Realistic modelling of hydrogen bonding of 2-cyclohexenone with H2O and H3BO3 in the outer coordination sphere of a chiral diene/Rh(i) complex by ab initio molecular dynamics

Abstract

A full-DFT Born-Openheimer MD (BOMD) study of the potential hydrogen bonding of 2-cyclohexenone π-complexed to Rh(I) in explicit 1,4-dioxane is presented. The complex is a key intermediate in the academically and industrially important asymmetric Rh-catalysed 1,4-addition of arylboronic acids to α,β-unsaturated ketones with the directing ligand phbod, a chiral bicyclic 1,4-diene. The ketone O atom (O_k) behaves as a single H-bond acceptor persistently throughout most of the simulation time while the donor is mobile and liable to exchange. Well-tempered metadynamics show that H-bonding with a (H₂O)₃ cluster is favorable by free energy but kinetically labile while with just H₃BO₃ is unfavorable but kinetically much more persistent. When both (H₂O)₃ cluster and H₃BO₃ are within H-bond distance from O_k, the non-H-bonded and the various H-bonded species are close in energy, implying the free energy surface is complex and quite flat. The most stable species features a H-bond with a water acceptor but not with H₃BO₃. The non-H-bonded state is 0.7 kcal mol⁻¹ higher in free energy. Model static DFT studies reveal that H-bonding with both (H₂O)₃ cluster and H₃BO₃ is favorable by enthalpy, but unfavorable by free energy when the entropy term is added.