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

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)(kappa2- -1 a)]BF4 (11) and [Ir(cod)(kappa2-(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) x 10(2) M(-1) s(-1) and k2=(4.3+/-0.6) x 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)(eta(6-)-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 eta6-bound compound [Ir(cod)(eta6-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 x 10(-4) s(-1) and k2=2.89 x 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.

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

“…The coordination behavior of amine 17 on iridium and its exchange kinetics with imine 14 have been the object of investigation in our laboratory and will be reported in due course. [36] Experimental Section General aspects: All experiments were performed under purified N 2 in a MBraun glovebox or under purified Ar using standard Schlenk techniques. All glassware and stirbars were oven-dried at 413 K. Pentane and hexane were distilled from Na/K (50 % w/w)/tetraglyme/benzophenone, THF and diethyl ether from Na/K, benzene from Na/benzophenone, toluene and heptane from Na, and CH 2 Cl 2 from Pb/Na alloy (Aldrich).…”

“…This is because imine 14 is a much better ligand for iridium than amine 17. [36] Note that amine 17 is a close model of MEA-amine (2). Complex (S)-(R)-13 reacted with amine (R)-17 to form complicated mixtures of hydrides.…”

Chiral Diphosphine ddppm‐Iridium Complexes: Effective Asymmetric Imine Hydrogenations at Ambient Pressures

“…Researchers have reported previously the facile racemisation of N-arylamine-iridium complexes via C À H activation. [16] In order to establish whether a similar process takes place with the Ir-ddppm catalyst the hydrogenation of imine 8a was followed over the course of several days. A slow drop in enantioselectivity from 94 to 88% was observed during the course of 3 days.…”

Synthesis, structure and catalytic activities for hydrogen transfer reaction of the carbonyl ruthenium(II) complex containing polypyridine and phosphine ligands

“…[1][2][3] Noyori and coworkers have developed highly active catalyst precursors of the transRuCl 2 (diphosphine)(1,2-diamine) type for the selective hydrogenation of ketones. [4,5] Morris's group have substantiated such a mechanism, particularly for a binap-tmen system (tmen=H 2 NC(Me) 2 C(Me) 2 NH 2 ) in which the key intermediates involve trans-dihydrido species [6].…”