Silver-Catalyzed Trifluoromethoxylation of Alkyl Trifluoroborates

Article abstract of DOI:10.1021/acs.orglett.0c01741

A silver-catalyzed trifluoromethoxylation of alkyl trifluoroborates with trifluoromethyl arylsulfonate as the trifluoromethoxylation reagent has been reported for the first time. This reaction is performed under mild reaction conditions and has wide functional group compatibility. In addition, the mechanism of this site-specific trifluoromethoxylation is proposed as a radical pathway.

Full text of DOI:10.1021/acs.orglett.0c01741

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Silver-Catalyzed Trifluoromethoxylation of Alkyl Trifluoroborates
Xiaohuan Jiang and Pingping Tang*
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ABSTRACT: A silver-catalyzed trifluoromethoxylation of alkyl
trifluoroborates with trifluoromethyl arylsulfonate as the trifluor-
omethoxylation reagent has been reported for the first time. This
reaction is performed under mild reaction conditions and has wide
functional group compatibility. In addition, the mechanism of this
site-specific trifluoromethoxylation is proposed as a radical
pathway.
a
Table 1. Optimization Study
he metabolic stability and lipophilicity of organic
T
molecules could be increased by introducing “OCF3”
groups, which, in turn, can significantly change their biological
properties (Scheme 1a).1−4 Accordingly, developing new
approaches to obtaining trifluoromethoxylated compounds
has become a hot topic in synthetic organic chemistry.5−42
Particularly, radical trifluoromethoxylation is becoming a
promising tool to synthesize trifluoromethoxy-containing
compounds.43−46 For example, with the new N-OCF3 reagents,
the radical trifluoromethoxylation of arenes under photo-
catalytic conditions was reported by Ngai and Togni,
respectively.43−45 Our group has reported the silver-promoted
oxidative benzylic C−H trifluoromethoxylation through a
radical mechanism.46 However, site-specific radical trifluor-
omethoxylation is still challenging.
b
entry
oxidant
silver salt
ligand
yield (%)
1
2
3
4
5
6
7
8
9
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
Selectfluor
K2S2O8
AgOTf
AgF
AgBF4
no
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
AgOTf
3,4,7,8-Me4-Phen
3,4,7,8-Me4-Phen
3,4,7,8-Me4-Phen
3,4,7,8-Me4-Phen
Phen
4,7-Ph2-Phen
4,7-Me2-Phen
dtbpy
91
88
88
0
50
33
61
75
0
no
10
11
12
3,4,7,8-Me4-Phen
3,4,7,8-Me4-Phen
3,4,7,8-Me4-Phen
0
0
0
Scheme 1. Silver-Catalyzed Trifluoromethoxylation of Alkyl
Trifluoroborates
PhIO
no
a
General conditions: alkyl trifluoroborates (1a) (0.05 mmol),
oxidants (0.10 mmol), silver salts (30 mmol %), ligands (30 mmol
%), KF (0.15 mmol), TFMS (2a) (0.15 mmol), 18-C-6 (0.15 mmol),
b
anisole (0.40 mL), N2 atmosphere, 30 °C. Yields were obtained
according to the 19F NMR spectrum, while benzotrifluoride was used
as an internal standard.
Organoboron compounds are of great use in many areas,
such as medicine, industry and basic science, because of their
low toxicity, stability, and easy preparation.47−49 For example,
boronic acid derivatives are valuable cross-coupling partners of
the Chan−Lam−Evans reaction50,51 and the Suzuki−Miyaura
Received: May 22, 2020
https://dx.doi.org/10.1021/acs.orglett.0c01741
© XXXX American Chemical Society
Org. Lett. XXXX, XXX, XXX−XXX
A

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Letter
a
Scheme 2. Scope of the Substrates
a
Reaction condition: alkyl trifluoroborates (0.50 mmol), AgOTf (30 mmol %), 3,4,7,8-Me4-1,10-Phen (30 mmol %), KF (1.50 mmol), TFMS (2a)
(1.50 mmol), Selectfluor (1.00 mmol), 18-Crown-6 (1.50 mmol), anisole, N2 atmosphere, 30 °C. bYields were obtained according to the 19F NMR,
while benzotrifluoride was used as an internal standard. AgOTf (10 mmol %) and 3,4,7,8-Me4-1,10-Phen (10 mmol %) were used. KF (2.00
mmol) and TFMS (2a) (2.00 mmol) were used.
c
d
reaction.52 In addition, organoborates have been demonstrated
to be excellent precursors for alkyl radical species.53,54
However, transformations about trifluoromethoxylation of
organoboron compounds are limited, and only one example
was reported. In 2011, Tobias successfully converted aryl
stannanes and arylboronic acids into the corresponding aryl
trifluoromethyl ethers through a silver-mediated cross-coupling
reaction (Scheme 1b).55 According to what we know, no
reaction about the trifluoromethoxylation of alkylboronic acid
derivatives has been reported so far. Thus, we developed a first
example of trifluoromethoxylation of alkyl trifluoroborates in
Ag-catalyzed system with trifluoromethyl arylsulfonate (TFMS,
2)56−58 (Scheme 1c).
We recently reported the silver-mediated oxidative trifluor-
omethoxylation of alkylsilanes through a radical mechanism.59
We wondered whether the trifluoromethoxylation of alkyl
trifluoroborates could also be implemented with a catalytic
amount of silver salts. Thus, using potassium (3-
phenylpropyl)trifluoroborate 1a as model substrate and
trifluoromethyl arylsulfonate (TFMS, 2) as trifluoromethox-
ylation reagent, we performed this reaction in a silver-catalyzed
system (see Table 1; more details can be found in the
Supporting Information). Different silver salts were screened
and AgOTf gave the highest yield (Table 1, entries 1−3). If
there were no silver salts, no ideal product 3a was detected
(Table 1, entry 4). The choice of oxidant was important. The
use of strong oxidants, such as Selectfluor, was effective, and
other oxidants, such as K2S2O8, or PhIO, did not give the
product (Table 1, entries 10−12). The ligand also has great
impact on this reaction (Table 1, entries 5−9). Both
phenanthroline and bipyridine could promote this trifluor-
omethoxylation. Crown ethers were added to this reaction as
phase transfer catalysts, which could be used to improve the
solubility of KF and alkyl trifluoroborates in anisole. 18-
Crown-6 was proven to be the most suitable additive for this
transformation, probably because the cavity of 18-Crown-6 and
the size of K+ were well matched. After further optimization of
the reaction conditions (Supporting Information), we found
the yield can reach up to 91% under the optimal condition of
2.0 equiv Selectfluor, 30 mmol % AgOTf, 30 mmol % 3,4,7,8-
B
https://dx.doi.org/10.1021/acs.orglett.0c01741
Org. Lett. XXXX, XXX, XXX−XXX

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molecule, such as ezetimibe derivative (1aa, 46% yield).
However, only a trace amount of desired product was observed
when tertiary alkyl trifluoroborate was applied as a substrate. In
addition, gram-scale reaction was performed under the
standard condition, and an 80% isolated yield of 3d was
gained. Furthermore, when the equivalents of silver salt and
ligand were both reduced to 10 mmol % in this transformation,
the yields decreased slightly (3e, 3f, 3h, 3p, 3q, 3t).
Scheme 3. Mechanistic Experiments and Potential Reaction
Pathway
In order to study the mechanism of this transformation, we
performed several experiments. In the first, 4.0 equiv of
TEMPO was added as the radical trapper, and the TEMPO
adduct 4 was observed (Scheme 3a). Furthermore, a radical
probe 1bb was designed and applied in the reaction, and the
ring-opening product 5 was achieved in 20% yield (Scheme
3b). These observations indicated that alkyl radicals were
generated from alkyl trifluoroborates in this reaction. In
addition, no trifluoromethoxylated product was observed when
2−3 equiv of AgF2 was used instead of AgOTf and Selectfluor,
which suggested that a higher valence state of Ag species might
be involved in this reaction. Based on the previous reports and
above results,60 a possible mechanism was proposed and
described in Scheme 3c. The Ag(I) species is oxidated by
Selectfluor to afford Ag(III)-F species,61−65 then this silver
species goes through the ligand exchange to form FAg(III)-
OCF3. The later reacts with alkyl trifluoroborates to give alkyl
radicals and Ag(II)OCF3 via single-electron oxidation.
Subsequently, the desired product was obtained by grabbing
OCF3 radical from Ag(II)OCF3,60,61,66−68 and Ag(I) was
regenerated at the same time. Further invetigations about
mechanism are needed to better understand the above
reaction.
In conclusion, we have reported, for the first time, about the
silver-catalyzed trifluoromethoxylation of alkyl trifluoroborates
with TFMS as the trifluoromethoxylation reagent. The
highlight of this method is its good tolerance for various
functional groups, compared with previous methods. Fur-
thermore, complex small molecules can also be successfully
converted to the corresponding trifluoromethoxylated com-
pounds in the late stage. In addition, preliminary mechanistic
research was investigated and suggested that the trans-
formation might undergo a free radical pathway. Our next
challenge is to achieve asymmetric trifluoromethoxylation of
sp3-hybridized carbon centers.
Me4-1,10-Phen, 3.0 equiv TFMS (2a), 3.0 equiv KF, and 3.0
equiv 18-Crown-6 in anisole under N2 atmosphere at 30 °C.
Moreover, if any of these reagents was missing, no desired
product would be obtained.
We then investigated the substrate scope for this silver-
catalyzed trifluoromethoxylation of alkyl trifluoroborates
(Scheme 2). Delightly, a broad scope of alkyl trifluoroborates
reacted smoothly and achieved 32% to 91% yields (3a−3z).
Substrates with aryl rings bearing electron-rich and electron-
deficient substituents reacted smoothly. Variously important
functional groups, such as alkene, hydroxyl, cyano, nitro,
amide, ketone, aldehyde, bromo, ether and ester moieties, were
well-tolerated, which highlight the compatibility of this
method. Notably, substrates with heteroaromatic rings were
also successfully processed to give the corresponding products
(3p, 3q). What is noteworthy is that the chemoselective
trifluoromethoxylation of alkyl trifluoroborates in the presence
of alkyl chloride or alkyl p-toluenesulfonate was observed. For
example, the alkyl trifluoroborate group in substrates 1g, 1t
was selectively converted to the corresponding “OCF3” group,
while other functional groups, such as alkyl chloride, or OTs
remained intact (3g, 3t), which could be used as good leaving
groups for further synthetic manipulation. The 40% desired
product 3w was observed, along with 20% ditrifluoromethoxy-
lated byproduct, which is generated by nucleophilic trifluor-
omethoxylation of alkyl iodide. Moreover, aryl azide was also
tolerated (3k), and the corresponding product could be further
applied in click reaction. Secondary alkyl trifluoroborates were
also tested, but lower yields were gained (3y, 3z). The main
side product observed was β-H elimination product.
Optimization was tried to improve the yield, such as changing
ligands, solvents, reaction temperature, and additives, but
failed. To our delight, this method could also be applied to the
trifluoromethoxylation of a structurally more-complicated
ASSOCIATED CONTENT
■
sı
* Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.orglett.0c01741.
Experimental procedures and characterization of all new
1
compounds including H, 13C, and 19F NMR spectra
(PDF)
AUTHOR INFORMATION
■
Corresponding Author
Pingping Tang − State Key Laboratory and Institute of
Elemento-Organic Chemistry, College of Chemistry, Nankai
University, Tianjin 300071, China; orcid.org/0000-0002-
8296-5695; Email: ptang@nankai.edu.cn
C
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(16) Chen, C.; Chen, P.; Liu, G. Palladium-Catalyzed Intramolecular
Aminotrifluoromethoxylation of Alkenes. J. Am. Chem. Soc. 2015, 137,
15648−15651.
(17) Zhang, Q.; Brusoe, A. T.; Mascitti, V.; Hesp, K. D.; Blakemore,
D. C.; Kohrt, J. T.; Hartwig, J. F. Fluorodecarboxylation for the
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Xiaohuan Jiang − State Key Laboratory and Institute of
Elemento-Organic Chemistry, College of Chemistry, Nankai
University, Tianjin 300071, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.orglett.0c01741
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
This work was supported by the National Key Research and
Development Program of China (No. 2016YFA0602900),
NFSC (Nos. 21672110 and 21925105), the Natural Science
Foundation of Tianjin (Grant No. 18JCJQJC47000), and the
Fundamental Research Funds for the Central Universities.
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