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-diethox y-2,5-dihydropyrazines rac-2a (R = Me) and lac-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-te trahydro-1H-3a,6-diaza-2-sila-inden-4-one (rac-3a) and rac-7-ethoxy-2,2-dip henyl-2,3,5,7a-tetrahydro-1H-3a, 6-diaza-2-sila-inden-4-one (rac-3b), respe ctively. Compounds rac-2b, rac-3a, and rac-3b were structurally characteriz ed by NMR studies and crystal structure analyses. In addition, the kinetics of the cyclization reaction rac-2a --> rac-3a were investigated. (C) 2000 Elsevier Science B.V. All rights reserved.