1 Introduction
Hypervalent iodine compounds (HICs) possess remarkable advantageous characteristics, including facile synthesis, low toxicity, and environmental friendliness, making them versatile tools in a wide range of organic reactions, such as functionalizations, rearrangements, aryl-coupling and cyclization reactions.1-3 The notable feature of HICs is its ability to exhibit similar reactivity to the transition metal reagents due to the changes in valence states, thereby providing cost-effective alternatives to expensive organometallic catalysts and toxic heavy metal-based oxidants.4,5 Within this category of HICs, diaryliodonium salts, also referred to as diaryl-λ3-iodines following IUPAC nomenclature,6 stand out as a unique class of HICs renowned for their utility as highly electrophilic arylating reagents under mild reaction conditions.7 Apart from their synthetic applications, diaryliodonium salts play a significant role in semiconductor photolithography. For instance, the bis(4-(1,1-dimethylethyl)phenyl)iodonium salt is a type of useful ionic photoacid generators (PAGs). Additionally, the aryliodonium salts find widespread usage in photo-redox catalysis and serve as cationic photo-initiators in polymerization reactions, contributing to the production of diverse materials such as adhesives, coatings, composites, dental materials.8-11
In 1983, Gross et al. pioneered the use of mass spectrometry to investigate the gas-phase reaction between benzene radical cations and alkyl iodides, leading to the proposal of the C9H13I+intermediate.12 Six years later, the same group described the six fragmentation processes involving iodonium radical cations.13 In 1989, Busch et al. employed fast atom bombardment (FAB) and secondary ion mass spectrometry (SIMS) to provide further insights.9,10 Then, in 1992, Busch group combined FAB with semi-empirical calculations to examine iodonium salts and pointed out that ortho -hindered iodonium salts lacked the rearrangement process.11 In 1995, Sam et al. made significant contributions by employing electrospray ionization mass spectrometry to investigate the oxidation of PhIO in methanol.14 In 1999, Ochiai et al. made a noteworthy discovery concerning the dimerization of iodine(III) reagents using FAB.15 Additionally, Silva and colleagues conducted detailed studies on the disproportionation reaction of (diacetoxyiodo)benzene (DAIB) in acetonitrile and methanol by employing high-resolution ESI-MS/MS techniques.16,17 Our group intercepted and characterized the transient reactive α-λ3-iodine alkyl acetophenone intermediate using ESI-MS/MS, thus validating the reaction mechanism proposed by Ochiai.18 Rissanen’s group dedicated to explore the role of iodonium ions in supramolecular chemistry.19-22 Niu et al. identified the photoproducts of bis(4-tert-butylphenyl)iodonium salts using GC-MS and proposed a mechanism for the photolytic reaction.8
The Smiles rearrangement is an intramolecular nucleophilic aromatic substitution, in which the aryl group transfers within an aromatic ring to an adjacent carbon atom.23 It is becoming increasingly recognized that the gas-phase Smiles rearrangement is not confined to the classical five-membered ring transition state. For examples, both Wang and Pan proposed transition states of six-membered rings, supported by theoretical calculations.24-28Additionally, Pan’s group highlighted that carbon monoxide (CO) could be eliminated through a three-membered ring transition state.29 Concerning the extrusion of sulfur dioxide (SO2), Lu and Irikura suggested that this process occurred through a five-membered ring transition state.30,31 In this case, Li proposed a three-membered ring pathway.32 Wu et al. proposed a Smiles rearrangement involving a four-membered ring, providing an explanation for the loss of formaldehyde (H2CO) following this skeletal rearrangement.33
Scheme 1 Schematics illustrating. (a ) Mass spectrometric behaviors of nitroarenes in EI-MS: ortho effect ofo -nitrotoluene and rearrangement of nitrobenzene; (b ) Our previous work: the gas-phase rearrangement reaction from [CF3-I+-Ph] to [PhCF3]+• by loss of I; (c ) This work: gas-phase reduction elimination of [Ar1-I+-Ar2] to aryl coupling product ion of [Ar1-Ar2]+•with loss of an iodine atom and the generation of O-atom transfer product ion [Ar1O]+ from [Ar1-I+-(o -NO2-Ar2)] in ESI-MS/MS.
The ortho effect and rearrangement of nitroarenes in EI-MS have previously been investigated (Scheme 1a ),34,35 and we have studied on gas-phase intramolecular aryltrifluoromethylation of phenyl(trifluoromethyl)iodonium cation via loss of an iodine atom (Scheme 1b ).36 However, our recent findings have unveiled that the introduction of NO2 to diaryliodonium salts had introduced an entirely new dimension to the fragmentation patterns, resulting in the unexpected O-atom transfer product ion [Ar1O]+ from [Ar1-I+-(o -NO2-Ar2)] (Scheme 1c ). Surprisingly, the exploration of the fragmentation patterns of nitro-substituted diaryliodonium cations was limited in the literature. Hence, we comprehensively investigated various diaryliodonium cations using high-resolution electrospray ionization tandem mass spectrometry (ESI-MS/MS) and provided mechanistic insights into the understanding of their gas-phase reactions through theoretical calculations. The research results would be beneficial for the research for the chemical and mass spectrometric studies of these diaryliodonium salts.