Imidazole-catalyzed decomposition

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>. 

The imidazole complexes are unstable in aqueous solution and decompose rapidly to TCO2. At lower pH the complexes undergo acid-catalyzed decomposition. None of the complexes exhibited oxidation or reduction processes by cyclic voltammetry. However, it was chemically possible to reduce the imidazole complex with Zn in IM HCl. A defined complex formed that did not contain the Tc=0 group probably reduction to Tc complexes occurred. " ... 

Product studies in certain buffers of various concentrations furthermore demonstrate that the decomposition of the neutral tetrahedral intermediate TH from N-phenyliminotetrahydrofuran to aniline and butyrolactone, is general acid-base catalyzed (without affecting the rate of disappearance of the substrate) (Cunningham and Schmir, 1966). Effective buffers are acetate, phosphate, bicarbonate, arsenate, phenyl-phosphate, and 3,3-tetramethyleneglutaric acid ineffective buffers are tris-(hydroxymethyl)-aminomethane, imidazole andp-nitrophenol. The capability of the effective buffers, which are all bifunctional, can be attributed to a cyclic internal acid-base catalyzed decomposition of the neutral intermediate TH (transition state 16). 

The reaction of acylimidazoles with imidazole is subject to both imidazole and imidazolium ion catalysis (Fife, 1965). The latter reaction is no doubt due to the imidazole-catalyzed hydration of acetylimidazolium ion, as in 18, and fully analogous to the N-methylimidazole-catalyzed hydrolysis of N-acetyl,N -methylimidazolium ion. The mechanism of the former reaction is undefined at the present time, since no 0 exchange studies have been performed with acylimidazoles in more alkaline solution where imidazole catalysis occurs. The leaving group, the imidazole anion, is quite basic (piC = 14-5) therefore it is possible that general base-catalyzed decomposition of the neutral tetrahedral intermediate (24) or general acid-assisted decomposition of the anionic tetrahedral intermediate (25) may occur. The general base-catalyzed alkaline hydrolysis of amides most probably occurs by the... 

Despite general acceptance of the catalytic mechanism, the order of the microscopic steps through which the acid-base catalysis occurs remains arguable. An acid-base buffer system, with or without zinc ion, that promotes the chemical catalysis of cyclization and cleavage of phosphodiester bonds provides some insight into the mechanism. In this nonenzymatic system, acidic imidazolium (Im+) ion initiates the conversion to a pentacoordinate phosphorane intermediate then the basic imidazole (Im) catalyzes the decomposition of this intermediate to a... 

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