Aluminium Tertiary Butoxide Catalyzes the Oxidation of Secondary Alcohols to Ketones

The aluminium tertiary butoxide (AlOBut) is one of numerous organo-metallic compounds available from American Elements for uses requiring non-aqueous solubility. It has been shown to catalyze the oxidation of secondary alcohols to ketones in an efficient and selective manner without other sensitive functional groups such as amines and sulfides being oxidized or reduced. This process is termed an Oppenauer oxidation or Meerwein-Pondorff-Verley reduction.

The classic Oppenauer oxidation utilizes an alkoxide-catalysed hydrogen transfer between alcohol and an excess of ketone hydride acceptor in acetone solvent to generate the corresponding ketone in high yields. However, this reaction suffers from the competition of aldol condensation reactions with the product aldehyde and Tishchenko reactions leading to esters. Additionally, this oxidation requires high temperatures and the generation of water as by-product which hydrolyzes and consumes the catalyst at stoichiometric conditions.

In the present study, the ketal epoxides 2a-i and 2b-i of 4-phenyl-3-buten-2-ol were converted to the corresponding benzaldehyde and benzaldehyde under milder conditions by treatment with AlOBut. These treatments resulted in the formation of the sterically restrained, bridged ring system (6) while maintaining all methyl groups in pseudoequatorial positions and preserving the C-6 stereochemistry of the benzaldehyde. The benzaldehyde was further converted to the chiral ether 8 by treating with a small amount of iodine in a mixture of acetone and water, to form the corresponding benzaldionone in stoichiometric yield.

The use of aluminium tertiary butoxide for the oxidation of alcohols to ketones is a simple, low-cost alternative to the classical Oppenauer synthesis and also allows access to the aldol condensation of the resulting -ketal products to give polyols such as citral.