Hydrosilylation metal-carbene complexes

Metal/carbene complexes (30-34) have also proved fruitful in the hydrosilylation of alkynes, olefins, enones, and ketones. Lappert and Maskell showed that ketones and alkynes could be reduced to silyl ethers and vinylsilanes, respectively, using complexes 30 and 31 (Eqs. 33 and 35) [75]. The yields tended to be high (92 and 98%, respectively), but the reactions needed to be rim neat. Poor selectivity was observed in the hydrosilylation of alkynes producing a mixture of the -trans, (i-c/ s, and a products. 

Phosphorus is a key element in catalysis, and the last two Nobel prizes in molecular chemistry were awarded to Noyori, Sharpless and Knowles (2001) for their work on enantioselective catalysis and to Grubbs, Schrock and Chauvin (2005) for their work on the chemistry of transition metal carbene complexes and their applications in metathesis. In both cases the development of highly efficient, specifically tailored phosphorus based ligands are of paramount importance The book opens with an account of the recent studies on a new family of air-stable chiral primary phosphines based on the binaphthyl backbone and their applications in asymmetric hydrosilylations (Chap. 1). The concept of applying phosphorus ligands to enantioselective catalysis is also the main subject of Chaps. 5 and 10, dealing with P-based planar chiral ferrocenes and chiral phosphorus ligands for enantioselective enyne cycloisomerizations, respectively. 

Lappert MF, Maskell RK. (1984) Carbene transition metal complexes as hydrosilylation... 

A synthesis of ( )-l-triorganosilyl-l-alkenes by hydrosilylation of l-aUcynes can be accomplished in the presence of a carbene-Pt complex in which the metal is also ligated to the two double bonds of diallyl ether. ... 

In the same year, a series of Ci-symmetric chiral triazolium Pd(II) complexes were prepared by Enders et al. As typically reported with the use of Ci-symmet-ric ligands, their NHC-metal complexes were obtained as diastereomeric mixtures due to the restricted rotation around the carbene-metal bond [6]. Without further elaboration, the authors stated that these complexes were used in an enantioselective Heck-type reaction achieving low asymmetric inductions. Soon thereafter, the authors investigated the coordination behavior of chiral triazolium salts 35 with [Rh(COD)Cl]2 and obtained a mixture of axially chiral complexes 36 with a diastereomeric excess of up to 94% (Scheme 3.20). These complexes were used as catalysts in asymmetric hydrosilylation reactions, achieving up to 44% ee for aromatic and ahphatic ketones [38,39]. 

The aim of this Chapter is to examine the application of well-defined N-hetero-cyclic carbene (NHC) complexes as well as the systems prepared in situ which involve free NHCs or the precursor salt for the reduction of imsaturated organic molecules such as alkynes, alkenes and carbonyl compounds. The most active complexes for such reductions contain electron-rich, late transition metals in low oxidation states. Herein, reductions useful for organic synthesis will be classified into four types aeeording to reductants used (i) hydrogenations, (ii) transfer hydrogenation, (iii) hydrosilylation and (iv) hydroboration. For examples of reduction reactions with systems containing non-classical NHC ligands, the reader is referred to Chapter 5. 

The reactivity of A-heterocyclic carbenes (NHC) with iron has been presented through the various types of complexes that have been described. Their catalytic applications in cross-coupling, allylation, aziridination, hydrosilylation, and as hydrogenase mimics are detailed. A report on cross-coupling reactions catalysed by complexes of iron group metals with NHC features the reaction mechanisms. ... 

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