Chiral Xyliphos Complexes for the Catalytic Imine Hydrogenation Leading to the Metolachlor Herbicide: Isolation of Catalyst–Substrate Adducts

Abstract

Abstract 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 (μ‐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(I ) (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 I 2 to give the iodo‐bridged dinuclear species [Ir 2 I 2 (μ‐I) 3 {( S )‐( R )‐ 4 } 2 )]I ( 12 ). [Ir 2 (μ‐Cl) 2 (coe) 4 ] (coe=cyclooctene) reacted with ( S )‐( R )‐ 4 to yield the chloro‐bridged dinuclear complex [Ir 2 (μ‐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 I –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 κ 2 coordination mode of imine 14 .