Unveiling the Mechanism and Stereochemical Pathways for BF 3 - mediated [3+2] Cycloaddition of N-boranonitrone with Alkenes: A DFT Study

In this work, we present a density functional theory (DFT) investigation of the BF ₃ -mediated [3 + 2] cycloaddition (32CA) of N-boranonitrone ( 6a) with three representative alkenes (styrene 7a , n-hexene 7b , and ethyl methacrylate 7c ). Calculations were carried out using hybrid meta-GGA M06-2X functional using standard 6–31 + G(d,p) basis set in both gas phase and dichloroethane solution using the PCM model. The reaction proceeds through a concerted one-step mechanism with two stereoisomeric pathways (endo/cis and exo/trans). Activation energy analyses reveal that the exo approach consistently provides lower barriers (by 1.4–3.4 kcal·mol⁻¹) than the endo pathway, in agreement with the experimentally observed predominance of trans cycloadducts. Bond orders and distances indicate asynchronous bond formation, with small differences between C–O and C–C bond development depending on the substrate, while ELF analysis confirms electron localization patterns characteristic of this process. Global reactivity descriptors show that N-boranonitrone acts as the electrophile, whereas the alkenes act as the nucleophiles. Noncovalent interaction (NCI) analysis highlights stabilizing attractive interactions in the exo transition states that further rationalize the stereochemical preference.