Diphosphetes-substituent stabilized ring systems

The mechanism of the formation of sigma P-2, sigma P-4-1,2-diphosphetes fro m phosphaacetylene and phosphinocarbene has been explored by quantum chemic al methods. A stepwise mechanism has been established, and no transition st ate for a concerted pathway could be obtained. The electron distribution of the ring can be understood in terms of a MO model, where the two substitue nts at phosphorus act as a pseudo pi center. Amino substituents on the sigm a (4) phosphorus and silyl group on the neighbouring carbon change the stab ility order of the possible isomers, stabilizing the 1,2-diphosphete ring. Substitution at the sigma P-2-by increasing its valence-destabilizes the di phosphete ring, which remains stable only as a result of the amino and sily l substituents.