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

Abstract

A comparative photo-CI DN P 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, 3RPl, CH; + ArSO', 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 3RPl, 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 3RPl 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 P-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 l H 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-Cl DNP 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. 3 * Our e.s.r. results indicate that the 2-Et2NCOC,H,SO' radical formed photochemically from (lb) is moderately stable in 2methyltetrahydrofuran solution at 150 K. Its g factor is 2.0092.

Fipronil insecticide: Novel photochemical desulfinylation with retention of neurotoxicity

“…Fipronil probably exists in the cyclic hydrogen-bonded form allowing interaction of the sulfinyl and amino moieties, as noted previously in ortho-substituted arylsulfoxides (12) (Fig. 3).…”

“…Formation of the radical pair shown in Fig. 3 (Left), proposed in the direct photolysis of aryl and alkenyl methyl sulfoxides (12,13), may be the first step in extrusion of SO from fipronil to give desulfinylfipronil in a reaction analogous to that established before only for a few (three-and fourmembered) cyclic sulfoxides (14). Fipronil may form the initial radical pair shown on the right as well, although previously reported only in dialkyl sulfoxides (14), leading to the detrifluoromethylsulfinyl product.…”

Method for Hydroxylation and Esterification of Interior Sites of Polyolefinic Films

“…Photochemical Processes . The photochemistry of DBTO in various liquids has been studied for more than 25 years. − Most recently, Jenks and co-workers have performed several extensive mechanistic investigations of the processes involved . Irradiation of a DBTO molecule leads to efficient dissociation of the S−O bond, producing DBT and a ground-state oxygen atom ( 3 O P ) that is able to oxidize a wide range of solvents.…”

Generation and Decay of Aryl Sulfinyl and Sulfenyl Radicals: A Transient Absorption and Computational Study

“…This took the form of weak EPR signals of sulfinyl radicals at low temperature [34][35][36][37] and chemically induced dynamic nuclear polarization (CIDNP) signals attributed to the intermediacy of sulfinyls. [38][39][40] However, overinterpretation of such data without corroborating evidence can be misleading as they do not necessarily indicate radical pathways for the majority of material. Only a brief and qualitative report of a transient absorption spectrum attributed to phenylsulfinyl has appeared previously.…”