A green approach to heterogeneous catalysis using ligand-free, metal-loaded cross-linked cyclodextrins

Abstract

In this work, we describe the application of a new series of solid cross-linked cyclodextrin (α-, ß-, and γ-CD) based catalysts that are obtained by reticulation with hexamethylene diisocyanate (HDI) in solutions containing Pd(II) or Cu(I) cations. Diisocyanates are efficient cross-linking agents for CDs because of their high reactivity towards hydroxyl groups. Metal content in the polyurethane-bridged CD polymers has been analyzed using ICP-MS. Pd(II) based catalysts have been successfully used in C-C couplings (Heck and Suzuki reactions), and a Cu(I) based system has been used in alkyne/azide [3+2] cycloadditions. Because metal leaching is negligible, which allows it to be recycled, and preparation is facile, this protocol is truly sustainable. As confirmed by ICP-MS analysis, the crude products do not require further purification for metal contamination. This versatile catalyst, due to its polar structure, is particularly suitable for microwave (MW)-assisted reactions that, in this study, gave the best yields in shorter reaction times. Due to their polar structure, both native CDs and cross-linked derivatives are very sensitive to dielectric heating; this effect is enhanced by the embedded cations. The design of sustainable synthetic protocols can be efficiently accomplished using a multi-faceted strategy that combines innovative solid catalysts and suitable enabling technologies.

Keywords

• click chemistry
• cross-linked cyclodextrins
• heck and suzuki reactions
• heterogeneous catalysis
• pd(ii)/cu(i) catalysts