Competing Modes of Hydrogen Activation in Singlet Pyridinylidenes: π-Approach σ*-Approach Reaction Pathways.

Carbenes are promising reagents for the transition metal-free activation of molecular hydrogen. Depending on their multiplicity and electron configuration, carbenes can access different hydrogenation reaction mechanisms, with singlet carbenes usually leading to geminal hydrogenation products a π-approach trajectory. Our group has recently prepared 1-iodopyridine-2-ylidene, , introducing a new class of singlet -heterocyclic carbenes featuring σ/σ* instead of the usually encountered σ/π frontier orbitals. Carbene reacts with H a unique sideways σ*-approach, leading to pyridinium iodide formation with N-I bond cleavage. This study investigates how nitrogen substituents with varying bond strengths (I < Br < Cl < OCF < OMe < NMe < F < Me) influence the electronic structure of the carbene and its preference for σ*- or π-approach hydrogenation. Using high-accuracy DLPNO-CCSD(T) and NEVPT2 computations, we find that I-, Br-, Cl-, and OCF-substituted pyridinylidenes adopt a σσ configuration and favor σ*-approach hydrogenation. In contrast, OMe-, NMe-, and Me-substituted carbenes exhibit a σπ configuration and lower barriers for the π-approach hydrogenation. Interestingly, the fluorine-substituted carbene assumes a σπ electron configuration yet still preferentially undergoes σ*-approach hydrogenation.