Iron-Catalyzed Cyclization of Ketoxime Carboxylates and Tertiary Anilines for the Synthesis of Pyridines

Article abstract of DOI:10.1021/acs.orglett.6b00326

A novel and efficient iron-catalyzed cyclization of ketoxime carboxylates and N,N-dialkylanilines for the modular synthesis of diverse pyridines was developed. The reaction was initiated by Fe-catalyzed N-O bond cleavage of ketoxime carboxylates in the presence of tertiary anilines. The methylene carbon on N,N-dialkylanilines functioned as a source of one-carbon synthon in the reaction. The reaction used readily available starting materials, tolerated various functional groups, and afforded 2,4-disubstituted and 2,4,6-trisubstituted pyridines in good to high yields under mild conditions.

Full text of DOI:10.1021/acs.orglett.6b00326

Letter
pubs.acs.org/OrgLett
Iron-Catalyzed Cyclization of Ketoxime Carboxylates and Tertiary
Anilines for the Synthesis of Pyridines
Mi-Na Zhao, Zhi-Hui Ren, Le Yu, Yao-Yu Wang, and Zheng-Hui Guan*
Key Laboratory of Synthetic and Natural Functional Molecule Chemistry of Ministry of Education, Department of Chemistry &
Materials Science, Northwest University, Xi’an 710127, P. R. China
S
* Supporting Information
ABSTRACT: A novel and efficient iron-catalyzed cyclization of
ketoxime carboxylates and N,N-dialkylanilines for the modular
synthesis of diverse pyridines was developed. The reaction was
initiated by Fe-catalyzed N−O bond cleavage of ketoxime carboxylates
in the presence of tertiary anilines. The methylene carbon on N,N-
dialkylanilines functioned as a source of one-carbon synthon in the
reaction. The reaction used readily available starting materials, tolerated
various functional groups, and afforded 2,4-disubstituted and 2,4,6-
trisubstituted pyridines in good to high yields under mild conditions.
yridines represent an important class of heterocycles and are
ubiquitous scaffolds in medicinal chemistry, natural
Scheme 1. Transition-Metal-Catalyzed Synthesis of Pyridines
from Ketoxime Carboxylates
P
products, functional materials, and agrochemistry.1 Conse-
quently, much effort has been devoted to the development of
synthetic methods to access these useful pyridines.2,3 In the past
decades, diverse formal [3 + 3]-,4 [4 + 2]-,5 [2 + 2 + 2]-
cycloadditions6 and cycloisomerizations7 toward the pyridine
core have been developed. Many valuable substituted pyridines
were synthesized by these methods. However, versatile aryl-
substituted pyridines are still one of the most challenging
compounds. Since the importance of aryl-substituted pyridines
in organic synthesis has been established, a concise and flexible
method for the synthesis of aryl-substituted pyridines is highly
desirable.
Ketoximes and derivatives are readily available chemicals.
However, the chemistry of ketoximes was less developed in
traditional organic synthesis.8 In the past decade, transition-
metal-catalyzed transformations of ketoximes have been paid
extensive close attention. Narasaka−Heck amination with
ketoximes has been developed by Narasaka and co-workers.9
Pd(0)-catalyzed cyclization of ketoxime carboxylates has also
been developed by the groups of Abell,10 Zhu,11 Hartwig,12 and
Bower.13 Cu-catalyzed coupling reactions of ketoxime carbox-
ylates has been developed by our14 and other groups.15 In 2011,
we developed the first Cu-catalyzed condensation of ketoxime
acetates and aldehydes for the synthesis of symmetrical pyridines
(Scheme 1, eq 1).14a Subsequently, Cu/iminium-co-catalyzed
condensation of ketoxime acetates and α,β-unsaturated
aldehydes for the synthesis of 2,4-di- or 2,3,4-trisubstituted
pyridines was developed by Yoshikai and co-workers (Scheme 1,
eq 2).16 Despite these advances, the development of novel mode
for transition-metal-catalyzed transformation of ketoximes for
the synthesis of useful structures is still highly desirable.17
Recently, Fe-catalyzed oxidative methylenation of 1,3-dicarbonyl
compounds using N,N-dimethylaniline as methylene donor has
been developed by Li and co-workers.18b,c Fe-catalyzed acylation
of indoles using secondary anilines as the carbonyl source has
been developed by Su and Wu.18d We hypothesized that
environmentally friendly and abundant iron salts may prompt the
N−O bond cleavage of the ketoxime carboxylate, thus implying
that Fe-catalyzed coupling of ketoximes and the α-C(sp3)−H
bond of tertiary amines may occur. Interestingly, the Fe-catalyzed
reaction of ketoxime acetates and tertiary anilines directly gave
aryl-substituted pyridines. In this paper, we describe the
development of an unprecedented Fe-catalyzed cyclization of
ketoxime carboxylates and tertiary anilines for the synthesis of
diverse pyridines (Scheme 1, eq 3). The methylene carbon on
N,N-dialkylanilines functioned as a source of one-carbon
synthon in the reaction.
We began our study with the Fe-catalyzed reaction of
acetophenone oxime acetate 1a and N,N-dimethylaniline 2a.
Gratifyingly, when FeCl2 was used as the catalyst, 2,4-
diphenylpyridine 3aa was observed in 18% yield in toluene at
Received: February 1, 2016
© XXXX American Chemical Society
A
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Letter
120 °C (Table 1, entry 1). The 2,4-diphenylpyridine product
dimethylaniline 2b affords the desired pyridine 3aa in 74% yield,
whereas electron-withdrawing (−Br, −CN) groups on aniline
decreased the yield of 3aa. These results clearly indicate that the
choice of amines is crucial for the reaction. Subsequently,
different acetophenone oxime carboxylates were investigated to
test their reactivity. These results indicate that the carboxylate
group at the ketoximes has little effect on the cyclization reaction
(see Scheme S2).
should come from the cyclization of two molecules of
a
Table 1. Optimization of the Reaction Conditions
With the optimized reaction conditions in hand, the scope of
the reaction was investigated (Scheme 2). This novel Fe-
entry
[Fe]
FeCl2
oxidant
O2
solvent
toluene
temp (°C) yield (%)
1
120
120
120
120
120
120
120
120
120
120
120
120
120
100
140
18
22
16
0
2
FeCl2
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
1,4-dioxane
CH3CN
DCE
Scheme 2. Fe-Catalyzed Cyclization of Ketoxime Acetates and
a
b
3
FeCl2
TBHP
N,N-Dimethylaniline
c
4
FeCl2
H2O2
5
FeCl2
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
DTBP
52
60
26
20
0
6
FeCl3
7
Fe(OTf)2
Fe(OTf)3
8
9
10
11
12
13
14
15
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
FeCl3
16
74
d
86(68)
DCE
37
62
80
DTBP
DTBP
DCE
DCE
a
Reaction conditions: 1a (0.4 mmol), 2a (0.4 mmol), [Fe] (10 mol
%), oxidant (0.6 mmol), solvent (3 mL), 5 h, in air; isolated yields.
b
c
d
TBHP (5−6 M in decane). 30 wt % solution in water. Reaction was
performed under argon.
acetophenone oxime acetate 1a and a methyl group on N,N-
dimethylaniline 2a. The reaction indicated that N−O bond
cleavage of the ketoxime acetates occurred in the presence of Fe
catalyst and tertiary anilines. This interesting primary result
prompted us to optimize the reaction conditions to develop an
Fe-catalyzed reaction of ketoxime acetates and tertiary anilines
for the synthesis of substituted pyridines.
Various oxidants such as O2, tert-butyl hydroperoxide
(TBHP), H2O2, and di-tert-butyl peroxide (DTBP) were then
screened to optimize the reaction (Table 1, entries 2−5). DTBP
was found to be the most effective oxidant, providing 3aa in 52%
yield. Further experiments showed that FeCl3 was superior to
FeCl2 (Table 1, entry 6). Other iron catalyst precursors such as
Fe(OTf)2 and Fe(OTf)3 were less reactive in the reaction (Table
1, entries 7 and 8). Notably, no reaction occurred in the absence
of the iron catalyst or in the presence of copper catalyst,19
indicating that the iron catalyst is essential for the reaction (Table
1, entry 9). Next, various solvents such as 1,4-dioxane, CH3CN,
and DCE were screened (Table 1, entries 10−12). DCE was
found to be the optimal solvent for the reaction, affording
pyridine 3aa in 86% yield (Table 1, entry 12). Notably, the 2,4-
diphenylpyridine 3aa was obtained in 68% yield when the
reaction was performed under argon, and the 2,4-diphenylpyr-
idine 3aa was observed in 37% yield in absence of the DTBP
under air (Table 1, entry 13). These results indicate that oxygen
combined with DTBP acted as the oxidant in the reaction.
Moreover, the reaction temperature was also varied. The
optimum temperature for the reaction was found to be 120 °C
(Table 1, entries 14 and 15).
a
Reaction conditions: 1 (0.4 mmol), 2a (0.4 mmol), FeCl3 (10 mol
%), (t-BuO)2 (0.6 mmol, 1.5 equiv), DCE (3 mL), in air; isolated
yields. CH3CN was used as solvent.
b
catalyzed cyclization reaction exhibited good functional-group
tolerance and proved to be a general method for the facile
construction of 2,4-disubstituted pyridines. Ketoxime acetates
with electron-neutral or electron-donating groups on aryl rings
such as methyl, alkyl, and methoxyl afforded the corresponding
2,4-diarylpyridines 3ba−da,ga−ha in good yields (61−80%).20
Ketoxime acetates with electron-withdrawing substituents, such
as fluoro, chloro, and bromo, were well tolerated and afforded the
corresponding 2,4-diarylpyridines 3ia−ja,la in 70−74% yields.
Moreover, trifluoromethyl-substituted ketoxime acetate pro-
ceeded smoothly to afford the desired 2,4-di(4-
(trifluoromethyl)phenyl)pyridine 3ma in 68% yield in
CH3CN.21 These results indicated that the electronic nature of
To evaluate the potential of amines to function as a source of
one-carbon synthon, various tertiary and secondary amines were
investigated under the standard conditions (see Scheme S1). The
reaction of acetophenone oxime acetate 1a with 4-methyl-N,N-
B
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Letter
the ketoxime acetates has little effect on the reaction. However,
the steric effect plays a role in the reaction. o-Methyl- or chloro-
substituted ketoxime acetates afforded the desired 2,4-diary-
lpyridines 3ea−fa,ka in moderate yields. In addition, β-naphthyl
methyl ketoxime acetate 1n, furanyl methyl ketoxime acetate 1o,
and thiophene-2-yl methyl ketoxime acetate 1p also reacted well
to give the desired 2,4-disubstituted pyridines 3na−pa in 60−
66% yields, respectively. However, no reaction occurred when an
aliphatic ketoxime acetate such as 3,3-dimethylbutan-2-one
oxime acetate was employed as the substrate.
Scheme 4. Tentative Mechanisms for Fe-Catalyzed
Cyclization of Ketoxime Acetates and Tertiary Anilines
On the basis of the aforementioned results, it was expected
that 2,4,6-trisubstituted pyridines could be synthesized by using
alternative N,N-dialkylanilines. Expectedly, 6-methyl-2,4-diphe-
nylpyridine 3ai was obtained in 62% yield in the presence of N,N-
diethylaniline 2i (Scheme 3, eq 1). 2,4,6-Triphenylpyridine 3aj
Scheme 3. Construction of 2,4,6-Trisubstituted Pyridine by
Using Alternative N,N-Dialkylanilines
In the above reaction, the 2,4-diarylpyridines with identical
aryls were synthesized. To extend the flexibility of the reaction,
Fe-catalyzed cycloaddition of ketoxime acetate 1a and N,N-
dicinnamylaniline 2l was conducted under the above conditions.
Delightfully, the desired 2,4-diphenylpyridine 3aa was obtained
in 66% yield. Inspired by this result, we synthesized a series of
2,4-diarylpyridines with different aryls by this method using
different ketoxime acetates (Scheme 5). Notably, the procedure
Scheme 5. Fe-Catalyzed Cyclization of Ketoxime Acetates and
a
Dicinnamylaniline
was also obtained in 71% yield when N,N-dibenzylaniline 2j was
used as the substrate (Scheme 3, eq 2). These results also reveal
that the carbon at the 6-position of pyridine product 3 definitely
comes from the methylene carbon on the N,N-dialkylaniline 2.
To study the byproduct in the dealkylation of N,N-
dialkylanilines, the cyclization of acetophenone oxime acetate
1a and N-phenyl-1,2,3,4-tetrahydroisoquinoline 2k was per-
formed under the standard reaction conditions (Scheme 3, eq 3).
In this reaction, N-(2-(4,6-diphenylpyridin-2-yl)phenethyl)-
aniline 3ak was obtained in 45% yield, clearly indicating that
the dealkylation of N,N-dialkylanilines produced N-alkylaniline
species in the reaction.
On the basis of the above results,22 a tentative mechanism for
the Fe-catalyzed cyclization is proposed, as shown in Scheme 4.
The first step of the reaction is the reduction of Fe3+ by N,N-
dimethylaniline 2a to afford Fe2+ species and iminium ion
intermediate 2a′.23 Then, reductive cleavage of the N−O bond of
ketoxime acetate 1 by Fe2+ species via a two-step single electron-
transfer process forms the imine anion intermediate A and Fe3+
species. Rapid coordination of intermediate A and Fe3+ species
gives the imino−Fe3+ intermediate B. Next, nucleophilic addition
of B to iminium ion intermediate 2a′ produces the enamine
intermediate C. Addition of enamine intermediate C to a second
molecular of ketoxime acetate 1 gives the intermediate D
followed by the release of NH2OAc to afford the intermediate
E.14 Tautomerization of E gives intermediate F, and intra-
molecular deamination cyclization of the intermediate F assisted
by FeCl3 forms the intermediate G. Finally, oxidative
aromatization of intermediate G produces the 2,4-diarylpyridine
3.
a
Reaction conditions: 1 (0.2 mmol), 2l (0.24 mmol), FeCl3 (10 mol
%), (t-BuO)2 (0.6 mmol), DCE (3 mL), in air; isolated yields.
shows a good method for the synthesis of 2,4-diarylpyridines
with different aryls. Ketoxime acetates with methyl, alkyl, fluoro,
chloro, and bromo on the aromatic ring gave the corresponding
2,4-diarylpyridines 3bl−ll in 60−72% yields, indicating that the
reaction was insensitive to the electronic nature of the ketoxime
acetates. β-Naphthyl-substituted ketoxime acetate 1n proceeded
smoothly as well to give the corresponding 2-(naphthalen-2-yl)-
4-phenylpyridine 3nl in 56% yield. In addition, a heterocyclic
ketoxime acetate such as 1p was also tolerated and afforded the
desired 4-phenyl-2-(thiophene-2-yl)pyridine 3pl in 69% yield.
C
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The mechanism proposed for the reaction is illustrated in
Scheme 6. Nucleophilic addition of intermediate B to the
Scheme 6. Proposed Mechanism for the Synthesis of
Pyridines with Different Aryls
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iminium ion 2l′ affords intermediate H. The intramolecular
cyclization/deamination of intermediate H gives the intermedi-
ate I. Finally, oxidative aromatization of I produces the 2,4-
diarylpyridine 3.
In conclusion, we have developed an unprecedented Fe-
catalyzed cyclization of ketoxime carboxylates and N,N-
dialkylanilines for the synthesis of 2,4-disubstituted and 2,4,6-
trisubstituted pyridines. It is a novel reaction for Fe-catalyzed N−
O bond cleavage of the ketoxime carboxylates in the presence of
tertiary anilines. The methylene carbon on N,N-dialkylanilines
functioned as a source of one-carbon synthon in the reaction.
The catalytic system was allowed to extend to cyclization of
ketoxime carboxylates and N,N-dicinnamylaniline for the
synthesis of 2,4-diarylpyridines with different aryls. These
methods show good procedures for rapid elaboration of the
readily available ketoxime carboxylates into diverse valuable
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ASSOCIATED CONTENT
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.6b00326.
■
S
Detailed experimental procedures, characterization data,
and copies of NMR spectra for all products (PDF)
(16) Wei, Y.; Yoshikai, N. J. Am. Chem. Soc. 2013, 135, 3756.
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M. M.; Witek, J. A.; Weinreb, S. M. Tetrahedron Lett. 2010, 51, 3555.
(c) Nakanishi, S.; Hirano, T.; Otsuji, Y.; Itoh, K. Chem. Lett. 1987, 16,
2167.
AUTHOR INFORMATION
Corresponding Author
■
*E-mail: guanzhh@nwu.edu.cn.
(18) (a) Ouyang, K.; Hao, W.; Zhang, W.-X.; Xi, Z. Chem. Rev. 2015,
115, 12045. (b) Li, H.; He, Z.; Guo, X.; Li, W.; Zhao, X.; Li, Z. Org. Lett.
2009, 11, 4176. (c) Liu, W.; Liu, J.; Ogawa, D.; Nishihara, Y.; Guo, X.; Li,
Z. Org. Lett. 2011, 13, 6272. (d) Wu, W.; Su, W. J. Am. Chem. Soc. 2011,
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(19) No reaction occurred when a variety of copper catalysts such as
CuI, CuBr, CuCl, CuBr2, CuCl2, and Cu(OAc)2 were screened in the
reaction.
(20) The byproduct of the reaction was the corresponding ketone.
(21) All of the reactions listed in Scheme 2 were performed both in
DCE and CH3CN. DCE gave better yields in most cases, but CH3CN
gave better result for 3ea and 3ma.
(22) Fe-catalyzed three-component cyclization of ketoxime acetate,
ketone (or N-Ts-protected ketoimine), and N,N-dimethylaniline under
the standard conditions was not successful; see the Supporting
Information. This result indicated that the ketone is not an intermediate
in the reaction.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by generous grants from the National
Natural Science Foundation of China (21272183 and 21472147)
and the Fund of Raising Stars of Shanxi Province (2012KJXX-
26).
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