Design, Synthesis, and Application of Chiral Bicyclic Imidazole Catalysts.

ABSTRACT

ConspectusAsymmetric organocatalysis has been considered to be an efficient and reliable strategy for the stereoselective preparation of optically active chemicals. In particular, chiral tertiary amines as Lewis base organocatalysts bearing core structures including quinuclidine, dimethylaminopyridine (DMAP), N-methylimidazole (NMI), amidine, etc. have provided new and powerful tools for various chemical transformations. However, due to the limitations in structural complexity, synthetic difficulty, low catalytic efficiency, and high cost, the industrial application of such catalysts is still far from being widely adopted. Therefore, the development of new chiral tertiary amine catalysts with higher activity and selectivity is greatly desired.In order to address the contradiction between activity and selectivity caused by the ortho group, a bicyclic imidazole structure bearing a relatively large bond angle ∠θ was designed as the skeleton of our new catalysts. 6,7-Dihydro-5H-pyrrolo[1,2-a]imidazole (abbreviated as DPI) and 5,6,7,8-tetrahydroimidazo[1,2-a]pyridine (abbreviated as TIP) are two of the utilized skeletons. In addition to obtaining satisfactory catalytic activity, excellent enantioselectivity would also be expected because the stereocontrol R group is neither far nor close to the catalytic active site (sp2-N atom) and is adjustable. Based on this skeleton, a family of chiral bicyclic imidazole catalysts were easily prepared and successfully applied in several enantioselective reactions for the synthesis of a variety of valuable chiral compounds.6,7-Dihydro-5H-pyrrolo[1,2-a]imidazole (abbreviated as DPI) is the predominantly utilized skeleton. First, HO-DPI, the key intermediate of the designed chiral bicyclic imidazole catalysts, could be efficiently synthesized from imidazole and acrolein, then separated by kinetic resolution or optical resolution. Second, Alkoxy-DPI, the alkyloxy-substituted chiral bicyclic imidazole catalysts, were synthesized by a one-step alkylation from HO-DPI. This type of catalyst has been successfully applied in asymmetric Steglich rearrangement (C-acylation rearrangement of O-acylated azlactones), asymmetric phosphorylation of lactams, and a sequential four-step acylation reaction. Third, Acyloxy-DPI, the acyloxy-substituted chiral bicyclic imidazole catalysts, were synthesized with a one-step acetylative kinetic resolution from racemic HO-DPI or acylation from enantiopure HO-DPI. The catalyst AcO-DPI has been successfully applied in enantioselective Black rearrangement and in direct enantioselective C-acylation of 3-substituted benzofuran-2(3H)-ones and 2-oxindoles. Fourth, Alkyl-DPI was synthesized via a two-step reaction from racemic HO-DPI and separated easily by resolution. The catalyst Cy-DPI has been successfully applied in dynamic kinetic resolution of 3-hydroxyphthalides through enantioselective O-acylation. Cy-PDPI was synthesized through a Cu-catalyzed amidation from Cy-DPI and successfully applied in the kinetic resolution of secondary alcohols with good to excellent enantioselectivities. Finally, the carbamate type chiral bicyclic imidazole catalysts, Carbamate-DPI, were readily synthesized from HO-DPI, and the catalyst Ad-DPI bearing a bulky adamantyl group was successfully applied in the synthesis of the anti-COVID-19 drug remdesivir via asymmetric phosphorylation. Alongside our initial work, this Account also introduces four elegant studies by other groups concerning asymmetric phosphorylation utilizing chiral bicyclic imidazole catalysts.In summary, this Account focuses on the chiral bicyclic imidazole catalysts developed in our group and provides an overview on their design, synthesis, and application that will serve as inspiration for the exploration of new organocatalysts and related reactions.