THE HIGH REACTIVITY OF THE CS DOUBLE-BOND IN 1,3-DIPOLAR CYCLOADDITIONS OF NITRONES - A MOLECULAR-ORBITAL THEORETICAL-ANALYSIS

The CS double bond displays an unusually high reactivity in a number o f 1,3-dipolar cycloadditions. Ab-initio calculations on different leve ls are performed for cycloadditions of the parent nitrone to thioforma ldehyde and ethylene, respectively. An orientation complex (OC) is fou nd for both cycloadditions. In the case of thioformaldehyde, the natur e of the complex is of charge-transfer type as revealed by a perturbat ional analysis, whereas for ethylene as dipolarophile, the van der Waa ls character predominates. The transition structures (TS) for cycloadd ition resemble closely the OC with a shorter separation and a stronger distortion of the reactants. A negative energy of activation relative to the reactants (e.g., -2.5 kcal mol(-1) (Becke3LYP)) and a small po sitive barrier (+1.2 kcal mol(-1)) relative to the OC is calculated fo r thioformaldehyde as dipolarophile; the corresponding values for ethy lene are +13.7 and +15.5 kcal mol(-1) respectively. The perturbational analysis shows a strong HOMO(nitrone) - LUMO(thioformaldehyde) intera ction as the principal reason for the high thione reactivity. The obse rved equilibria in the cycloadditions of N-methyl-C,C-diphenylnitrone to 2,2,4,4-tetramethyl-3-thioxocyclobutanon and of N-methyl-C-phenylni trone to 2,2,6,6-tetramethylcyclohexanethione are found to be well rep roduced by PM3 calculations.