Photocatalytic trifluoromethoxylation of arenes and heteroarenes in continuous-flow

Article abstract of DOI:10.3762/bjoc.16.111

The first example of photocatalytic trifluoromethoxylation of arenes and heteroarenes under continuous-flow conditions is described. Application of continuous-flow microreactor technology allowed to reduce the residence time up to 16 times in comparison t

Full text of DOI:10.3762/bjoc.16.111

Photocatalytic trifluoromethoxylation of arenes and
heteroarenes in continuous-flow
Alexander V. Nyuchev1, Ting Wan1, Borja Cendón1,2, Carlo Sambiagio1,
Job J. C. Struijs1, Michelle Ho1, Moisés Gulías2, Ying Wang3 and Timothy Noël*1
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2020, 16, 1305–1312.
1Micro Flow Chemistry and Synthetic Methodology, Department of
Chemical Engineering and Chemistry, Eindhoven University of
Technology, Het Kranenveld, Bldg 14 − Helix, 5600 MB Eindhoven,
The Netherlands, 2Centro Singular de Investigación en Química
Biolóxica e Materiais Moleculares (CIQUS) and Departamento de
Química Orgánica, Universidade de Santiago de Compostela, 15782
Santiago de Compostela, Spain and 3Discovery Chemistry and
Technologies, AbbVie Inc., 1 North Waukegan, Road, North Chicago,
Illinois 60064, United States of America
doi:10.3762/bjoc.16.111
Received: 12 April 2020
Accepted: 24 May 2020
Published: 15 June 2020
This article is part of the thematic issue "Advances in photoredox
catalysis".
Associate Editor: B. Nay
Email:
Timothy Noël* - t.noel@tue.nl
© 2020 Nyuchev et al.; licensee Beilstein-Institut.
License and terms: see end of document.
* Corresponding author
Keywords:
C–H functionalization; continuous-flow; organic synthesis; photoredox
catalysis; trifluoromethoxylation
Abstract
The first example of photocatalytic trifluoromethoxylation of arenes and heteroarenes under continuous-flow conditions is de-
scribed. Application of continuous-flow microreactor technology allowed to reduce the residence time up to 16 times in compari-
son to the batch procedure, while achieving similar or higher yields. In addition, the use of inorganic bases was demonstrated to
increase the reaction yield under batch conditions.
Introduction
The number of fluorine-containing compounds and their influ- unique properties, for example, a high electronegativity
ence in medicine and agrochemistry are increasing every year (χ = 3.7), and a high lipophilicity (Hansch-Leo parameter
[1,2]. The introduction in a drug (or a bioactive compound in π = 1.04, intermediate between the -CF3 and the -SCF3 values,
general) of single fluorine atoms, or multiple fluorine-contain- respectively, 0.88 and 1.44) [4]. From a pharmaceutical
ing substituents, such as -CF3, -SCF3, -CF2H/-CF2-, has a range perspective, the former property provides stability of the drug
of effects on the pharmacokinetics and pharmacodynamics of under physiological conditions, and the latter ensures better
the molecule [3]. Among other fluorine-based substituents, the solubility and transport across the physiological lipophilic envi-
trifluoromethoxy group (-OCF3) has until recently remained ronment (e.g., dissolution in lymph and cell membrane penetra-
less explored and understood [4]. Nonetheless, it possesses tion) [5]. In trifluoromethoxylated aryls, the -OCF3 group
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assumes a peculiar orthogonal conformation relative to the tions result in a relative conformational flexibility, where the
aromatic ring, partially due to the hyperconjugation of an -OCF3 group can rotate between the two sides of the aromatic
oxygen lone pair with an antibonding C–F orbital, and partially ring (Scheme 1A). This property might be responsible for
to the steric hindrance of the -CF3 group [6,7]. These interac- stronger binding affinities of trifluoromethoxylated compounds
Scheme 1: A) Properties and B) synthesis of CF3O-bearing arenes; C) trifluoromethoxylation using the “second” Ngai reagent.
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with the active sites in enzymes, proteins, or other biomole- the direct introduction of the -OCF3 moiety into an
cules [8,9].
organometallic species [15], a diazo compound [16,17], or an
unfunctionalized C–H bond (Scheme 1B). The latter method is
Several procedures for the synthesis of trifluoromethyl aryl particularly interesting, partially due to the general interest of
ethers were reported from the mid-1900s, mostly based on the community into C–H functionalization methodologies
the transformation of pre-functionalized aryls, such as [18,19], and partially thanks to the synthetic possibilities
trichloromethyl ethers, fluoroformates, or carbonochlorido- enabled by photoredox catalysis, a field widely explored in
thionate derivatives. These procedures typically required recent years [20-22]. In 2018, Ngai [23,24] and Togni [25]
harsh conditions and the use of aggressive and toxic chemicals reported simple photoredox protocols for the radical trifluoro-
like BF3, HF, MoF6, SF4, and SbF3 [10]. Alternative ap- methoxylation of unfunctionalized (hetero)arenes by using
proaches were recently reported [8,9,11], for example specifically designed CF3O radical-releasing agents
the trifluoromethylation of hydroxyaryls [12-14], and (Scheme 1B).
Table 1: Reaction optimization under flow conditions.
Entry
Equiv of substrate
Solvent, conc. of 1 (M)
Light source
Cat. (%)
Yield (%)a
1
2
25
25
25
25
25
25
10
10
10
10
CH3CN/CH2Cl, 0.04
CH3CN/CH2Cl, 0.04
CH3CN, 0.04
CH3CN, 0.04
CH3CN, 0.08
CH3CN, 0.08
CH3CN, 0.08
CH3CN, 0.08
CH3CN, 0.2
465 nm, 18 W
400 nm, 10 W
400 nm, 10 W
400 nm, 10 W
400 nm, 10 W
400 nm, 10 W
400 nm, 10 W
400 nm, 10 W
400 nm, 10 W
400 nm, 10 W
1
1
1
2
1
2
1
1
1
1
40
52
53
68
65
64
57
63b
61
73c
3
4
5
6
7
8
9
10
CH3CN, 0.08
control experiments
11
12
13
25
25
25
CH3CN, 0.04
CH3CN, 0.04
CH3CN, 0.04
465 nm, 18 W
400 nm, 10 W
465 nm, 18 W
1
0
0
10d
17
0
a 19F NMR yields are reported; b Degassed solvent; c Freshly prepared; d [Ir(dtbbpy)(ppy)2]PF6 as catalyst.
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Following our long-standing interest in the development of con- using a lower excess of substrate (10 equiv), obtaining product
tinuous-flow approaches for C–H functionalization [18,26] and 2 in 57% yield (Table 1, entry 7). Little improvement was seen
photochemical methodologies [27-31], herein we report the first upon degassing the reaction mixture prior to the reaction (three
application of flow methods for the direct trifluoromethoxy- freeze-pump-thaw cycles, 63%, Table 1, entry 8). Performing
lation of arenes and heteroarenes.
the reaction at 0.2 M concentration gave 61% yield (Table 1,
entry 9). Last, we were particularly pleased to see that a
sensible increase in yield could be obtained by using a freshly
Results and Discussion
Among the three reagents shown in Scheme 1B, we decided to prepared reagent 1 in the reaction (the activity of reagent 1
use the “second” Ngai reagent (1, Scheme 1C) for our investi- persists at a primary level during near two weeks when stored in
gation, as less side-products (derived from alternative radical the dark at −20 °C). A yield of 73% could be achieved at
formation from the other two reagents [23,25]) and tolerance 0.08 M, without the tedious degassing procedure (Table 1, entry
towards water and air gives this reagent a certain advantage 10).
over the others [24]. To transfer the batch procedure into con-
tinuous-flow conditions, the concentration of 1 was initially Finally, a few control experiments were performed (Table 1,
decreased from the original 0.2 M to 0.04 M, and the amount of entries 11–13). A more expensive Ir photocatalyst was tested
substrate increased from 10 to 25 equivalents, while the under blue light irradiation, but a poor 10% yield was obtained.
same catalyst ([Ru(bpy)3](PF6)2) and reaction medium Experiments in the absence of photocatalyst under violet or
(CH3CN/CH2Cl2 1:1) were used. Upon irradiation with blue blue light gave respectively 17% and 0% yield. The little
light (465 nm, 18 W LED), the desired trifluoromethoxyben- product obtained with 400 nm light can be explained by the
zene (2) was obtained in 40% yield after a residence time of 1 h photodecomposition of 1, which generates small amounts of
(Table 1, entry 1). A 400 nm LED light led to a higher yield CF3O radical (this has been shown for the “first” Ngai reagent
(52%, Table 1, entry 2), thus this light source was used for [23]).
further investigations. As CH2Cl2 is not a recommended sol-
vent, especially for industrial and pharmaceutical applications Under the conditions shown in Table 1, entry 10, we then set to
[32], we tested the reaction in pure acetonitrile. As the same figure out the optimal residence time (tR) for the reaction
yield was obtained (Table 1, entry 3), this solvent was chosen as (Scheme 2). Yields of around 70% were observed for the tri-
reaction medium for the following experiments. Either doubling fluoromethoxylation of benzene even at 20 minutes residence
the amount of catalyst or reducing the concentration resulted in time, which demonstrates the effectiveness of the flow process
considerably higher reaction yields (68 and 65%, Table 1, (a yield of 63% was obtained in batch over 16 h [24]). Howev-
entries 4 and 5). Increased catalyst loading at 0.08 M produced er, unreacted 1 was observed at this residence time, and full
no improvement (Table 1, entry 6). We next tested the reaction conversion could be obtained only after 1 h. Moreover, a resi-
Scheme 2: Optimization of residence time. 19F NMR yields are reported.
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Beilstein J. Org. Chem. 2020, 16, 1305–1312.
Scheme 3: Scope of photoredox trifluoromethoxylation in continuous-flow. In case of different products, the major isomer is shown. The position of
the CF3O group in the minor isomer is marked with a star. 19F NMR yields are reported. a CH3CN/CH2Cl2 3:2, 0.08 M; b CH3CN/CH2Cl2 1:1, 0.05 M;
c CH3CN/CH2Cl2 5:3, 0.05 M; d CH3CN/CH2Cl2 3:2, 0.02 M; e CH3CN/CH2Cl2 5:4, 0.057 M.
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dence time of 20 minutes proved not sufficient for several other pounds. Different aromatic ketones, aldehydes, esters, and
substrates, thus 1 h was chosen as the standard for the following carboxylic acids were transformed to the corresponding trifluo-
scope. It is worth mentioning that a lower excess of substrate at romethoxylated compounds (12–23) with generally good to
20 minutes residence time gave much lower yields of 2 (63 and high yields, except for 13, 16, 19 and 22 which were obtained in
28% for 5 and 1 equiv, respectively).
moderate or poor yields. Other synthetically useful substituents,
including cyano, nitro, phenylsulphonyl, (pinacolato)boron, and
With the optimized conditions in hands, we proceeded to inves- alkyl groups could be tolerated in the reaction (24–30), leading
tigate the trifluoromethoxylation of different arenes and to a yield range between 26 and 65%. The trifluoromethoxy-
heteroarenes in acetonitrile. Solvent mixtures and lower concen- lation of pyridine, pyrimidine, and thiophene derivatives also
trations had to be used in some cases due to solubility issues. As took place smoothly, giving products 31–37 in good yields
can be seen in Scheme 3, trifluoromethoxylation occurs prefer- (50–65%).
entially at the most electron-rich site of the arenes, although
sensitivity to steric hindrance was observed. The reactions with Overall, the yields obtained for different products were similar
halogenated aryls occurred smoothly, leading to products 3–11 to those reported by Ngai [24] and Togni [25] using batch
in good yields. An exception is represented by iodobenzene, protocols. In addition, in comparison with Ngai’s protocol, this
which could be methoxylated in only 23% yield (product 6). flow method allows for a much reduced reaction (residence)
Similar results were obtained with carbonyl-containing com- time, therefore, demonstrating the potentials of this technology
Table 2: Influence of bases on the photoredox trifluoromethoxylation in batch and flow.
Entry
Equiv of substrate
Сonc. of 1 (M)
organic/soluble bases
Base
Yield (%)a
1
2
3
4
5
10
10
25
25
25
0.08
0.08
0.04
0.04
0.04
(Bu4N)H2PO4, 1 equiv
(Bu4N)H2PO4, 1 equiv
DIPEA, 1 equiv
20
6 (flow)
18
DBU, 1 equiv
14 (flow)
0 (flow)
(Bu4N)OAc, 1 equiv
inorganic bases
6
25
25
25
10
10
10
10
10
10
10
10
10
10
10
0.04
0.04
0.04
0.04
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
K2CO3, 1 equiv
K2HPO4, 1 equiv
K2HPO4, 1 equiv
K2HPO4, 1 equiv
K2HPO4, 1 equiv
K2HPO4, 0.5 equiv
K2HPO4, 0.25 equiv
K2HPO4, 2 equiv
KH2PO4, 1 equiv
KH2PO4, 1 equiv
KH2PO4, 0.5 equiv
KH2PO4, 0.25 equiv
KH2PO4, 2 equiv
K3PO4, 1 equiv
54
7
86
8
7b (flow)
9
68
10
11
12
13
14
15
16
17
18
19
78
78
77
70
89
75c (flow)
64
63
78
54
a 19F NMR yields are reported; b 100 equiv of H2O added; c 5 minutes of irradiation in batch before flow reaction.
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Beilstein J. Org. Chem. 2020, 16, 1305–1312.
for the synthesis of trifluoromethoxylated compounds. The use Conclusion
of acetonitrile as solvent, rather than the CH3CN/CH2Cl2 mix- In summary, we have developed the first continuous-flow tri-
ture previously used [24], further increase the value of this fluoromethoxylation of arenes and heteroarenes. The use of
protocol [32] for the synthesis of medicinally relevant mole- flow photochemistry allows the reaction to occur 16 times faster
cules.
than in batch, while at the same time achieving similar yields.
Thanks to the intrinsic properties of microflow systems, espe-
Based on the reaction mechanism proposed by Ngai and cially for the scale-up of photochemical processes, we antici-
co-authors (Scheme 1C) [24], we hypothesized that the pate this procedure could be useful in medicinal chemistry ap-
addition of base could potentially improve the reaction plications. In addition, although no beneficial effect could be
outcome. To prove this idea, we performed a series of experi- achieved in flow by the addition of bases, we proved this has a
ments under batch and flow conditions (Table 2). Organic or significant effect on the reaction in batch, leading to higher
soluble bases such as amines and tetrabutylammonium salts yields. This discovery will be useful for the development of
(Table 2, entries 1–5) proved unfortunately not successful, future trifluoromethoxylation methodologies.
neither in batch nor flow, so we extended the investigation to
inorganic bases. Among the bases tested, potassium hydrogen-
phosphates K2HPO4 and KH2PO4 led to increased yields,
reaching up to 86% or 89% in batch (Table 2, entries 7 and 14).
Because of the high yields obtained, the use of these bases in
flow was investigated. The addition of water [33] allows these
bases to be solubilized, preventing clogging; however, this
strategy led to a mere 7% yield (Table 2, entry 8). During
batch experiments we noted that after several minutes of
irradiation, a homogeneous solution was obtained. Thus, we
thought of using this trick for the flow process: irradiation of
the reaction mixture in batch for 5 minutes, followed by injec-
Supporting Information
Supporting Information File 1
Procedure for continuous-flow and batch
trifluoromethoxylation reactions and 19F NMR spectra of
compounds 2–37.
[https://www.beilstein-journals.org/bjoc/content/
supplementary/1860-5397-16-111-S1.pdf]
tion into the microcapillary, provided 2 in 75% yield (Table 2, Funding
entry 15). We acknowledge financial support from AbbVie. T.W.
acknowledges support from the China Scholarship Council
These conditions were also applied to a few other substrates, for (CSC). B.C. thanks MINECO for the FPI fellowship BES-2017-
the sake of comparison (Figure 1).
079784. C.S. acknowledges the European Union for a Marie
Curie European post‐doctoral fellowship (FlowAct, Grant No.
794072).
ORCID® iDs
Alexander V. Nyuchev - https://orcid.org/0000-0002-0460-0543
Ting Wan - https://orcid.org/0000-0002-3520-9723
Borja Cendón - https://orcid.org/0000-0003-1783-0839
Carlo Sambiagio - https://orcid.org/0000-0001-8536-103X
Job J. C. Struijs - https://orcid.org/0000-0002-6051-2927
Michelle Ho - https://orcid.org/0000-0002-8041-112X
Moisés Gulías - https://orcid.org/0000-0001-8093-2454
Ying Wang - https://orcid.org/0000-0001-9516-3483
Timothy Noël - https://orcid.org/0000-0002-3107-6927
Figure 1: Effect of KH2PO4 – other substrates. a Conditions as for
entry 15 (Table 2), 1 h residence time; b conditions as for entry 14 (Ta-
ble 2), 18 h reaction time; c 0.5 equiv KH2PO4, CH3CN/CH2Cl2 1:1;
d 2 equiv KH2PO4. 19F NMR yields are reported.
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