Oxidation of congested thiophene 1,1-dioxides with m-chloroperbenzoic acid. Formation of epoxides and thiete 1,1-dioxides and steric acceleration^†

A series of thiophene dioxides (3), including highly congested ones, were synthesized. Their oxidation with m-chloroperbenzoic acid (m-CPBA) was investigated either in the presence or in the absence of Na₂CO₃. The following conclusions were reached. (1) Oxidation in the presence of Na₂CO₃ affords the corresponding epoxides (4) in moderate to excellent yields. (2) Oxidation in the absence of Na₂CO₃ produces the ring-contracted thiete 1,1-dioxides (5) as the principal product, thus providing a novel synthesis of the sulfur-containing unsaturated four-membered ring system. If necessary, 5 can also be derived by treatment of 4 with BF₃·Et₂O. In an extreme case, the oxidation of 3,4-di(1-adamantyl)thiophene afforded the corresponding thiete dioxide 5b directly in 78% yield. (3) Oxidation takes place faster with a more congested 3, probably because of destabilization of the HOMO by steric repulsion between bulky substituents and also owing to relief of steric crowding on going from the ground to the transition state. (4) The formation of 5 from 3 in the absence of Na₂CO₃ can be explained by the occurrence of an acid-catalyzed rearrangement of 4 initially formed. However, a competitive pathway leading directly to 5 may also be operative.