Chromic Acid Oxidation of Cyclopropanols

Abstract

The chromic acid oxidation of secondary cyclopropanols is 103to 106times faster than that of other secondary alcohols. Tertiary cyclopropanols are even more reactive with 1,2,2,3,3-pentamethylcyclopropanol being the most reactive organic compound known toward this oxidant. The free hydroxyl group is a necessary prerequisite for the high reactivity; cyclopropyl alkyl ethers and cyclopropyl esters are relatively unreactive. Only ring cleavage products are obtained; secondary cyclopropanols form β-hydroxypropionaldehydes, and tertiary alcohols yield β-hydroxy ketones. The formation of free radical intermediates has been demonstrated by the initiation of polymerization of acrylonitrile and acrylamide and by trapping with oxygen. Oxidation of cyclopropanol in the presence of oxygen leads to the formation of a substantial amount of malonaldehyde; 1-alkylcyclopropanols yield the corresponding 3-ketoaldehydes. The proposed mechanism consists of a rate-limiting two-electron carbon-carbon bond ring cleavage of a chromate ester. The release of ring strain in the transition state of the oxidative decomposition is believed to be responsible for the high rates and for the preference toward carbon-carbon cleavage instead of the usual carbon-hydrogen bond oxidation. A free radical is formed by subsequent reaction between cyclopropanol and chromium(IV). © 1975, American Chemical Society. All rights reserved.