1,2-Thiaphosphetanes: The Quest for Wittig-Type Ring Cleavage, Rearrangement, and Sulfur Atom Transfer

The potential energy surface for opening and ring-enlargement reactions of 1,2-thiaphosphetane with different oxidation states and coordination at phosphorus has been computationally explored. The most favored [2 + 2] cycloreversions are the so-called normal Wittig-type reactions furnishing an alkene and a P═S-containing component. Somewhat unfavored are reactions involving the P-S bond cleavage and a C-to-P hydrogen shift as well as a ring enlargement to a 1,3,2-dithiaphospholane in the case of high coordinate derivatives. Apart from a two-step P-to-S oxygen transfer in 1,2σ⁴λ⁵-thiaphosphetane P-oxide, an interesting reaction is the P-S bond cleavage leading to a thiabetaine which, alternatively, can be formed by the C-attack of a phosphinidene oxide on a thiirane. A similar route, energetically more favored, was found for 1,2σ⁵λ⁵-thiaphosphetane starting from thiirane and the corresponding phosphane. For a wide set of P(III) reagents including HPR₂ (R = Me, Ph), PR₃ (R = Me, Ph, NMe₂, OMe, F, Cl), and HP═O, the competitive direct S-attack was found to be the preferred pathway for the desulfurization of thiiranes, and the relative tendency for these P(III) reagents to act as an S-atom acceptor, the thermodynamic S-transfer potential (TSP) scale, was defined using the S₈/S₇ couple as a point of reference.