Elucidation of the substitutent effects on the reaction pathway of the cycloaddition of 1,3-diazabuta-1,3-dienes with ketenes using DFT-based reactivity indexes

The two possible [2+2] and [4+2] reaction pathways of a series of cycloaddition reactions of N-substituted 1,3-diazabuta-1,3-dienes with ketenes are elucidated theoretically by means of DFT-based reactivity indexes. In the first step of the stepwise mechanism, the σ-bond formation is rationalized by the determination of the most favorable electrophile-nucleophile interaction using Domingo's polar model [L.R. Domingo et al., Tetrahedron 58 (2002) 4417]. In the second step - which corresponds to a ring-closure - the formation of either a four- or a six-membered heterocycle is justified by the electrostatic interaction between the more positively and more negatively charged centers of the zwitterionic intermediate. The calculations are performed at the B3LYP/6-31G(d) level of theory and the obtained results are in total agreement with experimental findings.