Synthesis and applications of polymeric organocatalysts

Abstract

Inspired by the prototypes of enzymes as the natural highly catalytic polymers, chemists have long had interest in designing and synthesizing polymeric catalysts for chemical reactions. The past decade has witnessed a number of new variations of polymeric organocatalysts and new applications in organic synthesis, particularly with the use of automation or parallel flow systems in multi-step sequences. Polymer-supported catalysts, both soluble and insoluble, have been extensively studied in organic chemistry in recent years as they could ease the handling, simplify product purification and often be simply recovered. Attempts have also been successfully made to design and synthesize more densely functionalized self-supported polymeric organocatalysts. In recent years, the polymer-supported superbases have been widely used, providing a new frontier in the rapid production of a large number of chemical libraries. In Chapter 2, the synthesis and application of a rasta resin-supported TBD (RR-TBD) catalyst were investigated. Herein, an improved synthesis of α,γ-allenones was presented through the Morita-Baylis-Hillman (MBH) reactions with RR-TBD as catalysts. A soluble bifunctional polymer bearing both 4-(dimethylamino)pyridine (DMAP) and diisopropylamine (DIPEA) was then prepared (Chapter 3). It was found that the bifunctional polymer was effective at catalyzing a range of esterification reactions, even at the gram-scale. Moreover, the catalyst was recycled over 20 times without lose of activity. In Chapter 4, a goal of designing densely functionalized polymeric catalysts led to the synthesis of self-supported N-heterocyclic carbene (NHC) precursors. The self-supported NHC organocatalyst precursors were highly efficient in a set of benzoin condensation and redox esterification reactions. Furthermore, the study of catalyst recycling was also involved. The fact that researchers have shown remarkable interest in developing halogen bond (XB) donors as organocatalysts in the last decade, and our experience with the self-supported polymeric catalyst facilitating product purification and affording good yield of the desired product, led us to design and evaluate new polymers that were functionalized with 2-iodobenzimidazolium core orthogonally positioned along a main chain (Chapter 5). In this context, a myriad of synthetic transformations using XB donors as catalysts are realized.