Reductive elimination of the alkenyl fragment and a phosphine ligand from [Rh(acac){(E)-CH=CHR}(PCy3)(2)]BF4 (R = Cy, Ph, H): preparation of [(E)-RHC=CHPCy3]BF4 from alkynes

In dichloromethane under reflux, the five-coordinate alkenyl complexes [Rh(acac){(E)-CHCHR}(PCy3)2]BF4 [R=Cy (1), Ph (2), H (3)] evolve into the alkenylphosphonium derivatives [Rh(acac){η2-(E)-CH(PCy3)CHR}(PCy3)]BF4 [R=Cy (4), Ph (5), H(6)], by reductive elimination reactions involving the one-electron alkenyl fragments and one of the two-electron phosphine ligands of 1–3. Complexes 4–6 react with carbon monoxide to afford Rh(acac)(CO)(PCy3) and [(E)-RHCCHPCy3]BF4 [R=Cy (8), Ph (9), H (10)]. In addition, we describe a new route for the preparation of alkenylphophonium salts starting from terminal alkynes, PCy3 and HBF4, and using the Rh(acac)(PCy3) unit as a template.
We thank the DGICYT (Projects PB 94-1186 and PB 95-0806, Programa de Promoción General del Conocimiento) for financial support. A. Martı́nez is grateful for the F.P.I. grant (M.E.C.).