EFFECT OF NAOH ON HYDROGENATION OF BENZIL CATALYZED BY BIS(DIMETHYLGLYOXIMATO)COBALT(II)-BASE SYSTEM

The hydrogenation reaction of benzil catalyzed by Co(dmgH)2-(n-C4H2,)3P, Co(dmgH)2-pyridine, Co(dmgH)2-CH3,NH2, Co(dmgH)2-(CH3)2NH, Co(dpgH)2-pyridine, Co(dpgH)2-CH3NH2, or,po(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-C4H9)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-C4H9)3P system in the presence of molecular hydrogen and benzil shows bands at 408, 513, and 614 nm in the visible region, which agree with those of Co(III) -hydride complex, while other amine system show the absorption bands due to Co(II) complexes. These results suggested that the reaction catalyzed by Co(dmgH)2-(n-C4H9)3P system proceeds with a mechanism being different from the hydrogenase model which was proposed to the hydrogenation 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]/[Co]>2) act as basic sites. © 1979, The Chemical Society of Japan. All rights reserved.