Organic Sulfur Mechanisms. 17. A Brief Study of the Photolysis of an Unsaturated Cyclic Sulfone

Photolysis of 3-phenyl-2H-thiopyran 1,1-dioxide (3) in methanol gives a mixture of 3-phenyl-5-methoxy-5,6-dihydro-2H-thiopyran 1,1-dioxide (4) and methyl 4-phenyl-2,4-pentadiene-1-sulfonate (5). Formation of 5 shows that the photochemical ring opening of cyclohexadienic sulfonyl compounds (1) does not require an atom bearing a free electron pair in the heterocyclic ring, and therefore argues in favor of a mechanism involving cycloreversion to the sulfene (1 → 2).