The autoxidation of aliphatic esters. Part 3. The reactions of alkoxyl and methyl radicals, from the thermolysis and photolysis of peroxides, with neopentyl esters

Abstract

This paper reports a study of the dimerisation of ester radicals arising from the thermolysis and photolysis of di-tert-butyl peroxide (DTBPO) and dicumyl peroxide [DCPO, bis(α,α-dimethylbenzyl) peroxide] in neopentyl butanoate and a selection of structurally related neopentyl esters, in the temperature range 298 to 438 K. The acyl moieties of these esters were chosen to incorporate a variety of structural types to provide mechanistic information about the reactions. At 438 K, the thermolyses of DTBPO and DCPO in neopentyl butanoate give six ester radical dimers (three pairs of diastereoisomers). The two major diastereoisomers threo- and meso-dineopentyl 2,3-diethylbutanedioate have been prepared and the crystal structure of the meso compound determined. Interestingly, the dimer product distribution is independent of the peroxide used. By contrast, at 298 K more than twice as many dimers are observed and the product distributions from the two peroxides are no longer the same. Similar results are also observed for the other neopentyl esters. Evidence is presented to show that the ester radicals arise from hydrogen atom abstraction from the esters by alkoxyl and methyl radicals; the latter being formed by the fragmentation of the alkoxyls. At 438 K the dimer product distributions are determined predominantly by a thermodynamically controlled equilibrium of ester radicals prior to dimerisation. Lowering the temperature leads to the increased importance of kinetic effects in determining the product distribution.