Examining the Role of Internal Electric Fields in Reducing Reaction Barriers in Cycloaddition Reactions of Oxygen-Bridged Al/P- and Si/P-Based Frustrated Lewis Pairs with Heterocumulenes.

Abstract

In this investigation, computational methods were employed to study the transition-metal-free trapping reactions of small organic molecules using two distinct intramolecular frustrated Lewis pairs (FLPs) via cycloaddition mechanisms. The reactivity of oxygen-bridged geminal FLP-type molecules Al/P-Rea (1) and Si/P-Rea (2) with CS₂, PhN═C═S, SO₂, and PhN═S═O was evaluated. Experimental results reported by Mitzel and coworkers indicate that FLPs 1 and 2 alone exhibit limited ability to activate these four small molecules, being hindered by endergonic barriers, reaction thresholds, or potential structural degradation. Evidence from activation strain model (ASM) analysis suggests that the interaction of a larger FLP-type molecule with a chemical bond in a smaller organic molecule, or its binding to a single bond within a heterocumulene, induces enhanced motion within the smaller molecule (or heterocumulene). This increased motion leads to a significant rise in structural strain energy, which substantially affects the activation energy required for bonding. Alternatively, our ASM findings emphasize that when an FLP-type molecule binds simultaneously to both ends of a similar-sized organic molecule, the activation barrier is primarily governed by the strain energies of both reactants. Notably, this study further suggests that introducing cations (e.g., Li⁺) or anions (e.g., Cl⁻) into the reaction system of otherwise unreactive or weakly reactive FLP-type molecules induces an internal electric field that dramatically reduces the activation energy and enhances the exergonicity of the process, thereby facilitating the overall reaction. We hope that this study provides experimental synthetic chemists with novel insights and valuable guidance for future synthetic efforts.