SYNTHESIS AND STRUCTURE OF POLYSILABRIDGED AND DOUBLY BRIDGED ALLENES

Polysilacycloalka-1,2-dienes 1, 2, and 3 are synthesized by reactions of the Ph2C3 dianion with corresponding dichloropolysilanes. X-ray cry stallographic analyses of the tetrasilacyclohepta-1,2-diene 1 and the trisilacyclohexa-1,2-diene 3 show allenic sp carbons to be bent by 6-d egrees and 19-degrees, respectively, and the torsional angles around t he allenic moieties are 85.3-degrees and 52.2-degrees, respectively. P hotoirradiation (254 nm) of 1 gives a 1,3-bridged cyclopropene (11) vi a photoinduced rearrangement. Synthesis and X-ray-determined structure of the doubly bridged allene octasila[4.4]-betweenallene 4 are also r eported. The between allene 4 is synthesized by the reaction of dichlo rooctamethyltetrasilane with hexachloropropene in the presence of magn esium. The optimized structures of their smaller ring systems are also reported by semiempirical PM3 calculations.