1,2-Dihydro-1,3,2-diazasilines, formed on reaction of 4-amino-1-azabutadienes (3-amino-2-propenylideneamines) with dichlorodiphenyl- or dichlorodimethylsilane, react with aliphatic and aromatic isocyanates to give high yields of substituted 2-imino-1,2-dihydropyrimidines.