Recent advances in heterobimetallic catalysis across a "transition metal-tin" motif.

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Recent advances in heterobimetallic catalysis across a ‘‘transition metal–tin’’ motif Debjit Das,a Swapna Sarita Mohapatrab and Sujit Roy*b Heterobimetallic catalysts, bearing a metal–metal bond between a transition metal (TM) and a tin atom, are very promising due to their ability in mediating a wide variety of organic transformations. Indeed the utilization of such catalysts is a challenging and evolving area in the field of homogeneous catalysis. Catalysis across a ‘TM–Sn’ motif is an emerging area in the broader domain of multimetallic catalysis. The present

Received 29th December 2014

review apprises the chemists’ community of the past, present and future scope of this versatile catalytic

DOI: 10.1039/c4cs00523f

motif. The TM–Sn catalyzed reactions presented include, among others, Friedel–Crafts alkylation, carbonylation, polymerization, cyclization, olefin metathesis, Heck coupling, hydroarylation Michael addition and

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tandem coupling. The mechanistic aspects of the reactions have been highlighted as well.

‘‘Alone we can do so little; together we can do so much.’’ –Helen Keller

1. Introduction Activation of an organic functionality by two different types of metal centers, main group and transition metals, is an important theme of research in catalysis.1 The relative ability of the metals (both main a

Centre for Applied Chemistry, Central University of Jharkhand, Brambe, Ranchi-835205, Jharkhand, India b Organometallics & Catalysis Laboratory, School of Basic Sciences, Indian Institute of Technology, Bhubaneswar-751013, India. E-mail: [email protected]

Dr Debjit Das earned his BSc with Honors in chemistry from Calcutta University in 2006 and MSc in chemistry in 2008 from Indian Institute of Technology Kharagpur, with a brilliant academic record. He then joined the research group of Prof. Sujit Roy to pursue doctoral studies in the area of organic and organometallic reactivity of palladium–tin heterobimetallic systems. After graduating with a PhD, he has been at the Central University Debjit Das of Jharkhand, Ranchi, since August 2013, where he is currently an Assistant Professor. His current research interests include the development of new synthetic methods using inorganic and organometallic compounds as homogeneous catalysts.

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group and transition) to make a s- or a p-complex with appropriate substrates becomes useful while selecting a catalyst for a desired transformation, especially in cases where bi- or polyfunctional substrates are involved. Recently, multimetallic catalysis has received much attention, since it offers reactivity and/or selectivity patterns different from those shown by monometallic systems. The metal centers in a multimetallic catalyst act synergistically during substrate activation and coupling steps in the catalytic cycle, leading to enhanced catalytic activity and selectivity.2 In a multimetallic catalyst, the metal centers may be grafted within a single motif, or may be added as separate molecular units. We would refer the former case as ‘intramolecular multimetallic catalysis’, and the latter as ‘intermolecular multimetallic catalysis’. In the intramolecular version, two or more metals are linked together by a metal–metal

Swapna Sarita Mohapatra

Swapna Sarita Mohapatra was born in Kanpur, Odisha, India in 1984. With a brilliant academic record in high school and college, she joined National Institute of Technology, Rourkela, and completed her Masters in Chemistry in 2011 securing the 1st position and receiving the Institute Silver Medal. Endowed with the DSTINSPIRE fellowship, she joined the research group of Prof. Sujit Roy to pursue her doctoral studies in organometallic chemistry and catalysis. Presently she is a Senior Research Fellow in the group.

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Fig. 1

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Schematic representation of intramolecular multimetallic catalysts.

bond (as type 1, Fig. 1) or via a ligand (as type 2, Fig. 1) and these metals participate in substrate activation directly or indirectly. Under the classification of intramolecular multimetallic catalysts, those having a metal–metal bond between a transition metal (TM) and a tin atom are very promising due to their ability in mediating a wide variety of organic transformations. On the basis of a literature survey covering the last 25 years, a rough distribution of publications pertaining to the corresponding ‘TM–Sn’ multimetallic complexes/ clusters/motifs has been illustrated in Fig. 2. Notably, excluding catalysis by the metal clusters, solid supported or nano bimetallic/ multimetallic species, nearly 10% of total ‘TM–Sn’ publications are related with the catalytic organic transformations by ex situ or in situ generated ‘TM–Sn’ multimetallic complexes (source Scifinders). Indeed the utilization of such multimetallic catalysts is a challenging and evolving area in the field of homogeneous catalysis.

2. Chemistry of stannylenes and ‘TM–Sn’ complexes The tin atom in stannylenes (:SnR2) uses two of its p-electrons in covalent bonding with the R group. The other two electrons constitute a lone pair, the latter can be used to form an adduct with soft Lewis acids as well as transition metals (TM). Stannylenes also have low-lying empty p- and d-orbitals, which can participate in

Sujit Roy completed his doctoral studies from IIT Kanpur. After successive stints at UWO London, IICT Hyderabad, CRC Sapporo and IIT Kharagpur, he joined IIT Bhubaneswar in 2009 as the founding Head of the School of Basic Sciences and Dean (Faculty Affairs). While working within the portal of catalysis for fine chemicals, the group addresses the issues of atom-economy, selectivity, and sustainable development. Prof. Sujit Roy Roy has over 110 publications including original research papers, book chapters and patents. He is a Fellow of the Indian Academy of Sciences, and is the recipient of CSIR Young Scientist Award and the CRSI Bronze Medal.

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Fig. 2 Literature survey of the TM–Sn motif (TM = transition and lanthanide elements).

hybridization with the orbitals of tin, and generate empty orbitals, the latter being suitable for complex formation with Lewis bases. Therefore stannylene can act both as a Lewis base and a Lewis acid. Indeed, the trichlorostannyl anion [SnCl3] is a better Lewis base than SnCl2 and can also act as an excellent ligand towards transition metals.3 As stannylenes possess one unshared lone pair of electrons and vacant d-orbitals, the trichlorostannyl species can be considered as a potential s-donor and p-acceptor ligand for transition metals (Fig. 3). Coordination of tin to a transition metal removes electron density from the tin atom and lowers the energy of its empty orbitals. The increasing electron deficiency at the tin center indeed enhances its p back-bonding interaction with the transition metal. Here it is important to mention that three electronegative chlorines in [SnCl3] assist the stronger p bonding interaction by withdrawing more electron density from the tin center. In a nutshell, both the s and p components are present in the bonding of tin–transition metal species.3

3. General characteristics of ‘TM–SnR3’ species A ‘TM–SnR3’ species primarily demonstrates the following characteristics (Fig. 4)3a,4

Fig. 3

Bonding model of [SnCl3] ligand with transition metal (TM).


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Scheme 1

Fig. 4 Characteristic features of a ‘TM–Sn’ complex.

(1) When R is chlorine, the SnCl3 ligand has high p-acceptor ability due to the presence of three electron withdrawing chloride ions. As a result, it prevents the reduction of the central transition metal ion to a lower oxidation state or to the corresponding metal. (2) The directing effect of the SnX3 ligand also makes the trans-ligand attached to the TM center in a ‘TM–Sn’ complex kinetically labile, thereby creating a vacant coordination site at the TM center during the reaction of a heterobimetallic ‘TM–Sn’ complex. The latter may further facilitate the organic activation in subsequent steps. (3) Due to its inherent Lewis acidity, the –SnX3 group may also act as a binding site for organic activation leading to novel heterobimetallic reactivity. (4) In cases where R is a bulky group, it may provide opportunity to explore selectivity (for example linear vs. branched selectivity in hydroformylation of alkene) due to steric effect. Besides the above, the electronic cross-talk between the two metals (TM and Sn) within a single scaffold would also provide opportunity for an interesting activation pathway of organic substrates across the bimetallic core. The above special characteristics, along with the wide variety of readily available organotin compounds, enhance the scope to generate many catalytic systems. The present article primarily aims to look into recent elaborations (concentrating mainly on the last two decades) in organic transformations catalyzed by ex situ or in situ generated ‘TM–Sn’ multimetallic complexes. This review will focus on the discussion on multimetallic reactivity, reaction parameters, selectivity, plausible catalytic activation and catalytic cycle, and mechanistic rationale. Catalysis by the metal clusters, solid supported or nano-dimensional bimetallic/multimetallic species has not been included in this review primarily due to the fact that the mechanistic understanding of organic activation in these systems is yet to mature.

Hydroformylation reaction of olefins.

involves formal addition of a formyl group (CHO) and a hydrogen atom to a carbon–carbon double bond (Scheme 1). A key consideration of hydroformylation is the ‘‘linear/normal’’ vs. ‘‘branched/iso’’ selectivity. Platinum(II) complexes with monodentate phosphines along with SnCl2 are widely studied as catalysts in hydroformylation of olefins ensuring high activity and selectivity.3a The Pt–Sn catalytic system has the advantage of improving hydroformylation yield in comparison to hydrogenation yield.6 In addition, platinum–tin catalysts with chiral ligands show promising enantioselectivity and diasteroselectivity in asymmetric hydroformylation.5 Schwager and Knifton proposed a catalytic cycle consisting of five elementary steps (Scheme 2).7 The cycle begins with the in situ formation of a bimetallic ‘Pt–Sn’ intermediate (A1, Scheme 2) via the insertion of SnCl2 into the Pt–Cl bond (step I, Scheme 2). This is followed by coordination and insertion of the olefin into the Pt–H bond (steps II and III) to generate the Pt-alkyl species (A4). Subsequent carbonylation (step IV) furnishes the Pt-acyl intermediate (A5). Finally hydrogenolysis of the resulting Pt-acyl intermediate leads to the product aldehyde and regeneration of the catalytically active H–Pt–SnCl3 intermediate A2 (step V).8 Control experiments with [PtCl2(PH3)2] or SnCl2 alone as the catalyst show no activity – a fact that strengthens the important role of the heterobimetallic intermediate in the catalytic cycle. The role of the trichlorostannyl ligand has been the subject of many experimental studies,9 although the exact role of the tin reagent remains unclear. There is prevalent acceptance that the initial action of the tin co-catalyst is oxidative addition across the Pt–Cl bond. However, it is not certain whether the resultant

4. Heterobimetallic ‘TM–Sn’ catalyzed organic transformations 4.1.

Carbonylation

4.1.1. Hydroformylation. Homogeneous hydroformylation of olefins represents a versatile pathway for the synthesis of commercially important aldehydes and alcohols.5 The reaction


Scheme 2

Proposed catalytic cycle for the hydroformylation of olefin.

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Scheme 3

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Possible active species in the Pt–Sn catalytic hydroformylation.

SnCl3 group remains bound to the platinum(II) or forms a stable ion-pair (Scheme 3).10 Though high pressure NMR spectroscopy has shown that substitution of [SnCl3] for CO provides an ionic pathway for hydroformylation,11 theoretical studies confirm the formation and retention of the Pt–Sn bond, and favor the stabilization of the pentacoordinated species.8 The absence of hydroformylation activity in the absence of SnCl2 is ascribed to the large energy barrier for the olefin coordination and insertion steps (steps II and III, Scheme 2). Computational studies also indicate that the strong trans-labilizing effect of the SnCl3 ligand indeed favors the olefin insertion and carbonylation steps (step III and IV, Scheme 2).8a–c DFT studies show that the ratio of linear to branched products, that is, the regioselectivity of the hydroformylation of alkene (propene as a case in point) is kinetically controlled and it is set at the olefin insertion step.8c,d The nature and bite angle of the ligand attached to the platinum centre have a strong effect on the reactivity and selectivity of the platinum/tin-catalyzed hydroformylation reaction.12a In a given series of diphosphine ligands like 1,1-bis(diphenylphosphino)methane (dppm), 1,2-bis(diphenylphosphino)ethane (dppe), 1,3-bis(diphenylphosphino)propane (dppp) and 1,4-bis(diphenylphosphino)butane (dppb), the latter is shown to be the best choice in Pt/Sn catalyzed hydroformylation (Table 1).12a The catalyst bearing a trans-substituted norbornane-derived ligand (like dppm-nor L2, Fig. 5) showed the highest TOF (turn over frequency), but increased rates were also observed with ligands containing a cyclic four-carbon bridge (e.g. dppm-cyb L1, Fig. 5).12a The xanthene backbone is an excellent scaffold for the construction of ligands with wide natural bite angles. For this purpose simple diphosphines as well as rigid diphosphines such as sixantphos, xantphos, xantarsine and mixed xantphosarsine were employed as ligands in the hydroformylation reaction (Fig. 5). Vogt et al. achieved the regioselective hydroformylation of methyl 3-pentenoate to obtain linear 5-formyl

Table 1

The effect of ligand bite angle in hydroformylationa

#

Ligand (L)

Bite angle (deg)

Relative rate

% of linear isomer (hexanal)

1 2 3 4

dppm dppe dppp dppb

72 85 91 98

1 10 120 400

90 90 71 91

a Reaction conditions: catalyst concentration: 1.4 104 M; solvent: benzene; Pt : L : SnCl2 = 1 : 2 : 5; 50 bar CO; 50 bar of H2; 100 1C; Pt = [PtCl2(PhCN)2].

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methyl pentanoate by using diphosphine ligands.12b Rigid bidentate phosphine ligands like 1,2-bis(3-(diphenylphosphino)-4-methoxyphenyl)benzene L3 and 1,2-bis(2-diphenylphosphino)benzene L4 (Fig. 5) have been applied in the platinum/tin-catalyzed hydroformylation of 1-octene and both ligands showed moderate TOF but fairly high regioselectivity.12c Leeuwen et al. compared the effect of xantphos L5, homoxantphos L8, xantarsine L6 and the mixed xantphosarsine ligand L7 in the platinum/tin-catalyzed hydroformylation.12d,e Xantarsine and xantphosarsine ligands imparted high catalytic activity and selectivity for the selective hydroformylation of 1-octene (Table 2). Although the calculated bite angles are only marginally different from each other, in terms of reactivity the xantarsine and the mixed xantphosarsine ligands were superior to xantphos ligands (entries 1–3, Table 2). Indeed ligand L8, having a natural bite angle of 1021, was shown to be 40 times active than ligand L5 (bite angle = 1101). A proper explanation for the higher activities of ligands L8 compared to L5, L6 and L7 is still lacking. The influence of bite angle on the selectivity of a linear aldehyde over a branched aldehyde is also explored. According to Vogt et al. a linear nonanal selectivity was obtained for the hydroformylation of 1-octane by using a sixantphos ligand based Pt/Sn catalyst system.12f For the internal octanes, a selective tandem isomerization–hydroformylation towards n-nonanal is observed with this catalyst system. It was suggested that widening of the bite angle will help to increase the steric congestion around the platinum centre and will facilitate the formation of a sterically less hindered linear aldehyde selectively ´r et al. reported via the formation of platinum alkyl species. Kolla that the platinum–xantphos–tin(II) chloride system acts as an active hydroformylation catalyst for styrene hydroformylation.12g The regio-selectivity towards the linear aldehyde was much higher, particularly at higher temperature. Remarkably, highly branched selectivity for styrene hydroformylation was reported later by the same group using platinum complexes of malonate-derived monodentate phosphines with SnCl2 or the platinum–PNP–tin(II)chloride [PNP = aryl-bis(diphenylphosphinomethyl)amine] system.12h,i Recently, five- and six-membered P-heterocycles have been investigated as ligands and they proved to be catalytically active in platinum–tin catalyzed hydroformylation of styrenes.12j In the asymmetric version of the hydroformylation reaction, many chiral ligands were tested successfully by several groups (Fig. 6).13 Such chiral ligands include diphosphites, diphosphines, aminophosphines, diphosphonites, aminophosphonitephosphites, bis-binaphthophospholes, cyclopentane-based C2 chiral bis(phosphine) ligands, heteroannularly bridged heterobidentate ferrocenyl diphosphine ligands and phosphindole based chiral ligands.


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Fig. 5

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Various types of ligands used in Pt/Sn catalyzed hydroformylation.

Table 2

Platinum/tin-catalyzed hydroformylation of octene at 60 1Ca

#

Ligand

Bite angle (deg)

TOFb,c

l/b ratio

1 2 3 4

L5 L6 L7 L8

110.1 112.9 111.4 102.0

18 210 350 720

230 4250 4200 4250

a

Reactions were carried out in dichloromethane at 60 1C under 40 bar of CO/H2 (1 : 1), catalyst precursor [Pt(COD)Cl2], [Pt] = 2.5 mM, Pt : SnCl2 : L : 1-octene = 1 : 2 : 4 : 255. b Determined by GC with decane as the internal standard. c Averaged turnover frequencies (TOF) were calculated as (mol of aldehyde) (mol Pt)1 h1 at 20–30% conversion.

A strong dependence of enantioselectivity on the temperature of the reaction has been a frequent observation in asym´r reported a very interesting metry hydroformylation. Kolla [PtCl(SnCl3){(2S,4S)-BDPP}] [BDPP = 2,4-bis(diphenylphosphino)pentane] catalyzed hydroformylation of styrene in which a change from S- to R-enantioselectivity was seen as a function of temperature.13a For example, hydroformylation of styrene, catalysis by [PtCl(SnCl3){(2S,4S)-BDPP}], gave the branched aldehyde with 64% ee (S-major) at 40 1C but 17% ee (R-major) ´r proposed that the reversal of enantioat 100 1C. While Kolla selectivity might be due to a temperature-dependent change in the conformation of the catalyst’s six-membered chelate ring, later Casey and co-workers demonstrated that reversal of enantioselectivity at higher temperature arises from a change in the ´gl investienantioselective-determining step.13b More recently Ke gated the Pt/Sn-catalyzed asymmetric hydroformylation of styrene in the presence of the model complex [PtH(SnCl3)-(chiraphosH)] [chiraphosH = (2S,3S)-2,3-bis(diphosphino)-butane] by means of DFT calculations.13c In this study the authors have addressed all the steps in the catalytic cycle that includes olefin insertion, CO


coordination and insertion, oxidative addition of hydrogen, and finally the reductive elimination of the product aldehyde. Among all these, the migratory olefin insertion step has been found to be the rate-determining step. While much attention has been focused on the variation of the supporting ligands at platinum, the corresponding variation at the tin center and their impact on the hydroformylation is less studied. In this regard, Coles et al. developed [PtCl{(Me2Si-{NAr}2)SnCl}(PPh3)2] 1 [Ar = 2,6-iPr2C6H3] (Scheme 4) and compared its activity with [PtCl(SnCl3)(PPh3)2] 2 with respect to the hydroformylation of 1-hexene.10 Catalyst 1 showed very low hydroformylation activity with negligible formation of heptanal (only 3%), but with catalyst 2, under the same reaction conditions 50% of heptanal was formed. The observed lower activity of 1 is assigned to the bulk of the stannylene, which indeed disfavors olefin coordination and formation of the five-coordinate platinum intermediate in the catalytic cycle (step III, Scheme 2). Wesemann et al. reported two Pt(dppp) complexes with the stannaborate ligand [SnB11H11] (Scheme 5).14 They have developed selective hydroformylation of 1-octene mediated by complexes 3 and 4. Due to isomerization of 1-octene during the catalysis, four different aldehydes are theoretically expected from this transformation (Scheme 6). It was observed that, with catalyst 3 and 4, 1-octene was selectively transformed into aldehydes A6 and A7. The selectivity is most likely due to large steric crowding of the [SnB11H11] ligand. It was also found that the TOF of the reaction is dependent on the temperature and CO/H2 pressure. The yields of the aldehydes rise with the increase in temperature or pressure (Table 3). Other ‘TM–Sn’ catalysts such as Sn[Co(CO)4]4 are also known for hydroformylation of 1-hexene albeit with lower catalytic efficiency.15

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Fig. 6

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Selected chiral ligands for Pt/Sn catalyzed asymmetric hydroformylation.

of monoterpenes.16 The catalytic activity of [Pd(H)(SnCl3)(PPh3)2] was examined in the cyclocarbonylation of isopulegol with CO in toluene under the different reaction conditions to provide the corresponding lactone. Scheme 4

Scheme 5

Structure of complexes 1 and 2.

It was found that the cyclocarbonylation is dependent on the temperature at a fixed pressure (entries 1 and 2, Table 4). The effect of temperature predominated since the yields remained unaffected upon decreasing the pressure from 30 bar to even 10 bar (entries 1 and 3). Increasing the electron density on palladium by substitution of the PPh3 by PCy3 resulted in the complete absence of reactivity under the same reaction conditions. The use of a bulky dppb ligand improved diastereomeric ratios (entry 4). It was also noted that the catalytic activity decreased drastically upon substitution of –SnCl3 by –SnBu3 (as in [Pd(H)(SnBu3)(PPh3)2] as catalyst). Probably the steric hindrance of SnBu3 disfavors the olefin coordination and CO migratory insertion.

Structure of complexes 3 and 4.

4.2.

Scheme 6 Products of the hydroformylation of 1-octene and the olefins 2-, 3- and 4-octene.

Table 3

Complex 3 and 4-catalyzed hydroformylation of 1-octenea

#

Catalyst

Temperature (1C)

CO/H2 pressure (bar)

TOFb [h1]

1

cat.3 cat.4 cat.3 cat.4 cat.3 cat.4 cat.4

100

50

11 19.3 62.1 34.1 72.8 73.9 25.4 41.4 63.7

2 3 4

120 140 140

20 30 40

Friedel–Crafts alkylation

4.2.1. Alkylation with acetate. Early studies of alkylation ˘ovsky´ demonstrated that the bimetallic complex reactions by Koc [Mo(CO)3(MeCN)2(SnCl3)Cl] 5 can catalyze allylic substitution reactions with allyl acetates. They have used silyl enol ethers and methanol to form C–C and C–O bonds, respectively, under mild conditions in dichloromethane (Scheme 7).17a Simultaneously, the group expanded the scope of allylation catalyzed by 5 with electron rich arenes and heteroarenes as nucleophiles.17b 4.2.2. Alkylation with alcohol and ether. ‘TM–Sn’ catalyzed Friedel–Crafts alkylation with alcohol was demonstrated by Roy et al. by using the isolable complex [Ir2(COD)2(SnCl3)2Cl2(m-Cl)2] 6 or the corresponding 6–SnCl4ArH adduct 7 (Scheme 8). Catalyst 7 can be prepared by reacting either 6 and SnCl4 in a Table 4

[PdHClL2/SnCl2/L]-catalyzed cyclocarbonylation of isopulegola

a

Reactions were carried out in 10 ml dichloromethane, octane/Pt catalyst = 250, reaction time 2 h. b Turnover frequencies were calculated as mol of aldehyde per mol of Pt per hour.

4.1.2. Alkoxycyclocarbonylation. Kalck et al. proposed [Pd(H)(SnCl3)(PPh3)2] (prepared in situ from [Pd(H)(Cl)(PPh3)2] and SnCl2) as the active species in the cyclocarbonylation

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#

Ligand (L)

PCO (bar)

Temperature (1C)

Yield of lactone (%)

deb (%)

1 2 3 4

PPh3 PPh3 PPh3 dppb

30 30 10 10

60 40 60 60

100 62 100 100

20 13 22 56

a

Solvent: toluene; time: 16 h.

b

de = {R(%) S(%)}/{R(%) + S(%)}.


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Scheme 7

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Few representative examples of allylic substitution with C- and O-nucleophiles.

Scheme 9 Alkylation of various nucleophiles by complex 6 as catalyst: few representative examples.

Scheme 8

Formation of catalyst 6 and 7.

1 : 2 molar ratio or [Ir(COD)(m-Cl)]2 and SnCl4 in a 1 : 4 ratio in arene at higher temperature (Scheme 8).18a The authors reported the isolation of 7 as a benzene adduct (6–SnCl4PhH) in the case of benzene as the arene. The isolated adduct is highly moisture sensitive, soluble in benzyl alcohol and behaves as a 1 : 1 electrolyte in solution. From spectroscopic and other studies on the adduct along with literature precedence, it has been suggested that adduct 7 may be formulated as [IrIII(COD)(SnCl3)(Cl)(ArH)(SnCl4)]+Cl. The nature of interaction between the metal center and benzene and SnCl4 remains to be established. The use of complex 6 as a catalyst has been demonstrated towards the activation of secondary benzyl alcohol. Highly efficient benzylation reactions have been achieved not only with carbon nucleophiles (arenes and heteroarenes) but also with oxygen (alcohol), nitrogen (amide and sulfonamide), and sulfur (thiol) nucleophiles in dichloroethane.18b Later with the same catalytic system propargyl and allyl alcohols were also activated and the corresponding C-alkylated or N-/O-/S-alkylated products were obtained in excellent yield (Scheme 9).18c–e During propargylation it was found that when alkyl or aryl substituted tertiary propargylic alcohols reacted with the bulky arenes, substituted indenes were formed. From a synthetic


point of view, the method is particularly useful for the preparation of indene from the easily available propargyl alcohols (Scheme 10).18c A preliminary proposal has been made for the formation of indene via the intermediacy of an allenyl species (Scheme 11). Due to steric reasons, a bulkier arene would attack at the less crowded acetylenic center of the propargyl alcohols, leading to the intermediate allene (A8). Indene would result via subsequent intramolecular ring closure of species A9 and rearomatization of species A10. Catalyst 7, generated in situ, showed a higher efficiency towards alkylation of arenes and heteroarenes with either alcohols18a or ethers (mainly primary).18f One-pot synthesis of tetrasubstituted pyrroles using catalyst 7 was also demonstrated (Scheme 12).18e More recently, the group has reported a multimetallic pianostool complex [Ir-(SnCl3)2Cp*{SnCl2(H2O)2}] 8, having diverse

Scheme 10

Indene formation from propargyl alcohol.

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Scheme 11 Plausible mechanism of indene formation.

Scheme 12 Synthesis of substituted pyrroles.

catalytic potential.18g–i For example, 8 promotes the intermolecular nucleophilic substitution reaction (or a-amidoalkylation reaction) of g-hydroxylactams with varieties of various arenes and heteroarenes as nucleophiles giving rise to the corresponding substituted decorated isoindolinones in excellent yields (Scheme 13).18g,h The intermolecular version of the reaction was extended to the intramolecular version as well, opening up opportunities for the syntheses of derivatives containing 5-substituted pyrrolidin-2-one cores and others which are present in many natural and/or biologically active candidate compounds (Scheme 14).18h The N-aryloxymethylamidals did not undergo the expected arylated product but rather preferred an unusual cascade type path to afford the oxazino [2,3-a]isoindole-6-one products in

good yields (Scheme 15). It was speculated that first the endocyclic N-acylimium ion was formed. Then oxocyclization produces the exocyclic N-acylimium ion, the latter concomitantly undergoes intramolecular arylation to produce the final heterocyclic product (Scheme 15).18h Catalytic amounts (1 mol%) of complex 8 have been employed effectively also in the alkylation reactions.18i Initial studies showed that the pre-activation of the alcohol across the ‘Ir–Sn’ catalyst involves prior coordination of the alcoholic –OH group at the hard Lewis acidic tin center. Though the exact role of Ir(III) is yet not clear, overall an electrophilic mechanism was proposed from Hammett studies.18a,c,i Roy and coworkers successfully employed the heterobimetallic ‘Pd–Sn’ system, namely [PdCl(COD)SnCl3] 9, for the intermolecular

Scheme 13 Complex 8 catalyzed intermolecular a-amidoalkylation reaction.

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Scheme 14

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Complex 8 catalyzed intramolecular a-amidoalkylation reaction.

Scheme 15 Complex 8 catalyzed heterocyclization reaction.

coupling of arenes, heteroarenes, 1,3-dicarbonyls and organosilicon nucleophiles with allylic, propargylic and benzylic alcohols.18j In situ 1H NMR studies provided initial insight into the preactivation of the alcohol and the plausible intermediacy of ether. Based on the above clue, the work was extended by employing symmetrical and unsymmetrical p-activated ethers as the alkylating partner (Scheme 16). More recently an intramolecular version of the alkylation reaction has been demonstrated for the synthesis of a less commonly studied 1,4-oxathiepane core. The intramolecular cyclization was mediated by 3 mol% of catalyst [PdCl(COD)SnCl3] 9 or by a combination of 9/AgPF6 (Scheme 17).19 4.2.3. Alkylation with aldehyde. Triarylmethane (TRAM) and their heterocyclic variants constitute a fundamental motif in a number of bioactive compounds, pharmaceuticals, prodrugs and dyes.20 Furthermore, diindolylmethane (DIM) are well known for their biologically activity and chemo-sensing property.21 Ir–Sn complexes have been used successfully employed as catalysts in the construction of the TRAMs to construct the above-mentioned motif. Electron-rich arenes and heteroarenes react with aldehydes in the presence of a


dual catalytic system [Ir(COD)Cl]2–SnCl4 affording the corresponding triarylmethanes in good yields with 100% TRAM selectivity.22a This is a simple method for the synthesis of a variety of triarylmethanes, including heterocyclic compounds (Scheme 18). Recently complex 8 is also shown to be a useful catalyst for the bisarylation of aldehyde to synthesize TRAM derivatives.22b Indenothiophenes and indenoindoles constitute an important class of heterocyclic ligands used for the design of organometallic catalysts.23,24 The oligothiophene skeleton bearing terminal unsubstituted or substituted indeno[1,2-b]thiophene represents a promising class of organic materials for p-type organic semiconductors.25 Besides this, 5,10-dihydroindeno[l,2-b]indoles exhibit a wide range of biological activities and are potential non-toxic antioxidant, and membrane stabilizing agents.26 As a follow-up to their achievement in aldehyde activation, the group recently demonstrated the utility of a catalytic combination of 9/AgPF6 (5 mol%) for tandem ring-closing of 2-heteroarylaldehyde for the synthesis of indenothiophenes and indenoindoles having varied substitution at the 8/10 positions respectively (Scheme 19). It is proposed that AgPF6 abstracts a

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Scheme 16

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Few representative examples of Pd–Sn catalyzed alkylation.

4.3.

Scheme 17

Synthesis of 1,4-oxathiepane cores.

halogen leading to a more reactive cationic bimetallic ‘Pd–Sn’ species.27 Other heterocycle-fused indenes, fluorenes, and benzofluorene A13 were also made via a stepwise protocol. The stepwise route involved prior synthesis of the heteroarene and arene substituted biaryl alcohols A12 from the corresponding aryl or heteroaryl aldehydes A11 by a Grignard reaction and the cyclization of the biaryl alcohols in the presence of 9/AgPF6 (Scheme 20).27 4.2.4. Alkylation with aldimines. The catalyst 6 also mediates the aza-Friedel–Crafts reaction of N-sulfonyl aromatic aldimines with 1,3,5-trimethoxybenzene and indole derivatives, which allows the selective preparation of functionalized diarylamines. Additionally, this bimetallic catalyst also allows the formation of symmetrical and unsymmetrical TRAMs via a consecutive Friedel–Crafts reaction pathway (Scheme 21).28

Scheme 18

Olefin metathesis

In 2006, Meyer et al. reported that the addition of tin(II) halide has a large influence on the performance of Grubbs first generation catalyst [Ru(PCy3)2Cl2(QCHPh)] especially in 1-octene self-metathesis.29a The addition of tin(II) halide suppress the decomposition of the catalyst which results in a higher life time and prevents olefin isomerization. The Grubb’s first generation catalyst [Ru(PhobCy)2Cl2(QCHPh)] [PhobCy = 9-cyclohexyl-9-phosphabicyclo-[3.3.1]-nonane] shows improvement in the reaction rate in 1-octene/styrene cross-metathesis together with tin(II) chloride or bromide addition. It is suggested that the formation of a new Ru–Sn bonded complex in the presence of tin halide as well as steric effects contribute to the observed higher activity. Later Dixneuf and coworkers have shown that the Ru–Sn bond containing complex 10 can catalyze the transformation of diester into the corresponding C11 nitrile ester via cross metathesis with acrylonitrile (Scheme 22).29b At 1 mol% of loading, catalyst 10 afforded the C11 nitrile ester in good yield (93%) along with only a small amount of unsaturated ester (6%). Complex 10 also showed excellent catalytic activity for the cross-metathesis of plant oil derivatives, the C11o-unsaturated

Synthesis of TRAMs.

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Scheme 19

Nucleophile assisted tandem ring-closing.

Scheme 20

Stepwise route to heterocycle-fused indenes, fluorenes and benzofluorene.

ester and aldehyde, with acrylonitrile and methyl acrylate (entries 1 and 2, Table 5). The corresponding a,o-bifunctional compounds were produced in good to excellent yield (entries 1–2). With 1-decene the corresponding C11 alkenylnitrile was obtained in 85% yield albeit with a longer reaction time of 16 h (entry 3). On the other hand, complex 11 at low concentration showed catalytic activity for the self-metathesis of C11 o-unsaturated aldehyde to produce C20 a,o-dialdehyde (Scheme 23).29b


4.4.

Cyclization reactions

Jang et al. have studied various cyclization reactions by using platinum complexes (usually 5 mol%) in the presence of phosphine ligands (5 mol%), SnX2 (25 mol%, X = Cl or Br), and dihydrogen (Scheme 24). The cyclization reactions consist of reductive coupling of activated alkenes, hydrogenative cyclization of yne–enones, yne–aldehydes, and yne–dienes, cycloisomerization of haloenynes, cycloreduction

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Scheme 21 TRAM synthesis by catalyst 6.

Scheme 22

Table 5

Complex 10 catalyzed cross metathesis reactions.

Complex 10 catalyzed cross metathesis of terminal alkenesa

# 1

n = 8, R = CHO, C11o-unsaturated aldehyde

2

n = 8, R = CO2Me, C11o-unsaturated ester

3

n = 7, R = Me, 1-decene

1

R R1 R1 R1 R1

= = = = =

CN CO2Me CN CO2Me CN

Temperature (1C)

Time (h)

80 rtb 80 rtb 80

3 1.5 6 1.5 16

91 85 98 89 85

a

Reaction conditions: 0.5 mmol of terminal fatty compound; 1 mmol of acrylonitrile/methyl acrylate; complex 10 (0.5 mol%); solvent: distilled toluene; dodecane as the internal standard. b rt = room temperature.

Scheme 23

Complex 11 catalyzed self-metathesis reactions.

of 1,6-enynes and intramolecular coupling of allyl halides and hydrazones.30 It has been suggested that the catalytic cycle begins with the formation of [PtHLn(SnX3)] from PtX2, phosphine, and SnX2 in the presence of hydrogen. The mechanism has been proposed with the help of deuterium labeling studies. For example, cyclization of A14 under D2, resulted in the formation of deuterio-A15 in 63% yield and as a mixture of two stereoisomers (A15a and A15b). A dipolar mechanism involving the metastable intermediate A17 was considered to rationalize the

Chem. Soc. Rev.

outcome of the deuterium labeling experiment (Scheme 25). The catalytic cycle begins with the coordination of platinum to the alkyne, and subsequent cis–trans isomerization between intermediates A16 and A18. In the follow-up steps, each intermediate undergoes cyclization, giving rise to intermediate A19. Finally A19 reacts with another molecule of deuterium to produce the cyclized products (A15a and A15b) with the regeneration of the catalysts. The formation of the two isomeric mixtures of deuterio-A15 (Scheme 25) can be explained considering intermediate A16 and its equilibration with intermediate A18.30b


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Scheme 24

Few representative examples of Pt(II)/Sn(II) catalyzed cyclization reactions.

Scheme 25

Plausible catalytic cycle for Pt/Sn catalyzed cyclization reactions.

Chung et al. have achieved the reductive cyclization of diynes and enynes using allylplatinum complexes of NHCs 12 leading to the formation of 2,5-dihydrofurans, -pyrroles, and cyclopentenes from the corresponding oxygen-, nitrogen- and carbon-tethered substrates respectively (Scheme 26). During catalyst screening it was observed that the reactivity of complex


13 having a distinct Pt–Sn bond is comparable to that of the 12–SnCl2 system.31 4.5.

Synthesis of acetic acid

Acetic acid is an important chemical used in the production of vinyl acetate, cellulose acetate, pesticides and various organic

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Scheme 26

Chem Soc Rev

Pt–Sn–NHC catalyzed reductive cyclization.

intermediates of importance to pharmaceutics.32a The main process for its production is the Monsanto process, which consists of the carbonylation of methanol using an iodide-promoted rhodium catalyst.32b An alternative route to acetic acid utilizing bimetallic Ru(II)–Sn(II) complexes as catalysts was first proposed by Shinoda and co-workers.4,32b–d The proposed mechanism involves two steps: (a) formation of formaldehyde by the dehydrogenation of methanol (plausible rate-determining step), and (b) conversion of formaldehyde to acetic acid (and methyl formate). a

b

2CH3 OH ! CH2 O ! CH3 COOH Some of the advantages of this promising route include: (a) the use of methanol as the reagent, (b) the exclusion of external CO and the corrosive iodide promoter, and (c) the replacement of expensive rhodium by ruthenium. Based on experimental and theoretical evidence, a possible catalytic cycle was proposed by Rocha et al. for the catalytic dehydrogenation of methanol to formaldehyde (Scheme 27).32b The catalytic cycle is initiated by the coordination of the methoxy anion to the ruthenium centre. Then the dissociation of the ligand generates a vacant coordination site around the ruthenium atom. In the next stage a b-hydrogen elimination takes place leading to the transfer of hydrogen to the metal, generating the metal–hydride species A22, where formaldehyde coordinates weakly to the ruthenium through the CQO double bond (Scheme 27). Thereafter, the formaldehyde dissociates from A22 to form A23. Finally the regeneration of the catalyst is accomplished by the displacement of the hydride ligand from A23 by a free phosphine ligand. The formation of acetic acid from formaldehyde could be accounted for involving the following key steps in sequence (path-a, Scheme 28)32d–f (i) formation of a methyl-formato complex from formaldehyde leading to methyl formate by reductive elimination, (ii) conversion of the methyl-formato complex into a hydridoacetato complex, and

Chem. Soc. Rev.

Scheme 27 methanol.

Possible reaction pathways for the dehydrogenation of

(iii) reductive elimination of acetic acid from the hydridoacetato complex. Usually, the hydride complex is thermodynamically more stable than the corresponding methyl complex which may provide the driving force for the relevant isomerization process.32g It was also proposed that methyl formate may be activated by the interaction between Ru(II) (borderline soft) and Sn(II) (borderline hard) with the soft CQO group and the hard –OMe group of methyl formate, respectively. It has been suggested that the four-center interaction through a ‘Ru–Sn’ bimetallic site may aid in the transfer of the methyl group from oxygen to carbon (path-b, Scheme 28).32d The authors suggested that the


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Scheme 28

Review Article

Possible reaction pathways for the acetic acid preparation from formaldehyde.

driving force for the migration of the methyl group is the stability of the m-acetato bridging (formation of a five-membered ring with allylic resonance of the carboxylate group). According to Shinoda et al. the heterobimetallic Ru–Sn complexes [RuCp(PPh3)2(SnX3)] are considerably more efficient compared to their monometallic analogues [RuCpP2X], indicating the importance of the Ru–Sn bond in the catalytically active species.32f,h The importance of the SnCl3 ligand in the formation of acetic acid from methanol is unambiguously demonstrated by comparing the catalytic abilities of a series of Ru(II) complexes containing zero, one and two SnCl3 ligands, namely [RuCl2{P(OMe)3}4], [RuCl(SnCl3){P(OMe)3}4], and [Ru(SnCl3)2{P(OMe)3}3]. It was found that the parent ruthenium complex [RuCl2{P(OMe)3}4] with no SnCl3 ligand showed catalytic activity for the dimerization of formaldehyde to methylformate (2HCHO - HCO2Me). However the formation of acetic acid from methanol, paraformaldehyde (HCHO) or methyl formate occurred only with catalysts such as [Ru(SnCl3)2{P(OMe)3}3] bearing two SnCl3 ligands. [RuCl(SnCl3){P(OMe)3}4] with one SnCl3 ligand exhibited an intermediate character, being able to catalyze both the reactions (2HCHO - HCO2Me, and HCO2Me - MeCO2H) but incapable to activate methanol.32d In this regard the Ru–Sn heteronuclear cluster complex [Ru(SnCl3)5(PPh3)]3 has been found to be catalytically active for the isomerization of methyl formate to acetic acid.32i The specific characteristics of the SnCl3 ligand (refer to Section 3) like high p-acceptor ability and Lewis-acid character may have a positive influence in this transformation. In a detailed study, Gusevskaya et al. looked into the catalytic activity of Ru–Sn bimetallic complexes of general formula [RuCpP2(SnX3)] (P = phosphine; X = F, Cl, Br) in the conversion of methanol into acetic acid (methyl acetate) and suggested that the catalytic activity depends on the electronic effect of the SnX3 ligand.32a For example, a comparison between the rate data of methyl acetate formation clearly shows better catalytic activity of [RuCpP2(SnF3)] compared to its chloro or bromo analogues (Table 6). The highest efficiency of the ‘Ru–Sn’ complex with SnF3 as the ligand compared to the chloro and bromo analogues is in agreement with the electron-withdrawing ability of the SnX3 ligands. Thus, the increased positive charge on ‘Ru’ in the order


Table 6

CpP2Ru(SnX3) catalyzed methyl acetate formationa

Rate of methyl acetate formation (104 mol l1 h1) # Catalyst (103 mol l1) P = PPh3

P = PPh2Me

P = dppe

1 [RuCpP2(SnF3)] 2 [RuCpP2(SnCl3)] 3 [RuCpP2(SnBr3)]

2.2 1.4 1.4

2.4 1.2 1.1

a

6.7 4.4 4.4

Reaction conditions: methanol/acetonitrile = 1/1 (v/v); 140 1C.

of SnBr3 o SnCl3 o SnF3 facilitates the catalytic reaction. It has been suggested that the less steric bulk of the ‘Ru–SnF3’ complex (as compared to that of SnCl3 and SnBr3 derivatives) may also facilitate catalyst–substrate interaction. The highest activity of the fluoro-analog could also be related to the possibility of formation of the intramolecular hydrogen bond (Scheme 29), which could favor the heterolysis of the C–H bond (A21 - A22 in Scheme 27). In addition, the basicity of ligand attached to the Ru centre plays a key role in tuning the catalytic activity of [RuCpP2(SnX3)]. Thus, when PPh3 is substituted by more basic ligands like PPh2Me or dppe, the reaction rate decreases significantly (Table 6).32a 4.6.

Heck-type coupling of alkyl iodides to olefins

Carreira et al. demonstrated that cobaloximes of the type 14 catalyze intramolecular alkyl Heck-type coupling reactions of alkyl iodides to alkenes upon irradiation with visible light (blue LED, lmax = 465) in the presence of a tertiary amine base. This protocol is compatible with a wide range of functional groups, such as amides, esters, ketones, and aldehydes (Scheme 30).33a This methodology has also been used for the synthesis of (+)-Samin and (+)-Daphmanidin E.33a,b

Scheme 29

Proposed intramolecular hydrogen bond.

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Scheme 30

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Intramolecular alkyl Heck-type coupling reactions catalyzed by 14.

Scheme 31 Intermolecular Heck-type coupling catalyzed by 14.

Scheme 32

Rh–Sn catalyzed asymmetric Michael addition of a-cyanopropionates and acrolein.

The ‘‘Co–Sn’’ complex 14 also catalyzes the synthesis of allylic trifluoromethanes from styrene derivatives using 2,2,2-trifluoroethyl iodide under photo-irradiation. The mild conditions of the reaction allow a wide range of functional groups to be tolerated with good yields (Scheme 31).33c 4.7.

Michael addition

Motoyama et al. found a novel catalytic system for the asymmetric Michael addition of a-cyanopropionates and acrolein by the use of a 2,6-bis(oxazolinyl)phenyl (abbreviated as Phebox)derived rhodium–tin catalyst prepared in situ by simply mixing [{RhCl(c-octene)2}2] and [SnMe3(tBu-Phebox)]. The reactions proceed under neutral and mild conditions with good enantioselectivity (Scheme 32).34 Detailed mechanistic studies showed that the chiral [Rh(tBuPhebox)(SnMe3)Cl] complex, generated by oxidative addition of [{RhCl(c-octene)2}2] to [SnMe3(tBu-Phebox)], is an active catalyst and the turnover number of this catalytic system is greater than 10 000 within a few hours. The observed (R) stereochemistry of the Michael

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adducts can be clearly explained by the N-bonded enol intermediate A24, formed by enolization of a-cyanopropionates bound to the Lewis acidic RhIII centre. In A24 both the enol plane and the Phebox plane are parallel to each other and the Si face of the enol is masked by the tBu group on the oxazoline rings and the bulky SnMe3 group (Scheme 33). So, the acrolein attacked the exposed face of the bound enol to furnish the product with (R) stereoselectivity.

Scheme 33

Proposed N-bonded enol intermediate.


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Scheme 34

Review Article

Substrate scope in the Michael addition reaction.

Recently, Roy et al. have developed a new heterobimetallic ‘Pd–Sn’ catalyst for the efficient Michael addition reactions of differently substituted enones.19 This selective C–C and C–heteroatom bond forming reactions were accomplished under mild conditions and at room temperature for most cases. The successful utilization of a large spectrum of carbon, sulfur, oxygen, and nitrogen nucleophiles is noteworthy (Scheme 34).19 In search for the nature of the catalytically relevant bimetallic species, spectroscopy and kinetic experiments have been carried out, which showed that the combination of [PdCl2(MeCN)2] and SnCl2 leads to the in situ formation of catalytically active species [Pd(MeCN)2Cl(SnCl3)]. On the basis of 13C NMR and DFT studies, a suggestion is made that the alkene and carbonyl functions are concurrently activated across the bimetallic assembly (Fig. 7).19

4.8.

‘Co–Sn’ catalyzed organic reactions

Besides the hydroformylation reaction (mentioned in Section 4.1.1), Petit et al. demonstrated that the bimetallic Co–Sn complex, [Sn{Co(CO)4}4], can also be used in other homogeneous catalytic reactions, such as the hydrolysis of cyclic carbonates (eqn (1), Scheme 35), direct synthesis of lactones by double carbonylation of (2-bromoethyl)benzene (eqn (2), Scheme 35), isomerization of terminal epoxides to aldehydes and ketones (eqn (3), Scheme 35) and ring opening of epoxides with alcohols (eqn (4), Scheme 35).15 The latter reaction was found to be highly facile in the presence of [Sn2{Co(CO)4}4]. It is believed that these complexes act as a source of Co(CO)4 species, which is responsible for the hydrolysis of cyclic carbonates and double carbonylation reaction.15,35 The formation of Co(CO)4 from [Sn{Co(CO)4}4] is assumed to result from the disproportionation equilibrium as shown below.

Fig. 7 Proposed enone activation mode.


Scheme 35

Various Co–Sn catalyzed reactions.

It was also suggested that the presence of the Co(CO)4 moieties strongly modify the electrophilicity of the tin atom and make it a strong Lewis acid that catalyzes the ring opening of epoxides by tertiary alcohols and isomerization of epoxides. The selectivity of the isomerization of epoxides by the [Sn{Co(CO)4}4] catalyst depends on the temperature and CO pressure and particularly in the case of propylene oxide the use of [Sn{Co(CO)4}4] under high CO pressure allows selective formation of the aldehyde product (Table 7).15

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[Sn{Co(CO)4}4] catalyzed isomerization of propylene oxidea

#

PCO (bar)

Temperature (1C)

CH3CH2CHO (mol%)

CH3COCH3 (mol%)

1 2 3

20 120 120

100 100 150

23 48 98

77 0 0

a


Reaction conditions: 0.05 mol% catalyst; [epoxide]/[Co] = 100; solvent: benzene; time: 16 h.

4.9.

Oxidative carbonylation of phenol

Diphenyl carbonate (DPC) is a starting material for the synthesis of polycarbonates, which are excellent engineering thermoplastics and substitutes for metals and glass.36 A facile synthesis of diphenyl carbonate by oxidative carbonylation of phenol, in the presence of complex [Pd2(dppm)2(SnCl3)Cl] 15 or [Pd2(dppm)2(SnCl3)2] 16 and redox catalyst [Ce(Trop)4] [Trop = tropolonate] under CO and air, was reported by Ishii and co-workers (Scheme 36).37a It was later shown that, in terms of TOF, [Mn(TMHD)3] [TMHD = 2,2,6,6-tetramethyl-3,5-heptanedionate] serves as the best redox catalyst for this Pd–Sn catalyzed oxidative carbonylation reaction.37b Both the rate and the yield of the reaction are greatly affected by metals, oxidation state of metals and ligands of the redox catalyst. For example, Mn3+ shows better catalytic efficiency than Ce and Fe (Table 8). Mn3+ is more active than Mn2+ (entries 2–4) and among the Mn3+ based redox catalyst, the TOF of DPC increased in the following order: [Mn(TMHD)3] 4 [Mn(Trop)3] 4 [Mn(acac)3] [Trop = tropolonate, acac = acetylacetonate] (entries 1–3, Table 8).37b Complex 15 or 16 reacts with CO to form [Pd2(dppm)2(m-CO)(SnCl3)X] [X = Cl or SnCl3] A25. As the Lewis acidic tin center can interact with oxygen of the Lewis basic phenol, a simultaneous binding of CO and phenol as in the proposed intermediate A26 (Scheme 37) is believed to facilitate the reaction.37 4.10.

Hydroarylation

´ska-Buzar and co-workers showed that [W(CO)4(m-Cl)3Szyman W(SnCl3)(CO)3] 17 (2 mol%) catalyzed the hydroarylation of

Scheme 36

Table 8

Oxidative carbonylation of phenol.

Oxidative carbonylation of phenol: screening of redox catalystsa

#

Redox catalyst

TOF (DPC/Pd) (mol/mol h)

1 2 3 4 5 6 7 8 9

[Mn(TMHD)3] [Mn(Trop)3] [Mn(acac)3] [Mn(Trop)2] [Mn(acac)2] [Ce(TMHD)4] [Ce(Trop)4] [Ce(acac)3, 3H2O] [Fe(acac)3]

4.46 3.53 2.62 2.80 0.52 2.45 2.80 0.74 0.19

a Reaction conditions: 32 mmol phenol; 0.006 mmol [Pd2(dppm)2(SnCl3)Cl] 15; 0.011 mmol redox catalyst; 0.50 MPa CO; 0.25 MPa air; 100 1C; 3 h.

Chem. Soc. Rev.

Scheme 37

Plausible catalytic cycle in oxidative carbonylation.

bicyclo[2.2.1]hept-2-ene (NBE = norbornene) with arenes (such as benzene, toluene, para-xylene, or mesitylene) at room temperature, when the reactions were conducted in arene as a solvent (Scheme 38).38a,b It should be noted that 17 is able to catalyze the hydroarylation reaction only in neat aromatic hydrocarbons. The addition of even a small amount of solvent such as CH2Cl2 or CHCl3 to the benzene solution drastically changed the reaction course, and mainly 2,2 0 -binorbornylidene is formed. These hydroarylation reactions were facilitated with arenes having electron donating groups. In certain cases, besides mono adducts, higher adducts (di and tri) were obtained. The reaction represents a useful synthetic protocol to form exo-2-arylnorbornane in one step from simple electron-rich arenes and norbornene. Mechanistically, it is suggested that the splitting of chlorine bridges, and the simultaneous Z2-coordination of NBE and arene to the tungsten atom, resulted in the formation of an intermediate ionic pair of the tungsten complex [W(SnCl3)(CO)3(Z2-arene)(Z2-C7H10)][WCl3(CO)4] A27 (Scheme 39). Elimination of the hydroarylation product from A27 regenerates 17. Roy et al. demonstrated the reactivity of [PdCl(COD)SnCl3] 9 towards intermolecular hydroarylation of electron-rich a-methyl substituted aryl alkenes with heteroarenes.38c During catalyst screening, unsurpassed catalytic efficiency was observed in the case of heterobimetallic complex 9 compared to various Brønsted acids, Lewis acids, metal triflate and monometallic catalysts thereby suggesting the importance of the bimetallic core (Fig. 8). A number of N-substituted (alkyl, benzyl, allyl, propargyl and phenyl) indoles, as well as differently ring substituted free indoles were alkylated by various a-methyl styrenes (Scheme 40). Significantly, with free indole the reactions are completely C3-selective, with no N-alkyl product being formed. The progress of the hydroarylation reaction with O, S-heteroarenes was also satisfactory with catalyst 9 and the corresponding alkylated O-, and S-heteroarenes were obtained in good yields (Scheme 40). The addition reaction is also highly regioselective yielding only the corresponding Markovnikov product in all cases. The ease of handling of the catalyst (insensitive towards air and moisture) is also noteworthy.


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Scheme 38

Review Article

Hydroarylation and dimerization of NBE.

4.11.

Scheme 39

Plausible hydroarylation reaction mechanism.

Fig. 8 Hydroarylation model study: the effect of catalyst.

Scheme 40

Dimerization

In the previous Section (4.10) we indicated facile hydroarylation of norbornene (NBE) using catalyst [W(CO)4(m-Cl)3W(SnCl3)(CO)3] 17 when the reactions were conducted with arene as a solvent (Scheme 38). On the other hand, when the reaction of NBE was carried out in chloroform, dichloromethane, or chlorobenzene in the presence of the 17, mainly 2,20 -binorbornylidene (a norbornene dimer containing a carbon–carbon double bond) was formed. Other by-products, such as hydroxyl-2,20 -binorbornyl, cyclotrimers and ring-opening metathesis polymer of norbornene, were also detected (Scheme 41).38b The proposed mechanism involves prior splitting of chlorine bridges of catalyst 17 followed by coordination of NBE to generate monoolefin (A28) or bisolefin (A30) species (Scheme 41). A 1,2-hydrogen shift gives the metallacarbene complex A29 from A28 and bimolecular decomposition of the carbine to yield 2,20 -binorbornylidene regenerating 17. In the presence of a large excess of NBE, the carbene species A29 may initiate the ROMP reaction. On the other hand, the close proximity of the two alkenes in A30 promotes the formation of metallacyclopentane A31. Hydrolysis of A31 by H2O generates the hydroxyl-2,20 -binorbornyl. The insertion of an NBE molecule into the metal–carbon bond of metallacyclopentane A31, followed by reductive elimination, affords cyclotrimers (Scheme 41). Another heterobimetallic complex [{W(SnCl3)(CO)3}2(m-Cl)3][Hpip]+ can also act as a catalyst for the dimerization of NBE.39a However, in contrast to 17, this catalyst does not result in a ROMP reaction or the formation of a polymer (poly-1,3cyclopentylenevinylene, poly-NBE) was not detected. Konelev et al. reported that vinyl ketones in the presence of catalytic amounts of [RhCl(C2H4)2]2 are transformed into unsaturated 1,6-diketone and diene 1,6-diketones at 80 1C in acetone, A32 being the main product (Scheme 42).39b Interestingly this ‘‘head-to-head’’ dimerization of vinyl ketones in the

[PdCl(COD)SnCl3] 9 catalyzed hydroarylation reaction.


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Scheme 41

Plausible routes to NBE conversion in the presence of catalyst 17.

Scheme 42

Effect of solvent on the product distribution.

presence of the catalytic system [RhCl(C2H4)2]2-SnCl2 leads to preferential formation of A33. The solvent plays an important role in the distribution of products. Thus, replacement of acetone by ethyl acetate in the RhI–SnII catalytic system causes a dramatic change in distribution of the product ratio of A32/A33 from 0.24 to 2.46. The suggested mechanism includes ligand exchange between [RhCl(C2H4)2]2 and vinyl ketone to give the bis-(vinyl ketone)rhodium adduct A34, which reacts with SnCl2 to give A35 (Scheme 43).39b The species A35 is then converted into A36 in the presence of a solvent or substrate. The catalytic reaction basically starts by oxidative addition of two vinyl ketones within A36 to form the metallacycle A37 which is converted to intermediate A38 via b-hydrogen elimination. A38 is a key intermediate which can liberate a,b unsaturated diketone A32 (pathway a) to regenerate A36. Alternatively the insertion of the double bond of the vinyl ketone in the species A38 into the Rh–H bond can afford the diakylrhodium intermediate A39, which successively liberates diketone A33 (pathway b) and methyl ethyl ketone to give back A36. The mechanism is also

Chem. Soc. Rev.

supported by the identification of methyl ethyl ketone in the reaction mixture by GC-MS studies. 4.12.

Polymerization

´ska-Buzar et al. showed that seven-coordinate heteroSzyman bimetallic molybdenum(II) and tungsten(II) complexes of the type [M(CO)4(m-Cl)3M(SnCl3)(CO)3] (M = W, Mo) and [MCl(SnCl3)(CO)3(NCMe)2] (M = W, Mo) could be used as catalysts for ROMP of cyclic olefins giving rise to high molecular weight polymers (Scheme 44).40a Complexes of the type [M(CO)4(m-Cl)3M(SnCl3)(CO)3] and [MCl(SnCl3)(CO)3(NCMe)2] (M = W, Mo) initiate the ROMP of NBE at room temperature but the reaction is very slow and suffers from low selectivity (Table 9). Compounds such as [MCl(SnCl3)(CO)3(Z4-NBD)] can be prepared directly from [M(CO)4(m-Cl)3M(SnCl3)(CO)3] or [MCl(SnCl3)(CO)3(NCMe)2] (M = W, Mo) and an excess of NBD. Catalyst [MCl(SnCl3)(CO)3(Z4-NBD)] is shown to initiate the ROMP reaction of NBD.40b Catalyst [MCl(SnCl3)(CO)3(NCMe)2] (M = W, Mo) promotes the ring-opening metathesis polymerization of halogenated


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Scheme 43

Review Article

Plausible reaction mechanism for dimerization.

Scheme 44 ROMP reactions catalyzed by seven-coordinate complexes.

norbornenes.40c When both cyclic olefin and alkyne are present in the reaction mixture, block polymers are formed with high molecular weights and better solubility in CHCl3 than homopolymers.40d

Table 9

Polymerization of alkynes occurs very smoothly at room temperature in the presence of seven-coordinate tungsten(II) and molybdenum(II) compounds of the type [MCl(SnCl3)(CO)3(NCR)2] (M = W, Mo; R = Me, Et) (ML7 in Scheme 45).41a,b The catalytic coupling of alkynes yields at least two types of products namely polymers with conjugated polymeric structures and cyclic oligomers especially the aromatic cyclotrimers (Scheme 45).41a–c The complexes such as [Mo(SnCl3)2(CO)2(NCEt)3] or [W(CO)4(m-Cl)3W(SnCl3)(CO)3] are also active for the polymerization of phenyl acetylene.41c,d

Metathesis polymerization of NBEa

#

Catalyst

Solvent

Temperature (1C)

Yield (%)

PDI

1 2 3 4 5 6

[Mo(CO)4(m-Cl)3Mo(SnCl3)(CO)3] [W(CO)4(m-Cl)3W(SnCl3)(CO)3] [MoCl(SnCl3)(CO)3(NCMe)2] [MoCl(SnCl3)(CO)3(NCMe)2] [WCl(SnCl3)(CO)3(NCMe)2] [WCl(SnCl3)(CO)3(NCMe)2]

Toluene Dichloromethane Dichloromethane Chlorobenzene Dichloromethane Chlorobenzene

25 25 25 75 25 75

45 17 04 17 04 76

2.5 3.6 3.2 2.5 1.8 2.8

a

Reaction conditions: catalyst : NEB = 1 : 100; reaction time 5 h at 75 1C and 24 h at 25 1C.


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Scheme 45

Chem Soc Rev

Polymerization of alkynes.


Scheme 47 Table 10

#

‘TM–Sn’ catalyzed miscellaneous reactions.

Cyclotrimerization reactions of monosubstituted alkynes

Alkyne

Catalytic system

Selectivity of cyclotrimerization

1 2

[Mo(CO)4(pip)2]/SnCl4a

99

[Hpip]2[Mo(m-Cl)2(SnCl3)(CO)3]2b

01

3

[Mo(CO)4(pip)2]/SnCl4a

98

4

[Hpip]2[Mo(m-Cl)2(SnCl3)(CO)3]2b

05

a

Reaction conditions: [Mo(CO)4(pip)2] : SnCl4 : alkyne = 1 : 2 : 50; reaction time 24 h; room temperature; solvent: dichloromethane. b Reaction conditions: [Hpip]2[Mo(m-Cl)2(SnCl3)(CO)3]2 : alkyne = 1 : 50; reaction time 24 h; room temperature; solvent: dichloromethane.

´ ska-Buzar et al. showed that the selectivity of the Szyman alkyne reaction depends on the catalytic systems.41e For example, with [Mo(CO)4(pip)2]/SnCl4 [pip = piperidine] as catalyst, cyclotrimerization occurred (entries 1 and 3, Table 10) selectively whereas catalysis with [Hpip]2[Mo(m-Cl)2(SnCl3)(CO)3]2 led to the formation of polyenes (entries 2 and 4, Table 10). The proposed mechanism for the formation of cyclotrimers from alkynes involves initial coordination of two alkynes to the metal followed by rearrangement to a metallacyclopentadiene A41 (Scheme 46). Insertion of the third alkyne into the M–C bond, followed by reductive cyclization furnishes the cyclotrimers (Scheme 46). The formation of linear conjugated polyenic polymers involves the formation of species A43 and A44, which facilitates chain growth.41a,c It may be noted that the proposal as

Scheme 46

shown in Scheme 46 does not account for the formation of other isomers. 4.13.

Miscellaneous reactions

‘TM–Sn’ catalyzed reactions which are represented by fewer examples and could not be categorized in the previous sections are briefly highlighted below (Scheme 47). The Pt/Sn catalyzed hydrogenation and isomerization reactions of olefins are well known. In the literature Pt/Sn catalysts are also effective for the water gas shift reaction.3a The trichlorostannyl iridium complexes were evaluated as catalyst precursors for the hydrogenation of a,b-unsaturated ketones, 2-cyclohexen-1one and cinnamaldehyde.42 Nickel complexes like [Ni(PPh3)3SnCl3] have been used as catalysts to isomerize 1-butene.3a The tin(II)coordinated ‘Ir–Sn’ complex trans-[IrCl2(SnCl3)4]3, trans[IrClH(SnCl3)4]3 as well as [IrH(SnCl3)5](NEt4)3 are found to be good catalysts in the dehydrogenation of acyclic alcohols.43 The Pd–Sn complex like [Sn(CH2CH2NC6F5)2-NMe]4Pd showed moderate activity in the Suzuki–Miyaura and Heck crosscoupling reactions with iodo or bromoarenes, however chloro derivatives remain inactive.44 [PdCl(PPh3)2SnCl3] can be used as a potential catalyst for Suzuki coupling between various aryl and heteroaryl substituted boronic acid with bromo derivatives.27 On the other hand, [PdCl(P{ p-Tol}3)2SnCl3] and [Pd2Cl2(P{ p-Tol}3)2(SnCl3)2] showed promising reactivity in the Heck-type arylation of vinylsiloxanes.45

Plausible activation of substrates and reaction mechanism.

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5. Concluding remarks In this review an effort has been made to highlight the catalytic utility of the ‘TM–Sn motif crafted on a single scaffold’ towards efficient and selective organic transformations. The subject is within the realm of multimetallic catalysis, in general, and heterobimetallic catalysis in particular. In the past two decades, cooperative multimetallic catalysis has attracted wide attention of the catalysis community. In terms of efficiency, selectivity and mechanistic intrigue, cooperative multimetallic catalysis offers myriad opportunities to the practitioners in the community – the reflection of which can be seen very clearly in catalysis within the TM–Sn single scaffold domain. There exist many challenges (hence opportunities) in the design of new TM–Sn motifs and exploiting their catalytic potential. Foremost among these will be to understand the exact nature of synergism (the electronic cross-talk) between the two metals (please refer to Sections 2 and 3). This synergy will direct the organic group towards the TM–Sn motif and lead to the desired bond-making and bond-breaking events.46 In this regard, isolation and structure determination of the reactive organometallic intermediate(s) will greatly aid in understanding the key roles of these metal centers during the reaction. The accrued knowledge from the above activities will be of great help in promoting newer methodologies such as those involving multiple cascades and tandem sequences. Finally, there remains an excellent scope to design chiral TM–Sn catalysts, which can potentially contribute towards the development of asymmetric catalysis.

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Acknowledgements 10 SR is indebted to all colleagues (including the current and former members of his group) who have contributed to the establishment of dual reagent/bimetallic catalysis in the TM/Sn regime. Generous research funding from DST, CSIR, and GAIL is duly acknowledged. DD and SSM thank CSIR and DST-INSPIRE respectively for fellowship.

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