ORGN 323

Phosphadioxirane (POOR₃, R=2-methoxyphenyl-) decomposes into phosphine oxide (POR₃) and triplet dioxygen (³O₂) with a half-life of about 2000s at -80^oC in 1:1 CD₂Cl₂/toluene solution.(Ho, D.G.; Gao, R.; Celaje, J.; Chung, H.-Y.; Selke, M. “ Phosphadioxirane: A Peroxide from an Ortho-Substituted Arylphosphine and Singlet Dioxygen”, Science, 2003, 302, 259-262.) Two mechanisms for this decomposition, shown in Schemes 1 and 2, are consistent with the kinetic data.  In the first, the decomposition proceeds via slow loss of ³O₂, followed by very fast scavenging reaction of the resulting free phosphine with another molecule of phosphadioxirane, producing two molecules of phosphine oxide. In the alternative pathway (Scheme 2), the first step slowly generates a triplet phosphine oxide – oxene complex, followed by rapid reaction of the resulting triplet species with another molecule of phosphadioxirane, leading to formation of phosphine oxide and triplet dioxygen.  The rate laws for both mechanisms would be the first-order in phosphadioxirane. The energetics of the two proposed mechanisms have been investigated theoretically with the B3LYP/6-31G* and CBS-QB3 method (R=Me). The modeling calculation suggests that the mechanism involving the phosphine oxide – oxene intermediate is more favorable than that involving the PR₃ intermediate.

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