4-Dimethylamino Pyridine (DMAP) Catalyst with Fluxional Chirality: Synthesis and Applications

Abstract

Organocatalysis using small organic molecules to catalyze organic transformations, has emerged as a powerful synthetic tool that is complementary to metal-catalyzed transformations and remarkably promote stereoselective synthesis. Our group has designed useful templates, ligands, and additives that use fluxional groups to control and/or enhance stereoselectivity in a variety of asymmetric transformations. A key feature of this strategy is that the size of the fluxional substituent can be varied readily. As an extension of this strategy we became interested in developing efficient and broadly applicable and adjustable 4-dimethylaminopyridine (DMAP) organocatalysts. In our design, we surmised that a fluxional group would be effective in relaying stereochemical information from the fixed chiral center to the catalytic center of DMAP. Presented herein the synthesis of novel fluxionally chiral DMAP catalysts and their application in the acylative kinetic resolution of secondary alcohols and axially chiral biaryls, dynamic kinetic resolution of chiral biaryls with low rotation barriers and allylic substitution reactions. In the beginning, a comprehensive study of the chiral relay concept in enantioselective transformations was reviewed and the historic and current story of the chiral relay concept is covered. The design and synthesis of fluxionally chiral 4-dimethylaminopyridine catalysts was introduced. The key issues addressed in this chapter include the design concept regarding a stereoselective fluxionally 4-dimethylaminopyridine catalyst and muti-step synthesis strategies developed for catalyst synthesis. The development of fluxionally chiral 4-dimethylaminopyridine catalysts in the acylative kinetic resolution studies of secondary alcohols as well as axially chiral biaryls is investigated. Six different secondary alcohols are resolved with good selectivity factors (6-37) and ten biaryl substrates are resolved with moderate to high selectivity factors (10-51). Dynamic kinetic resolution has more practical applications to organic synthesis than simple kinetic resolution. The dynamic kinetic resolution of atropisomeric biaryls using the novel fluxionally chiral 4-dimethylaminopyridine catalysts was explored and the corresponding acylated products were obtained with 11-80 %ee. The newly designed DMAP catalysts containing fluxional groups as a stereocontrol unit could also be effectively applied as a nucleophilic catalyst in asymmetric allylic aminations. A range of α-methylene-β-amino esters were obtained with good yields and selectivities (up to 72 %ee).