Investigating the Reactivity of Removing a Sulfur Atom from Propylene Sulfide with a Geminal Frustrated Lewis Pair and the Origin of Their Activation Barriers

Using density functional theory (B3LYP-D3(BJ)/def2-TZVP), we explored the origin of the reaction barriers and reactivity for the elimination of one sulfur atom from propylene sulfide by methylene-linked geminal intramolecular G13/G15-based (G13 = group 13 element and G15 = group 15 element) frustrated Lewis pair (FLP) molecules. In contrast to the traditional understanding validated by previous theoretical examinations, our B3LYP computational results demonstrate that the elimination reactions with intramolecular geminal FLP-type molecules in this study proceed through multiple steps rather than a one-step concerted process. Our B3LYP computations reveal that during the multistep process the second transition state, G13/G15-TS2, serves as the rate-determining step, featuring a nucleophilic attack by the Lewis base at the G15 center on the least hindered sulfur atom of propylene sulfide, leading to the release of a propylene molecule. Based on our theoretical investigations using the activation strain model, the activation barrier of the essential second transition step is primarily determined by the structural deformation energy of the intramolecular geminal G13/G15-type FLP fragment, characterized by its flexible linear bent structure, as opposed to the rigid structure of the propylene sulfide fragment.