Olefins as Steering Ligands for Homogeneously Catalyzed Hydrogenations

Abstract

Iridium(I) complexes containing a (5H-dibenzo[a,d]cyclohepten-5-yl)-phosphane (tropp^R; R = phosphorus-bound substituent = Ph, Cyc) as a rigid, concave-shaped, mixed phosphane olefin ligand were prepared and tested as catalyst precursors in the hydrogenation of imines. With the complex [Ir(tropp^Cyc)(cod)]OTf, turnover frequencies (TOFs) of >6000 h⁻¹ were reached in the hydrogenation of N-phenyl-benzylidenamine, PhNCHPh. Lower activities (TOF>80 h⁻¹) are observed with N-phenyl-(1-phenylethylidene)amine, PhNCMePh. Chiral tropp-type ligands were prepared in few simple steps. Monosubstitution of the olefinic unit in the dibenzo[a,d]cycloheptenyl moiety with (R)- or (S)-mentholate gave mixtures of diastereomers that could be separated and isolated in enantiomerically pure form. Iridium(I) complexes with these ligands are rare examples of side-on bonded enolether complexes. In catalytic imine hydrogenations, complete conversion (>98 %) was reached in all cases (conditions: p[H₂] = 50 bar, T = 50 °C, t = 2 h, substrate/catalyst 100:1). The best enantiomeric excess (ee = 86 % S isomer) was reached with PhNCMePh as substrate and the R,R form of the (10-menthyloxy-5H-dibenzo[a,d]cyclohepten-5-yl)diphenylphosphane ligand. The iridium(I) complex containing the same phosphane gave a 60 % ee (S isomer) with the enamide N-(1-phenylvinyl)acetamide as substrate (conditions: p[H₂] = 4 bar, T = 50 °C, t = 18 h, substrate/catalyst = 50:1). These reactions constitute the first examples in which chiral olefins have been used as steering ligands in catalytic enantioselective hydrogenations.