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