Cyanomethylation of chiral N-(tert-butylsulfinyl)imines with 2-trimethylsilylacetonitrile (TMSCH2CN) in the presence of a Lewis base such as tetrabutylammonium phenoxide (PhONn-Bu-4) proceeds smoothly to afford the corresponding beta-amino nitriles in good to high yields with excellent diastereoselectivities.