Deoxygenation of oxiranes by the action of phosphane derivatives (PX₃) and heavier pnictogens (PnX₃) was explored in detail with regard to energetics as well as to the stereochemistry and regioselectivity outcome. Two possible routes involve either oxygen or carbene transfer reactions. The thermodynamic oxygen and carbene-transfer potentials were put into context and compared for a wide set of reducing agents.

Abstract

The deoxygenation of parent and substituted oxiranes by λ³σ³-phosphorus reagents has been explored in detail, therefore unveiling mechanistic aspects as well as regio- and stereochemical consequences. Attack to a ring C atom is almost always preferred over one-step deoxygenation by direct P-to-O attack. In most cases a carbene transfer occurs as first step, leading to a phosphorane and a carbonyl unit that thereafter react in the usual Wittig fashion via the corresponding λ⁵σ⁵-1,2-oxaphosphetane intermediate. Betaines rarely constitute true minima after the first C-attack to oxiranes, at least in the gas-phase. Use of the heavier derivatives AsMe₃ and SbMe₃ as oxirane deoxygenating reagents was also mechanistically studied. The thermodynamic tendency of λ³σ³-phosphorus reagents to act as oxygen (O-attack) or carbene acceptors (C-attack) was theoretically studied by means of the thermodynamic oxygen-transfer potential (TOP) and the newly defined thermodynamic carbene-transfer potential (TCP) parameters, that were explored in a wider context together with many other acceptor centres.