Imines trichlorosilane enantioselective

A contribution by Matsumura and co-workers [33] in 2006 paved the way toward the development of a novel class of catalysts for trichlorosilane-mediated reductions, derived from chiral amino alcohol. This group identified a series of IV-picolinoyl-pyrrolidine derivatives able to activate trichlorosilane in the reduction of aromatic imines, leading to good enantioselectivity (up to 80%). The authors proposed that both the nitrogen atom of the picolinoyl group and the carbonyl oxygen atom are involved in the coordination and activation of the silicon atom (Scheme 15.9). 

Next to be investigated were the members of the homologous series, namely, P-imino nitriles 133 and P-imino esters 134, existing predominantly in the conjugated enamine forms 131 and 132, which cannot be reduced with trichlorosilane (Scheme 15.27) [97d]. However, Maikov and Kocovsky found that Bronsted acids can facilitate the enamine-imine equiUbrium and provide sufficient concentration of the desired imine form, whose reduction can now be accomphshed. In the case of Sigamide 114b, acetic acid (1 equiv) was identified as an optimal additive the reduction, carried out in its presence, afforded the expected P-amino nitriles 135 and P-amino esters 136 in high yields and enantioselectivities (Table 15.12, entries 1 and 2). 

Recently, the methodology was successfully employed for the enantioselective reduction of indoles 137 into indolines 138 using catalysts 113c and 118b (Scheme 15.28) [98gj. To faciUtate the enamine/imine tautomeric equiUbrium, the required Br0nsted acid was generated by a controlled hydrolysis of excess trichlorosilane with water. 

Onomura O, Kouchi Y, Iwasaki F, Matsumura Y. New organic activators for the enantioselective reduction of aromatic imines with trichlorosilane. Tetrahedron Lett. 2006 47 3751-3754. 

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