Bonding Evolution Theory Study of the [3+2] Cycloaddition Reaction Between Benzonitrile Oxide and Ethylenic Derivatives

Using the framework of the bonding evolution theory (BET), we investigated the molecular mechanism of the [3+2] cycloaddition reaction of benzonitrile oxide (1) with five ethylenic derivatives (2a–e). The global mechanism consists in the concerted attack of one of the O lone pairs of 1 on one of the atoms of the C–X double or triple bond of 2 (X = C, O, N), the attack of the C–X double or triple bond of 2 on the C of the C–N triple bond of 1, and the conversion of C–N triple bond of 1 into a double bond and the creation of a lone pair on N. The reaction proceeds through a one-step mechanism, the creation and modification of the electron basins occur at different places along the intrinsic reaction coordinate, as unraveled by the BET study. In the cases of dipolarophiles (2a–c), first the C–N triple bond of 1 is converted into a double bond with the creation of a lone pair on N. Then, a C–C single bond between the C–N triple bond of 1 and one of the C atoms of the C–C double or triple bond of 2 is created. Finally, the O–C single bond is formed. Concerning 2e, the meta-path follows the same reaction mechanism as 2a–c, while for the ortho-path, the formation of C–N and C–O bonds are synchronous and mediated by the N4 lone pair. A similar observation (but for the two C–O bonds) is found for 2d ortho-path while the mechanism for the meta-path is quite different to the other ones due to the formation of an O–O bond. The present work represents a new example showing how the use of BET can provide curly arrows and electron flow representation to unravel molecular mechanisms.