Iridium-imine and -amine complexes relevant to the (S)-metolachlor process:: Structures, exchange kinetics, and C-H activation by IrI causing racemization

Iridium complexes of DMA-imine [2,6-dimethylphenyl-1'-methyl-2'-methoxyethylimine, 1a) and (R)DMA-amine [(1'R)-2,6-dimethylphenyl-1'-methyl-2'-methoxyethylamine, 2a] 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(1a)(2)]BF4 and [Ag((R)-2a)(2)]BF4 afforded [Ir(cod)(kappa(2)-1a)]BF4 (11) and [Ir(cod)(kappa(2-)(R)-2a)]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 1a 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) x 10(2) M-1 s(-1) and k(2) (4.3 +/- 10.6) x 10(-2)s(-1) with the respective activation energies E-A1 = (7.5 +/- 0.6)kJmol(-1) and E-A2 = (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 1, to liberate quantitatively the DMA-iminium salt 14. On the other hand, the chiral amine complex (R)-19 formed the optically inactive eta(6)-bound compound [Ir(cod)(eta(6)-rac-2a)]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 k(2) = 2.89 X 10(-5) s(-1), featuring an optically active intermediate prior to sp(3) C-H activation. Compounds 11, 12, rac-18, and (R)-19 were structurally characterized by single-crystal X-ray analyses.