Sigma in sight

The intermediate σ complex presumed, but not seen, to be involved in nucleophilic aromatic substitution reactions has now been observed experimentally

Credit: © 2008 Wiley

The transition state of a nucleophilic aromatic substitution reaction has always been presumed to be a σ complex. However, the transitory nature of such species has meant that they have so far not been observed experimentally in these reactions, although they have been seen in cases in which the final elimination step is unfavourable, leading to a build up of the σ complex.

Now, Asuka Fujii and colleagues in Japan have observed¹ σ complexes in a successful reaction involving C₆F₆ — having no ortho, meta or para isomers, only one intermediate complex is likely, making it more easy to observe. They used coherent vacuum ultraviolet light to directly ionize C₆F₆ to form C₆F₅NH₂⁺, and measured the infrared spectra of the resulting species. They found peaks attributed to the (C₆F₆–NH₃)⁺ cluster cation, which forms an intermediate σ complex.

Density functional theory calculations suggested four possible stable structures for this complex, and their concomitant infrared spectra, only one of which matched the infrared spectrum obtained experimentally. In this structure, a covalent bond is formed by donation of the lone pair of electrons on the nitrogen atom of the ammonia species to a carbon atom on the phenyl ring, with the resulting bond lying out of the plane of the ring. Nucleophilic aromatic substitution reactions are very common, and knowledge of the structure and stability of the intermediate structure should aid more directed synthetic approaches.