Effect of NaOH on Hydrogenation of Benzil Catalyzed by Bis(dimethylglyoximato)cobalt(II)-Base System

Abstract

The hydrogenation reaction of benzil catalyzed by Co(dmgH)2-(n-C41-19)3P, Co(dmgH)2-pyridine, Co(dmgFI), -CH, NH Co(dmgH), --(CH, ), NH, Co(dpgH), -pyridine, Co(dpgH)2-CH₃NH₂, or, Co(dpgH)2--(CH₃)2NH system, where dmgH is monoanion of dimethylglyoxime and dpgH is monoanion of diphenylglyoxime, was studied at various NaOH concentration under an atmospheric hydrogen pressure and at 0 and 20° C. It was found that the rate of hydrogenation catalyzed by Co(dmgH)2-(n-C41-19)3P system is independent of NaOH concentration, where as that catalyzed by other amine system depends on NaOH concentration. The absorption spectrum of Co(dmgH)2-(n-C41-19)3P system in the presence of molecular hydrogen and benzil shows bands at 408, 513, and 614nm in the visible region, which agree with those of Co(111)-hydride complex, while other amine system show the absorption bands due to Co01 complexes.
These results suggested that the reaction catalyzed by Co(drngH)2-(n-C4H9)3P system proceeds with a mechanism being different from the hydrogenase model which was proposed to the 1.17- drogenation reaction catalyzed by Co(dmgH)2-amine system. The dependence of the rate of hydrogenation catalyzed by Co(dmgH)2-pyridine system upon NaOH concentration could be explained in terms of the enzymatic model, and in this case not only pyridine (at the alkali concentration of [NaOH]/[Co]=-0-2) but also dimethylglyoximate anion (at [NaOH]/[C0]2) act as basic sites.