Redox-Divergent Synthesis of Fluoroalkylated Pyridines and 2-Pyridones through Cu-Catalyzed N?O Cleavage of Oxime Acetates

Article abstract of DOI:10.1002/anie.201802311

Cu-catalyzed redox-divergent [3+3] coupling of oxime esters with β-CF₃ enones and acrylates is described. This redox-neutral coupling with enones and acrylates affords trifluoromethylated pyridines and pyridones, respectively. Under reductive c

Full text of DOI:10.1002/anie.201802311

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Accepted Article
Title: Redox-Divergent Synthesis of Fluoroalkylated Pyridines and 2-
Pyridones via Cu-Catalyzed N-O Cleavage of Oxime Acetates
Authors: Xingwei Li, Dachang Bai, Xueli Wang, and Guangfan Zheng
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201802311
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Redox-Divergent Synthesis of Fluoroalkylated Pyridines and 2-
Pyridones via Cu-Catalyzed N-O Cleavage of Oxime Acetates
Dachang Bai,[a] Xueli Wang,[b] Guangfan Zheng,[b] Xingwei Li[a,b]
*
Abstract: Cu-catalyzed redox-divergent [3+3] couplings of oxime
esters with β-CF3 enones and acrylates are described. The redox-
neutral coupling with such enones and acrylates afforded
trifluoromethylated pyridines and pyridones, respectively. Under
reductive conditions, difluoromethylated pyridines, difluoromethylated
pyridones, and trifluoromethylated dihydropyridones were selectively
obtained. The reactions occurred under mild conditions with broad
substrate scope and regio/redox-selectivity.
We commenced our investigation with optimization studies on
the coupling of oxime acetate 1a and -CF3 enone 2a[11] (eq 1 and
Table S1). It was found that 4-trifluoromethylpyridine 3a was
isolated in 96% yield when simply catalyzed by CuCl in DMSO at
60 oC (Conditions A). To our surprise, the CF2H-analogue 4a was
obtained in 68% yield when catalyzed by CuBr/PPh3 in the
presence of tBuOK with iPrOH as a reducing agent (Conditions B).
Organofluorine compounds are of vital significance in
pharmaceutical, agrochemical, and material studies, as
introduction of F atoms into organics has a profound impact on
their metabolic stability, solubility, and lipophilicity.[1] In particular,
the CF2H group serves as a bioisostere of hydroxyl and thiol
With the optimized conditions in hand, we next investigated
the scope of the coupling systems (Scheme 1). The conditions A
turned out to be broadly applicable for enones bearing diverse
electron-donating and -withdrawing groups at the para position of
the benzene ring (3a-g, 45-97%). The reaction also worked well
for meta- and ortho-tolyl enones (3h, 3i) and for various other
(hetero)aryl enones (3j-m). Replacement of the CF3 group in the
enone by C2F5 also led to smooth coupling, albeit at elevated
temperature (3n). In several cases, CuBr exhibited higher activity
than CuCl (3c,f). The scope of the oxime ester was next examined
in the coupling with a p-tolyl-substituted enone. A variety of
electronically and sterically different acetophenone oximes
reacted smoothly (3o−3aa), including those bearing an -
substituent (3v-3aa). This protocol is also applicable to the late-
stage functionalization of a natural product. Besides enones, -
groups and
a lipophilic hydrogen bond donor. Selective
introduction of CF2H into aromatics can also remarkably improve
these properties, which has received considerable attention.[2]
However, compared to the well-established trifluoromethylation
systems, methods of difluoromethylation remain limited.[3] On the
other hand, pyridines and pyridones are prevalent structural
motifs in natural products and bioactive molecules.[4] The
introduction of fluoroalkyls into these heterocycles can modulate
their basicity and binding properties.[1-2,5] Consequently,
fluorinated heteroarenes possess beneficial properties of both
units and are a promising family of pharmacores. For example,
Mefloquine is used to prevent or treat malaria, and Fluazinam is
a broad-spectrum fungicide.[1,6]
Recently, tremendous effort has been made in metal-
catalyzed synthesis of pyridines.[7] The strategy of using readily
available oximes to construct heterocycles is appealing.[8] Recent
advancements in Cu-catalyzed [3+3]/[3+2] annulation of oxime
esters with activated -bonds, which proceeds via a nucleophilic
Cu(II)-enamide, provided new routes for heterocycle synthesis, as
has been reported by Guan, Yoshikai, Jiang, Wei, and others.[9,10]
Recently, Yoshikai developed Cu-catalyzed synthesis of
alkyl/aryl-substituted pyridines starting from oxime esters and α,β-
unsaturated aldehydes/ketimines.[9c,i] However, methods for
synthesis of fluoroalkylated pyridines/pyridones have been rather
limited. In addition, all these systems are limited to redox-neutral
or oxidative conditions.[8f] Reductive coupling of oximes remains
underexplored, which substantially limited the accessible patterns
of heterocycle products. We now report redox-divergent synthesis
of five classes of heterocycles via Cu-catalyzed [3+3] annulation
of oxime acetates with β-CF3-substituted enones/acrylates.
CF3 acrylonitrile also coupled smoothly to afford
a 2-
aminopyridine (3ac) in good yield.
[a]
[b]
Dr. D.-C. Bai, Prof. Dr. X. Li
School of Chemistry and Chemical Engineering, Henan Normal
University, Xinxiang 453007, China
X. Wang, Dr. G. Zheng, Prof. Dr. X. Li
Dalian Institute of Chemical Physics, Chinese Academy of
Sciences, Dalian 116023, China, E-mail: xwli@dicp.ac.cn
Scheme 1. Pyridine synthesis via condensation of oxime esters with β-
fluoroalkylated enones (Conditions A). See Supporting Information for details.
Supporting information for this article is given via a link at the end
of the document.
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benzene ring (6a-6j, 47-93%). Introduction of an alkyl group to
the -position of the oxime, surprisingly, shifted the selectivity to
the reduction product (dihydropyridones 7b, 7c). It is likely that
the steric effect of these alkyl groups inhibits oxidative
aromatization. To highlight synthesis of dihydropyridones, the
conditions D were adopted. It was found that a truly large scope
of oxime esters coupled with 5a in good to excellent yield, with full
compatibility with EDGs and EWGs (7a-7r, 7v) and tolerance of
steric and electronic effects of the oxime ester substrate. The
reaction was equally efficient for alkenyl-substituted oxime esters
(7s, 7t). Introduction of a phenyl group to the -position of oxime
is also tolerated, albeit with lower yield (7u).
We next investigated the scope of reductive synthesis of 4-
difluoromethylpyridines (Scheme 2). CF3-substituted enones
bearing various electron-donating and -withdrawing groups at the
para position were fully tolerated (4a-4g, 32-68%). The reaction
also tolerated enones bearing o-Me, m-Me, 2-Naph, and
dimethoxy substituents (4h-4k). The coupling of 2-furyl and
ferrocenyl-substituted enones also proceeded smoothly (4l and
4m). Regarding the scope of oxime esters, introduction of EDGs
and EWGs to different positions of the oxime ester is tolerated
(4n-4s). Incorporation of a Me or Ph group to the -position of the
acetophenone oxime also allowed synthesis of tetra-substituted
pyridines (4t, 4u). In contrast to viability of β-C2F5 enone and the
oxime ester derived from 3-pentanone and in Scheme 1 (3n and
3x), no reaction occurred under the current conditions.
Scheme 2. Reductive coupling of oxime esters with β-CF3 enones. Conditions
i
B: 1a (0.2 mmol), 2 (0.3 mmol), CuBr (0.04 mmol), PPh3 (0.05 mmol), PrOH
(2.0 equiv), tBuOK (4.4 eq) in THF (2.0 mL), isolated yields.
To better define the scope of the present [3+3] annulation
reaction, we next examined the annulation using a less
electrophilic β-trifluoromethylated acrylate (5a) as a coupling
partner (eq 2 and Table S2). Extensive screening of the solvent,
temperature, base, and reductant revealed that redox-neutral
annulation occurred with NaOAc as an additive to give 4-CF3
pyridine 6a in high yield (Conditions C). Application of Zn as a
reductant afforded CF3-retentive reduction product 7a (Conditions
D). In addition, 4-CF2H-substituted pyridone 8a (X-Ray analysis)
Scheme 3. CF3-retentive annulation of oxime esters with a β-CF3 acrylate
Reaction. Conditions C: oxime 1 (0.2 mmol), 5a (0.4 mmol), CuCl (0.04 mol),
NaOAc (0.40 mmol), and DMSO (2.0 mL), 100 ℃, 24 h. Conditions D: 1 (0.2
mmol), 5a (0.4 mmol), CuCl (0.02 mol), Zn (25 mol%), DMSO (3.5 mL), 80 ℃,
24 h.
[12]
was isolated as the major product when DBU was employed
as a base as well as a reductant (Conditions E).
The scope of the fifth class of product, 4-difluomethylpyridone,
was accordingly examined under conditions E with DBU (Scheme
4). In line with the scope of other coupling systems, oxime esters
derived from acetophenones and other ketones all underwent
smooth coupling with acrylate 2a to afford the desired products in
moderate to excellent yield, although oxime esters bearing an -
alkyl/aryl group tend to react in lower efficiency (8k, 8l).
Synthesis of 4-trifluoromethylated pyridones was then
explored (Scheme 3). Various oximes derived from
acetophenones exhibited good functional group tolerance,
regardless of the presence of halogens, EWGs, and EDGs in the
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dehydrofluorination of a CF3-functionalized 3,4-dihydropyridine.
We also performed H/D exchange experiment for the coupling of
1a and enone 1d with tBuOD as a deuterium source (Conditions
B). NMR analysis of the isolated product revealed that both the 3-
and 5-positions are equally deuterated (79% D), and significant
H/D exchange was also detected at the difluoromethyl position
(Scheme 6d), which agrees with intermediacy of a CF3-
dihydropyridine
(see
Scheme
7)
with
subsequent
tautomerization[14] (Note that no post-coupling H/D exchange was
observed for 4d under the standard conditions with CD3OD or
tBuOD, as confirmed by our control experiments).
Scheme 4. Dehydrofluorinative coupling of oxime esters with a -CF3 acrylate
Conditions E: 1 (0.2 mmol), 5a (0.4 mmol), CuCl (0.04 mol), DBU (0.40 mmol),
DMSO (2.0 mL), 120 ℃, isolated yields.
The synthetic utility of (dihydro)pyridones has been briefly
demonstrated (Scheme 5). Hydrogenation of dihydropyridone 7a
afforded lactam 9 as a single diastereomer in nearly quantitative
yield. In addition, Rh(III)-catalyzed oxidative [4+2] coupling
between NH pyridone 6b and diphenylacetylene afforded a fused
heterocycle (10) in excellent yield.[13] 2-Chlorination of 8a gave 11,
and the O-triflation of 8a yielded 12. The presence of Cl and OTf
group allows further manipulation.
Scheme 6. Mechanistic studies.
Scheme 5. Derivatization Reactions.
Several experiments have been performed to probe the
mechanism of these coupling systems (Scheme 6). Addition of
TEMPO to the coupling of 1a and 2a caused no inhibition toward
formation of 3a (conditions A), indicating an ionic pathway. In fact,
this conclusion is consistent with observation of product 3w
(Scheme 1), as a radical pathway would lead to ring scission.[10a]
While formation of CF2H-substituted pyridine 4a was retarded with
addition of TEMPO (conditions B), the reaction was only slightly
affected when 1,1-diphenylethylene was introduced (Scheme 6a),
so no solid conclusion on C-centered -imino radical can be
drawn. Parallel reactions were then conducted using 1a and
labeled 1a-d8 or 1a-d3 for KIE studies. Significant kinetic isotope
effects have been observed for the coupling with an enone (KIE
= 10.8, conditions A) or with an acrylate (KIE = 5.7, conditions D).
These results suggest that cleavage of the -C-H bond is involved
in the turnover-limiting step (Scheme 6b). To identify
intermediates during the formation of 4-CF2H pyridone 8a, 4-CF3-
functinalized dihydropyridone 7a was treated with DBU in DMSO,
which led to clean formation of 8a (Scheme 6c), suggesting that
it was generated via elimination of HF followed by tautomerization.
In contrast, 3a proved not to be an intermediate for the formation
of 4a since no conversion of 3a was observed (GC-MS) under
conditions B. Likewise, 4a is likely generated via
Scheme 7. Proposed Mechanism.
On the basis of our studies and reports on Cu-catalyzed coupling
of oximes, a plausible pathway is given for formation of pyridines
(Scheme 7).[9c,i] Oxidation of Cu(I) by oxime ester 1 gives a
copper(II) enamide A together with a Cu(II) species. Conjugate
addition of A to enone 2a then yields an enolate species B, and
subsequent protolonysis leads to ketone C. Dehydrative
cyclization would produce a dihydropyridine intermediate D, and
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further oxidation of D by two copper(II) species furnishes CF3-
pyridine product 3 with regeneration of the copper(I) catalyst. In
presence of a suitable base and reductant, D undergoes E1cb to
afford intermediate F, which then isomerizes to the 4-CF2H-
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In summary, we have demonstrated redox-divergent access
to five classes of fluoroalkylated heterocycles via copper-
catalyzed [3+3] coupling of oxime acetates with β-
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The coupling using PrOH, Zn, and DBU as a reductant led to
selective formation of 4-CF2H pyridines, 4-CF3-dihydropyridones,
and 4-CF2H pyridones, respectively. The coupling systems cover
a particularly broad range of oxime acetates derived from aryl-
alkyl and dialkyl ketones in acyclic as well as cyclic settings. The
redox-diversity and elegant control of reaction selectivity may
provide insight for future studies of other Cu-catalyzed systems,
which are currently underway in our laboratories.
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Keywords: Copper • Pyridine • Pyridone • Oxime• Fluoroalkyl
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Entry for the Table of Contents (Please choose one layout)
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D. Bai, X. Wang, B. Liu, G. Zheng,X. Li*
Page No. – Page No.
Redox-Divergent
Synthesis
of
Fluoroalkylated Pyridines and 2-
Pyridones via Cu-Catalyzed N-O
Cleavage of Oxime Acetates
Text for Table of Contents
Redox-divergent Cu-catalyzed coupling of oxime acetates with -CF3-substituted enones/acrylates led to selective synthesis of
fluoroalkylated pyridines and (dihydro)pyridones via substrate control and redox-control.
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