Thermal [2+2] Cycloaddition Reactions of Perfluorobicyclo[2.2.0]hex-1(4)-ene with Ethylene, Benzene and Styrene: A MEDT Perspective.

Thermal [2+2] cycloaddition (22CA) reactions of perfluorobicyclo[2.2.0]hex-1(4)-ene (PFBHE) and bicyclo[2.2.0]hex-1(4)-ene (BHE) with ethylene, benzene and styrene were investigated through the Molecular Electron Density Theory (MEDT) at the UM06-2X/6-311G(d,p) level in benzene. Scrutiny of the DFT-based reactivity indices indicates that the presence of the eight fluorines in PFBHE notably expands the electrophilic nature of this species, participating in polar reactions. These 22CAs proceed through a stepwise mechanism, while the non-polar 22CA reaction of BHE with ethylene requires high energy around 26.6 kcal·mol-1, the polar 22CA reaction of PFBHE with styrene requires a low activation energy of 13.2 kcal·mol-1. The polar 22CA reaction of PFBHE with benzene presents the highest activation energy, 28.3 kcal·mol-1, because of the loss of its aromatic character. Scrutiny of the electron localization function (ELF) at the TSs associated with the first step points that the creation of the C1-C3 bond set about, while that at the TSs associated with the ring-closure means that the creation of the C2-C4 bond has not started yet. At the end, a Relative Interacting Atomic Energy (RIAE) study of these thermal 22CA processes shows that while at the non-polar TS1a-I both interacting frameworks are electronically destabilized, at the polar TS1a-IV, the hefty global electron density transfer (GEDT) goes ahead towards PFBHE, causing a strong electronic stabilization of this framework, markedly reducing the RIAE activation energy.