Iridium Complexes with Chiral and Achiral β‐Aminophosphane Ligands: Catalysts for >C=O Hydrogenation and H/D Exchange Involving both Homo‐ and Heterolytic H2 Activation

Chiral and achiral P,N‐chelated IrI complexes of the general type [(COD)Ir(P∩NR1R2)]BF4, where COD = η4‐1,5‐C8H12 and P∩NR1R2 = (1R,2R)‐, (1S,2S)‐, or (1R,2S)‐Ph2PC1H(Ph)C2H(Me)NR1R2 (NR1R2 = NH2, NHMe, NHCH2Ph, NHCHMe2, NMe2), Ph2PCH2CR2NH2 (R = H, Me), or 2‐Ph2PC6H4NHMe, have been prepared by treating [Ir(COD)2]BF4 with the required β‐aminophosphane in THF. The monolithiated ligands Ph2PCH2CMe2N(Li)H and 2‐Ph2PC6H4N(Li)Me interacted with [{(COD)Ir(μ‐Cl)}2] to give the neutral alkyl‐ and arylamido compounds [(COD)Ir(Ph2PCH2CMe2NH)] and [(COD)Ir(2‐Ph2PC6H4NMe)]. All IrI complexes [(COD)Ir(P∩NR1R2)]BF4 acted as catalysts for the direct hydrogenation of alkyl aryl ketones to the corresponding 1‐phenylalkanols, if combined with an alkaline or amine base in methanol under H2 (10−50 bar) between 25 and 50 °C. The reaction occurred with modest to moderate enantioselectivity (ca. 20−75% ee) if chelate complexes bearing the various optically active β‐aminophosphanes were used as catalysts. The base‐free amido complexes [(COD)Ir(P∩NR)] displayed similar catalytic activity to the combined systems [(COD)Ir(P∩NHR)]BF4−KOH (P∩NHR = Ph2PCH2CMe2NH2, 2‐Ph2PC6H4NHMe). The ability of both the cationic β‐amino‐ and the neutral β‐amidophosphane IrI complexes to undergo oxidative H2 addition and the observation of H2/D+ as well as H2/D2 exchange processes during catalysis provided evidence for a mechanism involving reversible “[IrIII(H)2−P∩NHR]+ ↔ [(η2‐H2)−IrIII(H)−P∩NR]+” proton‐to‐hydride transfer and heterolytic H2 cleavage on amino−dihydride and amido−dihydrogen−monohydride tautomers. The crystal structures of [(COD)Ir{(1S,2S)‐Ph2PCH(Ph)CH(Me)NHCH2Ph}]BF4·2THF, [(COD)Ir{1R,2S‐Ph2PCH(Ph)CH(Me)NHMe}]BF4·THF, and the orthometalated 18e IrI complex [(COD)Ir{(1R,2S)‐Ph2PCH(C6H4‐o)CH(Me)NHCHMe2}], which resulted from treatment of [(COD)Ir{(1R,2S)‐Ph2PCH(Ph)CH(Me)NHCHMe2}]BF4 with excess KOH, have been determined by single crystal X‐ray diffraction studies. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)