Regioselective Synthesis of N-Heteroaromatic Trifluoromethoxy Compounds by Direct O-CF3 Bond Formation

Article abstract of DOI:10.1002/chem.201505181

The first one-step method for the synthesis of ortho-N-heteroaromatic trifluoromethoxy derivatives by site-specific O-CF₃ bond formation using hydroxylated N-heterocycles and Togni's reagent is described. The approach enables the unprecedented syntheses of a wide range of six or five-membered N-heteroaromatic trifluoromethoxy compounds containing one or two heteroatoms from most commonly used hydroxylated N-heterocycles. Notable advantages of this method include its simplicity and mild conditions, avoidance of the need for metals or toxic reagents, and compatibility with a variety of functional groups. Furthermore, this method is especially suitable for the larger scale application.

Full text of DOI:10.1002/chem.201505181

DOI: 10.1002/chem.201505181
Communication
&
Trifluoromethylation
Regioselective Synthesis of N-Heteroaromatic Trifluoromethoxy
Compounds by Direct OÀCF3 Bond Formation
Apeng Liang,[a, b] Shuaijun Han,[a] Zhenwei Liu,[a] Liang Wang,[a] Jingya Li,[a, b]
Dapeng Zou,*[a, b] Yangjie Wu,*[a] and Yusheng Wu*[a, b, c]
lim is known as a potent potassium channel opener in human
Abstract: The first one-step method for the synthesis of
ortho-N-heteroaromatic trifluoromethoxy derivatives by
airway smooth muscles.[5] There are also many pesticides, such
as Triflumuron, Indoxacarb, and Thifluzamide, that contain
site-specific OÀCF3 bond formation using hydroxylated
N-heterocycles and Togni’s reagent is described. The ap-
proach enables the unprecedented syntheses of a wide
range of six or five-membered N-heteroaromatic trifluoro-
methoxy compounds containing one or two heteroatoms
from most commonly used hydroxylated N-heterocycles.
Notable advantages of this method include its simplicity
and mild conditions, avoidance of the need for metals or
toxic reagents, and compatibility with a variety of func-
tional groups. Furthermore, this method is especially suita-
ble for the larger scale application.
OCF3.[3b,6] Aside from the examples of compounds containing
the OCF3 group at an aromatic ring that have been used in
drugs or pesticides, aliphatic trifluoromethyl ethers have been
used in the field of liquid crystalline materials.[3c]
Despite the special properties and the wide applications of
the OCF3 group, its facile introduction into organic molecules
has always been a challenge. Therefore, the development of
an efficient trifluoromethoxylation process is currently a major
goal in organic synthesis. Most current routes to trifluorome-
thoxy compounds fall into two main categories: 1) the fluorine
atoms are introduced by chlorine–fluorine exchange, deoxy-
fluorination of fluoroformates, or oxidative fluorodesulfuriza-
tion under hazardous and harsh reaction conditions;[7] 2) tri-
fluoromethoxy compounds were synthesized, based on con-
struction of the CÀOCF3 bond, by nucleophilic trifluoromethox-
ylation,[8] metal-mediated trifluoromethoxylation,[9] radical addi-
tion trifluoromethoxylation,[10] or OCF3 migration.[11] Although
OCF3 can be introduced to organic molecules, most ap-
proaches either suffer from poor substrate scope or require
use of highly toxic and/or thermally labile reagents. In addi-
tion, in some approaches, regioselectivity is difficult to control.
Direct OÀCF3 bond formation has recently received much at-
tention as a new synthetic route to trifluoromethoxy com-
pounds, owing to the development and utilization of trifluoro-
methylating reagents in recent years.[12] Togni and co-workers
reported the successful OÀCF3 bond formation with primary
and secondary alcohols or N,N-dialkylhydroxylamines by using
electrophilic hypervalent iodine reagents.[13] However, exam-
ples of direct O-trifluoromethylation of phenols are still limited.
These examples include: 1) a conceptually important approach
reported by Umemoto et al. for the direct, electrophilic trifluor-
omethylation of alcohols, phenols, and amines by using O-(tri-
fluoromethyl)dibenzofuranium salts,[14] and 2) the synthesis of
1,3,5-trimethyl-2-(trifluoromethoxy)benzene by using Togni’s
reagent, as reported by Togni and co-workers.[15] The drawback
of approach (1), however, is that the highly reactive species
need to be generated in situ from diazonium salts containing
a trifluoromethoxy group and reaction conditions are harsh,[14]
whereas, in approach (2), the desired product was obtained as
a byproduct in only 15% yield and the major products were
a mixture of C-trifluoromethylated derivatives.[15]Among the ar-
omatic compounds containing OCF3 group(s), 2-(trifluorome-
thoxy)pyridine and 2-(trifluoromethoxy)quinoline derivatives
Fluorine plays a conspicuous and increasingly important role
within pharmaceuticals and agrochemicals, as well as in mate-
rials science, because one or a few fluorine atoms substituted
at specific sites in organic compounds can dramatically alter
their chemical and biological properties, such as solubility,
metabolic and oxidative stability, lipophilicity, and bioavailabil-
ity.[1] Among fluorine-containing functional groups, the trifluor-
omethoxy group (OCF3) has unique structural and electronic
properties,[2] so OCF3-containing compounds with higher lipo-
philicities show enhancement in their in vivo uptake and trans-
port in biological systems. Indeed, many OCF3-containing com-
pounds are of current interest for their use in materials, agri-
cultural, and pharmaceutical science (Figure 1a).[3] For example,
Riluzole is the first approved drug for the treatment of neuro-
logical diseases such as amyotrophic lateral sclerosis.[4] Celika-
[a] A. Liang, S. Han, Z. Liu, L. Wang, Dr. J. Li, Prof.Dr. D. Zou, Prof. Dr. Y. Wu,
Prof. Dr. Y. Wu
The College of Chemistry and Molecular Engineering
Zhengzhou University, Zhengzhou 450001, P. R. China
E-mail: wyj@zzu.edu.cn
zdp@zzu.edu.cn
[b] A. Liang, Dr. J. Li, Prof.Dr. D. Zou, Prof. Dr. Y. Wu
Tetranov Biopharm, LLC and Collaborative Innovation Center
of New Drug Research and Safety Evaluation, Henan Province
No. 75 Daxue Road, Zhengzhou 450052, (P. R. China)
E-mail: yusheng.wu@tetranovglobal.com
[c] Prof. Dr. Y. Wu
Tetranov International, Inc.
100 Jersey Avenue, Suite A340, New Brunswick, NJ 08901, (USA)
E-mail: yusheng.wu@tetranovglobal.com
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201505181.
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Figure 1. Examples of OCF3-containing compounds.
are especially useful for agrochemicals and drugs (Fig-
ure 1b).[16] However, approaches to synthesize these com-
pounds and, especially, other N-heteroaromatics containing
two or three heteroatoms, are very limited. Only Leroux
et al.,[17] in 2010, reported a general approach to synthesize 2-
method that we report herein does not require metal-mediat-
ed conditions. As an effective complement to their work, we
provide another choice for the synthesis of OCF3-containing
compounds. Optimization of the reaction parameters was ex-
plored by using 5-bromopyridin-2-ol (1a) as a model substrate
in conjunction with various CF3 sources (Table 1). Initially, non-
hazardous, widely used Me3SiCF3 (I) and Togni’s reagent[12b,13,19]
(II, III; 1.0 equiv) were tested as electrophilic CF3-transfer re-
agents in CHCl3 solution at reflux temperature for 48 hours
(Table 1, entries 1–3). When I was used, no product was ob-
tained (Table 1, entry 1). However, by using reagents II or III,
(trifluoromethoxy)pyridines by
a multistep sequence with
lower yields. Moreover, due to the poor substrate scope and
highly toxic reagents, application of this method is limited.
Herein, we report the first convenient, general, and efficient
method for the synthesis of OCF3-containing pyridine, quino-
lone, and isoquinoline derivatives, as well as other N-heteroar-
omatics containing two heteroatoms, through site-specific OÀ the desired product was obtained. A higher yield was obtained
CF3 bond formation (Scheme 1).
when using reagent II as the CF3-transfer reagent (Table 1, en-
tries 2 vs. 3). We next investigated a variety of solvents and
found that CH3NO2 was the best choice (Table 1, entry 7), al-
though the yield did not exceed 24%. We then explored the
effects of the bases,[20] Lewis acids,[12b,13a] and Brønst-
ed acids[21,12b] on the reaction. To our disappointment,
none of these additives were conducive to the reac-
tion (see the Supporting Information). However, to
our surprise, by increasing the amount of 1a, the
yield was greatly improved (Table 1, entries 10–12).
When the reaction temperature was increased to
1008C and the reaction time was shortened from 48
hours to 5 hours, a yield of 67% was obtained
(Table 1, entry 13). With these results in hand, the re-
action was also performed under UV light or with the
addition of radical initiators, such as azobisisobutyro-
nitrile (AIBN) or benzoyl peroxide (BOP), but the ef-
fects were adverse (see the Supporting Information).
Thus, the optimum reaction conditions were deter-
mined as the combination of 3.0 equivalents of 1a,
1.0 equiv II, in CH3NO2 at 1008C for 5 hours.
During the preparation of this paper, Qing and co-workers[18]
reported the an excellent study on silver-mediated oxidative
trifluoromethylation of phenols. In contrast to their study, the
Next, the substrate scope of the trifluoromethoxy-
lation of N-heteroaromatic compounds was investi-
Scheme 1. Site-specific OÀCF3 bond formation of 2-hydroxypyridine derivatives.
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Table 1. Representative results for the optimization of the trifluoromethy-
lation of 1a with 2.[a]
Table 2. Direct O-trifluoromethylation of N-heteroaromatics.[a,b]
Entry
CF3 source
T [oC]
t [h]
Solvent
CHCl3
Yield [%][b]
1
2
3
4
5
6
7
8
I
II
III
II
II
II
II
II
II
II
II
II
II
70
70
70
70
70
70
70
70
70
70
70
70
100
48
48
48
48
48
48
48
48
48
48
48
48
5
0
14
8
19
12
8
24
16
trace
41
46
47
67
CHCl3
CHCl3
DCE
dioxane
ethyl acetate
CH3NO2
CH3CN
9
DMF
10[c]
11[d]
12[e]
13[f]
CH3NO2
CH3NO2
CH3NO2
CH3NO2
[a] Reaction conditions (unless otherwise stated): 1a (0.1 mmol) and CF3
source (0.1 mmol) in the specified solvent (1.0 mL) were stirred at 708C
for 48 hours under air; [b] yields were determined by using GC (average
of two GC runs) with n-dodecane as an internal standard; [c] 1a
(0.2 mmol), II (0.1 mmol); [d] 1a (0.3 mmol), II (0.1 mmol); [e] 1a
(0.4 mmol), II (0.1 mmol); [f] 1a (0.3 mmol), II (0.1 mmol).
gated (Table 2). Using 2-hydroxypyridine derivatives containing
electron-neutral, electron-donating and electron-withdrawing
groups as substrates, the reaction proceeded smoothly to give
2-trifluoromethoxypyridine derivatives 3a–n in 23–78% yield.
A variety of important functional groups, including ester,
cyano, acetyl, ether, halide, alkyl, and aryl, were well tolerated
under the mild reaction conditions. Furthermore, complicated
2-hydroxypyridine derivatives containing multiple functional
groups (3d, 3h, 3l) were also transformed into trifluorome-
thoxy compounds in good yields by using this method. In gen-
eral, substrates containing electron-withdrawing groups were
found to give the desired products in lower yields than those
with electron-donating groups (3c, 3i vs. 3g). When using pyr-
idines with an amino substituent at the 3- or 5-position, the tri-
fluoromethylation mainly occurred on the aromatic ring, af-
fording the desired trifluoromethyl ether in very low yield,
which could only be detected by 19F NMR spectroscopy and
LC-MS.
[a] Reaction conditions: 1 (3.0 mmol), 2 (1.0 mmol), CH3NO2 (10 mL),
1008C, 5 h, under air. [b] yields of isolated product based on 2; [c] yields
were determined by using GC (average of two GC runs) with n-dodecane
as an internal standard; [d] yield determined by 19F NMR spectroscopy
with 2-(trifluoromethyl)benzenamine as the internal standard.
The reaction is also very suitable for application in larger
scale experiments. Treatment of 330 g of 1a with 200 g of re-
agent II in 6 L of CH3NO2 produced 84 g of 3a (55% yield;
Scheme 2), with 152 g of 1a recovered.
To expand the potential utility of this reaction for medicinal
chemistry, heterocycles, such as quinoline/isoquinoline deriva-
tives, and other N-heteroaromatics containing two heteroa-
toms, were also subjected to this reaction protocol (3p–w).
The desired trifluoromethoxylated products 3p–u were never-
theless produced in good yields under the developed condi-
tions. Five-membered N-heteroaromatic compounds can also
be applied under these conditions to give the desired product
in good yield (3w).
To gain insight into the reaction mechanism, we carried out
several experiments (see the Supporting Information). A stoi-
chiometric amount of the radical trap 2,2,6,6-tetramethyl-1-pi-
peridinyloxy (TEMPO) and 3,5-di-tert-butyl-4-hydroxytoluene
(BHT) were added to the reaction mixture of 1a and reagent II.
With the increase of the amount of TEMPO, the yield of the
product was decreased and the amount of TEMPO-CF3 was in-
creased. When BHT was used, the formation of BHT-CF3 adduct
could be detected by GC-MS & 19F NMR. The result is a strong
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cesses, laboratory methods, and applications in drug discovery
research. The introduction of an OCF3 group into a molecule of
interest at a late stage of a synthesis under mild reaction con-
ditions could accelerate the discovery of lead compounds.
Scheme 2. Scalability of the trifluoromethylation of 1a.
Experimental Section
A dried glass reaction tube equipped with a magnetic stir bar was
charged with 1 (3.0 mmol), 2 (316 mg, 1.0 mmol), and CH3NO2
(10.0 mL). The reaction mixture was then stirred at 1008C for 5 h.
The reaction progress was monitored by TLC. The reaction mixture
was then allowed to cool to room temperature, filtered through
a pad of celite, and washed with ethyl acetate (10 mL”3). The
combined organic solvent was removed under reduced pressure.
The residue was purified by silica gel flash chromatography to
afford the desired product. The products were characterized by
1H NMR, 13C NMR, 19F NMR, GC-MS, LC-MS and HRMS.
indication of a radical pathway with a CF3 radical as the key in-
termediate.Treatment of the sodium salt of 1a with reagent II
in CH3NO2 at 1008C did not afford the desired product 3a, in-
dicating that the proton of the phenolic hydroxy group is im-
portant for the reaction. It is possible that the proton transfer
is a necessary condition for the formation of the CF3 radical.
Togni and co-workers have discussed similar proton transfer
pathways in their work on trifluoromethylation.[12b,13b,21,22]
Rapid protonation of the carboxyl moiety causes a weakening
of the IÀO bond, thus freeing a coordination site at the iodine,
followed by the occupation of the substrate to form an iodoni-
um intermediate. After protonation of reagent II by either hy-
droxypyridine, a CF3 radical, from hemolytic cleavage of the IÀ
CF3 bond by a SET step, and simultaneously a phenoxyl radical
are formed. After recombination, the desired product is
formed.
Acknowledgements
We are grateful to the Technology Research and Development
Funds of Zhengzhou (141PRCYY516) for financial support.
Keywords: CÀO bond formation
·
N-heterocycles
·
hypervalent iodine reagents · trifluoromethylation
In light of these results, we assumed that the mechanism for
OÀCF3 bond formation is similar to that proposed by Togni[13b]
(Scheme 3). Firstly, through a proton transfer pre-equilibrium
between the phenolic hydroxy and reagent II, a charge-transfer
complex 2a is formed. Secondly, a pair of radicals are generat-
ed through a SET step which then afford the desired product
3a upon recombination.
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Received: December 28, 2015
Published online on February 25, 2016
Chem. Eur. J. 2016, 22, 5102 – 5106
www.chemeurj.org
5106
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