Synthesis, Isomerization, and Catalytic Transfer Hydrogenation Activity of Rhodium(III) Complexes Containing Both Chelating Dicarbenes and Diphosphine Ligands

Different rhodium(III) complexes [Rh(C,C)(P,P)X-2](+) bearing both a cis-chelating dicarbene and a diphosphine ligand were synthesized (C,C = methylene(4,4'-diimidazolylidene); P,P = 1,2-bis(diphenylphosphino)ethane (dppe), (R)-(+)-2,2'-bis(diphenylphosphino)-1,1'-binaphthalene (R-BINAP); X = halide, carbanion, NCMe). Solution analysis by NMR spectroscopy indicate a dynamic behavior of the complexes and cis/trans isomerization processes, likely through dissociation of the nonchelating ligands X (X = halide, NCMe), and eventually also involving the diphosphine ligand, identified by the formation of phosphine oxides. The presence of a diphosphine ligand in addition to the dicarbene substantially enhances the catalytic activity of the rhodium center in the transfer hydrogenation of ketones in iPrOH/KOH, reaching over 4000 turnover numbers and turnover frequencies around 1000 h(-1) vs 330 h(-1) for the phosphine-free analogue. Optimization of the catalytic conditions allowed transfer hydrogenation to be run with only 1 mol % base instead of the often used 10 mol %. The chiral R-BINAP ligand enhances catalytic activity, though no enantioselectivity was induced in the transfer hydrogenation of fluoroacetophenone as prochiral substrate.