Olefins as Steering Ligands for Homogeneously Catalyzed Hydrogenations

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Abstract
Iridium(I) complexes containing a (5H‐dibenzo[a,d]cyclohepten‐5‐yl)‐phosphane (troppR; 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(troppCyc)(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.