On the origin of regioselectivity in the photorearrangement of 4,4-diphenyl-2,5-cyclohexadiene systems. Synthesis and photochemistry of 4,4-diphenyl-γ-thiopyran and -γ-thiopyran-1,1-dioxide

Abstract

The synthesis from diphenylmethane of 4,4-diphenyl-γ-thiopyran and 4,4-diphenyl-γ-thiopyran-1,1-dioxide is described. Upon direct photolysis at 254 nm the γ-thiopyran derivative undergoes rearrangement to a mixture of 5,6-diphenyl-2-thiabicyclo-[3.1.0]hexene and 2,4-diphenyl-α-thiopyran. The corresponding 1,1-dioxide derivative on the other hand undergoes, upon direct photolysis at 254 nm or sensitized (acetone) photolysis at 350 nm, rearrangement to 6,6-diphenyl-2-thiabicyclo[3.1.0]hexene-2,2-dioxide accompanied by a solvent addition product: 3-methoxy-4,4-diphenyl-2,3-dihydro-γ-thiopyran-1,1-dioxide. The results, along with those observed for 4,4-diphenyl-γ-pyran studied in this series, and other literature results are rationalized in terms of Zimmerman's reacting chromophore simulation using the Möbius–Hückel concept to evaluate competing excited state energies of the isoconjugate chromophores.