Microwave-assisted generation of alkoxyl radicals and their use in additions, β-fragmentations, and remote functionalizations

Abstract

Microwave irradiation (2.45 GHz, 300–500 W) of N-(alkoxy)thiazole-2(3H)-thiones in low-absorbing solvents affords alkoxyl radicals, which were identified by (i) spin adduct formation (EPR-spectroscopy) and (ii) fingerprint-type selectivities in intramolecular additions (stereoselective synthesis of disubstituted tetrahydrofurans), β-fragmentations (formation of carbonyl compounds), and C,H-activation of aliphatic subunits, by δ-selective hydrogen atom transfer. C-Radicals formed from oxygen-centered intermediates were trapped either by Bu₃SnH, L-cysteine ethyl ester, the reduced form of glutathione (reductive trapping), or by bromine atom donor BrCCl₃ (heteroatom functionalization) The results suggest that microwave activation is superior to UV/Vis-photolysis and conductive heating for alkoxyl radical generation from N-(alkoxy)thiazolethiones. It offers by far the shortest reaction times along with the option to reduce the amount of trapping reagent significantly.