Phenoxaphosphonium Mixed Ylides in Ring Expansion Reaction

Treatment of mixed phosphonium–iodonium
ylides featuring
a six-membered phenoxaphosphonium fragment with aqueous tetrafluoroboronic
acid induces a rearrangement, resulting in expansion of the phosphacycle
and oxidation of the phosphorus atom. The target difficult-to-access
dibenzo[b,f][1,4]oxaphosphepine
oxides (3 examples) were isolated in excellent yields (up to 95%)
as mixtures of stereoisomers. Hydrolysis of a five-membered mixed
ylide, a dibenzophosphole derivative, predominantly preserves the
phosphole system with cycle expansion occurring as a side process.