Mechanism of Anionic [3 + 2] Cycloadditions. An ab Initio Computational Study on the Cycloaddition of Allyl-, 2-Borylallyl-, and 2-Azaallyllithium to Ethylene

The anionic [3 + 2] cycloaddition of allyl anions or allyllithium compounds to double or triple bonds is an elegant route both to carbocyclic and to heterocyclic five-membered rings. The mechanism of such reactions has not yet been established conclusively. In this computational study, the concerted 4π_s + 2π_s mechanism, expected on the basis of the Woodward−Hoffmann rules, is found to be less favorable than two-step pathways for the cycloadditions of ethylene to the allyl, 2-borylallyl, and 2-azaallyl anions and their lithiated counterparts at Becke3LYP/6-311+G** and MP2(fc)/6-31+G* levels of theory. Except for allyllithium, the 4π_s + 2π_s cycloadditions (in C_s symmetry) are not concerted, since only second-order saddle points, rather than true transition structures, are involved. The anisotropy of the reactant polarizabilities is responsible. Instead, two-step cycloaddition pathways are followed by all three model systems. In accord with experimental experience, 2-borylallyl and 2-azaallyl compounds are found to undergo this type of reaction more readily than the unsubstituted allyl anion or allyllithium. The second, ring-closing step is facilitated by the anion-stabilizing effect of nitrogen and the boryl substituent.