Coulomb repulsion versus cycloaddition: formation of anionic four-membered rings from sodium phosphaethynolate, Na(OCP)

Abstract

Carbon dioxide and two equivalents of Na(OCP) form, in an equilibrium reaction, a CO₂ adduct of the composition Na₂(P₂C₃O₄). The anion of this salt, [O₂C–P(CO)₂P]²⁻, is built up by a four-membered 1,3-diphosphetane-2,4-dione ring and a carboxylate unit attached to one of the phosphorus atoms. A remarkable π-delocalization was observed within the OCPCO moiety. The stepwise reaction mechanism leading to Na₂(P₂C₃O₄) was investigated with quantum chemical calculations. Accompanied by the release of CO₂, Na₂(P₂C₃O₄) reacts with both 2-iodopropane and 4,4′,4′′-trimethoxytriphenylmethyl chloride to form four-membered cyclic anions. For comparison the analogous reactions were performed with Na(OCP) instead of Na₂(P₂C₃O₄) and the results are discussed in detail.