Experimental and Computational Studies on the Formation of Three para‐Benzyne Analogues in the Gas Phase

Experimental and computational studies on the formation of three gaseous, positively‐charged para‐benzyne analogues in a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer are reported. The structures of the cations were examined by isolating them and allowing them to react with various neutral reagents whose reactions with aromatic carbon‐centered σ‐type mono‐ and biradicals are well understood. Cleavage of two iodine–carbon bonds in N‐deuterated 1,4‐diiodoisoquinolinium cation by collision‐activated dissociation (CAD) produced a long‐lived cation that showed nonradical reactivity, which was unexpected for a para‐benzyne. However, the reactivity closely resembles that of an isomeric enediyne, N‐deuterated 2‐ethynylbenzonitrilium cation. A theoretical study on possible rearrangement reactions occurring during CAD revealed that the cation formed upon the first iodine atom loss undergoes ring‐opening before the second iodine atom loss to form an enediyne instead of a para‐benzyne. Similar results were obtained for the 5,8‐didehydroisoquinolinium cation and the 2,5‐didehydropyridinium cation. The findings for the 5,8‐didehydroisoquinolinium cation are in contradiction with an earlier report on this cation. The cation described in the literature was regenerated by using the literature method and demonstrated to be the isomeric 5,7‐didehydro‐isoquinolinium cation and not the expected 5,8‐isomer.