Chemical and Spectroscopic Studies of Benzimidazolenium Ion: Reactivity of a Formally Antiaromatic endo-Heterocyclic Nitrenium Ion

Benzimidazolenium 1, the nitrenium ion derived from benzimidazole, was generated from the photolysis of N-benzimidazolyl-2,4,6-triphenylpyridinium ion 7. The products isolated from its photolysis are benzimidazole, triphenylpyridine, and an adduct wherein triphenylpyridine adduct combines with the nitrogen center of benzimidazolenium ion. Surprisingly, photolysis of 7 in the presence of nucleophilic traps such as chloride, methanol, and ethanol failed to yield any nucleophilic adduct as the major product though transient absorption spectra does show that these nucleophiles interact with the species generated by laser flash photolysis. In addition, nucleophilic trapping of nitrenium ion 1 with silyl enol ethers produced miniscule amounts of adduct to the nitrenium ion. However, photolysis of 7 in toluene generates bibenzyl, a dimeric product from coupling of solvent-derived radicals. These studies suggest that 1 and similar species are likely to be extremely reactive and will decay primarily through abstracting H atoms from the solvent and in-cage recombination with the pyridine leaving group.