Directional Diels-Alder Cycloadditions of Isoelectronic Graphene and Hexagonal-Boron Nitride in Oriented External Electric Fields: Reaction Axis Rule vs Polarization Axis Rule
The present study introduces the mechanisms of oriented external electric field (OEEF)-participated cycloadditions of nanographene and the analogues hexagonal boron nitride (h-BN) nanoflakes. Despite the C-C and B-N pairs are isoelectronic, their different ionicities give rise to distinct response to applied electric fields. For the nanographene models, the Diels-Alder addition obeys the reaction axis rule and the activation barrier changes under an OEEF perpendicular to the carbon skeleton for the enhanced/reduced intermolecular charge transfer, which provides a feasible strategy for the side-selective derivatization of graphene to obtain one-face-only adducts and Janus bifunctional products. By contrast, for the h-BN models, the variation of barrier is pronounced when the electric field is aligned along the in-plane N-B bond rather than the well-accepted reaction axis. Electronic structure analyses indicate that, because of the opposite electron withdrawing/donating nature of the reacting sites B/N, an OEEF along the N-B bond is capable of further enhancing the polarization via in-plane intramolecular charge transfer, resulting in the stabilized transition state and notable barrier reduction.