Chiral iridium xyliphos complexes for the catalytic imine hydrogenation leading to the metolachlor herbicide:: Isolation of catalyst-substrate adducts

Iridium complexes relevant to the catalytic enantioselective hydrogenation of 2-methyl-6-ethylphenyl-1'-methyl-2'-methoxyethylimine (MEA-imine, 1) in the Syngenta Metolachlor (3) process were prepared and characterized. Reaction of the diphosphane (S)-1-[(R)-2-(diphenylphosphanyl)ferrocenyl]ethyldi(3,5-xylyl)phosphane ((S)-(R)-Xyliphos, (S)-(R)-4) with [Ir(2)(mu-Cl)(2)(cod)(2)] (cod=1,5-cyclooctadiene) afforded [Ir(Cl)(cod){(S)-(R)-4}] (7), which reacted with AgBF(4) to form [Ir(cod){(S)-(R)-4}]BF(4) (8). Complexes 7 and 8 reacted with iodide to yield [Ir(1)(cod){(S)-(R)-4}] (9). When 9 was treated with one and two equivalents of HBF(4), two isomers of the cationic Ir(III) iodo hydrido complex [Ir(I)(H)-(cod){(S)-(R)-4}]BF(4) were isolated (10 and 11, respectively). Complex 9 was oxidized with one equivalent Of 12 to give the iodo-bridged dinuclear species [Ir(2)I(2)(mu-I)(3){(S)-(R)-4}(2)]I (12). [Ir(2)(mu-Cl)(2)(coe)(4)] (coe=cyclooctene) reacted with (S)-(R)-4 to yield the chloro-bridged dinuclear complex [Ir(2)(mu-Cl)(2){(S)-(R)-4}(2)] (13). Complexes 7-12 were structurally characterized by single-crystal X-ray diffraction and tested as single-component catalyst precursors for enantioselective hydrogenation of MEA-imine. Complex 10 and dinuclear complex 12 gave the best catalytic results. Efforts were also directed at isolating substrate- or product-catalyst adducts: Treatment of 8 with 2,6-dimethylphenyl-1'-methyl-2'-methoxyethylimine (DMA-imine, 14, a model for 1) under H(2) allowed four isomers of [Ir(H)(2){(S)-(R)-4}(14)]BF(4) (18-21) to be isolated. These analytically pure isomers were fully characterized by 2D NMR techniques. X-ray structural analysis of an Ir(1)-imine adduct, namely, [Ir(C(2)H(4))(2)(14)]BF(4) (25), which was prepared by reacting [IrCl(C(2)H(4))(4)] with [Ag(14)(2)]BF(4) (16), confirmed the kappa(2) coordination mode of imine 14.