ELECTRON SPIN RESONANCE INVESTIGATION OF INTRAMOLECULAR HYDROGEN TRANSFER AND ALKYL ATTACK IN ESTER CATION RADICALS

Article abstract of DOI:10.1021/j100258a041

Reactions of the cation radicals of a number of butyl and neopentyl ester cations have been investigated by electron spin resonance spectroscopy.Radicals are formed at cryogenic temperatures by γ-irradiation of dilute CFCl3 frozen solutions of the esters.Ester cation radicals have been shown to be especially reactive intermediates in previous work.However, in this work a number of neopentyl ester cations have been stabilized at 77 K.The major couplings in thise cations arise only from three of protons on the terminal methyl groups of the neopentyl portion of cation.Thus the charge is predominantly located in the neopentyl group.In this work it is shown that, on annealing, the cation radicals of certain neoppentyl, tert-butyl, and butyl esters form alkyl radical intermediates by intramolecular hydrogen transfer, presumably to the carbonyl oxygen.These ester radicals further undergo intramolecular alkyl attack on oxygen to form a rearranged radical via a therefore unknown mechanism.Neopentyl formate cation radical undergoes a different reaction first by loss of a tert-butyl cation which presumably attaches to the carbonyl oxygen as in hydrogen transfer.This is followed by fragmentation of this rearranged structure to form isobutene cation radical.Selective deuteration at various positions on the alkyl side groups is used to verify the assignment of hyperfine couplings to positions on the radical structures as well as to verify the rearrangement and fragmentation reactions.The reactivity of ester cation radicals is discussed and summarized.