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.