Novel 3H-1,2,3-Dithiaphosphole and Unprecedented 1,3,2,4-Dithiadiphosphinane Heterocycles: Unanticipated Cyclization Reactions of Lawesson's Reagent and Alkynes

The treatment of Lawesson's reagent with selected alkyl or aryl alkynes results in the formation of cyclization products such as 3H-1,2,3-dithiaphosphole-3-sulfide or unprecedented 1,3,2,4-dithiadiphosphinane-2,4-disulfide derivatives. The new heterocycles are characterized spectroscopically and in two cases by crystal structure analyses. Density functional theory (DFT) computations (PW6B95D3/6-311 + G(d,p)-PCM(toluene)//B97D3/6-31 G*) explain the formation of the dithiaphosphole product from alkynes and Lawesson's reagent via a sequence of [2 + 2] addition, electrocyclic ring opening, S-S bond forming ring closure, and P(III) oxidation by sulfur transfer from Lawesson's reagent. Based on DFT calculations, the formation of a 1,3,2,4-dithiadiphosphinane-2,4-disulfide is rationalized by a sequence of cyclization, electrocyclic ring opening, and dipolar cyclization steps.