Visible-light-driven difluoroacetamidation of unactive arenes and heteroarenes by direct C-H functionalization at room temperature

Article abstract of DOI:10.1021/ol502676y

The directed difluoroacetamidation of unactivated arenes and heteroarenes with bromodifluoroacetamides via visible-light photoredox catalysis has been efficiently achieved at room temperature. Broad utility of this transformation is presented, including e

Full text of DOI:10.1021/ol502676y

Letter
pubs.acs.org/OrgLett
Visible-Light-Driven Difluoroacetamidation of Unactive Arenes and
Heteroarenes by Direct C−H Functionalization at Room Temperature
Lin Wang,† Xiao-Jing Wei,† Wen-Liang Jia,† Jian-Ji Zhong,‡ Li-Zhu Wu,‡ and Qiang Liu*,†
†State Key Laboratory of Applied Organic Chemistry, Lanzhou University, 222 South Tianshui Road, Lanzhou 730000, P. R. China
‡Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry, the
Chinese Academy of Sciences, Beijing 100190, P. R. China
S
* Supporting Information
ABSTRACT: The directed difluoroacetamidation of unac-
tivated arenes and heteroarenes with bromodifluoroacetamides
via visible-light photoredox catalysis has been efficiently
achieved at room temperature. Broad utility of this trans-
formation is presented, including electronically deficient heteroaromatic and aromatic systems. The mechanistic pathway of the
difluoroacetamidation was discussed based on photoluminescence quenching, spin-trapping, and kinetic isotope effect
experiments.
rganofluorine compounds are of great importance in the
areas of materials, pharmaceuticals, and agrochemicals,
activating groups at various positions around an aromatic ring. In
this context, the direct difluoroacetamidation of inactive arenes at
room temperature would be an appealing alternative.
O
since fluorine atoms enjoy privileged roles in life science and
materials science-related applications.1 Compounds containing a
trifluoromethyl group have been studied extensively.2,3 Similar to
the trifluoromethyl group, compounds containing partially
fluorinated alkyl groups, such as a difluoromethyl group, can
often induce many intriguing effects on biologically active
molecules.4 In particular, arenes containing difluoroacetamide on
an aromatic ring are often found in molecules that exhibit
biological activities (Scheme 1).5 Typically, these valuable
Over the past five years, visible-light photoredox catalysis has
attracted widespread research interest owing to its attractive
features such as mild and green conditions, excellent functional
group tolerance, and high reactivity.9,10 The excited photo-
catalyst by visible light at room temperature populates a strongly
oxidizing or reducing catalyst that can activate a variety of
substrates via photoinduced electron transfer. Seminal works
have demonstrated that haloalkanes are efficient precursors for
the generation of carbon-centered radicals under photoredox
catalytic conditions.11 Although radical addition to arenes is a
promising approach for synthesizing substituted arenes without
the reliance on prefunctionalization, the direct difluoro-
acetamidation of aromatic compounds via visible-light photo-
redox catalysis has not been reported to date.12
Scheme 1. Biologically Active Molecules Containing a
Difluoroacetamidated Arene Structural Motif
Here we report a mild, operationally simple strategy for the
direct difluoroacetamidation of unactivated arenes and hetero-
arenes with bromodifluoroacetamides through a radical-medi-
ated mechanism by visible-light-driven photoredox catalysis. We
demonstrate the broad utility of this transformation through
addition of difluoroacetamide moieties to a number of arene and
heteroarenes. The benefit to medicinal chemistry is also shown
through examples of the direct difluoroacetamidation of widely
prescribed pharmaceutical agents.
The initial studies focused on difluoroacetamidation of
benzenes with bromodifluoroacetamides 1a under visible-light
irradiation. After carefully screening reaction conditions such as
photocatalysts, solvents, and bases (Table S1, Supporting
Information), we gladly found that the reaction of benzene
with bromodifluoroacetamides 1a in a CH2Cl2 solution
containing Ir(ppy)3 as a photocatalyst and potassium acetate
molecules could be prepared by difluorination of a carbonyl
moiety with aminosulfur trifluorides, such as diethylaminosulfur
trifluoride (DAST) or Deoxofluor,6 and Cu-mediated cross-
couplings between halodifluoromethylated reagents and aryl
metal species or aryl halides.7 The recent breakthrough in the
construction of difluoromethylated arenes was mediated by
palladium or nickel catalyzed cross-coupling between aryl
boronic acids and difluoromethylated halides.8 While these
methods have proven valuable for a variety of applications, they
suffer from the disadvantage of requiring aryl precursors bearing
Received: September 10, 2014
© XXXX American Chemical Society
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Letter
(KOAc) as a base provided the respective difluoroacetamidated
product in 87% yield. Control experiments established the
importance of both visible light and the photocatalyst, as no
desired reaction was observed in the absence of light and
Ir(ppy)3.
Having identified the optimal conditions, we next examined
the scope of arenes that participate in this difluoroacetamidation
reaction. As shown in Figure 1, a broad array of mono-, di-, and
electron density (Figure 1, 2e−2h). However, when 3,4-
dimethoxybenzaldehyde was employed as the substrate, only
amide 2n was isolated in 89% yield. The structure of amide 2n
was also established by X-ray crystal analysis. In addition, a gram-
scale reaction of 3,4-dimethoxybenzaldehyde is successful and 2n
was isolated in 78% yield.
We next turned our attention to heteroarenes whose
frameworks have been realized as valuable motifs in biological
and medical chemistry (Figure 2). Unsurprisingly, an array of
Figure 2. Scope of the difluoroacetamidation of heteroarenes using
bromodifluoroacetamide 1a. Reaction conditions: 1a (0.2 mmol, 1.0
equiv), heteroarene (2.0 mmol, 10 equiv), KOAc (0.6 mmol, 3.0 equiv),
and fac-Ir(ppy)3 (0.006 mmol, 3.0 mol %) in dry CH2Cl2 (2.0 mL) were
irradiated with a 3 W blue LED for 36 h at rt. Isolated yield. (a)
Irradiated for 24 h at rt.
Figure 1. Scope of the difluoroacetamidation of arenes using
bromodifluoroacetamide 1a. Reaction conditions: 1a (0.2 mmol, 1.0
equiv), arene (2.0 mmol,10 equiv), KOAc (0.6 mmol, 3.0 equiv), and
fac-Ir(ppy)3 (0.006 mmol, 3.0 mol %) in dry CH2Cl2 (2.0 mL) were
irradiated with a 3 W blue LED for 24 h at rt. Isolated yield. (a) Reaction
carried out on a gram scale.
trisubstituted benzenes can serve as competent substrates (2a−
2n, 51−95% yield). In general, arenes bearing electron-
withdrawing substituents afforded lower yields than those with
electron-donating groups. Many versatile functional groups, such
as halides, aldehyde, ester, nitrile, or ether, were all tolerated
under the reaction conditions, highlighting the potential of the
method in organic synthesis. For the arenes bearing different
substituents, the corresponding products were formed as a
mixture of regioisomers (Figure 1, 2b, 2e−2h), indicating a
radical substitution process may be involved. It seems that the
site selectivity of these reactions is effected by both the electron
density of arenes and the stability of radical intermediates (capto-
dative stabilization13). Because the aminocarbonyldifluoro-
methyl radical is electrophilic, the aminocarbonyldifluoromethyl
groups prefer to locate in the position where there is greater
thiophene, pyridine, pyrazine, pyridazine, and pyrimidine
substrates are compatible with this new difluoroacetamidation
approach (3a−3l, 48−95% yield), although a prolonged reaction
time (36 h) was needed. The electron-rich thiophenes, which are
sensitive in the presence of the high temperatures and/or strong
oxidants required for many previously reported C−H activations,
underwent difluoroacetamidation in moderate to good yield
under our photochemical conditions (3a−3b). The electroni-
cally deficient nitrogen-containing heteroaromatics (such as 2,6-
dibromopyridine) were also operative, but the yields were much
lower. The efficiency was notably improved when electron-
donating groups were located on the heteroaromatic ring; the
corresponding products were obtained with good selectivity. In
addition, the visible-light driven difluoroacetamidation was
applicable to complex aromatic rings such as napropamide14
B
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Letter
and pentoxifylline.15 Both of them could be difluoro-
acetamidated in good yield (Figure 2, 3m, 3n).
To evaluate the structure−reactivity relationship of bromo-
difluoroacetamides 1, we prepared structurally diverse bromo-
difluoroacetamides 1 (see Supporting Information) and applied
them in the visible-light-driven difluoroacetamidation with
benzene. It was found that both N,N-disubstituted amides
(Figure 3, 4a−4c) and N-monosubstituted amides (Figure 3,
Figure 3. Scope of the amides for the reaction. Reaction conditions: 1
(0.2 mmol, 1.0 equiv), benzene (2.0 mmol,10 equiv), KOAc (0.6 mmol,
3.0 equiv), and fac-Ir(ppy)3 (0.006 mmol, 3.0 mol %) in dry CH2Cl2 (2.0
mL) were irradiated with a 3 W blue LED for 24 h at rt. Isolated yield.
Figure 4. (a) Left: Photoluminescence quenching of fac-Ir(ppy)3 5.00 ×
10−5 M) with progressive addition of 1a in anaerobic CH2Cl2. Right:
ESR spectrum of a solution of fac-Ir(ppy)3 (3.0 × 10−3 M), 1a (0.10 M),
and DMPO (0.50 M) in anaerobic CH2Cl2. (b) KIE experiments. (c)
Proposed mechanism of the reaction.
4d−4f) proved to be efficient in the reaction. The broad utility of
this transformation is presented by various amino moieties of the
amides, such as piperidine, piperazine, cyclooctanamine, cyclo-
propylmethanamine, and aniline.
the desired difluoroacetamidated benzene. To understand the
rate-determining step for the photoreaction, the reaction of a 1:1
mixture of benzene and benzene-d6 with 1a was carried out under
the conditions described for Figure 4b. The intermolecular KIE
was found to be 1.05 after 26% conversion, indicating that the
deprotonation of intermediate 7 is not a rate-limiting step of the
difluoroacetamidation.
In summary, we describe a mild visible-light-driven method for
directed difluoroacetamidation of unactivated arenes and
heteroarenes with bromodifluoroacetamides. The present
protocol proceeds smoothly at room temperature, does not
require prefunctionalization, and displays a broad scope toward
aromatic and heteroaromatic systems with a wide range of
functional group tolerance. Additionally, the reaction mechanism
was well studied by photoluminescence quenching, spin-
trapping, and kinetic isotope effect experiments. Further
investigations of other fluoroacylations via visible-light photo-
catalysis are ongoing in our laboratory.
The mechanism of the fluoroacylation was studied by
photoluminescence quenching, electron spin resonance (ESR)
spin-trapping, and kinetic isotope effect (KIE) experiments. The
photoluminescence of fac-Ir(ppy)3 was quenched by bromo-
difluoroacetamides 1a with a rate constant of 6.97 × 103 L·mol−1
(Figure 4a, left). In contrast, no quenching took place with the
addition of benzene into the same solution of fac-Ir(ppy)3. As the
energy transfer from the excited fac-Ir(ppy)3 to 1a is negligible,
the photoluminescence quenching is therefore attributed to the
electron transfer (ET) from the excited fac-Ir(ppy)3 to
bromodifluoroacetamides 1a. The ET procedure should
generate electron-deficient radical 5 along with fac-IrIV(ppy)3.
To verify the generation of the aminocarbonyldifluoromethyl
radical 5 in the initial step of the reaction, 5,5-dimethyl-1-
pyrroline-N-oxide (DMPO) was employed as a spin trap. It is
clearly shown in Figure 4a (right) that when the solution of
DMPO, fac-Ir(ppy)3, and 1a in anaerobic CH2Cl2 was irradiated
with a 3 W-blue LED, an intense EPR signal (Figure 4b) was
observed that exhibited a general 1:1:1:1:1:1 pattern (g = 2.0065,
αN = 13.9 G, αH = 17.0 G). The value of αN/αH is 0.85, indicating
an electron-deficient carbon-centered radical was trapped by
DMPO. On the basis of our observations and literature reports,16
the above signal should be attributed to the adduct of DMPO
with difluoromethyl radical 5. In the visible-light-driven
difluoroacylation (Figure 4c), the addition of difluoromethyl
radical 5 to benzene gives radical intermediate 6. Further ET
from radical 6 to fac-IrIV(ppy)3 affords carbocation intermediate
7 and regenerates the photocatalyst. Finally, intermediate 7 is
deprotonated, regenerating the aromatic system and leading to
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental details including synthesis, characterization data of
products, and crystallographic data of 2n (CIF). This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: liuqiang@lzu.edu.cn.
Notes
The authors declare no competing financial interest.
C
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Letter
Int. Ed. 2014, 53, 1669. (c) Min, Q.-Q.; Yin, Z.; Feng, Z.; Guo, W.-H.;
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ACKNOWLEDGMENTS
■
We are grateful for financial support from NSFC (No.
21172102), the Ministry of Science and Technology of China
(2013CB834804), and the Fundamental Research Funds for the
Central Universities (lzujbky-2013-47). We also thank Prof.
Long-Min Wu (State Key Laboratory of Applied Organic
Chemistry, Lanzhou University, P. R. China) for helpful
discussions in ESR experiments.
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