The reduction of acetals with cobalt carbonyl catalysts

Abstract

When heated with a dicobalt octacarbonyl (Co 2 (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.