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

Abstract

973The synthesis, isolation, and characterization of stable Nheterocyclic carbenes (NHCs) first reported by Arduengo et al. 1) in 1991, have attracted attention for the use of NHCs as ancillary ligands for transition metal complexes. Due to their strong s-donating properties, NHCs can form metal complexes that have high stabilities toward heat, moisture, and air, and they have higher catalytic abilities than their phosphine counterparts. [2][3][4][5][6] NHCs are now used as ligands for transition metals in many important chemical transformations such as Pd-catalyzed coupling reactions, 7) Ru-catalyzed olefin metathesis, 8) Rh-catalyzed hydrosilylations, [9][10][11] and Cu-catalyzed conjugate addition reactions. [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 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.The addition reaction of organometallic reagents to imines is one of the most efficient procedures for the synthesis of diarylmethylamines, which are important subunits of biologically significant compounds. [30][31][32][33] Despite many studies on the Rh-phosphine complex-catalyzed addition of arylboronic acids to imines, 34,35) there is only one study, i.e., that by Charette and coworkers, in which the Rh-NHC complex is used.36) Here, we report our results on the addition of arylboronic acid to both N-sulfonylarylimines and N-phosphinoylarylimines by using a catalytic amount of Rh-NHC complexes. Results and DiscussionIn the study by Charette and coworkers, the Rh-NHC complex was prepared in situ using the NHC-transfer reagent Ag-NHC and used as a catalyst in the arylation of pmethylphenyl-N-phosphinoylimine with phenylboronic acid. 36) They showed only one reaction example. We examined the catalytic ability of the Rh-NHC complex generated in situ 37) from azolium salts and [RhCl(cod)] 2 for the arylation of imines. The Rh-NHC complex was prepared from [RhCl(cod)] 2 , 1,3-diadamantylimidazolium chloride 2a, and t-BuOK; then, N-sulfonylphenylimine 1a 38) and phenylboronic acid were added to the mixture. The reaction in dioxane at 70°C for 5 h afforded N-sulfonyldiarylmethylamine 3a in 91% yield (Table 1, entry 1). The reaction without 2a in dioxane afforded the desired product 3a in only 62% yield after 10 h at 70°C (entry 2). These results showed and indicated that the NHC ligand accelerated the addition reaction. The reaction in THF afforded 3a in 48% yield, whereas in toluene, the reaction was very sluggish and gave no adducts (Ta...

N-Heterocyclic Carbenes in Late Transition Metal Catalysis

“…Another "common" imidazolium salt, IAd • HCl, in conjunction with [RhCl-(COD)] 2 , proved active in the arylation of N-sulfonyl and N-phosphinoyl arylimines. 187 Buchmeiser, pursuing the examination of the potential in catalysis of tetrahydropyrimidine-based NHCs, 155,174 tested complexes 82, 84, and 85 (see Chart 10) in the arylation of benzaldehyde derivatives. 188 All these compounds were found to be active, and the best results (i.e., TONs up to 1230) were obtained with a modified catalyst of the type 84/85, where the halogen was replaced by a trifluoroacetato ligand.…”

“…Of note, vinyl boronic acids could be used with this catalyst and afforded the corresponding allylic alcohols. Another “common” imidazolium salt, IAd·HCl, in conjunction with [RhCl(COD)] 2 , proved active in the arylation of N -sulfonyl and N -phosphinoyl arylimines …”