| Abstract
| - The oxidation of thiophene (1) with peracids in a strongly acidic environment yielded thiophen-2-one (4) as the product of an apparent direct hydroxylation of the thiophene aromatic ring togetherwith the anticipated thiophene-S-oxide dimers, 2a,b, as the main products. Formation of the latterdimers can be rationalized in a straightforward manner by initial oxidation at the sulfur atom ofthiophene (1) to yield thiophene-S-oxide followed by subsequent dimerization in a Diels−Aldertype reaction. Trapping experiments in the presence of a competing dienophile indicated thatthiophen-2-one (4) did not originate from the monomeric thiophene-S-oxide but was the product ofan independent reaction pathway. The extent of thiophen-2-one (4) formation correlated with theacidity of the reaction medium and was suppressed in the presence of water, the latter presumablyacting as a competing base. As evidenced by the use of 2,5-dideuterated thiophene (1-D), itsmechanism of formation involved a 1,2-hydride shift, a feature commonly described in the peracid-mediated epoxidation of aromatic hydrocarbons and indicative for the occurrence of cationicintermediates. In agreement with all these observations we propose a mechanism involving initialprotonation of thiophene followed by nucleophilic attack of the peracid in position 2 of the thiophenering. Intramolecular epoxidation may lead to the formation of thiophene 2,3-epoxide as a highlyreactive intermediate that then undergoes heterolytic ring opening and a 1,2-hydride shift to yieldthiophen-2-one (4) after a final, acid-catalyzed, isomerization of the double bond.
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