axial chirality, was obtained in

Perspective

enantiomerically pure form by separation of its camphanic esters by column chromatography and was converted then into phosphite 28.47 It must be noticed that the [4]helicene unit of 27 displays an only moderate configurational stability, since it epimerizes in refluxing toluene. The helical alcohol 30b (R = H) in Scheme 16b has been prepared in optically pure form by cyclotrimerization of the optically pure triyne 29 which displays a stereogenic carbon centre. The reaction promoted by CpCo(CO)2/PPh3 gave a 87 : 13 ratio of diastereomers when performed at 140 °C, under thermodynamic control,48 while an equimolar ratio of diastereomers was obtained under kinetic conditions, at room temperature. Although the helical scaffold of the alcohol 30b can undergo thermal epimerization, the epimerization barrier is high enough (27.7 kcal mol−1) to prevent inversion at room temperature. 5.1.2 Screening in palladium promoted allylic alkylations. One of the first uses of helical phosphines as ligands in enantioselective catalysis has been reported by Reetz et al. in 2000.49 Palladium complexes of the [6]helicene derived diphosphine 17b, called PHelix, were formed in situ from [(η3-C3H5)PdCl]2, with a phosphine-to-palladium ratio of 2 : 1. According to 31P NMR analysis, the mixture contains several species, which founded the assumption that the diphosphine does not behave as a chelating ligand. Nevertheless, when this catalytic system was used in the allylic substitution between 1,3-diphenylpropenyl acetate and dimethyl malonate, total conversion was achieved in 4 h at room temperature and the final product was obtained in 81% ee (Scheme 17).

Scheme 17 Enantioselective allylic substitution promoted by palladium(II)PHelix complexes.

Scheme 16

Synthesis of helical phosphites.


In these reactions, authors also noticed a kinetic resolution process which operates in the first step, i.e. during formation of the intermediate palladium(II)-allyl complex from the racemic allylic acetate. Thus, the enantiomeric excess of the substrate was found to increase during the catalytic reaction, reaching a >97% ee at about 60% conversion. A similar behavior was observed in the analogous allylic substitution on 1,3diphenylallyl benzoate. 5.1.3 Screening in rhodium promoted hydrogenations and hydroformylations. Several helicene-like molecules with appended phosphorus functions have been evaluated in olefin hydrogenations as benchmark reactions (Scheme 18). The first

Dalton Trans.

View Article Online


Perspective

Dalton Transactions

Furthermore, matching of the helical and axial chirality seems to be the crucial issue here, while the configuration of the menthyl group plays a minor role in the stereochemical control. Similar studies have been conducted also on analogous 1-phenylethanol derived phosphites (R* = PhCH(Me)), but the hydrogenation reaction attained a maximum enantiomeric excess of only 77%. Overall, the rhodium catalysts based on these helical phosphites perform better than helical diphosphine-rhodium complexes in terms of enantioselectivity. They require however harsh reaction conditions (90 bar), which is likely due to the steric hindrance of the ligand and make these catalysts noncompetitive with other known systems. The hydroformylation of styrene was investigated by Starý, Stará and Eilbracht44 by using four helical phosphites, 31, in combination with Rh(acac)(CO)2 (Scheme 19). These ligands display partially saturated helical backbones and cyclic phosphite functions connected to the most hindered C1-position of the helicene. Although the catalytic activity and the branchedto-linear ratios of hydroformylation products were satisfying, the enantiomeric excesses were low, ranging from 0 to 29% ee. The highest ee value was obtained with a seven-member cyclic phosphite unit derived from (1,1′-biphenyl)-2,2′-diol.

Scheme 18 Enantioselective hydrogenation of dimethyl itaconate with rhodium complexes of helical phosphines and phosphites.

example was reported by Reetz in 1997,34 showing that the [6]helicene based diphosphine (R)-17b, combined with a stoichiometric amount of (COD)2RhBF4, promotes the hydrogenation of methyl itaconate in 39% ee. The optically active diphosphine 17b (ee > 98%) was obtained by HPLC separation of the enantiomers on a Chiralcel column. An additional example of hydrogenation of the same substrate by a rhodium-diphosphine complex was described then by Licandro.37 The catalyst is a preformed cationic rhodium complex of the optically pure thiahelicene-derived diphosphine (P)-32 (Scheme 18b). In this case also, a disappointingly low enantiomeric excess was obtained (31% ee). The same rhodium catalyst afforded a moderate 40% ee in the analogous hydrogenation of 2-acetamidoacrylate. Further studies from Yamaguchi on the same reaction were undoubtedly more successful.45 Rhodium complexes were formed in situ from the monodentate phosphites 28 which display both the helical chirality of the [4]helicene unit, the axial chirality of the binaphthyl unit and the central chirality of the menthol derived phosphorus substituent (Scheme 18c). Four distinct diastereomers of these ligands were screened, showing that phosphite 28 with (M,M,S,l) relative configurations of the stereogenic elements gives the highest ee (96% ee).

Dalton Trans.

Scheme 19

Rhodium promoted hydroformylation of styrene.

The same levels of enantioselectivity, with a maximum 20–25% ee, were obtained with the same catalysts in the hydroformylation of both a substituted styrene, 4-Cl–C6H4– CHvCH2, and vinyl acetate, MeCO2–CHvCH2. 5.1.4 Screening in iridium promoted allylic aminations. So far, one of the most successful applications of helicenes with appended phosphorus functions is the iridium-catalyzed amination of allylic carbonates. Starý, Stará and Eilbracht44 have screened the four phosphites 31 (Scheme 19) in the amination of cinnamyl carbonate and ( pyridinyl)allyl carbonate with primary and secondary amines (benzylamine, pyrrolidine, morpholine and piperidine) as shown in Scheme 20. Among these phosphites, compound 31b, which displays a tetramethyl-1,3,2-dioxaphospholane unit, afforded the best results.


View Article Online


Dalton Transactions

Scheme 20

Perspective

Enantioselective iridium promoted allylic aminations.

The catalyst generated in situ from [Ir(COD)Cl]2 and two equivalents of phosphite 31b converted the allylic carbonates in Scheme 20 into allylic amines in good yields, with branched to linear ratios higher than 95 : 5. Dichloromethane was found to be superior to THF in terms of conversion rate and the enantiomeric excesses, in this solvent, were of between 90% and 94%. The other phosphites 31 tested so far in the same reactions (see Scheme 19) gave overall lower ees, with rather unpredictable results. For instance with the 1,3,2-dioxaphospholanederived phosphite 31a, the ees varied from 0% to 79%, depending on the substrate. The very bulky ligand 31d proved inactive. The allylic aminations above unambiguously demonstrate, for the first time, the high potential of helically chiral phosphorus ligands in asymmetric catalysis. Surprisingly, in spite of these highly encouraging results, the use of phosphites 31 has not been extended so far to any other catalytic process. 5.1.5 Screening of helical phosphine oxides in asymmetric organocatalysis. As far as we know, a single literature report deals with the use of helical phosphorus derivatives as organocatalysts. The helical phosphine oxides 33, obtained in enantiomerically pure form by HPLC resolution, have been tested as Lewis-base catalysts in three reactions involving activation of silicon reagents: an aldol reaction, an imine reduction and a reductive aldol reaction on chalcone.38 As shown in Scheme 21, the phosphine oxides displayed moderate catalytic activity and uniformly low enantiomeric excesses in these reactions.


Scheme 21 Screening of helical phosphine oxides as Lewis base organocatalysts.

These preliminary results must be considered as starting points only, which validate the possible use of helical phosphorus derivatives as organocatalysts.

5.2

Phosphahelicenes

An alternative design for phosphorus containing helicenes involves the use of phosphorus heterocycles as part of the helical scaffold itself. The dinaphthophosphole 34 described independently by Gladiali and Wild in 1993 (Scheme 22),50–53 might be considered as the first representative of this class of compounds, as far as it displays a [5]helicene like structure. However, its helical conformation is configurationally stable only below ambient temperature.

Scheme 22

Dinaphthophosphole.

Dalton Trans.

View Article Online


Perspective

Dalton Transactions

After these initial and rather sketchy studies on dinaphthophosphole, the field of phosphahelicenes has been neglected for more than 15 years. Recently, however, it has experienced significant developments that are overviewed in the next paragraphs of this review. 5.2.1 Synthesis. Recent studies on phosphahelicene like molecules have been motivated more specifically by the physicochemical properties of these compounds and their potential uses as semiconducting materials. In the first series of experiments, phospha[7]helicenes have been prepared by Tanaka et al.54 via the rhodium promoted [2 + 2 + 2] cyclization of polyynes which is typified in Scheme 23a by the synthesis of the phosphine oxide 36. The cyclization reaction builds two of the aromatic rings and the fused phosphole ring in a single step.

The key substrates are the dialkynyl phosphine oxides 34 and the suitably designed tetraynes 35 and 37. Starting from 35, in the presence of rhodium/(R)-tol-BINAP, the phosphaoxahelicenes 36 were obtained in enantioenriched form, with up to 73% ee. The method has been applied to the synthesis of nine different oxa-phosphahelicenes displaying either seven or nine fused rings in their helical structures. Similarly, the biaryl-derived tetraynes 37, combined with diynes 34, produced the benzo-fused phospha[7]helicenes 38 in up to 75% ee, when using a rhodium/(S)-Segphos complex as the chiral catalyst (Scheme 23b).55 Subsequently, Nozaki reported a synthetic approach to phosphahelicenes based on the intramolecular palladium promoted coupling of a secondary phosphine with an aryl triflate unit (Scheme 24).56 The catalytic reaction generates the phosphole ring through formation of a P–C bond. Under Pd(OAc)2/DPPB catalysis (DPPB = 1,4-bis(diphenylphosphino)butane), the reaction afforded the desired racemic phosphahelicene 39 in 34% yield, after oxidation of the trivalent phosphine with H2O2.

Scheme 24 Synthesis of phosphahelicene 39 via an intramolecular palladium promoted coupling.

Shortly after the initial studies above, our group has disclosed a different synthetic approach in which the phosphole ring is introduced in the initial steps of the synthetic sequence, instead of being formed in the final stages. The other main divergence from previous studies is our targeting of phosphahelicenes in which the phosphorus center is located at the end of the helical sequence, as sketched in Scheme 25. This new design was motivated by the specific aim of using phosphahelicenes as chiral auxiliaries in asymmetric catalysis. Indeed, in these compounds, overlapping of the aro-

Scheme 23 Enantioselective synthesis of phosphahelicenes rhodium promoted [2 + 2 + 2] cyclizations of polyynes.

Dalton Trans.

by

Scheme 25 Simplified phosphahelicenes.

outline

of

the

two

known

series

of


View Article Online


Dalton Transactions

matic rings at the end of the helical sequence will create a three-dimensional, highly asymmetric environment around phosphorus. Such asymmetric environment was anticipated to be a key feature for chiral induction. Phosphine oxides of this class have been prepared by two methods. The first method takes advantage of the well-known photochemical oxidative cyclization of diarylolefins,57–59 which has been adapted to olefinic substrates displaying phosphindole units.60–62 For instance, in the reaction shown in Scheme 26, the starting material, 41, combines a 1H-phosphindole unit and a benzo[c]phenanthrene fragment. The unsymmetric nature of the aromatic groups of 41 means that the photochemical cyclization might afford four different isomers, depending on the carbon atoms that will be involved in the ring-closure. As shown in Scheme 26, either one or two out of four possible isomers have been isolated from the reaction mixtures as the major products. They are the [6]helicenes 42, displaying meta-fused phospholes as the terminal rings, and the [7]helicenes 43 with ortho-fused phospholes. [Note: The [6]helicene derivatives 42 are called ‘HelPHOS’].

Perspective

at phosphorus, have been separated by chromatography. Then, the synthetic sequence has been carried out on these enantiomerically pure substrates, so as to end up easily with enantiomerically pure phosphahelicenes. The synthetic strategy in Scheme 26 has been applied to the synthesis of a variety of phosphahelicenes, including helicenes with benzo-fused phosphole rings, such as 44 and 45,61 and the phospha-thiahelicene 46,63 called ‘HelPHOS-S’ (Scheme 27a). The synthesis of the phosphathia[7]helicene 46 performs particularly well, since the final photochemical step gives good yield (66% total yield including the [7]thiahelicene 46 and the corresponding [8]helicene displaying an ortho-fused phosphole ring) and total diastereoselectivity. It gives an isomer ratio of about 8 : 2 in favor of the desired [7]helicene derivative 46.

Scheme 27 (a) Examples of phosphahelicenes prepared by the photochemical method. (b) X-ray crystal structure of (SP,M)-42e.

Scheme 26

Photochemical synthesis of phosphahelicenes.

A key feature of this photocyclization reaction is its high diastereoselectivity. Indeed, in all the phosphahelicene oxides 42 and 43 isolated so far, the phosphorus substituent is oriented toward the helical scaffold and the oxygen atom on the opposite face. In other words, in these photocyclization reactions, the configuration of the phosphorus center in the olefinic substrate 41 dictates the sense of helical chirality in the final product. Among others, this photochemical method has been applied to the synthesis of enantiomerically pure phosphahelicenes by taking advantage of an L-menthyl substituent on phosphorus as the chiral auxiliary. The two epimers of the starting phosphindole oxide 40, with opposite configurations


The solid state structures of most of the phosphahelicene oxides prepared in these studies have been ascertained by X-ray diffraction, as illustrated in Scheme 27 for compound (SP,M)-42e. The second synthetic approach to phosphahelicenes with meta-fused terminal phosphole units is based on the transition metal promoted [2 + 2 + 2] cyclization reaction shown in Scheme 28,64 largely inspired by the work of Stará et al.40,65,66 In this case, the phosphindole building blocks have been functionalized with alkyne containing chains so as to build the desired cyclotrimerization precursors 47. The cyclotrimerization reaction has been carried out then through a very efficient procedure: with 20 mol% Ni(cod)2/PPh3 as the catalyst, the reaction was completed after 15 minutes at room temperature, giving the oxa[6]helicenes 48 in 68–81% yields. The reaction has been carried out on both racemic (R1 = Ph) and enantiomerically pure substrates (R1 = L-menthyl). The cyclotrimerization afforded mixtures of two epimeric phosphahelicenes, with opposite configurations of the helical

Dalton Trans.

View Article Online


Perspective

Dalton Transactions

Scheme 28 Synthesis of phosphahelicenes via Ni-promoted cyclotrimerization of alkynes.

scaffolds, with isomer ratios from 33 : 67 to 86 : 14, depending on the substitution patterns. Thus, compared to the photochemical method in Scheme 26, the cyclotrimerization method has the advantage of allowing the specific targeting of single isomers of phosphahelicenes. It gives however lower diastereoselectivity, that is lower control of the helical chirality. 5.2.2 Phosphahelicene-gold complexes in enantioselective enyne cycloisomerizations. Following to the synthesis of phosphahelicenes displaying phosphole rings at the end of the helical sequence, our group has demonstrated the significant potential of these compounds as ligands in enantioselective gold catalysis.67 Gold complexes have been prepared easily, after reduction of the phosphine oxides, by in situ addition of suitable gold(I) derivatives. The procedure is typified in Scheme 29.62 In the intermediate trivalent phospha[6]helicene, the helical scaffold is configurationally stable, while phosphorus epimerizes at room temperature since phospholes, as well as phosphindoles, have lower inversion barriers than other phosphines.68 Consequently, complexation to gold affords a mixture of the two isomeric complexes 49 with opposite configurations at phosphorus, in about 1 : 1 ratio. In Scheme 29, the term ‘endo’ refers to complexes in which the gold atom is oriented toward the helical end, i.e. the inner face of the helicene, while ‘exo’ refers to complexes with the opposite geometry. Individual diastereomers have been separated easily by column chromatography to obtain enantiomerically pure gold complexes. Several gold complexes have been prepared by the same method.62 They are typified in Scheme 30. The observed endo– exo ratios depend on both the nature of the helical scaffold, the substitution pattern and the relative configurations of the stereogenic elements.

Dalton Trans.

Scheme 29

Synthesis of HelPHOS-gold complexes.

Scheme 30

Phosphahelicene-gold complexes.

In initial investigations, various phospha[6]helicene-gold complexes have been screened in a model cycloisomerization reaction, the rearrangement of N-tethered 1,6-enynes into azabicyclo[4.1.0]heptenes,69 shown in Scheme 31. As for other gold promoted reactions, these reactions are known to be highly challenging, since the linear geometry of Au(I) complexes demands very extended chiral pockets to reach the coordination site opposite to the ligand. Asymmetric variants of this reaction have been reported previously by using iridium, platinum and rhodium complexes,70–72 as well as with gold-phos-


View Article Online


Dalton Transactions

Perspective

80% in the benchmark cycloisomerization above (entries 6 and 7). The highly effective enantioselection has been tentatively rationalized based on the geometrical features of the HelPHOS-gold complex. The phosphorus and gold atoms, which are located on the internal rim of the helical scaffold, benefit indeed from a highly asymmetric environment. According to the X-ray structure in Scheme 32, the chiral phospha[6]helicene ligand shields three space quadrants around gold. Only the bottom-left quadrant is available to accommodate the most sterically demanding unit of the substrate in the stereodetermining step. It has been postulated in previous studies75 that, in the case of the N-tethered enyne in Scheme 31, the most sterically demanding group should be the alkyne aryl substituent. According to this hypothesis, the approaching mode of the alkene moiety to the π-complexed alkyne which accounts for the observed stereochemistry, should be the one shown in Scheme 32.

Scheme 31 Screening of phosphahelicene-gold complexes in the cycloisomerization of an N-tethered 1,6-enyne.

phoramidite complexes73 and bimetallic gold complexes of biaryl-type diphosphines.74 Selected results of the catalytic screening of phosphahelicenes are shown in Scheme 31. The phosphahelicene-gold chloride complexes have been activated by addition of silver salts (AgBF4). Systematic studies demonstrated that phospha[6]helicenes with meta-fused terminal phosphole rings may afford very good catalysts, such as (SP,P)-49-endo (see Scheme 29), provided that some structural requirements are fulfilled. These requirements include, notably, an endo-position of the gold center, which induces much higher catalytic activity than the exo-coordination mode (entries 3 vs. 4 and 5 vs. 6). In terms of enantioselectivity, the best results are obtained when the ligands have a terminal phosphole unit (rather than a benzophosphole unit, as in 54 in Scheme 30) with aromatic substituents alpha-to phosphorus (rather than unsubstituted phosphole units), as in 49 (Scheme 29) and 51 (Scheme 30). Moreover, the relative configuration of the helical scaffold with respect to the L-menthyl group must be taken into account, since the two epimers with opposite helical configurations may give very different enantiomeric excesses (see for instance 50-endo (entry 3) vs. 49-endo (entry 6)). The best catalysts, ClAu[(SP,P)-49-endo], and ClAu[(SP,P)-51endo], generally designated as HelPHOS-gold complexes, afforded total conversions and enantiomeric excesses of over


Scheme 32 (a) X-ray crystal structure of ClAu[(SP,P)-49-endo]. (b) Proposed stereochemical model for the cycloisomerization reaction.

The initial encouraging results of the cycloisomerization reactions shown in Scheme 31 motivated then more extended substrate screening as well as further ligand tuning, including the synthesis and use of the thiahelicene-based phosphine 46 displayed in Scheme 27. The corresponding gold complex afforded a 74% ee in the model cycloisomerization reaction (entry 8 in Scheme 31). Substrate screening has been extended to 1,6-enynes with additional olefinic functions (Scheme 33). Thus, the cyclic dien-yne 55 in Scheme 33a has been converted into the corresponding azabicyclo[4.1.0]heptene 56 in high yield (91% isolated yield) and 93% ee by using the phosphathia[7]helicene gold complex (SP,P)-52 as the pre-catalyst. In this reaction, the enantiomeric excesses were strongly increased, from 78% to 88%, when replacing AgBF4 by AgNTf2 as the silver salt to generate the catalytically active, cationic gold complexes. A 93% ee could be attained then by lowering the reaction temperature to 0 °C.

Dalton Trans.

View Article Online


Perspective

Dalton Transactions

5.2.3 Conclusion. In conclusion, helicenes with appended phosphorus functions gave rather disappointing, low to moderate enantiomeric excesses in the few organometallic and organocatalytic processes investigated so far. The only exception is the iridium promoted allylic amination in Scheme 20. The chiral ligand (P,S)-31b used in these experiments displays a phosphite function linked to the most hindered C1-position, on the internal rim of the helical structure. On the other hand, phospha-helicenes proved to have broad potential in enantioselective gold catalysis. Complexes (SP,P)-49-endo and (SP,P)-52-endo afforded highly enantioselective pre-catalysts for some challenging cycloisomerization reactions. In these cases also, the phosphorus functions are located on the internal rim of the helical scaffold. Hopefully, the very promising results above will stimulate further developments and applications of these and analogous ligands to new asymmetric organometallic and organocatalytic processes.

Acknowledgements Authors thank the Paris-Sud University for grant to P.A. and the LabEx CHARMMMAT for supporting their work on catalysis.

Notes and references

Scheme 33

Enantioselective cycloisomerizations of dien-ynes.

Finally, dien-ynes 57 displaying conjugated enyne units have been subjected to cycloisomerization reactions under analogous conditions. Both the phospha-carbohelicene (SP,P)4962 and the phospha-thiahelicene (SP,P)-5263 have been evaluated. The best catalytic system for these reactions proved to be ClAu[(SP,P)-52], combined with AgNTf2. These cycloisomerization reactions afforded either the expected bicyclo[4.1.0]heptane 58 in 89% ee, or the rearranged tricyclic compound 59 in 96% ee, depending on the R substituent of the olefin (R = H or Ph). The vinylcyclopropane – cyclopentene rearrangement leading to 59 was postulated to be catalyzed by the gold complex, since the non-catalyzed isomerization would require much higher temperatures. The stereochemical control should take place however at the initial cycloisomerization step. The cycloisomerization reactions in Schemes 31 and 33 represent the first uses of helical phosphines in enantioselective gold catalysis and demonstrate that catalysts based on chiral phosphahelicenes might complement the previous successful design concepts in this very challenging field.

Dalton Trans.

1 Y. Shen and C.-F. Chen, Chem. Rev., 2012, 112, 1463–1535. 2 M. Gingras, Chem. Soc. Rev., 2013, 42, 968–1006. 3 M. Gingras, G. Félix and R. Peresutti, Chem. Soc. Rev., 2013, 42, 1007–1050. 4 M. Gingras, Chem. Soc. Rev., 2013, 42, 1051–1095. 5 J. Crassous, N. Saleh and C. Shen, Chem. Sci., 2014, DOI: 10.1039/C4SC01404A. 6 M. J. Narcis and N. Takenaka, Eur. J. Org. Chem., 2014, 21–34. 7 B. Ben Hassine, M. Gorsane, J. Pecher and R. H. Martin, Bull. Soc. Chim. Belg., 1985, 94, 597–603. 8 B. Ben Hassine, M. Gorsane, J. Pecher and R. H. Martin, Bull. Soc. Chim. Belg., 1985, 94, 759–769. 9 B. Ben Hassine, M. Gorsane, J. Pecher and R. H. Martin, Bull. Soc. Chim. Belg., 1987, 96, 801–808. 10 B. Ben Hassine, M. Gorsane, F. Geerts-Evrard, J. Pecher, R. H. Martin and D. Castelet, Bull. Soc. Chim. Belg., 1986, 95, 557–566. 11 B. Ben Hassine, M. Gorsane, J. Pecher and R. H. Martin, Bull. Soc. Chim. Belg., 1986, 95, 547–556. 12 I. Sato, R. Yamashima, K. Kadowaki, J. Yamamoto, T. Shibata and K. Soai, Angew. Chem., Int. Ed., 2001, 40, 1096–1098. 13 T. Kawasaki, K. Suzuki, E. Licandro, A. Bossi, S. Maiorana and K. Soai, Tetrahedron: Asymmetry, 2006, 17, 2050–2053. 14 S. D. Dreher, T. J. Katz, K.-C. Lam and A. L. Rheingold, J. Org. Chem., 2000, 65, 815–822.


View Article Online


Dalton Transactions

15 C. Goedicke and H. Stegemeyer, Tetrahedron Lett., 1970, 12, 937–940. 16 R. H. Janke, G. Haufe, E.-U. Würthwein and J. H. Borkent, J. Am. Chem. Soc., 1996, 118, 6031–6035. 17 J. Chen and N. Takenaka, Chem. – Eur. J., 2009, 15, 7268– 7276. 18 Z. Peng and N. Takenaka, Chem. Rec., 2013, 13, 28–42. 19 N. Takenaka, R. S. Sarangthem and B. Captain, Angew. Chem., Int. Ed., 2008, 47, 9708–9710. 20 J. Míšek, F. Teplý, I. G. Stará, M. Tichý, D. Šaman, I. Císařová, P. Vojtíšek and I. Starý, Angew. Chem., Int. Ed., 2008, 47, 3188–3191. 21 M. R. Crittall, H. S. Rzepa and D. R. Carbery, Org. Lett., 2011, 13, 1250–1253. 22 M. Šámal, J. Míšek, I. G. Stará and I. Starý, Collect. Czech. Chem. Commun., 2009, 74, 1151–1159. 23 A. C. Spivey, T. Fekner and S. E. Spey, J. Org. Chem., 2000, 65, 3154–3159. 24 V. B. Birman and X. Li, Org. Lett., 2006, 8, 1351–1354. 25 J. C. Ruble, H. A. Latham and G. C. Fu, J. Am. Chem. Soc., 1997, 119, 1492–1493. 26 M. R. Crittall, N. W. G. Fairhurst and D. R. Carbery, Chem. Commun., 2012, 48, 11181–11183. 27 N. Takenaka, J. Chen, B. Captain, R. S. Sarangthem and A. Chandrakumar, J. Am. Chem. Soc., 2010, 132, 4536– 4537. 28 M. J. Narcis, D. J. Sprague, B. Captain and N. Takenaka, Org. Biomol. Chem., 2012, 10, 9134–9136. 29 S. E. Denmark and G. L. Beutner, Angew. Chem., Int. Ed., 2008, 47, 1560–1638. 30 J. Chen, B. Captain and N. Takenaka, Org. Lett., 2011, 13, 1654–1657. 31 T. Lu, R. Zhu, Y. An and S. E. Wheeler, J. Am. Chem. Soc., 2012, 134, 3095–3102. 32 A. Terfort, H. Görls and H. Brunner, Synthesis, 1997, 79–86. 33 K. Paruch, L. Vyklický, D. Z. Wang, T. J. Katz, C. Incarvito, L. Zakharov and A. L. Rheingold, J. Org. Chem., 2003, 68, 8539–8544. 34 M. T. Reetz, E. W. Beuttenmüller and R. Goddard, Tetrahedron Lett., 1997, 38, 3211–3214. 35 F. Aloui, R. El Abed, A. Marinetti and B. Ben Hassine, Tetrahedron Lett., 2007, 48, 2017–2020. 36 F. Aloui, S. Moussa and B. Ben Hassine, Tetrahedron Lett., 2011, 52, 572–575. 37 M. Monteforte, S. Cauteruccio, S. Maiorana, T. Benincori, A. Forni, L. Raimondi, C. Graiff, A. Tiripicchio, G. R. Stephenson and E. Licandro, Eur. J. Org. Chem., 2011, 5649–5658. 38 S. Cauteruccio, D. Dova, M. Benaglia, A. Genoni, M. Orlandi and E. Licandro, Eur. J. Org. Chem., 2014, 2694– 2702. 39 J. Žádný, P. Velýšek, M. Jakubec, J. Sýkora, V. Církva and J. Storch, Tetrahedron, 2013, 69, 6213–6218. 40 F. Teplý, I. G. Stará, I. Starý, A. Kollárovič, D. Šaman, Š. Vyskočil and P. Fiedler, J. Org. Chem., 2003, 68, 5193– 5197.


Perspective

41 S. Graule, M. Rudolph, N. Vanthuyne, J. Autschbach, C. Roussel, J. Crassous and R. Réau, J. Am. Chem. Soc., 2009, 131, 3183–3185. 42 S. Graule, M. Rudolph, W. Shen, J. A. G. Williams, C. Lescop, J. Autschbach, J. Crassous and R. Réau, Chem. – Eur. J., 2010, 16, 5976–6005. 43 R. El Abed, F. Aloui, J.-P. Genêt, B. Ben Hassine and A. Marinetti, J. Organomet. Chem., 2007, 692, 1156–1160. 44 Z. Krausová, P. Sehnal, B. P. Bondzic, S. Chercheja, P. Eilbracht, I. G. Stará, D. Šaman and I. Starý, Eur. J. Org. Chem., 2011, 3849–3857. 45 D. Nakano and M. Yamaguchi, Tetrahedron Lett., 2003, 44, 4969–4971. 46 D. J. Weix, S. D. Dreher and T. J. Katz, J. Am. Chem. Soc., 2000, 122, 10027–10032. 47 D. Nakano, R. Hirano, M. Yamaguchi and C. Kabuto, Tetrahedron Lett., 2003, 44, 3683–3686. 48 P. Sehnal, Z. Krausová, F. Teplý, I. G. Stará, I. Starý, L. Rulišek, D. Saman and I. Císařová, J. Org. Chem., 2008, 73, 2074–2082. 49 M. T. Reetz and S. Sostmann, J. Organomet. Chem., 2000, 603, 105–109. 50 A. Dore, D. Fabbri, S. Gladiali and O. De Lucchi, J. Chem. Soc., Chem. Commun., 1993, 1124–1125. 51 A. A. Watson, A. C. Willis and S. B. Wild, J. Organomet. Chem., 1993, 445, 71–78. 52 S. Gladiali, D. Fabbri, G. Banditelli, M. Manassero and M. Sansoni, J. Organomet. Chem., 1994, 475, 307–315. 53 S. Gladiali, A. Dore, D. Fabbri, O. De Lucchi and G. Valle, J. Org. Chem., 1994, 59, 6363–6371. 54 N. Fukawa, T. Osaka, K. Noguchi and K. Tanaka, Org. Lett., 2010, 12, 1324–1327. 55 Y. Sawada, S. Furumi, A. Takai, M. Takeuchi, K. Noguchi and K. Tanaka, J. Am. Chem. Soc., 2012, 134, 4080–4083. 56 K. Nakano, H. Oyama, Y. Nishimura, S. Nakasako and K. Nozaki, Angew. Chem., Int. Ed., 2012, 51, 695–699. 57 M. Flammang-Barbieux, J. Nasielski and R. H. Martin, Tetrahedron Lett., 1967, 8, 743–744. 58 L. Liu, B. Yang, T. J. Katz and M. K. Poindexter, J. Org. Chem., 1991, 56, 3769–3775. 59 Photocyclization of stilbenes and related molecules, ed. F. B. Mallory and C. W. Mallory, John Wiley & Sons, New York, 1984. 60 K. Yavari, S. Moussa, B. Ben Hassine, P. Retailleau, A. Voituriez and A. Marinetti, Angew. Chem., Int. Ed., 2012, 51, 6748–6752. 61 K. Yavari, P. Retailleau, A. Voituriez and A. Marinetti, Chem. – Eur. J., 2013, 19, 9939–9947. 62 K. Yavari, P. Aillard, Y. Zhang, F. Nuter, P. Retailleau, A. Voituriez and A. Marinetti, Angew. Chem., Int. Ed., 2014, 53, 861–865. 63 P. Aillard, A. Voituriez, D. Dova, S. Cauteruccio, E. Licandro and A. Marinetti, Chem. – Eur. J., 2014, DOI: 10.1002/ chem.201402822. 64 P. Aillard, P. Retailleau, A. Voituriez and A. Marinetti, Chem. Commun., 2014, 50, 2199–2201.

Dalton Trans.

View Article Online


Perspective

65 I. G. Stará, I. Starý, A. Kollárovič, D. Šaman and M. Tichý, J. Org. Chem., 1998, 63, 4046–4050. 66 I. G. Stará, Z. Alexandrová, F. Teplý, P. Sehnal, I. Starý, D. Šaman, M. Buděšínský and J. Cvačka, Org. Lett., 2005, 7, 2547–2550. 67 Previously, gold complexes of racemic thiahelicene-phosphanes have been used in gold catalysis: S. Cauteruccio, A. Loos, A. Bossi, M. C. Blanco Jaimes, D. Dova, F. Rominger, S. Prager, A. Dreuw, E. Licandro and A. S. K. Hashmi, Inorg. Chem., 2013, 52, 7995–7800. 68 W. Egan, R. Tang, G. Zon and K. Mislow, J. Am. Chem. Soc., 1971, 93, 6205–6216. 69 A. Fürstner, H. Szillat and F. Stelzer, J. Am. Chem. Soc., 2000, 122, 6785–6786.

Dalton Trans.

Dalton Transactions

70 T. Shibata, Y. Kobayashi, S. Maekawa, N. Toshida and K. Takagi, Tetrahedron, 2005, 61, 9018–9024. 71 D. Brissy, M. Skander, H. Jullien, P. Retailleau and A. Marinetti, Org. Lett., 2009, 11, 2137–2139. 72 T. Nishimura, T. Kawamoto, M. Nagaosa, H. Kumamoto and T. Hayashi, Angew. Chem., Int. Ed., 2010, 49, 1638–1641. 73 H. Teller, M. Corbet, L. Mantilli, G. Gopakumar, R. Goddard, W. Thiel and A. Fürstner, J. Am. Chem. Soc., 2012, 134, 15331–15342. 74 A. Pradal, C.-M. Chao, P. Y. Toullec and V. Michelet, Beilstein J. Org. Chem., 2011, 7, 1021–1029. 75 H. Jullien, D. Brissy, R. Sylvain, P. Retailleau, J.-V. Naubron, S. Gladiali and A. Marinetti, Adv. Synth. Catal., 2011, 353, 1109–1124.


Asymmetric Catalysis Powered by Chiral Cyclopentadienyl Ligands.

Chiral poly-rare earth metal complexes in asymmetric catalysis.

Tunable asymmetric catalysis through ligand stacking in chiral rigid rods.

Cation-triggered switchable asymmetric catalysis with chiral Aza-CrownPhos.

Development of chiral sulfoxide ligands for asymmetric catalysis.

Asymmetric Catalysis with a Mechanically Point-Chiral Rotaxane.

Chiral metal phosphate catalysis: highly asymmetric hetero-Diels-Alder reactions.

Enantiomeric impurities in chiral synthons, catalysts, and auxiliaries. Part 3.

Chiral phase-transfer catalysis in the asymmetric α-heterofunctionalization of prochiral nucleophiles.

Enantio-relay catalysis constructs chiral biaryl alcohols over cascade Suzuki cross-coupling-asymmetric transfer hydrogenation.

Asymmetric enamine catalysis with β-ketoesters by chiral primary amine: divergent stereocontrol modes.

Tunable chiral metal organic frameworks toward visible light-driven asymmetric catalysis.

Asymmetric catalysis mediated by the ligand sphere of octahedral chiral-at-metal complexes.

Simple sulfur-olefins as new promising chiral ligands for asymmetric catalysis.

Catalytic asymmetric reactions by metal and chiral phosphoric acid sequential catalysis.

DNA-cellulose: an economical, fully recyclable and highly effective chiral biomaterial for asymmetric catalysis.

Asymmetric catalysis with substitutionally labile yet stereochemically stable chiral-at-metal iridium(III) complex.

A chiral sulfoxide-ligated ruthenium complex for asymmetric catalysis: enantio- and regioselective allylic substitution.

Additive Effects on Asymmetric Catalysis.

Asymmetric catalysis: a radical revolution in synthesis.

Advances in asymmetric borrowing hydrogen catalysis.

Tandem rhodium catalysis: exploiting sulfoxides for asymmetric transition-metal catalysis.

The influence of fluorine in asymmetric catalysis.

Phosphine-olefin ligands based on a planar-chiral (π-arene)chromium scaffold: design, synthesis, and application in asymmetric catalysis.