Unveiling the Substituent Effects in the Stereochemistry of [3+2] Cycloaddition Reactions of Aryl‐ and Alkyldiazomethylphosphonates with Norbornadiene within a MEDT Perspective

Abstract

AbstractThe [3+2] cycloaddition (32CA) reactions of norbornadiene with aryl and alkyl substituted diazomethylphosphonates (SDAPs) have been studied within the molecular electron density theory (MEDT). Electron localization function (ELF) shows the presence of apseudoradicalC3 center at the alkyl SDAPs, classifying them aspseudo(mono)radicalspecies, while the aryl SDAP is classified as a zwitterionic one, owing to the delocalization of the C3 electron density on the C3‐P bond. These 32CA reactions present high activation enthalpies in dichloromethane, between 17.3 and 21.6 kcal mol−1, being strongly exothermic by more than 33.8 kcal mol−1. While these 32CA reactions present a totalsynfacial selectivity, theendo/exostereoselectivity depends on the steric hindrance caused by the bulk substituents present in these SDAPs along the approach to norbornadiene. Interestingly the phenyl group provokes a lesser hindrance than that of the small methyl group, justifying the 95 % ofexostereoselectivity experimentally observed. ELF and the AIM (Atoms‐in‐Molecules) analyses account for the bonding changes along these 32CA reactions. The energy cost for the rupture of the norbornadiene C−C and that of the SDAPs N−N double bonds are the major contributors to the activation energies of these non‐polar 32CA reactions.