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 t he one hand, and its lowest triplet pyramidal diradical state, on the other, using ab initio MP4/6-31G*//MP2/6-31G** method. The singlet-tr iplet gap (16.9 kcal mol(-1)) is significantly lower than for the HN(C H2)(2) analogous (22.8 kcal mol(-1)). The same calculations have been performed on CH3, NH2 and BH2 P-substituted compounds; the latter exhi bit a, rather low singlet-triplet separation (9.4 kcal mol(-1)). The p hosphirane-bis(methylene)phosphorane interconversion has been studied at the GVB/6-31G* level. The transition state (TS) shows a disrotator y conformation, contrary to Woodward-Hoffmann rules, with, however, we ak rotational barriers of the CH2 groups. Indeed, the TS is strongly p yramidal, with a diradicalar dominent character, and the phosphorus lo ne pair, having a strong 3s character, does not participate significan tly in a homoaromatic system: the reaction is nonpericyclic. When the phosphorus is constrained to a planar structure, a conrotatory TS is f ound. In contrast, the TS or aziridine ring opening, at the same calcu lation level, shows a conrotatory conformation and significant flatten ing of the nitrogen group. The influence of substitution on the conrot atory or disrotatory path of the reaction is briefly discussed.