Heteroorganic betaines - 5. A DFT study of the structure and reactivity of6,6-dimethyl-6-silafulvene and silicon-containing organophosphorus betaine-C5H4SiMe2CH2PMe3+

The structures of 6,6-dimethyl-6-silafulvene C5H4SiMe2 (3). its donor-accep tor complex with ammonia, C5H4SiMe2. NH3, dimethylfulvene, a number of cycl opentadienylides. methylenetrimethylphosphorane (6), and silicon-containing organophosphorus betaine -C5H4SiMe2CH2PMe3+ (13), the product of nucleophi lic addition of 6 to 3, were calculated using the density functional approa ch. For compound 13. the potential energy minimum corresponds to the confor mation with gauche-arrangement of the cyclopentadienyl anionic and trimethy lphosphonium cationic centers and a C-Si-C-P dihedral angle of 30.5 degrees , which is due to the Coulomb attraction between these centers. According t o calculations, betaine 13 is rather stable toward decomposition into 3 and 6 (DeltaH degrees = 42 kcal mol(-1), DeltaG degrees = 30 kcal mol(-1)). Th e main channel of thermal decomposition of compound 13 involves an intramol ecular nucleophilic substitution. which proceeds with elimination of trimet hylphosphine and results in 1,1-dimethyl-1-silaspiro[2,4]hepta-4,6-diene, w hich then undergoes a ready and irreversible isomerization into 6,6-dimethy l-6-silabicyclo[3.2.0]hepta-1,3-diene owing to the [1,5]-sigmatropic shift of the C-Si bond.