Photochemical synthesis of pyrrole derivatives by desulfurization of 2,5-dihydro-1H-pyrrole-2-thiones and [2 + 2] cycloaddition of 2,5-dihydro-1H-pyrrole-2-thiones with alkenes

Abstract

The photochemical reactions of the 2,5-dihydro-1H-pyrrole-2-thiones 2 have been examined. Irradiation of 2,5-dihydro-1H-pyrrole-2-thiones 2 in the presence of triethylamine gives desulfurization products, the pyrroles 3, or reduction products, the pyrrolidine-2-thiones 4, depending on the substituent on the nitrogen atom of thione 2. This reaction can be explained by a sequential electron/proton-transfer mechanism from the amine to the excited pyrrolethione. Irradiation of thiones 2 in the presence of alkenes 5 gives 2-alkylated pyrroles 6–14. The formation of these photoproducts can be explained in terms of the intermediacy of spiroaminothietanes, which are derived by [2 + 2] photocycloaddition of the CS bond of thione 2 and the CC bond of alkene 5.