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.