Study on the degradation of the highly reactive hypervalent trifluoromethylation iodine reagent PhI(OAc)(CF3)

Article abstract of DOI:10.1002/cjoc.201500730

Degradation of the highly reactive hypervalent trifluoromethylation iodine reagent PhI(OAc)(CF₃), which can only be generated in situ with mixing PhI(OAc)₂ and TMSCF₃ in the presence of CsF, was studied by ESI-MS and GC-MS combined with ¹⁹F-NMR. The important transient intermediate PhICF₃⁺ was determined by ESI-MS, and the major volatile products containing CF₃ were identified with the authentic compounds by using GC-MS, such as trifluoromethylbenzene, 2-iodobenzotrifluoride, 3-iodobenzotrifluoride, 4-iodobenzotrifluoride. Meanwhile, more evidences obtained with ¹⁹F-NMR were given for such degradation reaction. A possible rapid CF₃ radical transfer reaction pathway was proposed to clarify such degradation progress based on the experimental results. Therefore, this study may be helpful in elucidating the intrinsic reactivity of PhI(OAc)(CF₃) and the possible competing side reactions caused by such self-degradation pathway. Degradation of the highly reactive hypervalent trifluoromethylation iodine reagent PhI(OAc)(CF₃), which can only be generated in situ with mixing PhI(OAc)₂ and TMSCF₃ in the presence of CsF, was studied by ESI-MS and GC-MS combined with ¹⁹F-NMR. The important transient intermediate PhICF₃⁺ was determined by ESI-MS, and the major volatile products containing CF₃ were identified with the authentic compounds by using GC-MS, such as trifluoromethylbenzene, 2-iodobenzotrifluoride, 3-iodobenzotrifluoride, 4-iodobenzotrifluoride. Meanwhile, more evidences obtained with ¹⁹F-NMR were given for such degradation reaction. A possible rapid CF₃ radical transfer reaction pathway was proposed to clarify such degradation progress based on the experimental results. Therefore, this study may be helpful in elucidating the intrinsic reactivity of PhI(OAc)(CF₃) and the possible competing side reactions caused by such self-degradation pathway.

Full text of DOI:10.1002/cjoc.201500730

FULL PAPER
DOI: 10.1002/cjoc.201500730
Study on the Degradation of the Highly Reactive Hypervalent
Trifluoromethylation Iodine Reagent PhI(OAc)(CF3)
a,b
b
b
,a
,b
Hui Zhu, Shusheng Zhang, Haoyang Wang, Bin Xu,* and Yinlong Guo*
a
Department of Chemistry, Innovative Drug Research Center, Shanghai University, Shanghai 200444, China
b
National Center for Organic Mass Spectrometry in Shanghai, State Key Laboratory of Organmetallic Chemistry,
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, Shanghai 200032, China
Degradation of the highly reactive hypervalent trifluoromethylation iodine reagent PhI(OAc)(CF
3
), which can
only be generated in situ with mixing PhI(OAc) and TMSCF in the presence of CsF, was studied by ESI-MS and
2
3
1
9
＋
GC-MS combined with F-NMR. The important transient intermediate PhICF3 was determined by ESI-MS, and
the major volatile products containing CF were identified with the authentic compounds by using GC-MS, such as
3
trifluoromethylbenzene, 2-iodobenzotrifluoride, 3-iodobenzotrifluoride, 4-iodobenzotrifluoride. Meanwhile, more
1
9
evidences obtained with F-NMR were given for such degradation reaction. A possible rapid CF radical transfer
3
reaction pathway was proposed to clarify such degradation progress based on the experimental results. Therefore,
this study may be helpful in elucidating the intrinsic reactivity of PhI(OAc)(CF ) and the possible competing side
3
reactions caused by such self-degradation pathway.
1
9
Keywords degradation, trifluoromethylation iodine reagent, mass spectrometry, F-NMR, CF radical reaction
3
Introduction
reaction mechanism and degradation pathway of these
reagents were performed via UV-vis spectra, NMR,
Due to the unique and inherent nature of fluorine
element, fluorination of an organic molecule usually
causes significant changes in its physical and chemical
property. Consequently, the development of fluoration
methodology has attracted much attention in the chemi-
[22-24]
HRMS and X-ray structural study.
However, the
studies on hypervalent trifluoromethylation iodine re-
agent C, which can only be formed in situ by mixing
PhI(OAc) and TMSCF in the presence of CsF, re-
2 3
mained rare because of its extreme instability. As a
powerful tool for analysis, mass spectrometry is of great
value in the study of the fragmentation patterns, reaction
[
1-9]
cal community.
As synthetically useful electrophilic
trifluoromethylating reagents, hypervalent trifluoro-
methylation iodine reagents have been widely applied to
the trifluoromethylation reactions. In particular, those
trifluoromethylation reactions occur in mild conditions
with high yields via hypervalent trifluoromethylation
iodine reagents. As a consequence, large efforts have
focused on the developing of trifluoromethylation reac-
tions using hypervalent trifluoromethylation iodine re-
[25-35]
mechanism and intermediates.
In 2012, Guo and
co-workers reported the iodine(III)/PhI(I) catalytic cycle
of iodobenzene-catalyzed α-acetoxylation reaction of
acetophenone by electrospray ionization tandem mass
[
36]
spectrometry. In addition, several kinds of interesting
gas-phase fluorine migration reactions were also studied
by mass spectrometry, showing the unique intrinsic re-
activity of flurine-containing compounds and re-
[
10-21]
agents (Scheme 1).
[37,38]
agents.
Especially in 2013, Guo and co-workers
Scheme 1 Hypervalent trifluoromethylation iodine reagents
described the study of the gas-phase reaction of PhICF
3
＋
[39]
F3C
F3C
by using MS/MS.
Almost at the same time, Liu
AcO
CF3
I
O
I
I
O
and co-workers reported the capture of the acyclic hy＋-
pervalent iodide trifluoromethylating species PhICF
derived from the hypervalent trifluoromethylation io-
O
3
[
17]
A
B
C
dine reagent C by HRMS.
As a useful electrophilic trifluoromethylating reagent,
Among these electrophilic hypervalent trifluoro-
methylation iodine reagents, Togni’s reagents A and B
are important for trifluoromethylation chemistry. The
reagent C can act as CF sources for the trifluoromethy-
3
[
17]
lation reactions.
Because of the high reactivity of
PhI(OAc)(CF ), it might decay to some other non-reac-
3
*
E-mail: xubin@shu.edu.cn, Tel.: 0086-021-66132830; ylguo@sioc.ac.cn, Tel.: 0086-021-54925300
Received October 14, 2015; accepted November 29, 2015; published online December 15, 2015.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/cjoc.201500730 or from the author.
Chin. J. Chem. 2015, 33, 1365—1370
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Zhu et al.
FULL PAPER
tive species rapidly in solution, which might compete
with the main reaction pathway. Thus, figuring out its
degradation products and the mechanism would be
beneficial for optimizing reaction conditions and under-
standing reaction mechanism. Here, we report the study
on the degradation of hypervalent trifluoromethylation
iodine reagent C by ESI-MS and GC-MS combined
split injection mode with a split ratio of 40:1 was em-
ployed.
General procedure
TMSCF
mixed mixture of PhI(OAc)
CsF (0.4 mmol, 2 equiv.) in CH
stirred at room temperature. Then use the F-NMR,
ESI-MS and GC-MS to detect the reaction mixture.
3
(0.4 mmol, 2 equiv.) was added to a pre-
(0.2 mmol, 1 equiv.) and
CN (1.0 mL) and then
2
3
19
19
with F-NMR. The previously unreported major vola-
tile degradation products were identified with the au-
thentic compounds. Influence factors of the degradation
reaction were also investigated. Meanwhile, more evi-
Results and Discussion
1
9
dences were given by F-NMR and a possible CF
radical transfer pathway was proposed for the fast decay
of PhI(OAc)(CF ) in solution.
3
Studying and monitoring the important intermediate
＋
PhICF3 by ESI-MS
3
We initially studied the self-degradation of hyperva-
lent trifluoromethylation iodine reagent C in CH
solution via ESI-MS at different reaction time scales
Figure 1). PhI(CF )(OAc) can be generated rapidly
3
CN
Experimental
All commercially available compounds were used as
received without further purification. Fluorine nuclear
magnetic resonance spectra ( F-NMR) were recorded at
room temperature on a Bruker 400 MHz by using
fluorobezene (δ −113.5) as internal standard.
(
3
from the in situ reaction by using PhI(OAc) with
2
1
9
TMSCF by anion exchange reaction in the presence of
3
＋
CsF. The important transient intermediate PhICF3 at
m/z 273 could be detected just at the beginning stage of
such reaction. Continuous monitoring of the reactio＋n
ESI-MS/MS conditions
solution by ESI-MS showed that the signal of PhICF
3
The ESI-MS and ESI-MS/MS experiments were
performed on a Thermo TSQ Quantum Access triple-
quadrupole mass spectrometer (Thermo Fisher Scien-
tific, Waltham, MA, USA) equipped with a standard ESI
ion source. Nitrogen was used as the sheath and auxil-
iary gases. Argon was used as the collision gas. The ba-
at m/z 273 began to fade rapidly after 1 h and disap-
peared at about 3 h (Figures 1a－1c). Interestingly, the
＋
MS/MS process of PhICF3[ suggested its two impor-
39]
tant degradation pathways:
(1) formation of radical
cation of PhCF at m/z 146 by CF substitution process
3
3
and loss of I radical; (2) giving radical cation of PhI at
m/z 204 by disassociation of CF radical (Figure S1).
−6
sic ESI conditions were as follows: vacuum, 2.8×10
Torr; spray voltage, 3800 V; capillary temperature, 270
; sheath gas pressure, 5 arbitrary units; and auxiliary
3
＋
＋
Besides, the signals of I
3
at m/z 381 and I at m/z
℃
gas pressure, 2 arbitrary units. The collision energy was
dependent on the dissociation capability of the precursor
ions. Data acquisition and analysis were carried out with
the Xcalibur software package (ver. 2.0; Thermo Fisher
Scientific). An injection speed of 5 μL/min of the di-
luted reaction solution by 100 times with CH CN was
3
used for all of the ESI-MS experiments.
GC-MS conditions
The GC-MS experiments were performed on a
GC-TOFMS AccuTOF 4G GC (JEOL, Tokyo, Japan),
V
operated in electron ionization (EI) mode (70 eV). The
detector voltage was 1750 V. Full scan mode was ac-
quired in the mass range m/z 35－500. The capillary
column was a fused silica HP-5 MS capillary column
(
30 m×0.25 mm×0.25 μm, J&W Scientific). The
source temperature was 230 ℃ and the transfer line
temperature was set to 250 ℃. The basic GC-MS con-
ditions were as follows: injection volume was 1 μL of
the diluted reaction solution by 100 times with CH CN
3
and a solvent delay of 2.5 min. The initial oven tem-
perature was 35 ℃, held isothermal for 5 min, rising at
2
0 ℃/min to 250 ℃ and then held isothermal for 3.75
min, resulting in a total run time of 19.5 min. Helium
99.999%) was used as carrier gas at 1.0 mL/min, and
Figure 1 The ESI-MS spectra of reaction solutions of hyperva-
lent trifluoromethylation iodine reagent C at different reaction
time: (a) 0.5 h, (b) 1 h and (c) 3 h.
(
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Chin. J. Chem. 2015, 33, 1365—1370

3
Degradation of the Highly Reactive Hypervalent Trifluoromethylation Iodine Reagent PhI(OAc)(CF )
1
27 (very weak) were observed in the expanded
hard to distinguish clearly. Therefore, we try to use
F-NMR to study such degradation reaction.
1
9
ESI-MS spectra, indicating that iodine cation and its
related species might be formed in the degradation
process (Figure S5). Therefore, in the next step we try to
identify the possible volatile degradation compounds
1
9
F-NMR study on the degradation reaction
1
9
As shown in Figure 3, the F-NMR experiment re-
related to PhCF
were hard to be detected by ESI-MS.
3
and PhI by using GC-MS, because they
sults of degradation reaction solution after 10 min re-
vealed a number of resonance signals of fluorine-con-
3
taining and CF -containing compounds in the character-
Identification of volatile degradation products by
GC-MS
istic region between (δ −30) and (δ −160), such as the
＋
possible signal of PhICF
3
(δ −39.7) which disappeared
[41]
Considering the volatile property of the degradation
after 6 h, CHF (δ −80) and TMSF (δ −157). Among
3
products related to PhCF
al method for detecting these compounds. The reaction
solution was diluted 100 times with CH CN and ana-
lyzed by GC-MS. To our delights, two major kinds of
degradation products were detected and assigned: (1)
small amount of PhCF and its further trifluoromethyla-
3
tion products; (2) large amount of PhI and its further
trifluoromethylation products. Such results are quite
3
and PhI, GC-MS was the ide-
these resonance signals, the signals of the main degra-
dation products such as PhCF 2-iodobenzotrifluoride,
3
,
3
3-iodobenzotrifluoride, 4-iodobenzotrifluoride, 1,3-bis-
(trifluoromethyl)benzene, and 1,4-bis(trifluoromethyl)-
benzene could be confirmed between (δ −62) and (δ −63)
through comparing with the signals of every standard
compound added in the original reaction solution, which
were marked and showed in Figure 4. The isomers
3-iodobenzotrifluoride and 4-iodobenzotrifluoride,
which were not isolated well by GC-MS analysis, now
similar with the gas-phase fragmentation patterns of
＋
3
PhICF . The results suggested that the MS/MS tech-
1
9
nique is valuable to probe the intrinsic reactivity of such
reactive species.
can be resolved and observed clearly with F-NMR.
[
40]
In order to confirm the structures of isomers of the
degradation products, authentic compounds were ana-
lyzed at the same GC-MS conditions to support our
structural assignment results. The major volatile degra-
dation products corresponding to each retention time are
marked in the total ion chromatograms (Figure 2 and
3
Table S1), such as PhCF , 1,3-bis(trifluoromethyl)ben-
zene, 1,4-bis(trifluoromethyl)benzene, PhI, 2-iodoben-
zotrifluoride, 3-iodobenzotrifluoride and 4-iodobenzo-
trifluoride. The close retention times of PhI, 3-iodoben-
zotrifluoride and 4-iodobenzotrifluoride caused their
peaks overlapping. Due to the similar EI mass spectra,
3
-iodobenzotrifluoride and 4-iodobenzotrifluoride are
19
Figure 3 The F-NMR spectra of the reaction mixture obtained
upon treatment of PhI(OAc)2 (1 equiv.), CsF (2 equiv.) with
TMSCF (2 equiv.) in MeCN under air condition at different re-
3
action time: (a) 10 min; (b) 6 h. * represents signals of CF -con-
3
taining aromatic degradation compounds. See Figure S14 for the
full spectrum.
In the next step, we tested and monitored various
reaction conditions such as base, extra reagent, solvents,
reaction time, concentration and inert gas protection by
1
9
Figure 2 (a) Total ion current (TIC) chromatogram of the major
using F-NMR (Table 1, Tables S2－S3 and Scheme 2).
The results showed that only CsF and KOAc could all
initiate the degradation reactions and the performance of
CsF was better than that of KOAc (Table 1, Entries 1, 2).
The possible reason is that more stable compounds
formed (such as TMSF). In obvious contrast, other
volatile degradation products of PhI(CF )(OAc) detected by
GC-MS. (b) Extracted ion chromatogram (EIC) of ion at m/z 272,
showing the degradation products of 2-iodobenzotrifluoride,
3
3
-iodobenzotrifluoride, 4-iodobenzotrifluoride detected by GC-
MS. The retention times of 3-iodobenzotrifluoride and 4-iodoben-
zotrifluoride were close.
4
bases such as NH OAc could not trigger the reaction
Chin. J. Chem. 2015, 33, 1365—1370
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Zhu et al.
FULL PAPER
tion (Table S2, Entries 5, 6). Increasing the amounts of
TMSCF (4 equiv.) led to the higher degradation ratio
3
up to 29.83% (Table S3 in Supporting Information) and
the degradation ratio did not increase significantly when
3
over 4 equiv. TMSCF was used.
19
The continuous F-NMR monitoring results showed
that 80% CF -containing aromatic degradation product＋s
were already formed after 1 h and the signal of PhICF3
3
(
δ −39.7) decayed rapidly. Such result was closely re-
lated to the ESI-MS monitoring result, where also show-
ing the fast decreasing of reactive intermediate of
＋
PhICF . In the following time, degradation rate was
3
slow (Scheme 2). As shown in Scheme 2, the degrada-
tion degree was higher in argon condition than that in
air condition. The possible reasons are that the CF free
3
＋
radical released from PhI(OAc)(CF
3
) or PhICF
3
might
play an important role in the degradation reaction and
the termination of this degradation reaction was follow-
Figure 4 CF
regions in the expanded F-NMR spectra.
3
-containing aromatic degradation compounds
1
9
ing the disappearance of the intermediate of hypervalent
＋
iodine PhICF3 (Figure 1 and Scheme 2). However, the
CF radical could not directly be detected by ESI-MS,
3
NMR or GC-MS. Therefore, we performed the TEM-
PO-trapping experiments (Figures 5 and S26) and the
(
Table 1, Entry 3). The solvents screening showed that
CH CN was more effective than DMF, CH Cl and EA
Table 1, Entry 1 vs. Entries 4－6). Besides, it was less
effective to promote the reactions by using extra reagent
such as CuI or CuI with Na (Table 1, Entry 1 vs.
3
2
2
(
results of obtaining the TEMPO-CF adduct showed
3
2 2 8
S O
3
that the CF radical was also an important reactive in-
termediate involved in the degradation process. It is
well known that the oxygen could quench free radicals
Entries 7, 8). We also found that the degradation ratio of
the reaction significantly increased in the heat condition
compared with that at room temperature (Table S2, En-
try 1 vs. Entry 4). Notably, the degradation ratio of the
reaction in the heat condition is higher than that in the
light condition by using xenon lamp or house-bulb (Ta-
ble S2, Entries 2－4). The higher degradation ratio of
the reaction was also obtained with increasing amounts
easily. Therefore, the oxygen acts as a CF radical in-
3
hibitor for such degradation reaction. Thus, according to
the previous references, the reactions referring to the
PhI(OAc)(CF ) are normally carried out in nitrogen or
3
[
17]
argon protection environment.
of TMSCF
Table S2).
3
equivalent (Entry 1 in Table 1 vs. Entry 1 in
Scheme 2 The degree of degradation over reaction time: (a) air
condition; (b) argon condition. Relative quantity represents the
ratios of the area of CF -containing aromatic compounds (0.3 mL)
to that of PhF (5 µL as internal standard).
3
Table 1 Screening results of the base, extra reagent and reaction
solvents (EA: ethyl acetate)
a
Extra
reagent
－
Degradation
ratio /%
Entry Solvent
Base
b
1
2
3
4
5
6
CH
CH
3
CN
CN
CsF
KOAc
18.68
8.94
0
1.78
4.26
6.22
13.30
7.32
3
－
CH CN NH OAc
－
3
4
CH Cl
DMF
EA
CsF
－
2
2
CsF
CsF
CsF
CsF
－
－
c
Probing reaction mechanism of the degradation re-
action
7
8
CH CN
CuI
3
c
CH CN
3
CuI＋Na S O
8
2
2
a
Reaction conditions: PhI(OAc)
2
(0.2 mmol), base (0.4 mmol, 2
To get more insights into the degradation pathway,
equiv.), TMSCF (0.4 mmol, 2 equiv.) in 1.0 mL of solvent at
the degradation reaction solution was monitored by
F-NMR at specified time intervals. When PhI(OAc)
2
3
b
19
room temperature under air condition for 6 h. Degradation ratios
1
9
were determined by F-NMR with fluorobenzene as an internal
and CsF were added into 1 mL MeCN, no signal was
c
19
standard. Addition of extra reagents (5% equiv.).
detected by F-NMR. When PhI(OAc)
2 3
and TMSCF
were added into 1 mL MeCN, only the signal of
1
9
In addition, the degradation ratio was affected by the
concentrations of reaction solution. It is found that the
TMSCF
TMSCF
3
(δ −67) was observed by F-NMR. But when
3
and CsF were added into 1 mL MeCN, bub-
degradation ratio of the reaction in 0.2 mol/L CH
3
CN
bles were generating and the solution got orange rapidly.
solution was higher than that in 0.1 mol/L CH CN solu-
3
Then the signal of CHF
3
was observed at (δ −80). Con-
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Chin. J. Chem. 2015, 33, 1365—1370

3
Degradation of the Highly Reactive Hypervalent Trifluoromethylation Iodine Reagent PhI(OAc)(CF )
sidering the formation of bis-trifluoromethyl aromatic
compounds, we propose that this degradation reaction is
Scheme 3 The possible CF
self-degradation reaction of PhI(CF
3
radical transfer pathway for the
)(OAc)
3
a radical reaction with the CF
in the degradation process.
3
radical being generated
CF3
I
In order to verify our hypothesis, TEMPO was used
to trap the possible radical intermediates. However,
when 2 equiv. TMSCF was added into a premixed mix-
3
ture of TEMPO (0.2 mmol, 1 equiv.) and CsF (2 equiv.)
in MeCN (1 mL) under air condition and keeping stir-
ring at room temperature for 6 h, there was not the sig-
AcO
OAc
AcO
CF3
F3C
nal of (δ −55.2) of TEMPO-CF
,2,6,6-tetramethyl-piperidine) detected via F-NMR.
On the other hand, with 2 equiv. TMSCF adding into a
premixed mixture of PhI(OAc) (0.2 mmol, 1 equiv.),
3
(1-trifluoromethoxy-
I
I
I
1
9
TMSCF
CsF
3
-
OAc
2
3
2
TEMPO (0.2 mmol, 1 equiv.), and CsF (2 equiv.) in
CF
3
MeCN (1 mL) for 6 h, the signal of (δ −55.2) of
19
TEMPO-CF
CF
tant role as reactive species to form the trifluoro-
methylation products of PhCF and PhI in the degrada-
3
was detected via F-NMR, indicating that
3
radical was indeed generated and played an impor-
I
CF3
3
[
15]
tion progress (Figure 5).
CF
3
I
CF3
CF3
I
CF3
I
and
and
CF3
CF3
CF3
CF3
1
9
＋
F-NMR. PhICF
3
was proposed to be the important
Figure 5 The strong signal at (δ −55.2) of 1-trifluorometh-
oxy-2,2,6,6-tetramethyl-piperidine obtained by F-NMR. See
Figure S25 for the full spectrum.
1
9
intermediate of such self-degradation reaction. It is im-
＋
portant to figure out that the live time of PhICF3 is
shorter than 3 h at typical solution conditions and
mostly decays at the reaction time of 1 h. The volatile
Based on the above analysis and considering the
[
42,43]
degradation products closely related to PhCF and PhI
AcOH is easy to remove,
reaction transfer pathway is shown in Scheme 3. First,
PhI(OAc)(CF ) was generated by anion exchange reac-
tion of PhI(OAc) with TMSCF in the presence of CsF.
TMSF and CHF were also formed by reaction of
TMSCF＋ with CsF and H O. Then the key intermediate
PhICF degraded into PhCF and PhI via two major
pathways, respectively. At the same time, PhI(OAc)(CF
a possible degradation
3
19
were detected and identified by GC-MS and F-NMR.
Finally a possible CF radical transfer mechanism for
the self-degradation of PhI(CF )(OAc) was proposed
3
3
2
3
3
based on the experimental results, which might be use-
ful for the related chemical reaction studies.
3
3
2
3
3
3
)
Acknowledgement
＋
and PhICF
homolysis of I-CF
ated PhCF and PhI would further react with reactive
CF radical to form the corresponding trifluoromethyla-
tion products, such as 1,3-bis(trifluoromethyl)benzene,
3
3
could release reactive CF radical by the
The authors are grateful for financial supports from
the National Natural Science Foundation of China
NSFC Grant Nos. 21475145 and 21472228) and Youth
3
bond. Therefore, the newly gener-
3
(
3
Innovation Promotion Association CAS (2013171).
1
3
,4-bis(trifluoromethyl)benzene, 2-iodobenzotrifluoride,
-iodobenzotrifluoride and 4-iodobenzotrifluoride.
References
＋
Meanwhile, the PhICF
to 2-iodobenzotrifluoride.
3
might also directly rearrange
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In summary, we studied the degradation reaction of
PhI(CF )(OAc) by using ESI-MS, GC-MS and
9
3
Chin. J. Chem. 2015, 33, 1365—1370
© 2015 SIOC, CAS, Shanghai, & WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
www.cjc.wiley-vch.de
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Zhu et al.
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