Palladium-catalyzed direct dehydrogenative annulation of ferrocenecarboxamides with alkynes in air

Article abstract of DOI:10.1021/om5002606

A novel method to synthesize racemic ferrocene[1,2-c]pyridine-3(4H)-ones via Pd-catalyzed direct dehydrogenative annulations of ferrocenecarboxamides with internal alkynes in air has been developed. Both alkyl and aryl ferrocenecarboxamides can be applied as effective substrates.

Full text of DOI:10.1021/om5002606

Communication
pubs.acs.org/Organometallics
Palladium-Catalyzed Direct Dehydrogenative Annulation of
Ferrocenecarboxamides with Alkynes in Air
Wucheng Xie,† Bin Li,† Shansheng Xu,† Haibin Song,† and Baiquan Wang*,†,‡
†State Key Laboratory of Elemento-Organic Chemistry, Collaborative Innovation Center of Chemical Science and Engineering
(Tianjin), College of Chemistry, Nankai University, Tianjin 300071, People’s Republic of China
‡State Key Laboratory of Organometallic Chemistry, Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences,
Shanghai 200032, People’s Republic of China
S
* Supporting Information
ABSTRACT: A novel method to synthesize racemic ferrocene[1,2-c]pyridine-
3(4H)-ones via Pd-catalyzed direct dehydrogenative annulations of ferrocene-
carboxamides with internal alkynes in air has been developed. Both alkyl and
aryl ferrocenecarboxamides can be applied as effective substrates.
rganometallic complexes have been proven to be
Oexcellent candidates as antitumoral agents. Among
them, ferrocene and its derivatives have aroused great interest
because of their promising bioactivities.1 Frequently, when a
ferrocene fragment is attached to or replaces a benzene ring of
biologically active molecules, their biological activities are
enhanced significantly (Figure 1).2 Recently, incorporating a
ferrocene moiety into purely organic drugs has become a
pioneering strategy in the enhancement of therapeutic
effectiveness.3
rhodium-,6,7 and ruthenium-catalyzed8,9 oxidative couplings of
various aromatic substrates with alkynes have been extensively
investigated, leading to diverse cyclic compounds (Scheme 1,
eq 1).
Scheme 1. Transition-Metal-Catalyzed Oxidative
Heterocycle Synthesis
To our knowledge, although a few examples of catalytic
ferrocene C−H bond functionalization have been reported,10
no catalytic oxidative annulation reaction with alkynes to
construct ferrocenyl heterocycles has yet been reported. Herein,
we report the first palladium-catalyzed oxidative annulations of
ferrocenecarboxamides with alkynes via N−H and C−H bond
activation (Scheme 1, eq 2).
Figure 1. Representative bioactive ferrocenyl analogues.
To optimize the reaction condition, N-phenyl ferrocenecar-
boxamide (1a) was chosen as the model substrate. As shown in
Table 1, by treatment of 1a (0.2 mmol) with diphenylacetylene
(2a) (0.4 mmol) in the presence of catalytic amounts of
Pd(OAc)2 (0.02 mmol) and Cu(OAc)2·H2O (0.1 mmol),
tetrabutylammonium bromide (0.2 mmol) additives, and
NaHCO3 (0.8 mmol) in toluene (1.0 mL) at 90 °C, open to
Heterocyclic cores are essential structures and widely exist in
a large proportion of drugs. Thus, it is highly desirable to
develop efficient catalytic methods to synthesize ferrocenyl-
fused analogues of heterocycles from simple and widely
available starting materials.
During the past decade, transition-metal-catalyzed C−H
functionalization has emerged as a powerful tool to build
diverse complex molecules, including various heterocyclic
compounds in more efficient ways in comparison with
conventional synthetic methods.4 In particular, the palladium-,5
Received: March 11, 2014
Published: April 22, 2014
© 2014 American Chemical Society
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Communication
a
a
Table 1. Optimization of Reaction Conditions
Table 2. Substrate Scope of Ferrocenecarboxamides
b
entry 1a:2a
solvent
base
temp (°C) yield (%)
1
2
3
4
5
6
7
8
1:2
1:1.1
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
1:2
toluene
toluene
t-AmOH
dioxane
DMF
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
none
90
90
90
90
90
90
90
90
90
90
70
110
90
90
73
68
48
68
trace
44
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
toluene
K2CO3
67
HCO2Na·2H2O
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
NaHCO3
72
c
9
trace
trace
61
d
10
11
12
13
14
45
e
f
36
a
58
Conditions: 1a (0.2 mmol), 2a (0.4 mmol), Pd(OAc)2 (0.02 mmol),
a
Conditions: 1a (0.2 mmol), 2a (0.22−0.4 mmol), Pd(OAc)2 (0.02
mmol), TBAB (0.2 mmol), Cu(OAc)2·H2O (0.1 mmol), base (4
TBAB (0.2 mmol), Cu(OAc)2·H2O (0.1 mmol), NaHCO3 (4 equiv),
toluene (1.0 mL), 90 °C, 12 h, in air. Isolated yields are shown. 100
°C, 24 h.
b
b
c
equiv), solvent (1.0 mL), heating in an open tube. Isolated yield. No
d
e
Cu(OAc)2·H2O. No TBAB. 2 equiv of Cu(OAc)2·H2O under Ar.
f
24 h.
(3ia, 3ka) has no detrimental effect on this reaction, giving a
mixture of products due to axial chirality. However, mesityl
ferrocenecarboxamide (3qa) gave no product. Alkyl ferrocene-
carboxamides (3oa, 3pa) successfully undergo oxidative
coupling with alkynes, although showing low yields (24%,
20%). After the reaction time was increased to 24 h and the
reaction was carried out at 100 °C, the product could be
isolated in moderate yield (42, 40%). The molecular structure
of 3pa was confirmed by single-crystal X-ray diffraction analysis
(Figure 2). Other aromatic amines were also tested under these
conditions; α-naphthyl amide afforded 55% yield.
air for 12 h, the desired annulation product 4,5,6-
triphenylferrocene[1,2-c]pyridine-3(4H)-one (3aa) was ob-
tained in 73% yield (entry 1, Table 1). The structure of 3aa
was confirmed by its H and 13C NMR spectra and mass
1
spectrometry data. The yield of 3aa decreased slightly to 68%
with 1.1 equiv of 2a. Other solvents such as dioxane, t-AmOH,
and DMF were less effective (entries 3−5, Table 1). When no
base or other bases such as K2CO3 and HCOONa·2H2O were
used instead of NaHCO3, the yield decreased (entries 6−8).
Cu(OAc)2·H2O and TBAB were essential to the reaction.
When the reaction was carried out in the absence of Cu(OAc)2·
H2O or TBAB, no annulation product was detected (entries 9
and 10). Notably, when the reaction was carried out under an
Ar atmosphere using 2.0 equiv of Cu(OAc)2·H2O as oxidant,
the product was obtained in 36% yield (entry 13). The reaction
temperature exerts a tremendous influence. When the temper-
ature was increased to 110 °C or reduced to 70 °C, the yield
decreased (entries 11 and 12). When the reaction time was
extended to 24 h, the product was isolated with 58% yield,
probably because the product could be consumed by oxygen
(entry 13). Therefore, the optimal reaction conditions were
determined: Pd(OAc)2 (10 mol %), NaHCO3 (4.0 equiv),
Cu(OAc)2·H2O (0.5 equiv), TBAB (1.0 equiv), toluene, 90 °C,
12 h, in air.
With the optimal reaction conditions in hand, various
substituted ferrocenecarboxamides (1a−r) were treated with
diphenylacetylene (2a) and produced the corresponding
ferrocene[1,2-c]pyridone derivatives (Table 2). In the present
catalytic reaction, halogen-substituted 4-fluoro-, 4-chloro-, and
4-bromoanilines 1b−d could also be tolerated, affording the
annulation products 3ba−3da in good yields (52−65%).
Amides bearing electron-donating or electron-withdrawing
groups at the N-phenyl ring, aniline substituted with −CF3
(3na), -OMe (3ga), and −COOEt (3ha), reacted to give good
yields. Moreover, the effect of steric hindrance was investigated.
Introduction of an ortho substituent into the N-phenyl ring
Figure 2. Molecular structures of 3pa and 3ah-1.
In addition to 2a, other symmetrical alkynes (2b−g) were
also tested for the present reaction (Table 3). Methyl- (2e),
methoxy- (2f), fluoro- (2d), chloro- (2b), and bromo-
substituted (2c) diphenylacetylenes reacted with 1a and
afforded the corresponding ferrocene[1,2-c]pyridines 3ab−
3ag in moderate to good yields (38−78%). To give evidence
for the regioselectivity of this reaction, a few unsymmetrical
alkynes were employed. 1-Phenyl-1-propyne (2h) and ethyl 3-
phenylpropiolate (2i) gave the mixed products 3ah (60%, 3:1)
and 3ai (50%, >8:1) in moderate to good yields. Attempts to
react terminal alkynes or dialkyl alkynes were not successful,
with no expected product being isolated.
The electrochemical behavior of the products 3 was studied
through cyclic voltammetry (CV). The CV curves exhibit the
regular wave corresponding to the reversible ferrocene/
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a
ferrocenecarboxamides can be applied as effective substrates.
A wide range of ferrocene[1,2-c]pyridines were successfully
constructed with moderate to good yields. This method may
contribute to the design and synthesis of new bioactive
molecules containing ferrocenyl moieties. Further investiga-
tions into a detailed reaction mechanism and the application of
this method in the synthesis of other ferrocene heterocycles are
in progress.
Table 3. Substrate Scope of Alkynes
ASSOCIATED CONTENT
* Supporting Information
■
S
Text, figures, a table, and CIF files giving detailed experimental
procedures, characterization data for all new compounds, and
X-ray structures of 3pa and 3ah-1. This material is available free
of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for B. Wang: bqwang@nankai.edu.cn.
■
a
Conditions: 1a (0.2 mmol), 2a (0.4 mmol), Pd(OAc)2 (0.02 mmol),
TBAB (0.2 mmol), Cu(OAc)2·H2O (0.1 mmol), NaHCO3 (4 equiv),
toluene (1.0 mL), 90 °C, 12 h, in air. Isolated yields are shown.
Notes
The authors declare no competing financial interest.
b
c
Determined by X-ray diffraction analysis and NOESY. Determined
by NOESY.
ACKNOWLEDGMENTS
■
We thank the National Natural Science Foundation of China
(Nos. 21372122, 21174068, and 21121002) and the Research
Fund for the Doctoral Program of Higher Education of China
(No. 20110031110009) for financial support.
ferrocenium (Fc/Fc+) redox couple (see the Supporting
Information). In comparison with the value for the simple
unsubstituted ferrocene, E1/2 values for 3 are shifted toward
negative potentials, slightly modified by different substituents.
As previously reported,5j activation of the C−H bond
proceeded first, and the ferrocene[1,2-c]pyridine-3(4H)-one
products were generated after the carbometalation and
reductive elimination processes. In our case, after Pd(OAc)2
was subjected to the reaction system, activation of the N−H
bond and the adjacent C−H bond to the amide via directing
group participation with the loss of two molecules of HOAc
gives the five-membered palladacycle A, which can readily
undergo the regioselective insertion of an alkyne into the Pd−C
bond of intermediate A to give the seven-membered pallada-
cycle B. After the reductive elimination the desired product was
obtained, the Pd(0) can be oxidized by Cu(II) to regenerate a
Pd(II) species for the next catalytic cycle, and the Cu(I) could
be oxidized to Cu(II) in air (Figure 3).
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