Imidazoles and benzimidazoles

Imidazoles and benzimidazoles (155) react with acid chloride and alkali to give compounds of type (157), but these are reactions of the cation (156). 1,2,4-Triazoles and tetrazoles similarly undergo ring opening. 

These data are for the nucleophilic catalysis of the hydrolysis of p-nitrophenyl acetate by imidazoles and benzimidazoles at pH 8.0. Tbe apparent second-order catalytic rate constants are defined by... 

The formation of quaternary salts by the action of the common quatemizing reagents on 1-substituted imidazoles and benzimidazoles (29) to give salts of type 30 is well known. Only N-3 possesses a free... 

As mentioned, the frequencies of proton jumps leading to establishment of the equilibrium 14a 14b in solutions of imidazoles and benzimidazoles... 

Fyrazoles and indazoles, imidazoles and benzimidazoles/ and benzotriazoles which possess a free NH group are associated and have been considered to exhibit mesohydric tautomerism see discussion in Volume 1, article I, Section I,C. 

Similarly, pyrrole, indole, and tetrahydrocarbazole [285] as well as diarylamines give the corresponding N-vinyl compounds [286]. Several improvements of yields and reaction rates were observed by conducting the reactions in the presence of additives [287, 288]. The vinylation of imidazole and benzimidazole was reported to be catalyzed by KOH in the presence of zinc or cadmium salts [289]. The above reactions were reviewed in 1965 [290]. 

Whereas pyrrole was reported not to give N/H insertion by ketocarbenoids, such a reaction mode does occur with imidazole Copper-catalyzed decomposition of ethyl diazoacetate at 80 °C in the presence of imidazole gives ethyl imidazol- 1-ylacetate exclusively (93 %) small amounts of a C-alkylated imidazole were obtained additionally under purely thermal conditions 244). N/H insertion also takes place at benzimidazole 245 a). The reaction is thought to begin with formation of an N3-ylide, followed by N1 - C proton transfer leading to the formal N/H insertion product. Diazomalonic raters behave analogously however, they suffer complete or partial dealkoxycarbonylation under the reaction conditions 244) (Scheme 34). N-alkylation of imidazole and benzimidazole by the carbenoids derived from co-diazoacetophenone and 2-(diazoacetyl)naphthalene has also been reported 245 b>. 

Carbenes are defined as molecular species with formally divalent and two-coordinate carbon atoms bearing various substituents X and Y and a lone pair of electrons. While the simple representatives are of low stability (such as CH2) and may only appear as short-lived reaction intermediates or in adducts with electron donors, some cyclic systems can be readily isolated. This is particularly true for many of the A-heterocyclic carbenes (NHCs), which are now widely applied as ligands to metals ( Wanzlick-Arduengo carbenes ). Such carbenes based on imidazol and benzimidazol have become the working horses in this branch of organogold chemistry (Scheme 54). 

The most common ligands are those derived from imidazole and benzimidazol (Scheme 54), followed by the (benz)thiazols. The free Wanzlick-Arduengo carbenes can be isolated and employed for the synthesis of the complexes, but often it is more convenient to prepare the carbenes in situ from the dimers or the corresponding onium salts, or to use carbene-transfer reactions.256-259... 

Grimmett, M. R. Imidazole and Benzimidazole Synthesis Academic Press London, 1997. 

Reppe has shown that secondary amines of low basicity, such as carbazole, diphenylamine, indole, imidazole, and benzimidazole, and amides such as pyrrolidone react with acetylene in the presence of strong alkali to give vinyl derivatives [81, 83, 84a, b] (Eq. 30). As described by Reppe, these reactions are... 

Bruice and Schmir (3) have shown that for a series of imidazole derivatives, klm depends on the base strength of the catalyst and since pKA is an approximate measure of base strength, the value of klm should increase with increase in pKA. Table I shows that this is indeed the case. Imidazole, pKA = 7.08, has a catalytic constant eight times larger than that of benzimidazole, pKA = 5.53. Bronsted and Guggenheim (2) have obtained a linear relationship between log k/ and pKA for a series of carboxylic acids in the pKA range of 2 to 5, where kB is the carboxvlate anion basic catalytic constant for the mutarotation of glucose and Ka is the acid dissociation constant of the acid. Our results for imidazole and benzimidazole fit fairly well into the Bronsted plot. 

The chemistry of imidazole and benzimidazole N-oxides has been reviewed, and methods of preparation have been summarized (88JCS(Pi)68i, 93CHE127). The oxides are thermally unstable and may be explosive (84CB30, 89S773). 

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