Asymmetric silylation

Numbers of reactions have been developed by utilizing the strong affinity of the fluoride anion to the silicon atom. In this context, the use of chiral non-racemic ammonium fluorides137 41 43 751 for asymmetric silyl mediated reactions will be further investigated in future. 

Another structurally modified guanidine was reported by Ishikawa et al. as a chiral superbase for asymmetric silylation of secondary alcohols [122]. Soon after, Ishikawa discovered that the same catalyst promoted asymmetric Michael additions of glycine imines to acrylates [123]. The additions were promoted in good yield and great asymmetric induction under neat reaction conditions with guanidine catalyst 250 (Scheme 68). The authors deduced that the high conversion and selectivity were due to the relative configuration of the three chiral centers of the catalyst in... 

The detailed mechanism of a similar asymmetric silylation of 80 to give 81 is not yet known.15... 

Optically active alcohols, amines, and alkanes can be prepared by the metal catalyzed asymmetric hydrosilylation of ketones, imines, and olefins [77,94,95]. Several catalytic systems have been successfully demonstrated, such as the asymmetric silylation of aryl ketones with rhodium and Pybox ligands however, there are no industrial processes that use asymmetric hydrosilylation. The asymmetric hydrosilyation of olefins to alkylsilanes (and the corresponding alcohol) can be accomplished with palladium catalysts that contain chiral monophosphines with high enantioselectivities (up to 96% ee) and reasonably good turnovers (S/C = 1000) [96]. Unfortunately, high enantioselectivities are only limited to the asymmetric hydrosilylation of styrene derivatives [97]. Hydrosilylation of simple terminal olefins with palladium catalysts that contain the monophosphine, MeO-MOP (67), can be obtained with enantioselectivities in the range of 94-97% ee and regioselectivities of the branched to normal of the products of 66/43 to 94/ 6 (Scheme 26) [98.99]. 

Concerning stereochemistry, reactions at silicon usually proceed with high stereoselectivity, with either retention or inversion of configuration, and only rarely with racemization. Moreover, asymmetric silyl radicals and silyl anions show significant optical stability. 

Table 4.10 Guanidine mediated asymmetric silylation of cyclic secondary alcohols...

It is interesting that the chiral pro-azaphosphatrane 8 was found to be ineffective in promoting selective asymmetric silylations [78]. This maybe ascribed to the considerable distance in the intermediate of the chiral region from the site of attack on silicon of the alcohol oxygen, which could well be opposite the phosphorus for steric reasons. 

Hoveyda and Snapper developed an excellent method for asymmetric silylation by employing a newly developed chiral imidazole catalyst, 45 (Scheme 22.10). By virtue of catalyst 45, silylative asymmetric desymmetrisation of meso-1,l-dioX 46 (Scheme 22.10A) and a-symmetric triol 48... 

Scheme 22.10B) were achieved in high selectivity, although a relatively high catalyst loading was required. This method for asymmetric silylation was successfully extended to the regiodivergent resolution of racemic 1,2-diols 50 (Scheme 22.10C). ... 

Scheme 22.10 Asymmetric silylation promoted by Hoveyda-Snapper s chiral imidazole catalyst 45...

Scheme 22.12 Catalytic asymmetric silylation by Tan s protocol, intramolecularity .

It is worth mentioning that other methodologies for the asymmetric silyl addition to a,p-unsaturated systems [116] do not allow the formation of quaternary centres. 

Scheme 41.39 Kinetic resolution of secondary alcohols by catalytic asymmetric silylation.

Scheme 41.40 Kinetic resolution of 1,2-diols by catalytic asymmetric silylation.

Scheme 11.8 NHC-Cu-catalyzed asymmetric silyl transfer on tosylimines.

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