3.4 Computations for gas phase rearrangements of nitro
diaryliodonium cations
To shed insight to the reaction mechanisms of nitro diaryliodonium
cations, density functional theory (DFT) calculations were performed
with the Gaussian 16 package.43 The geometries were
fully optimized at the M062x/Def2-TZVP level in gas
phase,44,45 and harmonic vibration frequency
calculations confirmed that the optimized structures were either minima
(no imaginary vibrations) or transition states (only one imaginary
vibration). The Gibbs free energies were given in kcal/mol and
geometrical coordinates could be referred to the supporting information.
The schematic potential energy surface and the optimized structures of
reactive species in ESI-MS/MS of
[5 –OTf]+ resulting from the theoretical
calculations were shown in Figure 5 . Two paths of
[5 –OTf]+ were considered, includingpath (a ): the Smiles rearrangement via a six-membered
ring transition state (IAr2-o-TS1 ); and path(b ): the cleavage of the C–I bond to form another transition
state (IAr2-o-TS3 ). The distance between
C(ipso) and I, C(ipso) and O atom of
nitro group in IAr2-o-TS1 were 3.130 and 2.436 Å, which
were both smaller than that of IAr2-o-TS3 (4.367 and
4.589 Å, respectively). These results suggested that Smiles
rearrangement of [5 –OTf]+ could facilely
occur via a six-membered ring transition state
(IAr2-o-TS1 ), leading to the formation of the
intermediate IAr2-o-INT1 .
Figure 5 Schematic of potential energy surface and the
optimized structures of reactive species in ESI-MS/MS of
[5 –OTf]+ (IAr2-o-R ). The
Gibbs free energies were given in kcal/mol.
The energy barrier for the Smiles rearrangement of path(a ) via a six-membered ring transition state
(IAr2-o-TS1 ) was only 22.7 kcal/mol and 9.5 kcal/mol
less than the energy required for the C–I bond cleavage energy barrier
of IAr2-o-TS3 (36.0 kcal/mol). Furthermore, the Smiles
rearrangement from the hypervalent iodine(III) complexIAr2-o-R to the iodane(I) intermediateIAr2-o-INT1 (-14.9 kcal/mol) was exothermic, but the
C–I bond cleavage to product complex IAr2-o-INT3 (30.0
kcal/mol, which might give [Mes]+ at m/z119) was endothermic. Thus, according to Figure 5 , the Smiles
rearrangement process of path (a ) formed
[Mes-I+-(o -NO2- C6H4)]
(IAr2-o-R ) to the intermediate
[Ar1-O-(o -NO-C6H4I)]+(IAr2-o-INT1 ) via IAr2-o-TS1 was a
favorable reaction pathway both kinetically and thermodynamically. Then,
the N–O bond cleavage of intermediate
[Ar1-O-(o -NO-C6H4I)]+(IAr2-o-INT1 ) gave [MesO]+ atm /z 135 by loss of neutral
[o -NO-C6H4I]
(IAr2-o-INT2 ) via transition stateIAr2-o-TS2 (energy barrier of only 12.4 kcal/mol). Such
calculation results supported our hypothesis proposed for path(a ) IAr2-o-R to intermediateIAr2-o-INT1 via Smiles rearrangement and finally
produced [MesO]+ at m /z 135 with
N–O bond cleavage of intermediate IAr2-o-INT1 .
Meanwhile, the C–I bond cleavage to product complexIAr2-o-INT3 (30.0 kcal/mol) might produce the
intermediate IAr2-o-INT4 (-10.2 kcal/mol) in path(b ) and such process was exothermic. The N–O bond cleavage of
intermediate IAr2-o-INT4 might also give
[MesO]+ at m /z 135 and neutral
[o -NO-C6H4I]
(IAr2-o-INT5 ) via transition stateIAr2-o-TS4 (energy barrier of only 14.4 kcal/mol). The
structures of the active species involved in the steps fromIAr2-o-INT3 to IAr2-o-INT5 were same as
those in the process from IAr2-o-INT1 toIAr2-o-INT2 , but the species with same structures had
minor differences in relative energies due to the different spatial
configurations in the finally optimized structures.
Figure 6 Schematic of potential energy surface and the
optimized structures of reactive species in ESI-MS/MS of
[6 –OTf]+ (IAr2-m-R ). The
Gibbs free energies were given in kcal/mol.
The path (b ) of C–I bond cleavage process proposed for
[5 –OTf]+ was unfavorable than Smiles
rearrangement of path (a ) (Figure 5 ). However,
the Smiles rearrangement via O-attacking was not accessible for
[6 –OTf]+ and
[7 –OTf]+ due to their distances between
C(ipso) and O atom of nitro group. Therefore, the
stepwise processes involved ion-neutral complexes were considered for
the explanation of the unexpected fragment ion
[MesO]+ at m/z 135 in ESI-MS/MS of
[6 –OTf]+ and
[7 –OTf]+. The schematic potential energy
surface and the optimized structures of reactive species in MS/MS of
[6 –OTf]+ and
[7 –OTf]+ resulting from the theoretical
calculations were shown in Figure 6 and 7 . Theoretical
calculations showed that [6 –OTf]+ and
[7 –OTf]+ dissociated via similar
mechanism as the path (b ) of C–I bond cleavage process
proposed for [5 –OTf]+, which was
unfavorable than Smiles rearrangement of path (a )
(Figure 5 ). However, the Smiles rearrangement via O-attacking
was not accessible for [6 –OTf]+ and
[7 –OTf]+ , due to the fact that the
distances between C(ipso) and O atom of nitro group inIAr2-m-R and IAr2-p-R were 4.783 and
7.265 Å, respectively, which were both much longer than that ofIAr2-o-R (3.007 Å).
Figure 7 Schematic of potential energy surface and the
optimized structures of reactive species in ESI-MS/MS of
[7 –OTf]+ (IAr2-p-R ). The
Gibbs free energies were given in kcal/mol.
Therefore, the stepwise processes involved ion-neutral complexes were
considered for explanation of the unexpected fragment ion
[MesO]+ at m/z 135 in ESI-MS/MS of
[6 –OTf]+ and
[7 –OTf]+. The energy barriers for the
formation of IAr2-m-TS1 and IAr2-p-TS1were 33.0 and 33.4 kcal/mol, respectively, which were both much higher
than that of IAr2-o-TS1 (22.7 kcal/mol). These results
proved that the Smiles rearrangement process inIAr2-o-TS1 was the most favorable path to give
[MesO]+ at m /z 135. Therefore, such
calculation results explained why the fragmentation ion
[MesO]+ at m /z 135 dominated the
MS/MS of
[Mes-I+-(o -NO2- C6H4)]
from diaryliodonium salts (5 and13 ~16 ) but only with tiny amount of
[Mes]+ at m/z 119.
To test the generality of the fragmentation patterns we proposed in this
article, we further performed MS/MS experiments on cations from salts17 ~54 (structures shown inScheme S1 ), and their tandem mass spectra and MS/MS data were
summarized in Figure S17 ~S54 andTable S1 ~S4 . At the same time,
pseudo-MS3 experiments were performed in order to
assign the origin of some ions (Figure S55~S57 ). These results showed the fragmentation
ion [MesO]+ from
[M –OTf]+ (M =17 ~22 ) completely dominated the
spectra (Figure S17 ~S22 ) and
diaryliodonium cations from salts17 ~36 also conformed well to the
fragmentation patterns proposed in Scheme 3 and 4 . The
fragmentation patterns of cyclic diaryliodonium cations from salts37 ~54 were also characterized by loss
of an iodine atom (Scheme 6 , Figure
S37 ~S54 ).
Scheme 6 Proposed fragmentation patterns of
[M –OTf]+ from salts 37 ,41 , 43 and 46 .