An electron spin resonance study of the regioselectivity of the cyclization of protonated and unprotonated pent-4-en-1-oxyl and pent-4-en-1-peroxyl radicals

Abstract

The regioselectivity of the ring closure of protonated and unprotonated pent-4-en-1-oxyl radicals has been determined by monitoring by e.s.r. spectroscopy the radicals which are formed when the corresponding pent-4-enyl peroxides (2; R = H or Me) are photolysed in solution. [graphic omitted] Under neutral conditions, the spectra of the corresponding tetrahydrofuran-2-ylmethyl radicals (6) are observed, confirming the preferential 5-exo mode of cyclization. When trifluoroacetic acid is present, the different spectra of the protonated tetrahydrofuran-2-ylmethyl radicals (8) are observed, and it is argued that this reflects the similar regioselectivity of the intramolecular addition of the protonated alkoxyl radicals. The identity of the radicals (6) and (8) was confirmed by generating them independently by abstracting bromine atoms from the corresponding bromides (9). Similar experiments with the hydroperoxides (1) failed to show the spectra of the dioxan-3-ylmethyl radicals (17), perhaps because they rapidly undergo γ-scission to the oxygen-centred radicals (18). However, (17) could be trapped with oxygen to afford, after Ph₃P reduction, 1,2-dioxanes (4). [graphic omitted] No evidence could be found for the protonation of alkylperoxyl radicals by trifluoroacetic acid.