Improving Catalytic Enantioselectivity of Hydrogenation through Swelling-Induced Molecular Tension in Polymer Networks

Herein, we report that molecular tension generated by the swelling of a polyacrylate network containing a chiral [Biphep]Rh(I) catalyst as a tension-bearing, bis-tethered cross-linker enhances the enantioselectivity of hydrogenation of methyl 2-acetamidoacrylate. Differential swelling of the network is achieved by changing the relative concentrations of toluene (low swelling) and dichloromethane (high swelling) cosolvents. Swelling is characterized by the ratio of final to initial length in a single dimension of the network (λ = <i>l</i>_f/<i>l</i>_i), noting that changes are the same in all dimensions; λ_<i>x</i> = λ_<i>y</i> = λ_<i>z</i>. The enantiomeric excess (ee) of the hydrogenation reaction increases monotonically with λ, from 22% ee at λ = 1.53 to 39% ee at λ = 1.77. Control networks in which the catalyst is appended as a tension-free pendant side chain functionality display a minor change in ee as a function of swelling (ee = -0.7 ± 3.1%) with no obvious correlation between the two. The change in enantioselectivity due to swelling of the mechanically coupled network for a given λ is greater than when the same λ_<i>x</i> and λ_<i>y</i> are generated from the uniaxial compression along the normal <i>z</i>-axis, a result that is attributed to the isotropic strain associated with swelling in three dimensions rather than stretching in two dimensions. The results suggest that solvent swelling may provide a previously untapped modality for exploiting force-coupled catalysis in practical and scalable platforms, including those adapted from current polymer-supported catalysts.