Benzimidazole catalytic processes

As with the synthesis of indoles, tandem catalytic processes have also received considerable attention. To this end, imidoyl chlorides have proved useful starting materials (Scheme 24.10, disconnection D-3) [71,72]. Scheme 24.14 presents an example from the Zhang group, where imidoyl chloride 31 can be combined with benzyl amine using copper catalysis to deliver the expected benzimidazole in excellent yield [71]. However, it was necessary to include an electron-withdrawing substituent on the imidoyl chloride substrate to ensure good reactivity for example, the trifluoromethyl substituent present in substrate 31. 

Like their benzimidazole counterparts, indazoles have also been a popular target for the development of synthetic routes incorporating intramolecular and intermolecular C—N bond formations as well as tandem catalytic processes. Indeed, many of the... 

The catalysis is initiated by reaction 20, which is too fast to be detected the catalytic cycle is composed by reactions 21 and 22. The efficiency of the reaction turnover is lower than for benzimidazole containing systems described above. The treatment of the rate data for the dicopper(II) complex of EthPy4, on the basis of the proposed model in which the oxygenation of the Cu 2 moiety is taken as r.d.s., provided the kinetic and thermodynamic parameters of the process. The parameters related to the oxygenation step (k = 7.6 AH = 47.6 kJ mol and AS = —76 J K mol ) are not... 

Basu and Mandal have presented a comprehensive review on various sustainable synthetic strategies leading to benzimidazoles, quinoxalines, and their congeners highlighting the scope, limitations, and plausible mechanisms. These heterocyclic scaffolds have already drawn considerable interests to the researchers at large for their well-established biological and pharmaceutical properties. The authors have given emphasis on the sustainable processes that involve solid-phase synthesis, biomimetic synthesis, combinatorial synthesis, various catalytic methods, and reactions promoted by different solid sinfaces, solvent-free or on-water conditions or xmder microwave irradiation. This overview would serve as a useful somce of information to the readers in this particular area of interest. 

In 2010, Zhu and coworkers first reported the direct intramolecular C(sp )-H amination of Af-arylpyridine-2-ammes using a combination of Cu(OAc)2 and Fe(N03)3 as a bimetalUc catalytic system to furnish pyrido[l,2-a]benzimidazoles in satisfactory yields [13]. The authors believe that a Cu(lll)-catalyzed electrophilic aromatic substitution (SEAr) pathway is operating in this process according to the results of mechanistic studies, wherein iron(lll) acts a unique role to facilitate the formation of the more electrophilic Cu(III) species because in the absence of iron(III), a much less efficient and reversible Cu(II)-mediated SEAr process takes place. 

With microwave-assisted technology, Porcheddu and colleagues [11] prepared benzimidazoles via one-pot C-H activation of tertiary amines/oxidative cycliza-tion with 1,2-phenylenediamine in the presence of Pd/C (Scheme 14.10). To ascertain the heterogeneous character of the catalytically active species in this process, further leaching studies were performed. Both Sheldons hot filtration test and atomic absorption spectroscopy analysis (ICP-MS) of the filtrate detected no significant quantities of leached Pd in solution, suggesting that the reaction was a heterogeneous process. 

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