Reductive elimination of [Ph2C=C=CHPR3]BF4 from the rhodium(III)-allenyl derivatives [Rh(acac){CH=C=CPh2}(PR3)(2)]BF4 (PR3=PCy3, PiPr(3))

The olefinic unit of the complexes Rh(acac)(cyclooctadiene)(PR3) (PR3 = PCy3 (1), PiPr(3) (2)) is displaced by 1,1-diphenyl-2-propyn-1-ol, to afford Rh(acac){eta(2)-HC=CC(OH)Ph-2}(PR3) (PR3 = PCy3 (3), PiPr(3) (4)). At 60 degrees C, in toluene as solvent, and in the presence of 1 equiv of phosphine, complexes 3 and 4 evolve into Rh(acac)H{C=CC(OH)Ph-2}(PR3)(2) (PR3 = PCy3 (5), PiPr(3) (6)). At -78 degrees C, the treatment of complex 5 with HBF4 . OEt2 leads to the allenylphosphonium compound [Rh(acac){eta(2)-CH(PCy3)=C=CPh2}(PCy3)]BF4 (7). The X-ray crystal structure analysis of 7 reveals that the coordination geometry around the rhodium center is almost square-planar with the CH(PCy3)=C bond disposed perpendicular to the coordination plane of the rhodium center. The allenylphosphonium ligand of 7 is easily displaced by carbon monoxide, to give Rh(acac)(CO)(PCy3) (8) and [Ph2C=C=CHPCy3]BF4 (9). At -78 degrees C, the protonation of complex 6 leads to the five-coordinate rhodium(III)-allenyl derivative [Rh(acac){CH=C=CPh2}(PiPr(3))(2)]BF4 (10), which evolves in solution into [Rh(acac){eta(2)-CH(PiPr(3))=C=CPh2}(PiPr(3))]BF4 (11). For this isomerization first-order constants k(obs) were obtained in CD2Cl2, which give activation parameters of Delta H-not equal = 23 +/- 2 kcal mol(-1) and AS(not equal) = 2 +/- 2 cal K-1 mol(-1). Similarly to 7, the reaction of complex 11 with carbon monoxide affords Rh(acac)(CO)(PiPr(3)) (12) and [Ph2C=C=CHPiPr(3)]BF4 (13).