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

Carbon dioxide and two equivalents of Na(OCP) form, in an equilibrium reaction, a CO2 adduct of the composition Na-2(P2C3O4). The anion of this salt, [O2C-P(CO)(2)P](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 pi-delocalization was observed within the OCPCO moiety. The stepwise reaction mechanism leading to Na-2(P2C3O4) was investigated with quantum chemical calculations. Accompanied by the release of CO2, Na-2(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 Na-2(P2C3O4) and the results are discussed in detail.