N-Heterocyclic Carbene Catalysis under Oxidizing Conditions

Abstract

N-heterocyclic carbene organocatalysis under oxidizing conditions provides a vast range of various synthetic procedures via diverse mechanisms. The available catalysts, bases, oxidants, and oxidizing methods afford numerous opportunities for developing this branch of organocatalysis. Furthermore, implementation of tandem reactions and cooperative catalysis in the described methodology significantly expands the possibilities of modern organic chemistry. This approach allows the synthesis of different structurally complex and often enantiomerically enriched substances, which can be interesting in terms of biological activity and natural product synthesis. Many esters, amides, thioesters, lactams, lactones, and other cyclic compounds obtained in oxidative or oxygenative reactions promoted by N-heterocyclic carbenes can be interesting precursors in advanced organic synthesis. Sophistication and broad applicability prove that the described synthetic approaches are exceptionally worthy of further development.

NHC-Catalyzed Organocatalytic Asymmetric Approach to 2,2-Disubstituted Benzofuran-3(2H)-ones Containing Fully Substituted Quaternary Stereogenic Center

“…With the catalyst screening accomplished, further optimization studies were undertaken ( Table 2). Various reaction parameters, including base ( Table 2, entries 1-23), solvent (entries [24][25][26][27][28][29][30][31][32][33][34][35][36][37], and reaction time, were evaluated. As can be seen from Table 2, all bases used in this model reaction were well tolerated, giving the benzofuran-3(2H)-one derivative 2a and 2a' in high yields with excellent enantioselectivities.…”

“…Despite these elegant contributions, the asymmetric synthesis of chiral 2,2-disubstituted benzofuran-3-one derivatives is still in its infancy and novel catalytic processes are highly desirable. Over the past decade, N-heterocyclic carbene (NHC) organocatalysts have received considerable attention due to their unique ability to catalyze a wide range of synthetic transformations [24][25][26][27][28][29]. The ability of NHCs to reverse natural reactivity of a functional group has led to intensive research on them, leading to unprecedented access to designed target molecules and a set of umpolung reactions [30][31][32].…”

Synthesis of (−)-Verbenone-derived Triazolium Salts and Their Application in Enantioselective Intramolecular Stetter Reaction

“…In recent years, N-heterocyclic carbenes (NHCs) catalysis proved to be one kind of the most reliable organocatalyst and versatile platform in organocatalysis, and plenty of complex carbo-and heterocycles have been constructed via various umpolung or non-umpolung strategies [13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28][29][30]. Many kinds of chiral NHCs have been developed to catalyze stereochemical reactions, in which one of the most popular and effective structural scaffolds proved to be the aminoindanol skeleton.…”

N‐Heterocyclic Carbene‐Catalyzed Enantioselective Intramolecular Annulations to Construct Benzo‐Fused Pyranones with Quaternary Stereocenter

“…Subsequent base screening ( Table 1, entries 10-16) revealed, that different organic bases, even extremely strong ones such as BEMP or P 2 -Et could give the products with high yields and without erosion of stereoselectivity, remaining at < 99%. Additional screening of the reaction solvents did not show further improvements in the reaction outcomes ( Table 1, entries [16][17][18][19][20]. Finally, identification of diisopropylethylamine and toluene as the best-suited reaction conditions were indicated in terms of enantioselectivity for the developed annulation.…”

“…[13,14] In recent years N-heterocyclic carbene catalysis has emerged as an attractive and ideal strategy to synthesize complex heterocyclic molecules via umpolung reactivity. [15][16][17][18][19][20][21][22][23][24][25][26][27][28][29] Among various asymmetric catalytic strategies for the stereoselective construction and selective functionalization of the six-membered motifs, the Stetter reaction proved to be one of the universal and fundamental tool of the N-heterocyclic carbene (NHC) catalysis [30][31][32][33][34][35][36] After Ciganek initial contribution on intramolecular NHC-catalyzed synthesis of chromanones, during next years many highly enantioselective approaches have been developed (Scheme 1, eq a). [30,[37][38][39][40] In the later stage, Rovis and coworkers disclosed a highly enantioselective and diastereoselective Stetter reaction of salicylaldehyde-derived bearing a tethered α,β-disubstituted Michael acceptors (eq b).…”