Multivalent Bifunctional Carbosilane Dendrimer-Supported Ammonium and Phosphonium Organocatalysts for the Coupling of CO2 and Epoxides

Abstract

Insertion of carbon dioxide into the structure of organic epoxides leads to the formation of valuable cyclic carbonates and polycarbonates. For the purpose of this process, we report here a series of recyclable organocatalysts with superior activity. Carbosilane dendrimers (first to third generation) are employed as a support to which periphery ammonium or phosphonium centers are covalently attached to form a dendritic ionic liquid (DIL). The obtained DILs were tested as homogeneous catalysts in the cycloaddition of CO₂ to epoxides with respect to their activity and recyclability. The catalytic performance of active sites is enhanced by the presence of a proximal hydroxyl group, playing the role of a hydrogen bond donor. An experimentally observed synergistic effect was confirmed by density functional theory calculations. The activity of DILs is affected more by the structure of onium salt than by the size of the support (dendrimer generation). The dendritic catalysts of all tested generations could be recovered by nanofiltration and repeatedly reused. In order to simplify the separation of the catalysts, composite materials were prepared by intercalating the more active ammonium dendrimers into the interlayer space of a natural montmorillonite. The resulting heterogeneous catalysts were easily separable by centrifugation while retaining a reasonable level of activity.