SUBSTITUENT EFFECTS ON THE C-C BOND STRENGTH .12. THERMOLYSIS OF AROMATIC PINACOL DIMETHYL ETHERS AND STABILIZATION ENERGIES OF ALPHA-METHOXYBENZYL RADICALS

The phenyl-substituted pinacol dimethyl ethers meso- and DL-5a-d have been prepared by reductive dimerisation of the dimethyl acetals 7a-d of the ketones 8a-d. H-1-NMR spectra and GC-retention times are used to distinguish between the diastereomers, and an X-ray structure analysis of meso-5a confirms these distinctive criteria. The products and the kinetics of the thermolysis reaction of 5a-d have been studied. A linear correlation between DELTAG(not-equal) (300-degrees-C) of thermolysis and strain enthalpies H(s) of 5, or its release D(s) during the dissoziation into radicals 6, is observed. These correlations do not differ from those of the thermolysis of tetraalkyl-diphenylethanes (1, R = alkyl, S = phenyl)(3, 20a]. Hence, the methoxy group does not influence the homolysis reaction of the C-C bond in 5 compared to an alkyl group. It is concluded that any stabilisation of the radicals 6 by the alkoxy group[1] is canceled by an extra stabilisation of 5 by the geminal phenyl/methoxy pairs.