An easy route to N,N-diarylhydrazines by Cu-catalyzed arylation of pyridine-2-carbaldehyde hydrazones with aryl halides

Article abstract of DOI:10.1002/ejoc.201201445

A copper-catalyzed C-N coupling reaction is described for the preparation of pyridine-2-carbaldehyde N,N-diarylhydrazones by the arylation of N-mono- or -non-substituted hydrazones with aryl bromides/iodides, and the subsequent conversion of the hydrazones into N,N-diarylhydrazines by a transimination process with an aqueous solution of H₂NNH₂. The reaction features the use of CuI as the catalyst without the need for external ligands. This methodology provides a convenient alternative to the synthesis of structurally diverse N,N-diarylhydrazines from simple, easily accessible precursors. Copyright

Full text of DOI:10.1002/ejoc.201201445

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SHORT COMMUNICATION
DOI: 10.1002/ejoc.201201445
An Easy Route to N,N-Diarylhydrazines by Cu-Catalyzed Arylation of
Pyridine-2-carbaldehyde Hydrazones with Aryl Halides
Wei Wu,[a,b] Xin-Le Li,[a,b] Xin-Heng Fan,[a] and Lian-Ming Yang*[a]
Keywords: Synthetic methods / Arylation / Hydrazines / Cross-coupling / Copper / Halides
A copper-catalyzed C–N coupling reaction is described for
the preparation of pyridine-2-carbaldehyde N,N-diaryl-
hydrazones by the arylation of N-mono- or -non-substituted
hydrazones with aryl bromides/iodides, and the subsequent
conversion of the hydrazones into N,N-diarylhydrazines by a
transimination process with an aqueous solution of H2NNH2.
The reaction features the use of CuI as the catalyst without
the need for external ligands. This methodology provides a
convenient alternative to the synthesis of structurally diverse
N,N-diarylhydrazines from simple, easily accessible precur-
sors.
Our motivation to develop a new copper-catalyzed strat-
egy for the synthesis of N,N-diarylhydrazines arose from a
chance finding: We observed that in some copper-catalyzed
cross-coupling reactions using pyridine-2-carbaldehyde N-
arylhydrazone as a ligand, the hydrazone readily underwent
an N-arylation reaction with arylating agents. This
prompted us to carry out a detailed investigation into the
copper-catalyzed cross-coupling reactions of hydrazone
substrates.
Introduction
N,N-Disubstituted hydrazines, particularly N,N-diarylhy-
drazines, are highly valuable molecules due to their bioac-
tivity[1] and intermediacy in the synthesis of pharmaceuti-
cally important N-arylindoles[2] (by the Fischer indole syn-
thesis[3]) and important organic photoelectrical materials.[4]
Typically, N,N-disubstituted hydrazines are prepared by the
reduction of the corresponding N-nitrosamine precursors,
which are, in turn, obtained by the treatment of secondary
anilines with HNO2.[5] The classical method suffers from
harsh reaction conditions, limited functional group toler-
ance, complicated work-up, and low yields. Over the past
decade, transition-metal-catalyzed cross-coupling reactions
of aryl halides with hydrazine-type nucleophiles have at-
tracted intense interest and become a useful alternative to
the synthesis of aryl-substituted hydrazines,[6–10] of which
successful examples related to N,N-diarylhydrazine synthe-
sis include the Pd-catalyzed cross-coupling of benzophe-
none hydrazone with aryl bromides,[8] Cu-mediated di-
arylation of N-acylhydrazines,[10a] Cu-mediated arylation of
Boc-protected hydrazines with bismuthane,[10b,10c] Cu-cata-
lyzed coupling of N-acyl-NЈ-arylhydrazines with aryl iod-
ides/bromides,[10d] and the CuI/PPAPM-promoted N-aryl-
ation of phenylhydrazine with activated aryl bromides/iod-
ides.[10e]
Results and Discussion
Pyridine-2-carbaldehyde (E)-N-phenylhydrazone (1a)
and p-bromoanisole were selected as model substrates to
determine the optimal reaction conditions for the catalytic
C–N coupling reaction (Table 1). We first surveyed the role
of copper in the reaction. As can be seen in Table 1, no
reaction occurred in the absence of copper catalysts (en-
try 2). Various Cu salts, whether mono- or divalent, were
effective to a certain degree (entries 1 and 3–6), with CuI
giving the best result with an almost quantitative conver-
sion and high isolated yield of the desired product (entry 1).
However, the use of copper metal powder substantially
slowed the reaction rate, likely due to heterogeneous cataly-
sis (entry 7). The nature of the base is also important for
this reaction. Moderately strong K3PO4 (entry 1), K2CO3
(entry 10), and Cs2CO3 (entry 11) seemed to be suitable
bases, whereas weaker bases like Na2CO3 (entry 9) or
stronger bases such as KOH (entry 12) and tBuOK (en-
try 13) were almost ineffective. Of the solvents, 1,4-dioxane
(entry 1) proved to be far superior to other solvents such as
toluene (entry 14), DMF (entry 15), acetonitrile (entry 16),
and ethanol (entry 17). Attempts to reduce the reaction
temperature led to a significantly lower yield (entry 8 vs.
[a] Beijing National Laboratory for Molecular Sciences (BNLMS),
Laboratory of New Materials, Institute of Chemistry, Chinese
Academy of Sciences,
Beijing 100190, P. R. China
Fax: +8610-62559373
E-mail: yanglm@iccas.ac.cn
Homepage: http://www.iccas.ac.cn
[b] Graduate School of the Chinese Academy of Sciences,
Beijing 100049, P. R. China
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201201445.
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W. Wu, X.-L. Li, X.-H. Fan, L.-M. Yang
SHORT COMMUNICATION
entry 1). It was also found that p-iodoanisole is more favor- Table 2. Effect of R groups at the hydrazone end on the reaction.[a]
able for the reaction (entry 18), but that p-chloroanisole was
unreactive under the conditions (entry 19).
Entry
R
Yield [%]
Table 1. Optimization of the copper-catalyzed coupling reaction of
pyridine-2-carbaldehyde N-phenylhydrazone (1a) with p-bromoani-
sole.[a]
1
2
3
4
5
6
7
8
9
2-pyridyl
phenyl
4-nitrophenyl
2-hydroxyphenyl
3-pyridyl
2-thienyl
2-furyl
4-(N,N-dimethyl)phenyl
4-(trifluromethyl)phenyl
83
0
0
0
0
0
0
0
0
Entry Variation from standard conditions
Yield [%][b]
[a] Reagents and conditions: hydrazone (1 mmol), p-bromoanisole
(1.5 mmol), CuI (0.05 mmol), K3PO4 (1.5 mmol), dioxane (5 mL),
100 °C, 12 h.
1
none
no [Cu]
83
0
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
CuBr instead of CuI
CuCl instead of CuI
CuBr2 instead of CuI
71
73
70
72
33
67
6
69
79
trace
Ͻ5
58
–
Cu(OAc)2 instead of CuI
Cu powder instead of CuI
80 °C instead of 100 °C
Na2CO3 instead of K3PO4
K2CO3 instead of K3PO4
Cs2CO3 instead of K3PO4
KOH instead of K3PO4
tBuOK instead of K3PO4
toluene instead of 1,4-dixoane
DMF instead of 1,4-dixoane
MeCN instead of 1,4-dixoane, sealed tube
EtOH instead of 1,4-dixoane, sealed tube
p-iodoanisole instead of p-bromoanisole,
6 h
2ab) and -withdrawing (2ad, 2af, 2ag, and 2ah) aryl brom-
ides gave excellent yields except in the case of 4-for-
mylphenyl bromide (2ad; the reason is unclear at present),
but bromides with steric hindrance (2ac, 2al, and 2am)
slowed the reaction, providing moderate yields with ap-
preciable amounts of the starting material 1a remaining.
Free amino and hydroxy groups in the bromide substrates
generally produced no adverse effect on the coupling reac-
tion (2aj and 2ak). Furthermore, heteroaryl bromides read-
ily coupled to 1a in high yields (2an and 2ao). Again, it
should be noted that iodides (2aa) perform better than the
corresponding bromides, but that chlorides are not suitable
substrates for the reaction (2ab).
Next, we explored the effect on the reaction of the aryl
group at the amine end of pyridine-2-carbaldehyde hydraz-
ones. Several N-aryl-substituted pyridine-2-carbaldehyde
hydrazones were prepared[12] and treated with aryl brom-
ides under the optimal conditions described above
(Table 4). The coupling reactions with electron-neutral
and -deficient aryl groups proceeded readily in most cases
(2ba–2bc and 2ca–2cd), whereas the yields of the reactions
with heteroaryl bromides were not very satisfactory but ac-
ceptable (2bd, 2be, 2cd, and 2ce). On the other hand, elec-
tron-rich aryl groups generally retarded the reaction (2da–
2de); the yields were particularly poor when the electro-
philic substrates were also electron-rich (2da) or sterically
hindered (2dc).
23
33
90
19
p-chloroanisole instead of p-bromoanisole
0
[a] Normal reagents and conditions: hydrazone (1 mmol), p-bromo-
anisole (1.5 mmol), CuI (0.05 mmol), base (1.5 mmol), solvent
(5 mL), 100 °C, 12 h. [b] Isolated yield.
This reaction does not need an external ligand, which
is unusual in homogenous copper-catalyzed C–N coupling
reactions performed under relatively mild reaction condi-
tions, particularly with aryl bromides as electrophilic cou-
pling partners.[9,10d,10e,11] We speculated that the catalytic
efficiency of copper salts is related to the presence of the 2-
pyridyl group in the hydrazone substrate. To confirm this,
a number of phenylhydrazones were prepared[12] and sub-
jected to the coupling reaction with p-bromoanisole under
the same conditions (Table 2). It can be observed from the
results in Table 2 that the electronic effects of the R group
did not affect the reaction of benzaldehyde hydrazones (en-
tries 2–4, 8, and 9) and that the reactions of phenylhydraz-
ones with a 3-pyridyl group (entry 5) or five-membered het-
erocyclic groups (entries 6 and 7) failed. Thus, only the 2-
pyridyl group demonstrates the ability to activate CuI in
this reaction.
Aqueous H2NNH2 is a readily available, convenient hy-
drazine source from which nonsubstituted pyridine-2-car-
baldehyde hydrazone (3) was readily prepared.[12] We
wanted to utilize hydrazone 3 as a starting material for the
synthesis of aryl-substituted hydrazones by a CuI-catalyzed
process similar to that described above. Initially, a 1:1 molar
ratio of bromobenzene/hydrazone 3 was used to obtain an
The scope of the electrophilic coupling partners in the N-monoarylated product. Unfortunately, hydrazone 3 failed
reaction was then examined under the optimized conditions to be arylated with bromobenzene; rather, cyclization of
(Table 3). In general, the reaction was insensitive to the elec- pyridine-2-carbaldehyde hydrazone took place to give a
tronic effects of aryl bromides but sensitive to their steric 1,2,3-triazolo heterocycle as a byproduct in a high yield of
effects. For example, both p-electron-donating (2aa and over 70%.[13] When bromobenzene was replaced with iodo-
2
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Cu-Catalyzed Synthesis of N,N-Diarylhydrazines
Table 3. CuI-catalyzed arylation of pyridine-2-carbaldehyde N-phenylhydrazone with aryl bromides.[a]
[a] Reagents and conditions: hydrazone (1 mmol), aryl bromide (1.5 mmol), CuI (0.05 mmol), K3PO4 (1.5 mmol), dioxane (5 mL), 100 °C,
12–20 h. [b] The iodide used. [c] The chloride used.
benzene, the coupling reaction occurred but afforded a ucts as well as other byproducts [Scheme 1, Eq. (1)]. Inter-
complex distribution of products. The product mixture in- estingly, diarylhydrazone 4a was the predominant product.
cluded mono- (both E and Z forms)[14] and diarylated prod- For comparison, benzophenone hydrazone 5, a precursor
Scheme 1.
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W. Wu, X.-L. Li, X.-H. Fan, L.-M. Yang
SHORT COMMUNICATION
Table 4. CuI-catalyzed coupling reactions of pyridine-2-carbaldehyde N-arylhydrazones with aryl bromines.[a]
[a] Reagents and conditions: hydrazone (1 mmol), aryl bromines (1.5 mmol), CuI (0.05 mmol), K3PO4 (1.5 mmol), dioxane (5 mL), 100 °C,
12–20 h.
in the palladium-catalyzed arylhydrazine synthesis,[6,8] was partner [Scheme 1, Eq. (2)], which indicates once again that
also tested. In this case, no cross-coupled product was de- the 2-pyridyl group of the hydrazone substrate plays a spe-
tected with either the bromide or iodide as the coupling cial role in the CuI-catalyzed N-arylation reaction.
Table 5. CuI-catalyzed diarylation of pyridine-2-carbaldehyde hydrazone (3).[a]
[a] Reagents and conditions: hydrazone (1 mmol), aryl iodide (2 mmol), CuI (0.05 mmol), K3PO4 (3.0 mmol), dioxane (5 mL), 100 °C, 6–
10 h. Isolated yields are provided. [b] 4d = 2da. [c] Aryl iodide (3 mmol); significant amounts of the monoarylated product were obtained:
22% of (E)-1e and 15% of (Z)-1e.[14]
4
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Cu-Catalyzed Synthesis of N,N-Diarylhydrazines
In view of the fact that the second arylation of pyridine- groups of substrates in metal-catalyzed coupling reactions,
2-carbaldehyde hydrazone (3) proceeds more rapidly than which can be exploited in future studies. The mechanism of
the first, we attempted to transform hydrazone 3 directly the reaction is under investigation and the results will be
into N,N-diarylhydrazones by using a 2:1 molar ratio of reported in due course.
aryl iodide/hydrazone 3 in the coupling reaction (Table 5);
Supporting Information (see footnote on the first page of this arti-
several aryl iodides were chosen as arylating agents. We
cle): Experimental details and characterization data for all com-
found that the diarylation reaction did not provide high
pounds prepared.
yields of the desired products 4a–d, mainly because there
exists a competing ring-closing side-reaction of the starting
hydrazone 3.[13] In the case of o-iodoanisole (4e), steric fac-
tors further lower the yield of the diarylated product with
Acknowledgments
appreciable amounts of the monoarylated product formed.
We then turned our attention to the cleavage of the pro-
tecting group in pyridine-2-carbaldehyde N,N-diarylhydraz-
ones to release the corresponding N,N-diarylhydrazines. For
this, acidic hydrolysis of the aldehyde/ketone hydrazones is
the most commonly employed procedure. Unfortunately, we
found that even strong acids such as hydrochloric acid or
p-toluenesulfonic acid failed to hydrolyze pyridine-2-carbal-
dehyde N,N-diarylhydrazones to release the corresponding
diarylhydrazines. However, N,N-diarylhydrazines can read-
The authors thank National Natural Science Foundation of China
(NSFC) (project number 20872142) for financial support of this
work.
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drazine (85%, 2 mL), 2-methoxyethanol (3 mL), reflux, 5–15 h.
8686.
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Conclusions
We have presented an easy route to N,N-diarylhydrazines
by the CuI-catalyzed arylation of pyridine-2-carbaldehyde
hydrazones followed by a hydrazine exchange reaction. Our
protocol offers several advantages over previously used
methods: The simple copper salt works efficiently without
the need for additional ligands, the catalyst is cheaper, lower
loadings are required, and mild bases and relatively mild
reaction temperatures are employed. It is worth noting that
the 2-pyridylmethylene moiety in the hydrazone substrates
serves not only as a “blocking group” for regioselectivity,
but also as a “promoter”, activating the copper precatalyst.
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SHORT COMMUNICATION
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[14]
The E and Z isomers of the compound can be isolated and
purified by chromatography.
Received: October 30, 2012
Published Online: ᭿
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Cu-Catalyzed Synthesis of N,N-Diarylhydrazines
Cross-Coupling
W. Wu, X.-L. Li, X.-H. Fan,
L.-M. Yang* ..................................... 1–7
An Easy Route to N,N-Diarylhydrazines by
Cu-Catalyzed Arylation of Pyridine-2-
carbaldehyde Hydrazones with Aryl Hal-
ides
A Cu-catalyzed C–N coupling reaction is
sion of the hydrazones into N,N-diaryl-
hydrazines by a transimination process
with an aqueous solution of H2NNH2. The
reaction features the use of CuI as catalyst
without external ligands.
described for the synthesis of pyridine-2-
carbaldehyde N,N-diarylhydrazones by the
arylation of hydrazones with aryl brom-
ides/iodides, and the subsequent conver
Keywords: Synthetic methods / Arylation /
Hydrazines / Cross-coupling / Copper /
Halides
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