Skeletal Rearrangement during Rhodium-Promoted Ring Opening of 1,2-Diphenyl-3-vinyl-1-cyclopropene. Preparation and Characterization of 1,2- and 2,3-Diphenyl-3,4-pentadienediyl Rhodium Complexes and Their Ring Closure to a 1,2-Diphenylcyclopentadienyl Complex

Under conditions of kinetic control, 1,2-diphenyl-3-vinyl-1-cyclopropene undergoes ring opening with the [Rh(Cl)(PMe 3 ) 2 ] fragment to give two isomeric η 3:η 1 -1,3-pentadienediyl compounds: the expected 1,2-diphenyl isomer, and the 2,3-diphenyl isomer resulting from an apparent skeletal rearrangement reaction. The latter complex has been characterized by X-ray crystallography. Both complexes underwent ring closure to give the same 1,2-diphenylcyclopentadienyl complex on treatment with silver ion. Addition of a third equivalent of trimethylphosphine to the 2,3-diphenyl isomer produced two meridional rhodacyclohexadienes, which exhibit facile solvent-dependent chloride dissociation. In contrast, phosphine added reversibly to the 1,2-diphenyl isomer to give only the chloride-dissociated compound, and the tris(phosphine) product could only be isolated after anion exchange with hexafluorophosphate. No deprotonation to give rhodabenzene complexes could be achieved. The mechanism of rearrangement is proposed to involve a carbocation rearrangement during the ring-opening reaction and is compared to other metal-promoted reactions of vinylcyclopropenes.