Visible-Light Photoredox-Catalyzed and Copper-Promoted Trifluoromethoxylation of Arenediazonium Tetrafluoroborates

Article abstract of DOI:10.1002/anie.201901447

We report the development of photoredox-catalyzed and copper-promoted trifluoromethoxylation of arenediazonium tetrafluoroborates, with trifluoromethyl arylsulfonate (TFMS) as the trifluoromethoxylation reagent. This new method takes advantage of visible-light photoredox catalysis to generate the aryl radical under mild conditions, combined with copper-promoted selective trifluoromethoxylation. The reaction is scalable, tolerates a wide range of functional groups, and proceeds regioselectively under mild reaction conditions. Furthermore, mechanistic studies suggested that a Cs[Cu(OCF₃)₂] intermediate might be generated during the reaction.

Full text of DOI:10.1002/anie.201901447

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Accepted Article
Title: Visible-Light Photoredox-Catalyzed and Copper-Promoted
Trifluoromethoxylation of Arenediazonium Tetrafluoroborates
Authors: Shaoqiang Yang, Miao Chen, and pingping tang
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201901447
Angew. Chem. 10.1002/ange.201901447
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10.1002/anie.201901447
Angewandte Chemie International Edition
DOI: 10.1002/anie.201((will be filled in by the editorial staff))
Trifluoromethoxylation
Visible-Light Photoredox-Catalyzed and Copper-Promoted
Trifluoromethoxylation of Arenediazonium Tetrafluoroborates
Shaoqiang Yang,+ Miao Chen+ and Pingping Tang*
Dedicated to Professor Qingyun Chen on the occasion of his 90th birthday
fluoroformates,[6] and fluorodesulfurization of sulfonate esters.[7]
However, harsh reaction conditions were required. Recently,
several new strategies had been developed to synthesis
trifluoromethyl aryl ethers.[8] For examples, Ritter and coworkers
Abstract:
A
photoredox-catalyzed
and
copper-promoted
trifluoromethoxylation of arenediazonium tetrafluoroborates have
been developed for the first time. Trifluoromethyl arylsulfonate
(TFMS) was used as the trifluoromethoxylation reagent. This
reaction is scalable and proceeds regioselectively under mild
reaction conditions. Furthermore, mechanistic studies suggested
that Cs[Cu(OCF3)2] intermediate might be generated during the
reaction.
reported
stannanes
a
silver-mediated trifluoromethoxylation of aryl
and aryl boronic acids with
tris(dimethylamino)sulfonium trifluoro-methoxide (TASOCF3) as
the trifluoromethoxylation reagent.[9] Qing and coworkers reported
a silver-mediated trifluoromethylation of phenols to prepare
trifluoromethyl aryl ethers.[10] The fluorodecarboxylation for the
synthesis of trifluoromethyl aryl ethers were reported by Hartwig,
Hu, Sammis, and Prakash, respectively.[11] Hu and coworkers
T
rifluoromethyl ethers are increasingly being investigated for
pharmaceutical, agrochemical, and materials science, because
trifluoromethoxy group (OCF3) has strongly electron-withdrawing
nature and high lipophilicity.[1] Therefore, the development of new
methods for the synthesis of trifluoromethyl ethers has received
much attention.[2] However, due to the reversible decomposition
of trifluoromethoxide anion and limited trifluoromethoxylation
reagents, methods for the synthesis of trifluoromethyl aryl ethers
remain extremely rare.[3] Herein, we reported the first example of
developed trifluoromethyl benzoate (TFBz) as
a
new
trifluoromethoxylation reagent for the synthesis of trifluoromethyl
aryl ethers.[12] The catalytic C(sp2)-H trifluoromethoxylation of
arenes with the new N-OCF3 reagents under photocatalytic
conditions were reported by Ngai and Togni, respectively.[13]
Although the progress has been made, strong oxidants or
regioisomers were observed with few exceptions. Therefore, the
development of a selective trifluoromethoxylation method for the
synthesis of trifluoromethyl aryl ethers without strong oxidants is
highly desirable.
a
photoredox-catalyzed
and
copper-promoted
trifluoromethoxylation of arenediazonium tetrafluoroborates under
mild reaction conditions, with trifluoromethyl arylsulfonate (TFMS,
2)[4] as the trifluoromethoxylation reagent (Scheme 1).
Table 1. Optimization of the reaction conditions.a
Yield
Entry
Photocatalyst
Copper salts
Fluoride
(%)f
32
76
0
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
CsF
KF
1
2
fac-Ir(ppy)3
[Ir(ppy)2(dtbbpy)]PF6
Eosin Y
CuOTf
CuOTf
CuOTf
CuOTf
CuOTf
CuOTf
CuOTf
CuOTf
CuOAc
CuI
3
Scheme 1. Photoredox-catalyzed and copper-promoted
trifluoromethoxylation of arenediazonium tetrafluoroborates.
4
Fluorescein
0
5
[Ru(bpz)3](PF6)2
[Ru(phen)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
[Ru(bpy)3](PF6)2
none
3
6
68
78
28
11
0
7
8b
9
The traditional synthesis of trifluoromethyl aryl ethers had been
achieved by nucleophilic substitution of the corresponding
trichloromethyl aryl ethers with fluoride,[5] deoxyfluorination of
10
11
12
13
14
15
16c
17d
18
19
20e
Cu2O
38
0
Cu(OTf)2
CuOTf
CuOTf
CuOTf
CuOTf
CuOTf
none
0
[]
S. Yang,+ M. Chen,+ Prof. P. Tang
AgF
none
CsF
CsF
CsF
CsF
CsF
32
0
State Key Laboratory and Institute of Elemento-Organic
Chemistry, College of Chemistry, Nankai University
Tianjin 300071, China
61
67
0
E-mail: ptang@nankai.edu.cn
CuOTf
CuOTf
3
[+]
These authors contributed equally.
[Ru(bpy)3](PF6)2
0
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/anie.201xxxxxx.
[a] General conditions: 1a (0.05 mmol), 2a (0.25 mmol), photo-
catalyst (1 to 2 mol%) , copper salts (1.0 equiv), fluoride (2.0 equiv),
degassed CH3CN, 25 to -40 °C, 12 h, 15 W white LED. [b] 1.0 equiv
1
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10.1002/anie.201901447
Angewandte Chemie International Edition
of Cu(OTf) and 1.0 equiv of CsF were used. [c] copper salts (0.4
equiv). [d] copper salts (0.6 equiv). [e] no light. [f] Yields were
determined by 19F NMR with benzotrifluoride as a standard.
1. It was crucial to use a photoredox catalyst for achieving an
efficient transformation in the presence of Cu(OTf) and CsF, and
[Ru(bpy)3](PF6)2 was found to give the highest yield comparing to
other photocatalysts fac-Ir(ppy)3, [Ir(ppy)2(dtbbpy)]PF6 or organic
photocatalysts (entries 1-7). Next, different copper salts were
evaluated. Cu(OTf) was found to give the highest yield while no
desired product was observed with Cu(OTf)2 (entries 7-12). The
yield was also sensitive to fluoride sources, and CsF was most
effective (entries 7, 13, 14). Interestingly, the ratio of the amount
of Cu(OTf) and CsF significantly influenced the reaction yield, and
the highest yield was obtained when 1.0 equiv of Cu(OTf) and 2
equiv of CsF were used (entries 7, 8). In addition,
substoichiometric amount of Cu(OTf) ( 0.4 or 0.6 eq) could be
applied in the reaction, while it led to slightly lower yields (entries
16, 17), which might be due to the decomposition of active copper
species in the reaction. Importantly, photoredox catalyst, copper
salts, CsF, and visible light were all critical for this transformation.
Significantly reduced yields of 3a (<3%) were observed in the
absence of these components (entries 18-20). Finally, extensive
optimization with various TFMS (2), solvents and temperatures
(See more details in the supporting information), revealed that
product 3a could be obtained in the highest yield when the
reaction of 1a was conducted with 1.0 mol% [Ru(bpy)3](PF6)2, 1.0
equiv of Cu(OTf), 2.0 equiv of CsF and 5.5 equiv of TFMS (2a) in
MeCN under N2 atmosphere using 15W White LED light
irradiation for 12 hours.
Inspired by Kolomeitsev and Vicic reports that CuOCF3 could
be generated from trifluoromethane sulfonate (TFMT),[14] but no
copper-catalyzed/mediated trifluoromethoxylation reaction was
reported to date, we were wondering whether synthesis of
trifluoromethyl aryl ethers was achieved from aryldiazonium salts
via the combination of photocatalysis and copper salts (Scheme
1).[15] We envisioned that aryl radical (A) could be generated by
photoredox reaction of aryldiazonium salts (1),[16] and CuOCF3
(B) was prepared in situ with TFMS in the presence of copper
salts and fluorides, we hypothesized the merger of visible-light
photocatalysis with copper salts might provide a mild method for
the synthesis of trifluoromethyl aryl ethers (3), effectively taking
advantage of CuIII’s propensity (C) to undergo facile reductive
elimination from its high valency oxidation state.[17] Our initial
investigations focused on the trifluoromethoxylation of 4-
fluorobenzenediazonium tetrafluoroborate 1a in the presence of a
photoredox catalyst and copper salts with TFMS (2a). The
desired product 3a was obtained when a Cu(OTf) solution in
acetonitrile was stirred with TFMS (2a) and cesium fluoride for 1
hour at room temperature, followed by addition of the diazonium
salts and photoredox catalyst continued stirring at -40 °C for 12
hours under irradiation of white LED, as briefly illustrated in Table
2
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10.1002/anie.201901447
Angewandte Chemie International Edition
Scheme 2. Substrate scope for trifluoromethoxylation of aryldiazonium saltsa. [a] Reaction conditions: 1a (1.0 equiv), 2a (5.5 equiv),
[Ru(bpy)3](PF6)2 (1 mol%) , CuOTf (1.0 equiv), CsF (2.0 equiv), CH3CN, N2 atmosphere, 25 to -40 °C, 12 h, 15 W White LED. Yields of isolated
products are given, unless otherwise noted. [b] Yields were determined by 19F NMR with benzotrifluoride as a standard. [c] The isolated yield
was shown in parentheses.
Having optimized the reaction conditions, we next investigated
the substrate scope of the transformation (Scheme 2). Various
Scheme 3. Mechanistic investigations.
arenediazonium tetrafluoroborates derivatives 1a-1aa bearing
electron-donating to electron-withdrawing substituents were
successfully converted to the desired products (3a-3aa) with
yields ranging from 27% to 81%. Only for the low-boiling
was added, no trifluoromethoxylated product was obtained, and
substrates, the products could not be isolated in good yields
observed by in situ spectroscopic analysis due to their volatility.
These results indicated that an aryl radical was generated in the
Moreover, a good range of functional groups, such as nitro, cyano,
ketone, ester, amide, amine, sulfone groups was tolerated in this
was estimated to be 0.38, which indicated that a radical chain
photoreaction. Notably, halogen-substituted aryl diazonium salts
To get some insight into the mechanism, some preliminary
studies were conducted (Scheme 3). When 4.0 equiv of TEMPO
the TEMPO-trapped intermediate (13) was isolated in 74% yield.
reaction. To verify the effect of photoirradiation, the value of Φ
process was unlikely in the reaction. To probe the electron
(1a to 1f, 1j) successfully underwent trifluoromethoxylation
transfer among the catalyst and substrates,
a series of
leaving the C-X bond intact, which is useful for further synthetic
elaboration. However, the efficacy of the reaction was hindered
by ortho substitution (1f, 27% yield). The yield of fluorination
byproducts were less than 6% in all cases. It was worth
luminescence quenching experiments were investigated (See
more details in the supporting information). The aryl diazonium
salt (1) and CuOTf could quench the excited photocatalyst,
respectively.18 Furthermore, monitoring of the reaction between
TFMS (2a) and CuOTf in the presence of CsF by 19F NMR
spectroscopy showed that a new intermediate (-21.9 ppm) was
generated compared to the CsOCF3 (-20.9 ppm) and CuOCF3 (-
23.5 ppm) (Scheme 3a). Based on the ratio of amount of Cu(OTf)
(1.0 equiv) and CsF (2.0 equiv), the new intermediate could be
postulated to be Cs[Cu(OCF3)2].19 In addition, the highest yield of
product 3a was observed with Cs[Cu(OCF3)2] compared to
CsOCF3 or CuOCF3, which suggested that the proposed
Cs[Cu(OCF3)2] was the reactive species for this reaction (Scheme
3b).
mentioning
that
trifluoromethoxylation
always
occurred
regioselectively, no other regioisomers were obtained. To further
investigate the synthetic utility and generality of this reaction, we
tested the sulfamethoxazole derivative (1bb), which is an
antibiotic, and estrone derivative (1cc) under the reaction
conditions, the desired products (3bb, 3cc) were achieved with
moderate yields, which highlighted the good potential of this
reaction for complex molecular synthesis. A limitation of this
method is that no desired products were observed with heteroaryl
diazonium salts. Furthermore, in order to demonstrate the
scalability of this method, we prepared 3m on a gram scale under
the standard reaction conditions in 76% isolated yield.
a)
Scheme 4. Proposed mechanism.
Based on these mechanistic studies, our proposed mechanism
was outlined in Scheme 4.20 Under exposure to visible light, the
photocatalyst [Ru(bpy)3]2+ should produce excited triplet state
species [Ru*(bpy)3]2+, which was oxidatively quenched by the
arenediazonium tetrafluoroborates (1) to deliver an oxidizing
[Ru(bpy)3]3+ and the aryl radical (Ar•) via single-electron transfer.
Concurrently, the anionic Cu(I) trifluoromethoxide (I), which was
generated in situ via Cu(I) and TFMS (2) in the presence of CsF,
was oxidized to Cu(II) species (II) by the [Ru(bpy)3]3+ through
single electron transfer. Aryl radical should quickly undergo
radical trapping by Cu(II) species (II) to yield the key aryl-Cu(III)-
OCF3 adduct III. Subsequently, reductive elimination from this
high-valent Cu (III) intermediate (III) would yield the desired
product 3 and generate the Cu(I) species.
3
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10.1002/anie.201901447
Angewandte Chemie International Edition
Int. Ed. 2018, 57, 13266; e) F. Wang, P. Xu, F. Cong, P. Tang, Chem.
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In conclusion, we had developed the first example of a
photoredox-catalyzed
and
copper-promoted
trifluoromethoxylation of arenediazonium tetrafluoroborates with
trifluoromethyl arylsulfonate (TFMS) as the trifluoromethoxylation
reagent. This new method took advantage of visible-light
photoredox catalysis to generate the aryl radical under mild
conditions and merged it with copper-promoted selective
trifluoromethoxylation reaction. This method tolerated a wide
range of functional groups and was applicable to late-stage
trifluoromethoxylation of complex small molecules. Furthermore,
mechanistic studies indicated that proposed Cs[Cu(OCF3)2]
intermediate might be generated, which could lead to the
development of a new trifluoromethoxylation reaction in the future.
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This work was supported by the National Key Research and De-
velopment Program of China (2016YFA0602900), NFSC
(21522205, 21672110), the Natural Science Foundation of Tianjin
(Grant No. 18JCJQJC47000), and the Fundamental Research
Funds for the Central Universities.
Conflict of interest
The authors declare no conflict of interest.
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Keywords: trifluoromethoxylation · trifluoromethyl arylsulfonate ·
photoredox · copper · synthetic methods
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[18] Interestingly, the possible Cs[Cu(OCF3)2] generated by mixing CsF,
CuOTf and TFMS could not quench the excited photocatalyst.
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K. Kase, K. Shimoda, J. Organomet. Chem. 2005, 690, 6001; b) R.
4
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10.1002/anie.201901447
Angewandte Chemie International Edition
Giri, A. Brusoe, K. Troshin, J. Y. Wang, M. Font, J. F. Hartwig, J. Am.
Chem. Soc. 2018, 140, 793. Proposed equation to generate
Cs[Cu(OCF3)2] was shown below:
[20] We tried to prepared Cu(OCF3)2 intermediate but we failed due to the
decomposition of this intermediate (See more details in the supporting
information) .
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10.1002/anie.201901447
Angewandte Chemie International Edition
Entry for the Table of Contents
Trifluoromethoxylation
Shaoqiang Yang,+ Miao Chen+ and
Pingping Tang* __________ Page –
Page
Visible-Light Photoredox-Catalyzed and
Copper-Promoted
Trifluoromethoxylation of
With trifluoromethyl arylsulfonate (TFMS) as the trifluoromethoxylation reagent, a
photoredox-catalyzed and copper-promoted trifluoromethoxylation of
arenediazonium tetrafluoroborates has been reported for the first time. The reaction
tolerates a wide range of functional groups and is applicable to late-stage
trifluoromethoxylation of complex small molecules. Preliminary mechanistic studies
suggest that proposed Cs[Cu(OCF3)2] intermediate might be generated during the
reaction.
Arenediazonium Tetrafluoroborates
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