Olefins as steering ligands for homogeneously catalyzed hydrogenations

Iridium(i) complexes containing a (5-H-dibenzo[a,d]cyclohepten-5yl)-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[H-2] = 50 bar, T = 50degreesC, 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)diphenylphos- phane 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[H-2] = 4 bar, T = 50degreesC, 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.