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

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Abstract
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 [Ir2Cl2(cod)2] (cod=1,5-cyclooctadiene) with [Ag(1 a)2]BF4 and [Ag((R)-2 a)2]BF4 afforded [Ir(cod)(κ2--1 a)]BF4 (11) and [Ir(cod)(κ2-(R)-2 a)]BF4 ((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 k1=(2.6±0.3)×102 M−1 s−1 and k2=(4.3±0.6)×10−2 s−1 with the respective activation energies EA1=(7.5±0.6) kJ mol−1 and EA2=(37±3) kJ mol−1. Furthermore, complex 11 reacted with H2O to afford the hydrolysis product [Ir(cod)(η6--2,6-dimethylaniline)]BF4 (12), and with I2 to liberate quantitatively the DMA-iminium salt 14. On the other hand, the chiral amine complex (R)-19 formed the optically inactive η6-bound compound [Ir(cod)(η6-rac-2 a)]BF4 (rac-18) upon dissolution in THF at room temperature, presumably via intramolecular CH activation. This racemization was found to be a two-step event with k′1=9.0×10−4 s−1 and k2=2.89×10−5 s−1, featuring an optically active intermediate prior to sp3 CH activation. Compounds 11, 12, rac-18, and (R)-19 were structurally characterized by single-crystal X-ray analyses.

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