The reduction of acetals with cobalt carbonyl catalysts

Abstract

When heated with a dicobalt octacarbonyl (Co2(CO)8) catalyst under synthesis gas, acetals of aromatic aldehydes (in contrast to aliphatic acetals) gave good yields of the products of reductive cleavage, but no products of hydroformylation. A mechanism is proposed to explain the products of cyclic acetals of benzaldehyde which yielded not only the expected hydroxy-ethers, but also considerable amounts of dibenzyl ethers. The room temperature reductive cleavage of aromatic acetals by pure HCo(CO)4 in organic solvents indicates that HCo(CO)4 might be the active species in the catalytic reductions. Since acetals of benzaldehyde para-substituted with electron-releasing groups cleaved faster than those substituted with electron-withdrawing groups, the mechanism probably involves a carbonium ion intermediate.The previously observed formation of dibenzyl ether during benzaldehyde reduction with cobalt carbonyl catalysts can now be explained via the reductive cleavage of hemiacetal or acetal intermediates.