Gas-phase Smiles rearrangement of ortho-nitro-substituted diaryliodonium cations and their mechanistic insights

Rationale: Diaryliodonium salts are useful electrophilic reagents in organic chemistry, finding extensive applications in arylations and photo-induced polymerizations. However, comprehensive mechanistic investigations, particularly concerning the mass spectrometric behaviors of diaryliodonium salts, are relatively scarce in the literature. Methods: Diaryliodonium salts could be readily ionized using electrospray ionization mass spectrometry (ESI-MS) to give [Ar-1-I+-Ar-2], and high-resolution ESI-tandem mass spectrometry (MS/MS) experiments were conducted to investigate their gas-phase chemical reactions. Results: Investigations on ESI-MS/MS of [Ar-1-I+-Ar-2] revealed two major fragmentation patterns: (1) reductive elimination resulting in the diaryl coupling product ion [Ar-1-Ar-2](+center dot) by the loss of I; (2) generation of aryl cations [Ar-1](+) or [Ar-2](+) through cleavage of the C-I bonds. We revealed that the introduction of NO2 into Ar-2 of [Ar-1-I+-Ar-2] could lead to an unexpected fragmentation ion [(ArO)-O-1](+) in MS/MS, arising from an O-atom transfer process from NO2 to Ar-1. Particularly, when NO2 was ortho-positioned to the iodine in Ar-2, the [(ArO)-O-1](+) sometimes exhibited dominant behavior. Conclusions: Comprehensive ESI-MS/MS studies and theoretical calculations provided strong support for the O-atom transfer mechanistic pathway: [Ar-1-I+-(o-NO2-Ar-2)] initially underwent a Smiles rearrangement to the intermediate [Ar-1-O-(o-NO-(ArI)-I-2)](+), which subsequently dissociated to [(ArO)-O-1](+) or [o-NO-(ArI)-I-2](+center dot). Herein, we proposed an unexpected "ortho effect" in the gas-phase fragmentation reaction of [Ar-1-I+-(o-NO2-Ar-2)], in which the crucial determinant factor for the aryl migration was identified as the Smiles rearrangement reaction.