Nonafluoro- tert-butoxylation of Diaryliodonium Salts

Article abstract of DOI:10.1021/acs.orglett.9b01813

A highly efficient method to incorporate the nonafluoro-tert-butoxy group into various arenes is developed. This C-O cross-coupling reaction proceeds smoothly in the absence of transition-metal catalyst with good functional group tolerance and scalability

Full text of DOI:10.1021/acs.orglett.9b01813

Letter
pubs.acs.org/OrgLett
Cite This: Org. Lett. XXXX, XXX, XXX−XXX
Nonafluoro-tert-butoxylation of Diaryliodonium Salts
Huan Meng,†,‡ Lixian Wen,‡ Zhenchuang Xu,‡ Yipeng Li,‡ Jian Hao,*,† and Yanchuan Zhao*,‡,§
†Department of Chemistry, Innovative Drug Research Center, Shanghai University, Shanghai 200444, China
‡Key Laboratory of Organofluorine Chemistry, Center for Excellence in Molecular Synthesis, Shanghai Institute of Organic
Chemistry, Chinese Academy of Sciences, 345 Ling-Ling Road, Shanghai 200032, China
§Key Laboratory of Energy Regulation Materials, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences, 345
Ling-Ling Road, Shanghai 200032, China
S
* Supporting Information
ABSTRACT: A highly efficient method to incorporate the
nonafluoro-tert-butoxy group into various arenes is developed.
This C−O cross-coupling reaction proceeds smoothly in the
absence of transition-metal catalyst with good functional group
tolerance and scalability. In comparison with the conventional
approach, this method avoids the use of nonafluoro-tert-butyl
alcohol as the reaction solvent and does not require handling of
hazardous diazonium salts. A series of OC(CF3)3-containing
analogues of 19F NMR-based probes targeting various biologically relevant analytes are prepared.
atoms and the intense singlet 19F NMR signal. For instance,
luorine-containing molecules have attracted increasing
Fattention owing to their unique biological functions and
the α-(nonafluoro-tert-butoxy)carboxylic acids have been used
superior physical properties.1 In addition to being extensively
explored in the pharmaceutical industry and material sciences,
fluorine-containing molecules also have striking applications in
molecular sensing and imaging.2 For instance, the 18F-labeled
radiotracers have been widely employed in positron emission
tomography (PET) to identify the abnormal metabolism in
biological processes.3 On the other hand, the 19F-labeled
molecular probes have enabled the investigation of recognition
processes and conformational changes of biomolecules, such as
DNAs, RNAs, peptides, and proteins, under physiologically
relevant conditions.4 In addition, the activity of an enzyme is
readily determined by monitoring the enzymatic reaction using
19F-decorated substrates.2a−c,5 On the basis of the character-
istic recognition-induced perturbation of 19F chemical shifts,
various cations, anions, and neutral organic analytes can be
unambiguously identified in complex mixtures.2a−c,6 It is
noteworthy that the scarcity of naturally occurring 19F NMR
signals and the high penetration of the harmless radio
frequency irradiation to large and opaque objects make the
19F NMR-based sensing techniques well suited for the
noninvasive detection of biological systems with high fidel-
ity.2a,b Despite their robust sensory power, one limitation still
hampering the wide application of many NMR-based
techniques is their relatively low sensitivity.2a,b Routine NMR
analysis often requires samples in the millimolar concentration
range, which is significantly higher than that of unenriched
real-world samples. The incorporation of multiple magnetically
equivalent fluorine atoms is an efficient way to increase the
sensitivity of 19F NMR-based detection. In this regard, the
nonafluoro-tert-butoxy group [OC(CF3)3] is a privileged
moiety owing to its large number of equivalent fluorine
as sensitive 19F NMR-based solvating agents for the
discrimination of chiral amines.7 The SN2 substitution of
aliphatic halides or tosylates with nonafluoro-tert-butyl alcohol
is most frequently used to incorporate the OC(CF3)3 group,
where a flexible methylene unit (−CH2−) usually serves as the
linkage.8 This approach has successfully led to tracer agents for
sensitive 19F MRI applications (Scheme 1, a).9 Unlike
molecular tracers, many 19F-labeled probes, especially those
used in the chemical-shift based sensing scheme, require the
installation of fluorinated moieties on arenes because the
precise positioning of 19F and the effective electron trans-
mission between 19F and the recognitive moiety are crucial to
the success of the detection.6 Unfortunately, methods to access
the nonafluoro-tert-butyl aryl ether are scarce. The only known
approach relies on the thermal decomposition of aryl
diazonium salts.10 The use of expensive nonafluoro-tert-butyl
alcohol as the solvent and the need to handle hazardous
chemicals hampers its wide adaptation (Scheme 1, b). Herein,
we report an efficient method to introduce the nonafluoro-tert-
butoxy group onto various arenes (Scheme 1c), which gives
access to sensitive 19F NMR probes targeting diverse
biologically relevant analytes.
Previous investigations on copper-based reactions for C−O
bond formation have revealed that the couplings between aryl
iodides and alcohols proceed smoothly in the presence of
suitable ligands, such as 1,10-phenanthroline (Table 1, L1) and
ethyl 2-cyclohexanonecarboxylate (Table 1, L4).11 Similar
ligands also effectively promote the copper-mediated or
Received: May 24, 2019
© XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.9b01813
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
1, entries 1−5) or under more forcing conditions (Table 1,
entry 6). Mechanistically, the oxidative addition of the CuOR
(R = aryl, alkyl) species to aryl iodide to form ArCu(III)ORI is
often involved in the copper-catalyzed C−O couplings.12 We
envisioned that this crucial step might become sluggish due to
the low electron-donating ability of the OC(CF3)3 group.
Therefore, diaryliodonium salt 2a, which has a much higher
reduction potential compared to that of 1a, was attempted with
the aim of facilitating the oxidative addition.13 To our delight,
the use of 2a as the substrate furnished the desired product 3a
in 51% yield (Table 1, entry 7). Nevertheless, control
experiments disclosed that the reaction proceeded even better
in the absence of both copper and ligand (Table 1, entry 8),
which indicated that a transition-metal-free coupling between
2a and nonafluoro-tert-butyl alcohol occurred. It is noteworthy
that the couplings between diaryliodonium salts and alcohols
are known;14 however, the nonafluoro-tert-butoxylation has
not been achieved.
Scheme 1. Methods To Introduce the Nonafluoro-tert-
butoxy Group
We next tested the influence of other reaction parameters on
this reaction. Bases such as K2CO3 and K3PO4 could also
promote the current reaction, albeit with lower efficiency
(Table 1, entries 9 and 10). The best yield was achieved in the
reaction using CsF as the base, despite the fear of the
competitive fluorination reaction (Table 1, entries 11 and
17).15 No fluoroarenes were produced in these reactions,
which indicates that the nucleophilicity of the fluoride ion is
significantly weakened due to the hydrogen bonding with
nonafluoro-tert-butyl alcohol. Yields were substantially dimin-
ished when the reactions were performed at lower temper-
atures (Table 1, entries 15 and 16) or in polar solvents such as
dimethylformamide and acetonitrile (Table 1, entries 13 and
14). When the reaction time was extended to 12 h, a
quantitative yield of 3a was obtained. Having identified the
suitable reaction conditions (Table 1, entry 17), we next
explored the scope of this nonafluoro-tert-butoxylation
reaction. Unsymmetrical iodonium salts 2 bearing the
dummy 1,3,5-trimethoxyphenyl (TMP) group were used as
the substrates to achieve a chemoselective coupling of the
desired aryl moiety.16 As summarized in Scheme 2, the
OC(CF3)3 group can be readily incorporated into a variety of
arenes in good to excellent yields. Common functionalities,
such as ester, nitrile, ketone, and nitro, are compatible. The
reaction is also amenable to substrates (2f, 2i−k, and 2o−q)
containing a carbon−bromine bond, providing opportunities
for further structural elaborations. The heterocyclic iodonium
salts were smoothly converted to the corresponding OC-
(CF3)3-substituted products (3d, 3v, 3x) under the same
conditions. Azide-containing building block 3y can be obtained
through the current method, which is useful for the rapid
access to OC(CF3)3-labeled molecules through a click
reaction.17 Highly sterically congested substrate 2o could
participate in the reaction as well, furnishing product 3o in
excellent yield. The nonafluoro-tert-butoxy group was readily
introduced to the protected tyrosine, which could serve as a
sensitive 19F-labeled unit to study the function of proteins.10c
The reaction is readily scaled up, providing more than 1 g of
products 3q and 3u in 78% and 90% yields, respectively.
Notably, other highly fluorinated tertiary alcohols (Scheme 3,
4, 6, and 8) are also suitable substrates for this C−O coupling,
affording structurally diversified fluoroalkyl aryl ethers
(Scheme 3, 5, 7, and 9), which are otherwise difficult to access.
To gain more insights into the reaction mechanism, the
diaryliodonium nonafluoro-tert-butoxide 2-BrC6H4(TMP)-
Table 1. Survey of Reaction Conditions
a
entry substrate catalyst
base
solvent
T (°C) yield (%)
b
1
2
3
4
5
6
7
8
9
1a
1a
1a
1a
1a
1a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
2a
CuI/L1
CuI/L2
CuI/L3
CuI/L4
CuI/L5
CuI/L1
CuI/L1
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
Cs2CO3
K2CO3
K3PO4
CsF
CsF
CsF
CsF
CsF
CsF
CsF
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
dioxane
CH3CN
DMF
120
120
120
120
120
180
120
120
120
120
120
120
120
120
60
ND
ND
ND
ND
ND
ND
51
82
79
39
95
b
c
b
b
b
b
10
11
12
13
14
15
16
17
78
47
34
11
toluene
toluene
toluene
80
120
52
d
e
99 (94)
a
Reactions were conducted on 0.17 mmol scale. Yields were
determined by 19F NMR analysis using PhCF3 as an internal
b
standard. CuI (0.1 equiv) and ligand (0.2 equiv) were employed.
c
d
CuI (0.1 equiv) and L3 (1.2 equiv) were employed. Reaction time
e
= 12 h. Isolated yield. ND denotes not detected.
catalyzed trifluoroethoxylation of aryl bromides and iodides.12
In the light of these advances, we commenced our investigation
by screening various ligands and bases for the copper-catalyzed
C−O coupling reaction between 2-iodonaphthalene (1a) and
nonafluoro-tert-butyl alcohol. However, no desired product
was observed when the reaction was carried out in the
presence of copper iodide and various ligands at 120 °C (Table
B
DOI: 10.1021/acs.orglett.9b01813
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
product 3f in 95% yield (for details, see Figure S1). This result
suggests the reaction pathway is similar to what was observed
in other C−O couplings with diaryliodonium salts, which
involves a ligand exchange followed by a reductive elimination-
like ligand coupling.14
Scheme 2. Substrate Scope
To illustrate the application of the current method for the
construction of sensitive 19F NMR-based probes, we selected a
series of previously reported probes targeting diverse bio-
logically relevant analytes and decorated them with the
OC(CF3)3 moiety for enhanced sensitivity. Sando and co-
workers have shown that p-aminophenyl trifluoroethyl ether
reacted selectively with hypochlorite ion (−OCl) through ipso-
substitution.18 This highly specific reactivity was utilized to
create a hypochlorite ion responsive 19F MRI agent.18 The
structural analogue of this probe with the replacement of the
OCH2CF3 group by a more sensitive OC(CF3)3 was readily
obtained by a simple reduction of 3w, followed by alkylation to
increase its solubility in aqueous solution (Scheme 4, a). The
Scheme 4. Preparation of OC(CF3)3-Containing Analogues
of 19F NMR-Based Probes
fluorinated aryl boronic acids have been widely utilized as
selective sensors for hydrogen peroxide and various carbohy-
drates.6d,e,19 These sensors usually suffer from low sensitivity
due to the quite limited amounts of equivalent fluorine atoms
appended to the probe. The OC(CF3)3-containing boronic
acid is therefore of interest for achieving superior sensitivity. It
was found that the OC(CF3)3 group is robust under the
lithium bromide exchange conditions, thereby allowing the
desirable boronic acid 12 to be quickly prepared through the
synthetic route depicted in Scheme 4b.
a
b
With 200 mg of 2. Yields of isolated products are given. The low
c
yields are partially due to the high volatility of the products. The
reaction was carried out at 80 °C for 20 h.
Scheme 3. Reactions with Other Highly Fluorinated
Tertiary Alcohols
In summary, we have developed an efficient method for the
installation of the OC(CF3)3 group onto various arenes. A
variety of functional groups are tolerated, and the reaction
gives access to the OC(CF3)3-containing version of NMR
probes targeting diverse biologically relevant analytes. We
expect this method will significantly promote the creation of
novel sensitive 19F-labeled probes and widen the scope of
NMR-based detection. A systematic exploration of the
application of OC(CF3)3-containing probes is currently
underway in our laboratory.
ASSOCIATED CONTENT
* Supporting Information
■
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.or-
glett.9b01813.
IOC4F9 (2f′) was prepared through ion exchange of 2f with
potassium nonafluoro-tert-butoxide. Thermal decomposition of
2f′ in toluene at 80 °C for 4 h gave the expected coupling
Experimental details, characterization of new com-
pounds, and NMR spectra (PDF)
C
DOI: 10.1021/acs.orglett.9b01813
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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AUTHOR INFORMATION
■
Corresponding Authors
̈
Gorls, H.; Bellstedt, P.; Schiller, A. J. Am. Chem. Soc. 2017, 139,
*E-mail: zhaoyanchuan@sioc.ac.cn.
*E-mail: jhao@shu.edu.cn.
̈
11413−11420. (e) Axthelm, J.; Gorls, H.; Schubert, U. S.; Schiller, A.
J. Am. Chem. Soc. 2015, 137, 15402−15405. (f) Guo, L. E.; Hong, Y.;
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W. Y.; Liu, G.; Van Zijl, P. C. M.; Bulte, J. W. M. M.; McMahon, T. J.
Am. Chem. Soc. 2013, 135, 12164−12167. (i) Bar-Shir, A.; Yadav, N.
N.; Gilad, A. A.; Van Zijl, P. C. M.; McMahon, M. T.; Bulte, J. W. M.
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ORCID
Yanchuan Zhao: 0000-0002-2903-4218
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is financially supported by National Natural Science
Foundation of China (Grant Nos. 21871291, 21421002, Y.Z.;
21672139, J.H.). We thank Dr. Haoyang Wang at SIOC for
assistance in collecting mass spectra.
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DOI: 10.1021/acs.orglett.9b01813
Org. Lett. XXXX, XXX, XXX−XXX