The photochemistry of sulphoxides. A CIDNP study of carbon–sulphur bond cleavage paths

Abstract

A comparative photo-CIDNP study of triplet benzophenone sensitized C–S photocleavage of sulphoxides reveals significant structural dependence of the cage recombination and escape processes. In the photoreactive ortho-substituted phenyl methyl sulphoxides the triplet spin-correlated methyl–arylsulphinyl radical pair, ³RP1, [graphic omitted], is formed by triplet benzophenone sensitization or in some molecules, by direct excitation and intersystem crossing. Polarized methane and ethane are formed by the escape path of ³RP1, while the methyl- and aryl-H polarized starting molecule is obtained by the recombination path. The methyl-H polarized thioanisole product is obtained by deoxygenation of ArSO˙ escaped from ³RP1 and subsequent reaction of ArS˙ with a polarized methyl radical. In the absence of an active ortho-substituent, phenyl methyl sulphoxides give by direct excitation singlet state methyl–sulphinyl bond cleavage and form polarized methane as a minor escape product. The benzophenone triplet sensitized path takes place as a very inefficient process. The photocleavage reactivity of the methyl vinyl sulphoxides (3) and (4) is similar to that of the ortho-substituted methyl phenyl sulphoxides (1). In the methyl β-substituted ethyl sulphoxide (5a) photocleavage takes place only at the ethyl–sulphinyl bond. Polarized (5a) is obtained by the recombination path, while the escape path leads by the rare SO extrusion process to a ¹H methyl polarized n-propyl product (5b). Fluorenyl cinnamyl sulphoxide (6) undergoes two parallel bond cleavage processes, at the fluorenyl–sulphinyl and at the cinnamyl–sulphinyl bonds. The in-cage recombination in both radical pairs gives rise to (6*) polarized on both fluorenyl and cinnamyl moieties. The present work indicates that the recently described complex photorearrangement of the closely related cyclic sulphoxides (7a–d) is not a free radical reaction but rather a concerted electrocyclic process. The bond cleavage processes of (6) were not observed in these systems. 4-Methylpentadienyl phenyl sulphoxide (9a) and its sigmatropic rearrangement isomer (9b) undergo photocleavage at the pentadienyl–sulphinyl bond. In the two isomers the alternating sign polarization pattern due to the recombination path was evidenced at 90 MHz and more clearly so in the recently obtained 270 MHz photo-CIDNP spectra. Good correlation can be established between the photocleavage reactivity and ground state bond dissociation energies for C–S bonds of sulphoxides, while the escape path reactivity can be correlated with the C–H bond dissociation energies of the hydrocarbon product.