oxaphosphetane

Oxaphosphetane refers to a class of four-membered cyclic intermediates described in the mechanism of the Wittig reaction. The ring incorporates a phosphorus atom and an oxygen atom linked to two carbon atoms derived from the reacting phosphorane (ylide) and the carbonyl component. In the commonly accepted mechanism, a [2+2] cycloaddition between an alkylidenephosphorane and a carbonyl compound forms the oxaphosphetane, a strained intermediate that collapses to give an alkene and a phosphine oxide byproduct.

Although the oxaphosphetane is typically short-lived and not isolable under standard conditions, evidence from low-temperature spectroscopy, trapping experiments, and computational studies supports its existence. Its structure is consistent with a four-membered P–O–C–C ring, with a propensity to eliminate to the alkene and oxide.

Substituent effects on the ylide and carbonyl influence the rate of formation and the stereochemical outcome of the Wittig reaction. While there has been debate about the exact mechanistic path, the oxaphosphetane sequence is a central feature of many descriptions. In some cases, stabilized ylides or specialized conditions allow observation or trapping of oxaphosphetane-related species, providing insights into the factors that govern selectivity.