HOMOLYTIC FRAGMENTATIONS OF BENZYL METHYL-SUBSTITUTED BENZYL CARBINYLOXY RADICALS AND THE RELATED REACTIONS - LIMITATIONS OF THE LEFFLER-HAMMOND POSTULATE AND REACTIVITY SELECTIVITY PRINCIPLE/

Photolyses of benzyl methyl substituted-benzyl carbinyl hypochlorites produced the corresponding carbinyloxy radicals, which undergo fragmen tations. Oxidations of 1-aryl-2,3-diphenylpropan-2-ols by cerie ammoni um nitrate also proceeded through quite similar homolytic beta-scissio ns. The relative rates and Hammett p(+) values are independent of temp eratures. The differential activation parameters indicate that the ent ropic factors are important for the reactions. The differential entrop ic term (Delta S double dagger(Y-H)) could be related to the differenc e in the extent of bond cleavages caused by the different sigma(+) val ues of the substituents, Y and H, in the polar transition state (TS). The selectivities (relative rates) remain constant or even increase wi th increasing temperature, indicating that the reactivities are contro lled by entropy. The polar TS can be caused by the perpendicular effec t to invite the dual substituent effects with breakdowns of the Leffle r-Hammond postulate. The solvent effects on the reactions of the three oxygen radicals are compared. A new scheme is proposed for hydrogen a bstractions by solvated tert-butoxy and cumyloxy radicals.