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 4πs +
2πs 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 4πs + 2πs 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.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.