New bicyclic sila-heterocycles: syntheses and crystal structure analyses of rac-7-ethoxy-2,2-diorganyl-2,3,5,7a-tetrahydro-1H-3a,6-diaza-2-sila-inden-4-ones

Metalation of 3,6-diethoxy-2,5-dihydropyrazine with one molar equivalent of n-BuLi and subsequent treatment with one molar equivalent of R2Si(CH2Cl)2 (R=Me, Ph) yielded the racemic 2-R2Si(CH2Cl)CH2-substituted 3,6-diethoxy-2,5-dihydropyrazines rac-2a (R=Me) and rac-2b (R=Ph). Heating of rac-2a and rac-2b led almost quantitatively (along with the formation of EtCl) to the new bicyclic sila-heterocycles rac-7-ethoxy-2,2-dimethyl-2,3,5,7a-tetrahydro-1H-3a,6-diaza-2-sila-inden-4-one (rac-3a) and rac-7-ethoxy-2,2-diphenyl-2,3,5,7a-tetrahydro-1H-3a,6-diaza-2-sila-inden-4-one (rac-3b), respectively. Compounds rac-2b, rac-3a, and rac-3b were structurally characterized by NMR studies and crystal structure analyses. In addition, the kinetics of the cyclization reaction rac-2a→rac-3a were investigated.