Carboxylation of 2-methyladamantan-2-ol and 2-(1-adamantyl)ethanol: evidence for the intermolecular nature of hydride transfer reactions in rearrangements involving adamantyl cations

Abstract

Three experimental procedures are described for Koch–Haaf carboxylation of 2-methyladamantan-2-ol: at normal concentrations at 0° the major products are 2-methyladamantane-1-carboxylic acid and syn- and anti-4-methyladamantane-1-carboxylic acid; at normal concentrations at 50° the major product is 3-methyladamantane-1-carboxylic acid; but with high dilution conditions at 0° 2-methyladamantane-2-carboxylic acid is formed exclusively. Carboxylation of 2-(1-adamantyl)ethanol at room temperature with either normal or high dilution conditions gives predominantly 3-ethyladamantane-1-carboxylic acid, whereas at 50° equal amounts of this acid and adamantane-1-carboxylic acid are produced. These rearrangements are interpreted in terms of stereochemical inhibition of intramolecular 1,2-hydride shifts and dilution-controlled intermolecular hydride shifts.