From a Fluorenyl Substituted Ylide-Functionalized Phosphine to a Neutral Phosphide via P-C Bond Cleavage

Bulky ylide-substituted phosphines have recently found application as potent ligands in homogeneous catalysis. The attempted synthesis of the ylide-substituted fluorenylphosphine Y_Ph P(Cy)Flu [Y_Ph =Cy₃ P(Ph)C; Flu=9-methylfluorenyl] now resulted in the unexpected elimination of 9-methylenefluorene during the deprotonation step of the intermediary α-phosphino phosphonium salt to yield the secondary ylide-substituted phosphine Y_Ph P(Cy)H. This phosphine underwent formal H₂ elimination under basic conditions to form a cyclic phosphonium ylide with a P-C-P-C four-membered ring via deprotonation of one cyclohexyl group of the PCy₃ moiety. Upon coordination to transition metals the secondary ylidylphosphine forms a neutral phosphide ligand by shift of the proton into the ylide-backbone and formation of zwitterionic metal complexes.