Stereoelectronic and Steric Effects in the Synthesis and Recognition of Diastereomeric Ethers by NMR and EPR Spectroscopy

(2,6‐Di‐ tert ‐butyl‐4‐hydroxyphenyl)‐alkyl‐carbinols I are easily dehydrated in the presence of catalytic amounts of a mineral acid to form the corresponding phenyl‐alkyl‐carbocations Ia . These cations can be reversibly deprotonated to the quinone methides Ib , therefore, an equilibrium between both reactive compounds is assumed. In the absence of nucleophiles a reaction of these intermediates with the starting carbinol is observed leading to symmetrical ethers Id . In alcoholic solution the solvent acts as a nucleophilic compound and the formation of an unsymmetrical ether Ie is observed predominantly. If the nucleophile contains a chiral carbon atom diastereomers are formed in this reaction which are observed in variable concentrations depending on the reaction time. The assignment of these isomers to the meso and racemic compound has been achieved by NMR investigations in solution in combination with their solid‐state structures determined by X‐ray crystallography. These results indicate a remarkable thermodynamic stabilization of the symmetrical ethers in the racemic form whereas the meso compounds are favored if the products are formed under kinetic control. In both cases the diastereomer ratio is determined by steric repulsion and the generalized anomeric effect. – A bonding interaction of lone pairs of the ether oxygen with the π system of the corresponding phenoxyl ring was also observed by EPR investigations. This electron delocalization contributes remarkably to the mechanism responsible for the distinction of the diastereomeric radicals by different β‐proton coupling constants.