Thia-Diels–Alder reactions of hetaryl thioketones with nonactivated 1,3-dienes leading to 3,6-dihydro-2H-pyrans: evidence for a diradical mechanism

 Dihetaryl thioketones possessing thiophen-2-yl and selenophen-2-yl rings react as “superdienophilic” reagents with nonactivated 1,3-dienes such as 2,3-dimethylbuta-1,3-diene, cyclopentadiene, and mixtures of isomeric hexa-2,4-dienes to produce the expected 2H-thiopyrans in moderate to excellent yields. In the latter case, the corresponding cis-2,2-dihetaryl-3,6-dimethyl-3,6-dihydro-2H-thiopyrans are formed as the sole products in a stereoconvergent thia-Diels–Alder reaction. A stepwise mechanism via delocalized diradical intermediates is postulated to rationalize the observed reaction course. Treatment of 4,5-dimethyl-2,2-di(thiophen-2-yl)-3,6-dihydro-2H-thiopyran with excess of m-CPBA at room temperature leads to the oxidation of the C=C bond and the sulfur atom in the six-membered ring.