Heteroorganic betaines 6. Structure and reactivity of silicon-containing organophosphorus betaines with the S--Si-C-P+ fragment: a DFT study

The structures of silicon-containing organophosphorus betaines S--SiR21-CR2 2-P+-R-3(3) and their ylide isomers were calculated using the density funct ional approach with the gradient-corrected PBE functional and extended TZ2P basis set. Three possible pathways of thermal decomposition of these betai nes were analyzed. These are (i) cleavage of the central C-Si bond with the formation of a Wittig ylide and silanethione, (ii) intramolecular nucleoph ilic S-N-substitution with elimination of phosphine PR33 and the formation of silathiirane (the Corey-Chaikovscky transformation), and (iii) a Wittig- type decomposition followed by the formation of substituted silaethylene. The structures of products and transition states of these reactions were ca lculated. The cis-gauche conformation of the S--Si-C-P+ fragment of betaine s was found to be the most stable. This is in agreement with the results of X-ray diffraction study and can be rationalized by strong Coulomb attracti on between the cationic and anionic centers. The betaines are stable toward retro-Wittig thermal decomposition. The Corey-Chaikovscky formation of thi irane is preferable under conditions of thermal decomposition. Retro-Wittig -type decomposition of betaines followed by the formation of silanethione i s favored by intra- and intermolecular coordination of donor ligands.