| 15378634 |
Dorta R, Broggini D, Kissner R, Togni A: Iridium-imine and -amine complexes relevant to the (S)-metolachlor process: structures, exchange kinetics, and C-H activation by Iri causing racemization. Chemistry. 2004 Sep 20;10(18):4546-55.
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. |
81(1,1,1,1) |