Unveiling the [3+2] cycloaddition between difluoromethyl diazomethane and 3-ylideneoxindole from the perspective of molecular electron density theory

Abstract

The [3+2] cycloaddition (32CA) reaction between difluoromethyl diazomethane (DFDAM) and 3-ylideneoxindole (3YOI) has been studied using molecular electron density theory at the M06-2X/6-311+G(d,p) computational level. ELF topological analysis of DFDAM characterizes this three-atom-component (TAC) as a pseudoradical species participating in the pseudomonoradical (pmr)-type 32CA reactions. Analysis of the conceptual DFT reactivity indices indicates that along a polar reaction 3YOI will act as the electrophile while DFDAM will act as the nucleophile. This 32CA reaction presents low activation enthalpy in dichloromethane, 8.2 kcal mol⁻¹. The reaction is endo selective and completely ortho regioselective in agreement with the experimental outcome. The strong exergonic character of this 32CA reaction, −18.9 kcal mol⁻¹, makes it irreversible. The high global electron density transfer found at the most favorable ortho/endo TS, 0.24e, accounts for the highly polar character of this 32CA reaction, classified as the forward electron density flux. BET analysis along the most favourable ortho/endo reaction path asserts the pmr-type reactivity of this TAC. This 32CA reaction takes place through a non-concerted two-stage one-step mechanism characterized by the initial C–C single bond formation involving the pseudoradical C3 carbon of DFDAM.