The photolytic reaction of thiols with diphenylvinylphosphine

The thiols RSH react with diphenylvinylphosphine under u.v. irradiation to give Ph2 PC2H4SR when R is Et, Prn, Bun, Ph, or C6 F5. If R is But or PhCH2, however, Ph2(vinyl)PS and RH are formed instead. The bahaviour of PriSH is intermediate, and it produces Ph2PC2H4 Spri, Ph2(vinyl)PS, and Ph2P(S)C2H4SPri. Excess of thiol leads to ph2P(S)C2H4SR in every case. The formation of the different products can be accounted for by RS˙ radical attack on either olefin or phosphorus. The favoured route appears not to depend on the nature of RS˙, however, but on the ease of elimination of R˙, a step involved in the route to the phosphorus(V) sulphide. This mechanism demands either that RS˙ attack on PIII is readily reversible, or that the intermediate [Ph2(vinyl)PSR]˙ readily undergoes intramolecular rearrangement. An equilibrium between the intermediates Ph2(vinyl) P˙SR and Ph2PC˙HCH2SR is probably established.