Photoreduction and photoarylation of O-ethyl S-n-propyl phenylphosphonothioate

Abstract

U.v. irradiation (254 nm) of O-ethyl S-n-propyl phenylphosphonothioate (1) in EtOH leads to a rapid photoreductive cleavage of the phosphorus–sulphur (ϕ= 0.14) or the sulphur–carbon (ϕ= 0.05) bond, as evidenced by the formation of O-ethyl phenylphosphinate, n-propanethiol, O-ethyl phenylphosphonothioic acid, and propane as the major reaction products. The concomitant photo-oxidation of EtOH to acetaldehyde is also observed. The rate of photoreduction of (1) is independent of the hydrogen-donor capacity of the solvent (MeOH, EtOH, or PrⁱOH). Hence, the reaction proceeds without prior hydrogen abstraction, probably by direct homolysis of the excited state of (1). In accordance with this, in situ spin-trapping experiments show the intermediacy of the phosphonyl radical EtO(Ph)P(O).

Sensitizers with a triplet energy >80 kcal mol^–1 sensitize the photoreduction of (1). Sensitization by aromatic hydrocarbons, e.g. benzene, is found to be complicated by simultaneous photoarylation of (1), which leads to the formation of O-ethyl diphenylphosphinate.