Kinetics and Mechanism of Nitrile Hydration Catalized by Unhindered Hydridobis(phosphine)platinum(II) Complexes. Regioselective Hydration of Acrylonitrile

Article abstract of DOI:10.1021/ja00264a025

The reaction between trans-PtHCl(PR3)2 (R = Me and Et) and 1 equiv of NaOH in 50-50 water/acetonitrile solutions yields a species that catalyzes the hydrolysis of acetonitrile to acetamide at rates of 178 and 70 mol/(mol of catalyst h) for the R = Me and Et derivatives, respectively, at 80 deg C.These catalysts remain active for days and give as many as 6000 turnovers.The PMe3 derivate catalyzes hydrolysis of acrylonitrile but exhibits low regioselectivity between the olefin and nitrile functionalities at 80 deg C; at 25 deg C, it hydrates 21 mol of acetonitrile/(mol of catalyst h) and regioselectively (97percent) hydrates 6.1 mol of acrylonitrile/(mol of catalyst h) to acrylamide.The catalytic intermediates, ⁺, ⁺, and PtH(NHC(O)Me)(PEt3)2, have been intercepted and stereoscopically characterized and their interconversions demonstrated.Execpt at low hydroxide concentrations, the rates of catalysis were independent of hydroxide concentration, and proton transfer from solvating water to coordinated N-carboxamido, rather than nucleophilic attack of hydroxide on coordinated nitrile, limits the rate.Rate constants of ca. 8 and ca. 20 s^-1 were determined for the proton-transfer process for the PEt3 and PMe3 systems, respectively.A kinetic isotope effect of 3.4 was observed in reactions using D2O.Adjusting the catalytic solutions to pH 8.5 resulted in a linear hydroxide dependence, and rate constants of 4.9 +/ -0.3 and 7.0 +/- 0.4E3 M^-1s^-1 were measured for nucleophilic attack of hydroxide on coordinated acetonitrile for the PEt3 and PMe3 systems, respectively.Deuterium labeling experiments using trans-PtDCl(PMe3)2 to catalyze acrylonitrile hydration showed that olefin hydration proceeds through a coordinated olefin intermediate and, unlike nitrile hydration, involves a reductive elimination step in the catalytic cycle.