Formation of Dimeric Products during the Hydrogenation of Benzaldehyde with β-Trithiobenzaldehyde Catalyzed by Molybdenum Trisulfide

Abstract

During the hydrogenation of benzaldehyde catalyzed by molybdenum trisulfide (MoS₃) at 120-230C under hydrogen pressure of 50 kg/cm², an appreciable amount of dimeric products, viz., benzoin, trans-stilbene and dibenzyl were obtained. ln order to elucidate the course of the reaction and the mechanism of dimerization, benzaldehyde and related intermediates were subjected to heat-treatment or hydrogenated under various conditions.
It was found that the dimeric products formed mainly in the absence of selvent, and in particular, trans-stilbene was noteworthily obtained as a main product under a relatively low pressure of hydrogen or at comparatively high hydrogenation temperatures. These results were quite similar to that obtained with the hydrogen-poor Raney nickel catalyst. Furthermere, during the hydrogenation of p-dimethylaminobenzaldehyde which scarcely underwent benzoin condensation, the corresponding dimeric product, 4, 4'-tetramethyldiaminedibenzyl, was obtained in the yield up to ca, 20%.
It was suggested that the formation of dimeric product might take place on the catalyst surface through radicals, and the chemisorbed-dihydroxy trans-stilbene might be a main intermediate in the formation of trans-stilbene.
During the hydrogenation of β-trithiobenzaldehyde at 230C, dibenzyl was obtained as a main product, and trans-stilbene or dibenzyl sulfides might be the main intermediates in the reaction. Whereas, benzyl mercaptan did not seem to play important roles in the formation of dimeric products under these conditions.
The hydrogenation reaction course and its mechanism were discussed, and the suggested path for the formation of dimeric preducts during the hydrogenation of benzaldehyde is illustrated in Fig.1, schematically.