Alkoxide transfer from aluminium alkoxide to aldehyde in the Tishchenko reaction

Abstract

Tishchenko reaction of benzaldehyde with various aluminium alkoxides has been studied kinetically in benzene at 20°. The aluminium t-butoxide catalysed reaction shows a long induction period and gives benzyl benzoate (75–90%) together with a small amount of t-butyl benzoate and benzyl alcohol. The initial rate of reaction is decreased in the order: aluminium benzyl oxide > di-t-butoxyaluminium benzyloxide [a reaction mixture of PhCHOAl(O-t-Bu) 3] > aluminium isopropoxide > aluminium t-butoxide. The aluminium benzyloxide catalysed reaction is second-order in benzaldehyde. These results support a mechanism involving an alkoxide transfer from catalyst to aldehyde. A t-butoxide transfer to benzaldehyde is much slower than the benzyloxide transfer because of a large steric interaction between bulky t-butoxide and benzene ring. On the contrary, hydride transfer is rate-determining with aluminium benzyloxide. Electron-withdrawing substituents of aldehyde promote the Tishchenko reaction. Crossed Tishchenko reaction is discussed in terms of the alkoxide transfer and the hydride transfer.