Figure 2 High-resolution ESI-MS/MS of
[M –OTf]+: (a ) 5 ,
(b ) 6 , (c ) 7 and (d )8 .
Scheme 4 Proposed rearrangement patterns of
[M –OTf]+ of salts5 ~7 .
Based on the observation of O-atom transfer product ion
[t Bu-C6H4O]+at m/z 149 from nitro-substituted diaryliodonium cation
[4 –OTf]+, we extended to more
diaryliodonium cations with salts 5 ~7as shown in Scheme 1 , and the spectra of the salts5 ~7 with nitro substituent in
different position (ortho , meta and para ) of
Ar2 were depicted in Figure
2a ~c . To our delight,
[Mes-I+-(NO2-C6H4)]
from salts 5 ~7 all exhibited the
O-atom transfer product ion [MesO]+ atm /z 135 with high abundances in their respective MS/MS. As
a comparison, we also tested cations from salt 8 simultaneously
(Figure 2d ). However, the product ion
[MesO]+ at m /z 135 could not be
observed in the MS/MS of [Mes-I+-Mes] atm/z 365. The accurate mass measurements (in Table S1 andS2 ) supported our explanations. These results collectively
demonstrated that the introduction of a nitro group into
Ar2 of
[Ar1-I+-Ar2]
could give rise to new type fragmentation ions
[Ar1O]+ by O-atom transfer
process from the nitro group in Ar2 to
Ar1 in ESI-MS/MS of nitro-substituted diaryliodonium
cations.
Most importantly, when the nitro group was in ortho -position of
Ar2, the fragmentation ion
[MesO]+ at m /z 135 dominated the
tandem mass spectrum of
[Mes-I+-(o -NO2- C6H4)]
at m /z 368 from salt 5 , with only a minimal
presence of [Mes]+ at m/z 119
(Figure 2a ). Two possible reaction pathways were proposed for
the gas-phase O-atom transfer rearrangement of
[5 –OTf]+ (Scheme 4 ):path (a ) Smiles rearrangement via a six-membered ring
transition state (TS 5a ) to intermediate 5b[Mes-O-(o -NO-C6H4I)]+,
which finally dissociated to [MesO]+ atm /z 135; path (b ) Cleavage of the C–I
bond to form another transition state (TS 5a ) and ion-neutral
intermediate 5d , which could directly give to
[Mes]+ at m/z 119 or further lead to
intermediate 5b and finally forming
[MesO]+ at m /z 135 (Scheme
4 ). Due to the distance problems, the
[Mes-I+-(m -NO2-C6H4)]
and
[Mes-I+-(p -NO2-C6H4)]
from salt 6 or 7 might follow a stepwise process
involving ion-neutral intermediates [Mes+ &
(m -NO2-C6H4I)]
or [Mes+ &
(p -NO2-C6H4I)]
to generate the intermediate 6b[Mes-O-(m -NO-C6H4I)]+or 7b[Mes-O-(p -NO-C6H4I)]+,
respectively. These intermediates ultimately resulted in the O-atom
transfer product ion [MesO]+ at m /z135 (here Ar1 = Mes) with loss of neutral species
[m -NO-C6H4I] or
[p -NO-C6H4I] (Scheme
4 ).