The effects of methyl substituents on the trimethylenemethane diradical

Abstract

A simple, semiquantitative model for the effect of methyl substituents on the trimethylenemethane (TMM) diradical is advanced. In this, as in previous models, each methyl substituent is assumed to stabilise the diradical by ca. 2.8 kcal mol–1 per odd electron. It is further proposed that the steric factors associated with methyl substituents can be estimated from the same parameters as are used in the conformational analysis of cyclohexane derivatives. One prediction of this model is that the chemistry of singlet hexamethyl-TMM should be dominated by species adopting mono- and bis-orthogonal geometries. Existing evidence in support of the prediction is surveyed and further supporting evidence is drawn from studies of the flash vacuum pyrolysis of 4-isopropylidene-3,3,5,5-tetramethyl-4,5-dihydro-3H-pyrazole (9b), 3-isopropylidene-2,2,4,4-tetramethylthietane 1,1-dioxide (12b), and their deuteriated derivatives. The kinetic product ratio for thermal dediazenation of 4-([2H6]isopropylidene)-3,3,5,5-tetramethyl-4,5-dihydro-3H-pyrazole (9a) is ca. 71 : 29 {3-[2H6]isopropylidene-1,1,2,2-tetramethylcyclopropane (10a) to [1,1-dimethyl-2H6]-3-isopropylidene-1,1,2,2-tetramethylcyclopropane (11a)}.