Benzimidazole carbene insertion

Carbenes insert into the N=N bond of azo compounds and the unstable adducts rearrange spontaneously to produce benzopyrazoles (e.g. Scheme 7.31) [37,38], The reactions of azo compounds with dichlorocarbene is also catalysed by tertiary amines [36] and, under such conditions, azoxyarenes produce the benzimidazoles, presum-... 

Imidazoles react with chloroform at high temperature to form azines by carbene insertion and trichloromethyl radicals behave similarly, but carbenes do not always induce ring expansion. In alkaline medium, chlorodifluoromethane converts benzimidazole and its 2-methyl analogue into the 1-difluoromethyl derivatives 365. Dichlorocarbene under basic conditions N-alkylates benzimidazole, and 1-methylbenzimidazole couples under the influence of the same reagent (Scheme 69), perhaps involving initial attack of the carbene at N(3). 

Carbenoid addition of diazocarbonyl compounds to pyrrole, A -alkylpyrroles, indole, N-alkylindoles, imidazole, and benzimidazole does not result in cyclopropanation, but leads to the formal products of carbene insertion into a heterocyclic C-H bond (see Houben-Weyl, Vol.E19b, ppll58 and 1334). However, Af-acylpyrroles, and Af-acylindoles - " have successfully been converted into 2-azabicyclo[3.1.0]hex-3-ene-exo-6-carboxylates and alkyl-1,la,2,6b-tetrahydrocyclopropa[ ]indole-exo-l-carboxylate, respectively (for an experimental procedure, see Houben-Weyl, Vol.E19b, pll60). 

A new stable ciystalline carbene l,3-bis(l-adamantyl)benzimidazol-2-ylidene 133), was synthesized [90], by decomposition of l,3-bis(l-adamantyl)-2,3-dihydro-l/f-benzimidazol-2-ylacetonitrile 142) on heating under reduced pressure. Heteroaromatic stable carbene 133 reacted with acetonitrile to give the corresponding insertion products 142. The geometric parameters of l,3-bis(l-adamantyl)benzimidazol-2-ylidene 133), determined by X-ray analysis. 1,3-Bis(l-adamantyl)benzimidazol-2-ylidene 133) reacted with molecular sulfur in benzene to give l,3-bis-(l-adamantyl)-2,3-dihydro-l/f-benzimidazole-2-thione 143). 

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