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.