ZnO-supported Pd nanoparticle-catalyzed ligand- and additive-free cyanation of unactivated aryl halides using K4[Fe(CN)6]

Article abstract of DOI:10.1021/jo500820q

The use of a new ZnO-supported palladium(0) nanoparticle catalyst for the cyanation of aryl halides using a relatively benign cyanide source, K ₄[Fe(CN)₆], is described. This catalyst has been applied for the efficient cyanation of a variety of functionalized aryl bromides and activated aryl chlorides. This process circumvents the need for an additive and a ligand for the reaction and offers the advantages of high product yields, low catalyst loading (0.2 mol % Pd), and recyclability of the catalyst.

Full text of DOI:10.1021/jo500820q

Note
pubs.acs.org/joc
ZnO-Supported Pd Nanoparticle-Catalyzed Ligand- and Additive-
Free Cyanation of Unactivated Aryl Halides Using K [Fe(CN) ]
4
6
Tanmay Chatterjee, Raju Dey, and Brindaban C. Ranu*
Department of Organic Chemistry, Indian Association for the Cultivation of Science, Jadavpur, Kolkata-700032, India
S Supporting Information
*
ABSTRACT: The use of a new ZnO-supported palladium(0) nanoparticle catalyst for
the cyanation of aryl halides using a relatively benign cyanide source, K [Fe(CN) ], is
4
6
described. This catalyst has been applied for the efficient cyanation of a variety of
functionalized aryl bromides and activated aryl chlorides. This process circumvents the
need for an additive and a ligand for the reaction and offers the advantages of high
product yields, low catalyst loading (0.2 mol % Pd), and recyclability of the catalyst.
romatic nitriles are of much importance, as they are
present in many natural products, pharmaceuticals,
palladacycle in DMF in the presence of K CO under
2 3
20 4b
A
microwave heating, Pd(PPh ) /DBU in t-BuOH/H O,
a
3
4
2
1
−3
8x
agrochemicals, and dyes.
Moreover, nitriles are useful
palladium complex in NMP in the presence of Na CO , under
2 3
2
1
building blocks in the synthesis of heterocycles, as they are
easily converted into a variety of functional groups such as
homogeneous conditions with Pd/C in Bu N/Na CO , and
3 2 3
Pd/C in poly(ethylene glycol) (PEG)-4000/H O in the
2
4
22
carboxylic acid, ketone, oxime, amine, amidine, etc. Thus, the
presence of NaF under heterogeneous conditions. Although
development of an efficient procedure for the synthesis of these
these methods are quite satisfactory, many of them use
expensive ligands and additives, produce metal waste, and do
not offer recyclability of the catalyst. Thus, a more efficient and
convenient protocol for this important transformation would be
appreciated.
compounds is of much interest.
5
The classical synthesis of aromatic nitriles by the Sandmeyer
6
and Rosenmund−von Braun reactions are associated with
serious drawbacks, including the use of high temperatures
(
150−250 °C) and stoichiometric amounts of copper(I)
We recently reported cyanation of aryl iodides by K [Fe-
4
23a
cyanide as the cyanating agent, which leads to equimolar
amounts of heavy metal waste. The transition-metal-catalyzed
cyanation of aryl halides has received wide attention in recent
10
(CN) ] catalyzed by hydroxyapatite-supported Cu(I)
and
6
23b
CuO-supported palladium nanoparticles (NPs).
However,
although aryl iodides underwent reactions successfully, aryl
bromides remained inert under these reaction conditions. With
our continued interest in heterogeneous supported transition-
metal-catalyzed cyanation reactions, we report here cyanation
of aryl bromides and chlorides catalyzed by ZnO-supported Pd
NPs (ZnO-Pd NPs) without using any ligand, base, or additive
7
8
9
times. Several transition metals, including Pd, Ni, and Cu,
have been involved in cyanation reactions using a variety of
metal cyanide sources that are primarily toxic, such as alkali-
14
8
e,f,10a,11
8e,16
12
13
metal cyanides,
Zn(CN)2,
CuCN, TMSCN,
1
5
KCN, and NaCN,
as well as relatively less toxic non-
1
7a
17b
metallic agents
such as acetone cyanohydrins,
alkyl
(
Scheme 1).
1
7c
17d
17e
nitriles,
phenyl cyanate,
cyanobenzimidazole,
benzyl thiocyanate,
and N-
17f
usually in the presence of several
Scheme 1. ZnO-Pd NP-Catalyzed Cyanation of Aryl
Bromides and Chlorides Using K [Fe(CN) ]
nitrogen- and phosphorus-containing ligands, which are often
hazardous and expensive. Although Pd catalysts are highly
efficient for cyanation, their use is also associated with the
formation of inactive palladium cyanide species due to the
exposure of Pd to high concentration of cyanide ions, which
4
6
18
significantly inhibits the catalytic cycle. This problem is
partially controlled with certain special measures related to the
4b
The catalyst, palladium-doped ZnO, was prepared by
microwave irradiation of a solution of zinc nitrate hexahydrate,
palladium acetate, and PEG-6000 in aqueous ethanol. Filtration
of the solid material followed by washing with water and
ethanol and drying provided the ZnO-Pd catalyst. The Pd
content was determined to be 0.1109 mmol g− by atomic
absorption spectroscopy (AAS). The catalyst was characterized
solvent and cyanide source. Significant improvement in the
cyanation reaction was achieved with introduction of a
relatively nontoxic and inexpensive K [Fe(CN) ] as a cyanide
4
6
8i
source by Beller and co-workers. Since then, a number of
procedures using this reagent in the presence of several metal
1
catalysts have been developed. They include Pd(OAc) in
2
8o
dimethylacetamide (DMAC) in the presence of Na CO ,
2
3
Pd(OAc) in combination with a phosphorus ligand in DMF in
2
8i
the presence of K PO , Pd(OAc) in the presence of NaF and
Received: April 12, 2014
3
4
2
19
TBAB in H O under microwave irradiation in a sealed tube,
a
2
©
XXXX American Chemical Society
A
dx.doi.org/10.1021/jo500820q | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
by transmission electron microscopy (TEM), scanning electron
microscopy (SEM), and powder X-ray diffraction (PXRD). The
TEM image of the catalyst (Figure 1) shows rod-shaped Pd
Table 1. Standardization of the Reaction Conditions for the
Cyanation of 4-Bromoanisole
a
ZnO-Pd NPs
(mol % Pd)
temp.
(°C)
time
(h)
yield
b
entry
solvent
(%)
1
2
3
4
5
6
7
8
9
5
DMSO
NMP
140
140
140
140
100
110
65
15
15
15
15
15
15
15
15
15
15
15
15
15
14
12
12
65
71
76
62
−
5
5
DMF
5
PEG-600
5
H O
2
5
toluene
THF
−
−
5
1
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
140
140
140
140
130
120
130
130
130
76
76
76
65
76
70
76
69
49
0.5
0.2
0.1
0.2
0.2
0.2
0.2
0.2
1
1
1
1
1
1
1
0
1
3
4
5
6
7
Figure 1. (a) TEM and (b) HR-TEM images of the ZnO-Pd catalyst.
NPs supported on ZnO. The average diameter and length of
the palladium nanorods were found to be 15 and 96 nm,
respectively. The SEM image (Figure 2) shows a rough
morphology of the catalyst. The PXRD pattern of the catalyst
c
a
Reactions were carried out using 0.17 mmol of K [Fe(CN) ], unless
otherwise noted. Yields of isolated pure products. 0.34 mmol of
4
6
(
Figure 3) reveals a hexagonal lattice for ZnO and a face-
b
c
centered cubic (fcc) lattice for Pd.
K [Fe(CN) ] was used
4
6
considerable extent (Table 1, entry 17), which implies that the
excess concentration of cyanide ion deactivates the catalyst.
Thus, in a typical reaction procedure, a mixture of aryl halide (1
mmol) and K [Fe(CN) ] (0.167 mmol) was heated in DMF at
4
6
1
30 °C in the presence of ZnO-Pd NPs (0.2 mol %) under
argon for the required period of time (TLC). Standard workup
of the reaction mixture followed by column chromatography
provided the pure product.
A wide variety of diversely substituted aryl bromides
underwent cyanation by this procedure to produce the
corresponding nitriles. The results are summarized in Table
Figure 2. SEM image of the ZnO-Pd catalyst.
2
2
. Both electron-donating (CH , OCH , NH ) (Table 2, entries
3 3 2
−4) and electron-withdrawing (CHO, COCH , Cl) (Table 2,
3
entries 5−7) group-substituted aryl bromides smoothly
participated in this reaction. However, the yields of products
obtained from the electron-withdrawing-group-substituted aryl
bromides were slightly higher than those of the electron-
donating-group-substituted aryl bromides. Sterically hindered
2
-bromotoluene also underwent the reaction without any
difficulty (Table 2, entry 2). Significantly, 4-bromochloroben-
zene reacted with K [Fe(CN) ] (0.34 mmol) under these
4
6
reaction conditions to provide 4-chlorobenzonitrile selectively,
leaving the chloro functionality unaffected (Table 2, entry 7).
However, the yield of this product was comparatively low,
which may be due to catalyst poisoning to some extent by the
higher concentration of cyanide ions in the reaction mixture.
Figure 3. PXRD pattern of the ZnO-Pd catalyst.
1
,4-Dibromobenzene produced the corresponding dicyano
To optimize the reaction conditions, a series of experiments
were performed for a representative reaction of 4-bromoanisole
and potassium ferrocyanide with variation of solvent, catalyst
loading, temperature, and time. The best result was obtained
using 0.2 mol % ZnO-Pd NPs, 0.17 mmol of K [Fe(CN) ], and
product without any difficulty (Table 2, entry 8). 1-
Bromonaphthalene was also cleanly converted to 1-cyanonaph-
thalene (Table 2, entry 9). Significantly, heteroaryl bromides
such as 3-bromoquinoline (Table 2, entry 10) and 5-acetyl-2-
bromothiophene (Table 2, entry 11) successfully participated in
the cyanation reaction to furnish the corresponding nitriles.
trans-Styrenyl bromide produced (E)-styrenylnitrile selectively,
4
6
DMF as the solvent at 130 °C for 14 h (Table 1, entry 15). Use
of 0.34 mmol of K [Fe(CN) ] decreased the yield to a
4
6
B
dx.doi.org/10.1021/jo500820q | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
Table 2. ZnO-Pd NP-Catalyzed Cyanation of Aryl Bromides
moderate yield (Scheme 3, compound A), chlorobenzenes
substituted with electron-withdrawing groups (F, CHO, CN,
a
Using K [Fe(CN) ]
4
6
NO ) provided better yields (Scheme 3, compounds B−E),
2
while electron-donating substituents (OCH , NH ) completely
3
2
arrested the cyanation reaction.
In general, the cyanation reaction is clean. The pure products
are obtained by a simple workup and column chromatography.
Several useful functionalities and sensitive heterocyclic moieties
are compatible with this procedure. Although the reaction was
performed on a 1 mmol scale, reactions on a 10 mmol scale also
provided uniform results.
The reusability of the ZnO-Pd catalyst in subsequent
cyanation reactions was investigated. The performance of the
recovered catalyst for the representative reaction was tested in
up to five successive runs, and the corresponding yields are
shown as a histogram in Figure 4. Although the catalytic activity
of the recovered catalyst decreased gradually, the yield was still
7
0% even after the fourth run. The palladium loading of the
catalyst after the fourth cycle was found to be 0.0963 mmol/g,
compared with 0.1109 mmol/g in the fresh catalyst (as
detected by AAS).
In conclusion, we have developed an improved methodology
for ligand-, base-, and additive-free cyanation of unactivated aryl
halides catalyzed by palladium nanoparticles supported on
ZnO. The notable advantages offered by this methodology are
cyanation of aryl bromides and chlorides using a very low
catalyst loading (0.2 mol % Pd), operational simplicity, general
applicability to the synthesis of a wide range of nitriles
(
including heterocyclic ones), recyclability of the catalyst, and
considerably good product yields. Certainly, this would make
an important addition to the existing methods for cyanation.
EXPERIMENTAL SECTION
■
Preparation of Palladium-Doped Zinc Oxide (ZnO-Pd)
Catalyst. To a solution of Zn(NO ) ·6H O (2.6 g, 8.7 mmol) and
3
2
2
PEG-6000 (12 g) in rectified spirit (125 mL) and water (60 mL) was
added Pd(OAc) (112 mg, 0.5 mmol) with constant stirring, and 10
2
(N) NaOH was added dropwise with stirring until the pH of the
resulting solution became 10. The solution was then irradiated in a
domestic microwave oven (LG, Korea) at 360 W (30% of 1200 W; the
maximum temperature recorded was 90 °C as measured by a
thermometer inside the reaction vessel) for 20 min (4 × 5 min). The
reaction mixture was filtered, and the solid mass was washed with
water and ethanol successively and then dried in the oven at 70 °C to
provide the catalyst ready for use.
a
Reaction conditions: Aryl halide (1 mmol), K [Fe(CN) ] (0.167
4
6
mmol), ZnO-Pd NPs (15 mg, 0.2 mol % Pd), DMF (3 mL), unless
otherwise noted. Yields of isolated pure products. 0.34 mmol of
K [Fe(CN) ] was used.
b
c
4
6
General Experimental Procedure for Cyanation of Haloar-
enes: Representative Procedure for the Cyanation of 4-
Bromoanisole with K [Fe(CN) ] (Table 2, entry 3). A mixture of
4
6
whereas cis-styrenyl bromide provided a mixture of the E and Z
isomers (Table 2, entries 12 and 13).
It is likely that the cyanation reaction follows a general
catalytic cycle via oxidative addition and reductive elimination
at the metal center (Scheme 2), in accordance with the related
4
-bromoanisole (187 mg, 1 mmol), K [Fe(CN) ] (123 mg, 0.34
4 6
mmol), and ZnO-Pd catalyst (15 mg, 0.2 mol % Pd) in DMF (4 mL)
was heated with stirring at 130 °C under argon for 14 h (TLC). The
reaction mixture was filtered to separate the solid catalyst, which was
recycled. The filtrate was extracted with EtOAc (4 × 15 mL). The
17b,24
reactions reported earlier.
trans-Styryl bromide leads to
extract was washed with water and brine and then dried (Na SO ).
2 4
Evaporation of the solvent left the crude product, which was purified
by column chromatography over silica gel (60−120 mesh), eluting
with hexane/ethyl acetate (90:10), to afford pure 4-methoxybenzoni-
trile (101 mg, 76%) as a white solid. Mp 60−62 °C; IR (KBr) 3087,
thermodynamically more stable (E)-styrenylnitrile in a
straightforward way. However, in the case of cis-styryl bromide,
the corresponding intermediate I (NC−Pd−styryl complex)
may undergo β-elimination to form alkyne−Pd−CN inter-
mediate II, which is rather free to lead to the mixture of cis and
trans isomeric products. The probable reaction pathway is
outlined in Scheme 2. This is a usual trend in related reactions
25b,c
−
1 1
2
854, 2221, 1612, 1543, 1467, 1234, 1142, 1024 cm ; H NMR (500
MHz, CDCl ) δ 3.88 (s, 3H), 6.95 (d, J = 8.5 Hz, 2H), 7.57 (d, J = 8.5
3
13
Hz, 2H); C NMR (125 MHz, CDCl ) δ 55.6, 103.9, 114.8 (2C),
3
1
13
1
19.2, 134.0 (2C), 162.9. The spectroscopic data ( H and C NMR)
25a
as observed and reported earlier by us and other groups.
are in good agreement with those reported for an authentic sample.
This procedure was followed for all of the reactions listed in Table 2.
All of the products listed in Table 2 are known compounds, and they
Aryl chlorides also participated in the cyanation reaction.
Although chlorobenzene was converted to benzonitrile in
C
dx.doi.org/10.1021/jo500820q | J. Org. Chem. XXXX, XXX, XXX−XXX

The Journal of Organic Chemistry
Note
a
Scheme 2. Proposed Mechanistic Pathways for the Cyanation of Styryl Bromides
a
Reactions with aryl bromides also follow the same pathway.
Scheme 3. ZnO-Pd NP-Catalyzed Cyanation of Aryl
Chlorides using K [Fe(CN) ]
ASSOCIATED CONTENT
■
4
6
*
S
Supporting Information
Copies of H and 13C NMR spectra of all products listed in
Table 2 and Scheme 3. This material is available free of charge
via the Internet at http://pubs.acs.org.
1
AUTHOR INFORMATION
Corresponding Author
E-mail: ocbcr@iacs.res.in.
■
*
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We are pleased to acknowledge the financial support from
DST, New Delhi, for the award of the J. C. Bose Fellowship to
B.C.R. (Grant SR/S2/JCB-11/2008). T.C. and R.D. thank
CSIR for their fellowships.
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