Lewis base catalyzed imine reduction

The experiments reported by Malkov and Kocovsky [12h] and by Sun [lla,b,e,f], indicate that the Lewis base catalyzed imine reduction with trichlorosilane is not affected by isomeric nonhomogeneity of the starting imines. Thus, for example, imines 9h-9j, which exist as 5 2 to 5 3 E/Z mixtures, were reduced to the corresponding amines with 94 97% ee (Table 4.5, entries 8 10). Appar ently, traces of HCl, naturally present in the moisture sensitive Cl3SiH, trigger an E/Z equilibration of imines 6-10, which is faster than the reduction (Scheme 4.5). 

Reduction of Imines with Trichlorosilane Catalyzed by Chiral Lewis Bases... 

Asymmetric hydrosilylation of ketones and ketoimines has been demonstrated in the absence of transition metal catalysts. Using catalytic amounts of chiral-alkoxide Lewis bases such as binaphthol (BINOL), Kagan was able to facilitate the asymmetric reduction of ketones (eq 19). This process is believed to arise from activation of the triethoxysilane by mono-alkoxide addition to give an activated pentavalent intermediate, which can undergo coordination of an aldehyde. This highly ordered hexacoordinate transition state directs reduction in an asymmetric manner, with subsequent catalyst regeneration. Brook was able to facilitate a similar tactic for asymmetric reduction by employing histidine as a bi-dentate Lewis base activator of triethoxysilane. A similar chiral lithium-alkoxide-catalyzed asymmetric reduction of imines was demonstrated by Hosomi with the di-lithio salt of BINOL and trimethoxysilane. ... 

Table 15.8 Asymmetric reduction of imines 106 derived from aromatic amines with trichlorosilanes catalyzed by chiral Lewis bases 110,112-115,118, and 120-124 (Scheme 15.25 and Figures 15.5 and 15.6).

P. and Stoncius, S. (2010) Reduction of imines with trichlorosilane catalyzed by chiral Lewis bases, in Chiral Amine Synthesis (Methods, Developments and Applications) (ed. T. Nugent), John Wiley Sons, Ltd, Chichester, p. 131 ... 

In 2009, the imidazole-derived Lewis base catalyst 92, which was prepared by Jones et al., was employed for the reduction of ketimines with trichlorosilane as the reducing agent Interestingly, low catalyst loading (as low as 1% mol) works well for this reduction (entry 13, Table 32.1) [57]. During their studies, the authors found that 92 is able to selectively reduce the imine while it is inactive for ketones. Therefore, in 2011, the same group developed an asymmetric reductive amination of ketones 16 catalyzed by 92 with trichlorosilane as the hydride donor (Scheme 32.19). However, the yields for the threealkyl ketimines, a two-step, one-pot procedure plus microwave irradiation is needed to secure a useful synthetic yield [60]. 

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