Heterocyclizations at the Isocyanide Carbon: Mechanistic Insights into the 2-Isocyanoaniline to Benzimidazole Conversion

Benzo-fused heteroazacycles can form through spontaneous cyclizations of phenylisocyanides bearing some electron donor substituents at the <i>ortho</i> position. Although such reactions have been known since the 1970s, a clear mechanistic understanding of these transformations remains absent. Here, we disclose a detailed computational analysis aimed at elucidating the more plausible mechanism for one of these cyclizations, the <i>o</i>-isocyanoaniline to benzimidazole conversion. Our results show that a transient 1,4-diazabutatriene could play a key role in this cyclization by enabling a reaction pathway of lower barrier than the unimolecular concerted process.