N-Heterocyclic Carbene Ligands in Copper-Catalyzed Addition of Diethylzinc to N-Sulfonylimines

Abstract

N-Heterocyclic carbenes (NHCs) are strong s-donating species, showing coordination properties similar to those of phosphines.1,2) Initially, the widespread use of catalysts containing carbene ligands was limited due to their relatively difficult preparation. Since the discovery of stable carbenes by Arduengo 3) in 1991, interest in the use of N-heterocyclic carbene-metal complexes have increased, and it has been demonstrated that they are efficient catalysts in important chemical transformations such as Pd-catalyzed coupling reactions, 4) Ru-catalyzed olefin metathesis, 5) and Rh-catalyzed hydrosilylations. [6][7][8] NHCs have also attracted great attention as organocatalysts in several reactions (e.g., benzoin condensation, [9][10][11][12][13][14] Stetter reaction, [15][16][17] transesterification/acylation reactions, 18,19) and nucleophilic substitution reactions). [20][21][22] Recently, our research group as well as other groups have reported the cyanosilylation of aldehydes catalyzed by NHCs. [23][24][25][26] In the course of our studies on the use of NHCs in organic synthesis, we have found that copper-NHC complexes exhibit the ligand acceleration effect (LAE) in the addition of diethylzinc to N-sulfonylimines. The LAE of copper-NHC complexes was first reported in the addition of diethylzinc to cyclohexenone, 27) and later the asymmetric version of these conjugate addition reactions was accomplished by using chiral NHC ligands. 28,29) In this study, we report our results on the addition of diethylzinc to N-sulfonylimines by using catalytic amounts of copper-NHC complexes. Results and DiscussionIn order to examine the catalytic abilities of the copper-NHC complexes, an addition reaction of diethylzinc to N-(benzylidene)-p-methylbenzenesulfonamide (N-sulfonylimine) 1a 30) was carried out in the presence of 5 mol% of imidazolium salt 2a as a ligand source, 5 mol% of CuI, and 5 mol% of t-BuOK in toluene (Table 1). After reacting at room temperature for 1.5 h, the addition product 3a (50%) and the reduction product 4a (28%) were obtained (Table 1, entry 1). The formation of 4a could be due to the transfer of the b-hydrogen of the ethyl functionality to the CϭN bond of the N-sulfonylimine 1a. In the absence of 2a, the reaction afforded the desired product 3a in only 15% yield and afforded 4a as the major product (entry 2). The reaction without CuI at room temperature for 6 h afforded 3a and 4a in 31% and 22% yields, respectively (entry 3). These results indicate that the LAE of NHC is involved in the addition reaction. To increase the yield of 3a and to suppress the byproduct formation, the reaction was carried out at 0°C. It was observed that though the yield of 3a was increased to 65%, byproduct 4a (12%) was still obtained. Further lowering of the reaction temperature to Ϫ5°C led to the exclusive formation of 3a (entry 5).At Ϫ5°C, the addition reactions of diethylzinc to 1a were carried out by using imidazolium and imidazolinium salts 2b-d as ligand precursors, and the addition products 3a were isolated in go...

PFAM catalyzed enantioselective diethylzinc addition to imines

“…6 Since then different groups have studied this reaction with different chiral catalysts; for example, Gong et al used copper-bidentate and tridentate bisoxazolines, 7 Wang et al used copper-chiral ferrocenyl amidophosphines, 8 Shi et al used copper-chiral binaphthylthiophosphoramides, 9 and Suziki et al used copper-N -heterocyclic carbenes. 10 As examples of diethylzinc addition to N-diphenylphosphinoyl arylaldimine, Charatte et al used copper-chiral phosphine ligands, 11 Ha et al used copper-diphosphine and thiophosphoramide ligands, 12 Wang et al used copperferrocenyl amino ketones, 13 Liao et al used copper-chiral tert-butanesulfinylphosphines, 14 and Yus et al used polymer-supported L-prolinol catalyst. 15 Besides N -sulfonyl and N -phosphinoyl imines, this reaction was also studied with BOC and formyl protected imines by Aleksakis et al 16 and Feringa et al 17 respectively.…”

Rh-N-Heterocyclic Carbene (NHC) Complex-Catalyzed Addition of Phenylboronic Acid to N-Sulfonyl and N-Phosphinoyl Aldimines

“…[12][13][14] Moreover, NHCs have attracted considerable attention as organocatalysts in several reactions such as benzoin condensation, [15][16][17][18][19][20] Stetter reaction, [21][22][23] transesterification/acylation reactions, 24,25) and nucleophilic substitution reactions. [26][27][28] In our study on the use of NHCs as ligands in organic transformations, we have recently reported the addition of diethylzinc to N-sulfonylarylimines catalyzed by Cu-NHC complexes, 29) where NHCs exhibit a strong ligand acceleration effect (LAE). This finding has prompted us to investigate the applicability of a similar methodology to the Rh-NHC complex-catalyzed addition of phenylboronic acid to imines.…”