Iridium–Imine and –Amine Complexes Relevant to the ( S )‐Metolachlor Process: Structures, Exchange Kinetics, and CH Activation by Ir I Causing Racemization

Iridium complexes of DMA‐imine [2,6‐dimethylphenyl‐1′‐methyl‐2′‐methoxyethylimine, 1 a ) and ( R )‐DMA‐amine [(1′ R )‐2,6‐dimethylphenyl‐1′‐methyl‐2′‐methoxyethylamine, 2 a ] that are relevant to the catalytic imine hydrogenation step of the Syngenta ( S )‐Metolachlor process were synthesized: metathetical exchange of [Ir 2 Cl 2 (cod) 2 ] (cod=1,5‐cyclooctadiene) with [Ag( 1 a ) 2 ]BF 4 and [Ag(( R )‐ 2 a ) 2 ]BF 4 afforded [Ir(cod)(κ 2‐ ‐ 1 a )]BF 4 ( 11 ) and [Ir(cod)(κ 2 ‐( R )‐ 2 a )]BF 4 (( R )‐ 19 )), respectively. These complexes were then used in stopped‐flow experiments to study the displacement of amine 2 a from complex 19 by imine 1 a to form the imine complex 11 , thus modeling the product/substrate exchange step in the catalytic cycle. The data suggest a two‐step associative mechanism characterized by k 1 =(2.6±0.3)×10 2   M −1  s −1 and k 2 =(4.3±0.6)×10 −2  s −1 with the respective activation energies E A1 =(7.5±0.6) kJ mol −1 and E A2 =(37±3) kJ mol −1 . Furthermore, complex 11 reacted with H 2 O to afford the hydrolysis product [Ir(cod)( η 6‐ ‐2,6‐dimethylaniline)]BF 4 ( 12 ), and with I 2 to liberate quantitatively the DMA‐iminium salt 14 . On the other hand, the chiral amine complex ( R )‐ 19 formed the optically inactive η 6 ‐bound compound [Ir(cod)( η 6 ‐ rac ‐ 2 a )]BF 4 ( rac‐ 18 ) upon dissolution in THF at room temperature, presumably via intramolecular CH activation. This racemization was found to be a two‐step event with k ′ 1 =9.0×10 −4 s −1 and k 2 =2.89×10 −5 s −1 , featuring an optically active intermediate prior to sp 3 CH activation. Compounds 11 , 12 , rac ‐ 18 , and ( R )‐ 19 were structurally characterized by single‐crystal X‐ray analyses.