The conversion of furans into phosphinines.

The [4+2] cycloadducts between furan compounds and a methylenechlorophosphane pentacarbonyltungsten complex are converted into the corresponding 2-hydroxy- or 2-bromophosphinine complexes by treatment with BBr(3) and triethylamine. The X-ray crystal structure of the parent 2-phosphaphenol complex shows that the hydroxy substituent is coplanar with the ring and that the conjugation between the π lone pair of electrons on the oxygen atom and the ring leads to lengthening of the P-C(OH) bond. This complex is methylated at the phosphorus atom by methyl iodide with disruption of the ring aromaticity. The complex is further silylated, acylated, and triflated at the oxygen atom with retention of the aromatic structure, and decomplexation by 1,2-bis(diphenylphosphino)propane (DPPE) leads to the free parent 2-phosphaphenol. The comparison of the X-ray crystallographic structural analysis of the 2-bromophosphinine complex with an earlier structure of the analogous 2-chlorophosphinine complex suggests that 2-bromo species is a better ligand than 2-chlorophosphinine. When the [4+2] adducts are treated with BBr(3) and water, a 2-hydroxy-3-bromo-1,2,3,6-tetrahydrophosphinine derivative is obtained, which yields a seven-membered 2,1-phosphaoxepin when treated with an amine.