CLOSED-SHELL VS DIRADICAL STATES OF BIS(METHYLENE)PHOSPHORANE - IS THE PHOSPHIRANE-BIS(METHYLENE)PHOSPHORANE INTERCONVERSION A PERICYCLIC REACTION - A THEORETICAL-STUDY

The energy and structure of bis(methylene)phosphorane HP(CH2)(2) have been calculated in its singlet closed shell planar ground. state, on the one hand, and its lowest triplet pyramidal diradical state, on the other, using ab initio MP4/6-31G**//MP2/6-31G** method. The singlet-triplet gap (16.9 kcal mol(-1)) is significantly lower than for the HN(CH2)(2) analogous (22.8 kcal mol(-1)). The same calculations have been performed on CH3, NH2 and BH2 P-substituted compounds; the latter exhibit a, rather low singlet-triplet separation (9.4 kcal mol(-1)). The phosphirane-bis(methylene)phosphorane interconversion has been studied at the GVB/6-31G** level. The transition state (TS) shows a disrotatory conformation, contrary to Woodward-Hoffmann rules, with, however, weak rotational barriers of the CH2 groups. Indeed, the TS is strongly pyramidal, with a diradicalar dominent character, and the phosphorus lone pair, having a strong 3s character, does not participate significantly in a homoaromatic system: the reaction is nonpericyclic. When the phosphorus is constrained to a planar structure, a conrotatory TS is found. In contrast, the TS or aziridine ring opening, at the same calculation level, shows a conrotatory conformation and significant flattening of the nitrogen group. The influence of substitution on the conrotatory or disrotatory path of the reaction is briefly discussed.