Bond homolysis / Thermolysis / Kinetics / Radical stabilisation Substituent Effects on the C-C Bond Strength, 12[11. -Therma Energies of a-Methoxybenzyl Radicalst2]
The phenyl-substituted pinacol dimethyl ethers mesoand DL-5a-d have been prepared by reductive dimerisation of the dimethyl acetals ?a-d of the ketones 8a-d. 'H-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 AG* (300°C) of thermolysis and strain enthalpies H, of 5, or its release Ds during the dissoziation into ilysis of Aromatic Pinacol Dimethyl Ethers and Stabilisation radicals 6, is observed. These correlations do not differ from those of the thermolysis of tetraalkyl-diphenylethanes (1, R = alkyl, S = phenyl)[3f20a1. 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 ['] is canceled by an extra stabilisation of 5 by the geminal phenyl/methoxy pairs.
In vorangehenden Arbeiten[',3,41 wurde gezeigt, daI3 sich das Studium der Thermolyse 1,2-disubstituierter Ethane 1 dazu eignet, die Stabilisierungsenergie (HR oder RSE) der Alkyl-Radikale 2 durch Substituenten S zu bestimmen.