Unveiling [3 + 2] cycloaddition reactions of benzonitrile oxide and diphenyl diazomethane to cyclopentene and norbornene: a molecular electron density theory perspective

A molecular electron density theory (MEDT) study is performed for the [3 + 2] cycloaddition (32CA) reactions of benzonitrile oxide (BNO) and diphenyldiazomethane (DPDM) to cyclopentene (CP) and norbornene (NBN) with the objective to analyse the experimentally observed acceleration in NBN reactions relative to the CP ones. The activation enthalpy of the 32CA reaction of NBN with BNO is lowered than that of CP by 2.1–2.9 kcal mol in gas phase, DMSO, acetonitrile and THF, while the corresponding differences are 1.3–1.8 kcal mol with DPDM. The 32CA reactions of DPDM show lower activation parameters compared to that of BNO consistent with the respective pseudo(mono)radical and zwitterionic type characters of DPDM and BNO. The syn diastereofacial approach of NBN is energetically feasible compared to the anti one. The global electron density transfer (GEDT) is identified to anticipate the minimal electron density flux at the TS entity, which is otherwise not accounted by the global electrophilicities of the separated regents. This MEDT study allows comprehending that for these non-polar reactions, the acceleration in NBN cycloadditions compared to the CP ones is due to the relatively lower energy cost demanded for the depopulation and subsequent rupture of C–C double bond of NBN followed by sequential bonding changes along the reaction paths, rather than the “predistorted geometry towards the TSs” as previously proposed in the 32CA reactions of norbornene derivatives.