Boranes as Reducing Reagents

The reduction of the chiral /3-hydroxy sulfinyl imines with HBcat or LiBHEt3 provided syn- and anti- 1,3-amino alcohols, respectively, with very high diastereomeric ratios (Equations (255) and (256)).1072 

The high chemoselectivity of BHCl2 SMe2 was demonstrated in the selective reduction of azide group in the presence of an ester, halide, nitrile, and nitro group (Equation (257)).1073 The reduction of azides with BHCl2-SMe2 was faster than the hydroboration of alkenes. 

The catalyzed reduction has a wide scope for various ketones with excellent enantioselectivity (Table 6). The protocol was successfully applied to the reduction of a-siloxy and tetahydropyranyloxy ketones (entry l),1074 cr-bromo and cr-chloroacetophenne (entry 2),1091-1094 trihalomethyl ketones (entry 3),1095-1099 a-sulfonyl ketones (entry 4),1100 

The oxazaborolidine-catalyzed reduction of 1,2-diketones (Equation (258)),1116-1120 1,2-keto-imines (Equation (259)),1121-1123 and 1,2-diimines (Equation (260))1124 provided optically active 1,2-diols, amino alcohols, and 1,2-diamines with both high enantiomeric and diastereomeric excess. The method can be also effective for (3- and y-analogs. 

Only limited success has been reported in the reduction of ketimines due to the low electrophilicity of the imine carbon and the rapid equilibration between the (E)- and (Z)-isomers. However, high enantioselectivity was achieved in catalytic reduction of imines of keto esters (Equation (261))1125 and oximes of acetophenone (Equation (262))1089,1125-1131 cyclic ketones (Equation (263)),1127 and a ketone possessing a boryl group (Equation (264)).1128 

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