Microwave-assisted generation of alkoxyl radicals and their use in additions, beta-fragmentations, and remote functionalizations.
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), beta-fragmentations (formation of carbonyl compounds), and C,H-activation of aliphatic subunits, by delta-selective hydrogen atom transfer. C-Radicals formed from oxygen-centered intermediates were trapped either by Bu(3)SnH, L-cysteine ethyl ester, the reduced form of glutathione (reductive trapping), or by bromine atom donor BrCCl(3) (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.
Hartung
Jens
J
Fachbereich Chemie, Organische Chemie, Erwin-Schrödinger Strasse, Technische Universität Kaiserslautern, D-67663 Kaiserslautern, Germany. hartung@chemie.uni-kl.de
Daniel
Kristina
K
Gottwald
Thomas
T
Gross
Andreas
A
Schneiders
Nina
N