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Concerted vs Stepwise Mechanism in 1,3-Dipolar Cycloaddition of Nitrone to Ethene, Cyclobutadiene, and Benzocyclobutadiene. A Computational Study
journal contribution
posted on 2000-08-23, 00:00 authored by Cristiana Di Valentin, Mauro Freccero, Remo Gandolfi, Augusto RastelliThe problem of competition between concerted and stepwise diradical mechanisms in 1,3-dipolar
cycloadditions was addressed by studying the reaction between nitrone and ethene with DFT
(R(U)B3LYP/6-31G*) and post HF methods. According to calculations this reaction should take
place via the concerted cycloaddition path. The stepwise process is a viable but not competitive
alternative. The R(U)B3LYP/6-31G* study was extended to the reaction of the same 1,3-dipole
with cyclobutadiene and benzocyclobutadiene. The very reactive antiaromatic cyclobutadiene has
an electronic structure that is particularly disposed to promote stepwise diradical pathways.
Calculations suggest that its reaction with nitrone represents a borderline case in which the stepwise
process can compete with the concerted one on similar footing. Attenuation of the antiaromatic
character of the dipolarophile, i.e., on passing from cyclobutadiene to benzocyclobutadiene, causes
the concerted 1,3-dipolar cycloaddition to become once again prevalent over the two-step path. Thus,
our results suggest that, in 1,3-dipolar cycloadditions that involve normal dipolarophiles, the
concerted path (Huisgen's mechanism) should clearly overwhelm its stepwise diradical (Firestone's
mechanism) counterpart.