On the Rapid Oxidation of Allene-Containing Phosphines

Allene-containing phosphines have recently been shown to serve as effective ligands in transition metal-catalyzed enantioselective reactions. Surprisingly, (2-allenylphenyl)­diphenyl phosphines rapidly oxidize when exposed to air, whereas many other triaryl phosphines are stable under ambient conditions. Here we describe experiments designed to understand the origin of this behavior. Stereochemical probes and an isolated phosphonium complex support the hypothesis that phosphines can cyclize onto pendant allenes and that the resultant zwitterion undergoes rapid oxidation with molecular oxygen