Study of alkyl radicals fragmentation from 2-alkyl-2-propoxyl radicals

The reaction of a series of 2-alkyl-2-propoxyl radicals 2 in cumene has been studied. Alkoxyl radicals 2 were generated from the thermolysis of the corresponding tert-alkyl peroxypivalates, and underwent several modes of unimolecular reactions, that is, beta-scission to give methyl radical, beta-scission to give alkyl radicals and 1,5-H shift, which competed with hydrogen abstraction from cumene. The ratios of the rate constant for alkyl radicals elimination to that for methyl radical elimination, k(beta)(R)/k(beta)(Me), were determined by using the radical trapping method employing TEMIO as the scavenger. The logarithm of the relative rate was satisfactorily correlated with the heat of formation of the leaving alkyl radicals containing the steric parameters of neopentyl radical 2d. Then the plot of ln [k(beta)(R)/k(beta)(Me)] vs. the SOMO energies of the corresponding alkyl radicals showed a linear relationship with a slope of -7.6 eV(-1), which is comparable to that of formolysis of alkyl bromides. The mechanism involving a fairly polar transition state of the alkyl radicals fragmentation from tert-alkoxyl radicals 2 is discussed in terms of the MO diagrams and the solvent effects.