Concerted vs stepwise mechanism in 1,3-dipolar cycloaddition of nitrone toethene, cyclobutadiene, and benzocyclobutadiene. A computational study

The problem of competition between concerted and stepwise diradical mechani sms in 1,3-dipolar cycloadditions was addressed by studying the reaction be tween nitrone and ethene with DFT (R(U)B3LYP/G-31G*) and post HF methods. A ccording to calculations this reaction should take place via the concerted cycloaddition path. The stepwise process is a viable but not competitive al ternative. The R(U)B3LYP/G-31G* study was extended to the reaction of the s ame 1,3-dipole with cyclobutadiene and benzocyclobutadiene. The very reacti ve antiaromatic cyclobutadiene has an electronic structure that is particul arly disposed to promote stepwise diradical pathways. Calculations suggest that its reaction with nitrone represents a borderline case in which the st epwise process can compete with the concerted one on similar footing. Atten uation of the antiaromatic character of the dipolarophile, i.e., on passing from cyclobutadiene to benzocyclobutadiene, causes the concerted 1,3-dipol ar cycloaddition to become once again prevalent over the two-step path. Thu s, our results suggest that, in 1,3-dipolar cycloadditions that involve nor mal dipolarophiles, the concerted path (Huisgen's mechanism) should clearly overwhelm its stepwise diradical (Firestone's mechanism) counterpart.