Benzimidazolium salts, oxidation

Methylbenzimidazole with DMAD in acetonitrile gave134 the yellow 360, readily oxidized by bromine in perchloric acid to a pyrido-[l,2-a]benzimidazolium salt, and an isomeric red compound originally thought to have structure 357, and consequently it was surprising that oxidation did not follow the same course as for 360. The red compound is best prepared from the reactants in toluene and has now been identi-... 

Benzimidazoles, 2,3-dihydro-, by Nu addition at C-2, 56, 203 Benzimidazole, 2-fluoro-, 59, 272 Benzimidazole, 1-methyl-, ring expansion to chloroquinoxalines, 59, 305 Benzimidazole, l-methyl-2-(5 -methyl-2 -thienyl)-, bromination, 57, 319 Benzimidazole 3-oxides, chloro-deoxygenation, 59, 270 Benzimidazoles, 2-perfluoroalkyl-, 60, 25 Benzimidazoles, 2-(l-pyridinio)-, 60, 204 Benzimidazoles, 2-(2-pyridinioethyl)-, elimination reactions, 60, 246 Benzimidazoles, tetrafluoro-, 59, 14 Benzimidazole-2-carboxylic ester 3-oxide, 7-nitro-5-trifluoromethyl-, 60, 17 Benzimidazolium salts, l-amino-3-alkyl-, reaction with activated acetylenes, 56, 138... 

Although isolated in small amounts, the pyridinium salts 282 (arising from the iodine-mediated oxidation of DHPs 281) do not afford benzimidazolium salts 279 in the presence of isocyanides. A likely pathway may start with the iodo-dihy-dropyridinium intermediate 280, which can be formed by interaction of iodine with the reactive double bond of the DHPs (Scheme 44). The isocyanide may subsequently attack these species to generate the nitrilium ion T, which interacts with a second equivalent of isocyanide to form cation U. This intermediate undergoes an intramolecular trapping by the enamine unit to close the bicyclic system V... 

Further investigations by Crabtree showed oxidative addition of imidazolium C H bond possible for Pd(0). Reaction of pyridine-substituted imidazohum salts (140) (Scheme 25) with Pd2dba3 afforded the neutral or cationic complexes (141) and (142). Apparently, the geometry cis/trans of the formed complex is a function of the steric pressure of the NHC-nitrogen substituents. Remarkably, this behavior is not related to the pyridine moiety, which could act as a base, since the oxidative addition of benzimidazolium salts to [Pd2dba3] affording [(NHC)2PdCl2] was achieved in moderate yield. ... 

There are a number of examples in which 1-alkoxybenzimidazoles, l-alkoxy-3-methyl-benzimidazolium salts, and 1 -alkylbenzimidazole 3-oxides react with anionic sulfur species to give... 

The use of /r-hydroxo or ju-alkoxo bridged polynuclear complexes of chromium, molybdenum, tungsten, or rhenium in this route leads to the formation of monomeric bis(NHC) complexes, to the elimination of hydrogen, and to the partial oxidation of the metal [Eq.(ll)]. Chelating and nonchelating imidazolium salts as well as benzimidazolium and tetrazolium salts can be used. 

Reaction of 68 with protected galactose (59) and benzimidazolium triflate as activator followed by oxidation or sulfurization, deprotection, and conversion into the sodium salt provided bis(phosphonate) 70 and bis(phosphonothioate) 71 (Figure 2.9) in a total yield of 5 (70) and 25% (71). 

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