Hydrophosphination of Styrene and Polymerization of Vinylpyridine: A Computational Investigation of Calcium-Catalyzed Reactions and the Role of Fluxional Noncovalent Interactions

A computational investigation of the intermolecular hydrophosphination of styrene and 2-vinylpyridine, catalyzed by the heteroleptic β-diketiminato-stabilized calcium complex [(PhNC(Me)CHC(Me)NPh)CaPPh2], is presented. Alkene insertion does not proceed via the traditional route as proposed by experimental and theoretical research related to intermolecular hydroamination catalyzed by alkaline earth or lanthanide complexes. In contrast, for the hydrophosphination mechanism, insertion proceeds via outer sphere, conjugative addition where there is no direct interaction of Ca with the vinyl functionality. Following the initial rate-determining alkene insertion, two distinct mechanisms emerge, protonolysis or polymerization. Polymerization of styrene is energetically less favorable than protonolysis, whereas the reverse is determined for 2-vinylpyridine, thereby providing strong evidence of outcomes observed experimentally. The vinylarene ring is important as it allows for preferential coordination of the unsaturated substrate through numerous noncovalent Ca···π, CH···π, and Ca ← E (E = P or N) interactions; moreover, the vinylarene ring counteracts unfavorable charge localization within the activated transition state. The additional stability of the Ca ← N over Ca ← P dative interaction in vinylpyridine provides a rationalization for the experimentally observed enhanced reactivity of vinylpyridine, particularly in the context of the almost identical local alkene insertion barriers. Previously, little emphasis has been placed on the involvement of noncovalent interactions; however, our calculations reveal that Ca···π, CH···π, and Ca ← donor interactions are critical, stabilizing key intermediates and transition states, while also introducing numerous competitive pathways