Microwave-enhanced cyanation of aryl halides with a dimeric ortho-palladated complex catalyst

Article abstract of DOI:10.1007/s11243-011-9525-8

A new dimeric ortho-palladated complex of 2-methoxyphenethylamine was synthesized and characterized and its application as a cyanation catalyst was investigated. The main advantages of this catalyst are its easy preparation, handling, stability, and moisture insensitivity. Thus, [Pd{C₆H ₃(CH₂CH₂NH₂)-4-OMe-5-κ ²-C,N}(μ-Br)]₂ showed excellent catalytic activity for the cyanation of aryl iodides and bromides with K₄[Fe(CN) ₆], in DMF in the presence of K₂CO₃ under microwave irradiation and conventional heating at 130 °C to give the desired cyanoarene products in good to high yields. The less reactive aryl chlorides also react with K₄[Fe(CN)₆] to give moderate yields of the aromatic nitriles. The effects of various parameters such as solvent, base, and amount of catalyst were studied. The reaction is suitable for a wide variety of substituted aryl halides with different electronic properties. Application of microwave irradiation improved the yields of the reactions and reduced the reaction times from hours to minutes. Graphical Abstract: Microwave-enhanced cyanation of aryl halides with a dimeric ortho-palladated complex catalyst [Figure not available: see fulltext.]

Full text of DOI:10.1007/s11243-011-9525-8

Transition Met Chem (2011) 36:725–730
DOI 10.1007/s11243-011-9525-8
Microwave-enhanced cyanation of aryl halides with a dimeric
ortho-palladated complex catalyst
Abdol-Reza Hajipour
Ghazal Tavakoli
•
Fatemeh Abrisham
•
Received: 18 June 2011 / Accepted: 1 August 2011 / Published online: 25 August 2011
Ó Springer Science+Business Media B.V. 2011
Abstract A new dimeric ortho-palladated complex of
-methoxyphenethylamine was synthesized and character-
Introduction
2
ized and its application as a cyanation catalyst was inves-
tigated. The main advantages of this catalyst are its easy
preparation, handling, stability, and moisture insensitivity.
Aromatic nitriles find applications as dyes, herbicides,
agrochemicals, pharmaceuticals, and natural products [1].
Furthermore, the nitrile group can undergo a wide variety of
transformations to give compounds such as benzoic acids,
amidines, amides, imidoesters, benzamidines, amines, het-
erocycles, and aldehydes [2]. Nitriles can also be transformed
into a variety of medicinally important groups such as tetra-
zoles, triazoles, oxazoles, thiazoles, and oxazolidinones [3].
A variety of methods for the introduction of the cyanide
functional group into aromatic compounds have been
described [4]. The Sandmeyer reaction (involving diazonium
salts plus copper(I) cyanide) and Rosenmund–Von Braun
reaction, in which aryl halides react with stoichiometric
amounts of copper(I) cyanide, provide early examples of
the conversion of aryl halides to aryl cyanides [5]. Drawbacks
of these reactions are the high temperature required
(150–250 °C) and the use of stoichiometric amounts of cop-
per(I) cyanide, which leads to equimolar amounts of heavy
metal waste. On an industrial scale, aryl cyanides are mainly
produced via ammoxidation of the corresponding toluene
derivatives [6]. Since the discovery of transition metal cata-
lyzed cross-coupling reactions, a great deal of interest
has been dedicated to the development of a practical cata-
lytic version of this transformation [7]. The first palladium-
catalyzed cyanation of aryl halides was introduced in 1973 by
Takagi et al., using aryl bromides and iodides with potassium
cyanide as cyanating agent [8]. In addition to palladium,
transition metal complexes based on nickel [9] and copper
have been employed for the coupling of aryl halides with
cyanide [10]. The cyanation reaction can also be performed
using ligand-free catalysts [11, 12]. However, palladium
compounds have some advantages over the other metal
complexes, such as less sensitivity to air and humidity
2
Thus, [Pd{C H (CH CH NH )-4-OMe-5-j -C,N}(l-Br)]
6
3
2
2
2
2
showed excellent catalytic activity for the cyanation of aryl
iodides and bromides with K [Fe(CN) ], in DMF in the
4
6
presence of K CO under microwave irradiation and con-
2
3
ventional heating at 130 °C to give the desired cyanoarene
products in good to high yields. The less reactive aryl
chlorides also react with K [Fe(CN) ] to give moderate
4
6
yields of the aromatic nitriles. The effects of various
parameters such as solvent, base, and amount of catalyst
were studied. The reaction is suitable for a wide variety of
substituted aryl halides with different electronic properties.
Application of microwave irradiation improved the yields
of the reactions and reduced the reaction times from hours
to minutes.
Keywords Cyanation reaction ꢀ Palladium ꢀ
K [Fe(CN) ] ꢀ Aromatic nitrile ꢀ Catalyst
4
6
A.-R. Hajipour (&) ꢀ F. Abrisham ꢀ G. Tavakoli
Pharmaceutical Research Laboratory, Department of Chemistry,
Isfahan University of Technology,
84156-83111 Isfahan, I.R. Iran
e-mail: haji@cc.iut.ac.ir
A.-R. Hajipour
Department of Pharmacology, University of Wisconsin Medical
School, 1300 University Avenue, Madison,
WI 53706-1532, USA
123

7
26
Transition Met Chem (2011) 36:725–730
compared with nickel, more activity than copper and tolera-
tion of a wider variety of functional groups [1]. Typical
cyanide sources are alkali metal cyanides [10, 13], Zn(CN)2
55 °C in acetonitrile (20 mL) for 6 h. After cooling, the
resulting suspension was filtered through a plug of MgSO4;
the solvent was removed under reduced pressure using a
rotary evaporator to give complex 2 as a yellow solid. Solid
NaBr (165 mg, 1.6 mmol) was added to a suspension of
complex 2 (500 mg, 0.79 mmol) in acetone (30 mL), and
the mixture was stirred for 8 h. The solvent was removed,
and CH Cl (10 mL) was added. The resulting suspension
[14–16], CuCN [17], trimethylsilyl cyanide (TMSCN) [18],
and acetone cyanohydrins [19]. These reagents have disad-
vantages that impede wider applications; for example, alkali
metal cyanides are highly poisonous, zinc cyanide leads to
heavy metal waste, and both TMSCN and acetone cyanohy-
drin can easily liberate hydrogen cyanide [20].
2
2
was filtered through a plug of MgSO , concentrated to 2 mL
4
In 2004, Beller et al. introduced potassium ferrocyanide,
K [Fe(CN) ], as a novel cyanation reagent with lower
under reduced pressure using a rotary evaporator and n-hex-
ane (15 mL) was added to precipitate a yellow solid, which
was filteredoff, washed with water and n-hexane and air-dried
4
6
toxicity and without special precautions for handling [21].
This reagent is used industrially for metal extractions for
the fabrication of a range of advanced materials. It is also
used in low doses in some food preparations [11] and as an
anti-agglutinating auxiliary for NaCl [6].
to afford 0.41 mg of complex 3 (77% yield) as a yellow solid,
1
Decomp.: 142 °C. H NMR (400 MHz, CDCl ): d = 7.22 (t,
3
1H, C H ), 6.89(d, 1H, C H ), 6.86(d, 1H, C H ), 3.82(s, 3H,
6
3
6
3
6 3
OMe), 3.09 (m, 2H, CH CH ), 2.84 (m, 2H, CH CH ), 2.24
2
2
2
2
1
3
Microwave-assisted heating under controlled conditions
is an alternative to traditional heating, with the possible
advantages of reducing the reaction time, increasing the
yield, selectivity and purity of the products [22]. This
method has been established to be very effective in
accelerating cyanation reactions [23].
(br s, 2H, NH2). C NMR (100 MHz, CDCl ): d = 156.10,
3
129.37, 126.90, 126.79, 119.23, 109.14 (C aromatic), 54.08
(OCH ), 43.63 (CH CH NH ), 35.90 (CH CH NH ). IR
2
3
2
2
2
2
2
-1
(KBr, cm ): m 3228, 3225, 2936, 1599, 1571, 1494, 1460,
1244, 1031, 754. Anal. Calcd for C H Br N O Pd : C, 32.1;
18
24
2
2
2
2
H, 3.6; N, 4.2. Found: C, 32.4, H, 3.5; N, 4.2.
Herein, followingourpreviouswork[24–27], wereportthe
synthesis ofa new ortho-palladated complex, [Pd{C H (CH -
General procedure for the cyanation reactions
6
3
2
2
CH NH )-4-OMe-5-j -C,N}(l-Br)] (3), from 2-methoxy-
2
2
2
phenethylamine (1) and Pd(OAc)2 using the method
reported by Vicente et al. [28], and its application for
cyanation of various aryl halides under microwave irradi-
ation and conventional heating using K [Fe(CN) ] as a
A mixture of the appropriate aryl halide (1 mmol), potassium
hexacyanoferrate(II) (0.22 mmol), palladacycle complex 3
(0.4 mol%), and K CO (2 mmol) was added to DMF (3 mL)
2
3
in a round-bottom flask equipped with a condenser and placed
into a Milestone Microwave. Initially, the microwave irradi-
ation was set at 500 W, and the temperature was ramped from
room temperature to the desired temperature of 130 °C. Once
this was reached, the reaction mixture was held at this tem-
perature until the reaction was completed. During this time,
the power was modulated automatically to keep the reaction
mixture at 130 °C. The mixture was stirred continuously
during the reaction. After the reaction was completed, the
mixture was cooled to room temperature and diluted with
water (20 mL) and n-hexane (20 mL). The organic layer was
4
6
cyanide source.
Experimental
Melting points were determined in open capillaries with a
Gallenkamp instrument and are uncorrected. FT-IR spectra
were run on a Jasco-680 spectrophotometer (Japan) as KBr
1
13
disks or as smears between salt plates. H and C NMR
spectra were recorded on a Bruker spectrometer at 400 and
1
00 MHz, respectively, using CDCl as solvent at room
3
dried overCaCl , thenfiltered, and the solventwas evaporated
2
temperature (TMS was used as an internal standard).
A BEIFIN 3420 Gas Chromatograph equipped with a
Varian CP SIL 5CB column- 30 m, 0.32 mm, 0.25 lm was
used to monitor completion of the reactions. All reagents and
solvents used in this study were of reagent grade quality,
obtained from commercial suppliers (Acros, Merck, and
Aldrich), and used without further purification.
using a rotary evaporator. The residue was purified by
recrystallization using ethanol and water.
Results and discussion
The reaction of 2-methoxyphenethylamine (1) with
Pd(OAc) in a 1:1 molar ratio in acetonitrile gave the
2
2
Synthesis of [Pd{C H (CH CH NH )-4-OMe-5-j -
cyclometalated complex [Pd{C H (CH CH NH )-4-OMe-
2
6
3
2
2
2
6
3
2
2
2
C,N}(l-Br)] (3)
5-j -C,N}(l-Br)] (3) (Scheme 1).
2
2
The application of this complex as a catalyst for the
A mixture of 258 mg (1.7 mmol) of 2-methoxyphenethyl-
cyanation of various types of aryl halides with K [Fe(CN) ]
4
6
amine and 382 mg (1.7 mmol) of Pd(OAc) was heated at
2
was examined (Scheme 2).
1
23

Transition Met Chem (2011) 36:725–730
727
Scheme 1 The synthesis route
for preparation of ortho-
palladated complex 3
1
1). We found that K CO is the best base for this reaction,
2
3
while Cs CO , NaOAc, KOH, and Na CO led to 97, 90,
2
3
2
3
80, and 60% conversions to benzonitrile, respectively. NaF
was not efficient in this reaction.
We examined the effect of different molar ratios of iodo-
benzene to K [Fe(CN) ] on the cyanation reaction. A study
4
6
4 6
Scheme 2 Cyanation of aryl halides using K [Fe(CN) ]
of the literature suggested that the best results were obtained
with 1:0.22 ratio of aryl halide to K [Fe(CN) ] [29]. Using
4
6
In order to improve the efficiency of the reaction, the
solvent and base were both optimized. Initially, the cyana-
tion reaction conditions were optimized for the reaction
of iodobenzene with potassium hexacyanoferrate(II) in the
presence of various solvents and bases using 0.5 mol% of
catalyst (Table 1). Several organic solvents such as toluene,
0.4% palladacycle and a 1:1 molar ratio of iodobenzene to
K [Fe(CN) ] led to complete conversion to the benzonitrile
4
6
product. Furthermore, since each mole of K [Fe(CN) ] con-
4
6
tains six cyanide ions, decreasing the ratio of K [Fe(CN) ] to
4
6
iodobenzene to 0.22:1 did not decrease the reaction yield.
The loading of catalyst was also optimized by employ-
ing various amounts of catalyst, and 0.4 mol% gave the
best results. (Table 2).
0
acetonitrile, N,N -dimethylformamide, N-methyl-2-pyrroli-
done (NMP), and methanol were examined. According to
data given in Table 1, DMF was the most efficient solvent for
this reaction (Table 1, entry 2).
We then used the optimal reaction conditions (aryl halide,
1 mmol; K [Fe(CN) ], 0.22 mmol; K CO , 2 mmol; and
4
6
2
3
After choosing DMF as the solvent, we examined sev-
eral different bases. Our experiments showed that the base
was necessary for the cyanation reaction (Table 1, entry
DMF as solvent) for cyanation of different aryl halides under
both microwave irradiation and conventional heating condi-
tions, and the results are shown in Tables 3 and 4.
Table 1 Optimization of reaction condition of iodobenzene using palladacycle 3 under microwave irradiation
a
Conversion (%)
Entry
Base
Solvent
Temperature (°C)
Time (min)
1
2
3
4
5
6
7
8
9
K
K
K
K
K
2
2
2
2
2
CO
CO
CO
CO
CO
3
3
3
3
DMF
NMP
MeOH
130
5
5
5
5
5
5
5
5
5
5
5
5
5
100
90
0
130
Reflux
Reflux
Reflux
130
CH
3
CN
0
3
Toluene
DMF
DMF
DMF
DMF
DMF
DMF
DMF
DMF
0
Na CO
2
3
60
97
90
20
80
0
Cs CO
2
3
130
NaOAc
NaF
KOH
–
130
130
1
1
1
1
0
1
2
3
130
130
b
c
K
2
CO
CO
3
3
130
100
35
K
2
130
4 6
Reactions were carried out with iodobenzene (1 mmol), K [Fe(CN) ] (0.22 mmol), base (2 mmol) in solvent (3 mL)
a
Determined by GC
b
4
K [Fe(CN)
6
] (1 mmol)
c
4
K [Fe(CN)
6
] (0.16 mmol)
123

7
28
Transition Met Chem (2011) 36:725–730
Table 2 Optimization of loading of catalyst in cyanation of iodo-
benzene with K [Fe(CN)
case and steric hindrance in the vicinity of the Br as leaving
group in the latter case. In general, however, using these
reaction conditions, different types of aryl halides even the
less reactive aryl chlorides produced the desired cyanated
products (Table 3, entries 3 and 12); these reactions
required longer reaction times and their yields were lower
than for the other substrates. Also, the aryl iodides reacted
more rapidly than similar aryl bromides, reflecting the
higher reactivity of I as leaving group. The reaction of 1,4-
diiodobenzene as a difunctional aryl halide proceeded
quantitatively in a short reaction time using double the
amount of other reagents. Reactions were performed with
both electron-donating and electron-withdrawing groups,
and the results clearly showed that there is no sensitivity to
substituents on the aryl ring. The effect of chemoselectivity
of the method was investigated using ortho-, meta-, and
para-bromochlorobenzene as test substrates. In each case,
only Br was substituted, and the chloro-group did not enter
into the reactions.
4
6
a
Entry
Mol% catalyst
Time (min)
Conversion (%)
1
2
3
4
5
6
None
0.2
0.3
0.4
0.5
1
5
5
5
5
5
3
0
85
93
98
100
100
Reactions were carried out with iodobenzene (1.0 mmol), K
0.22 mmol), K CO
cycle 3 in DMF (3 mL)
4
[Fe(CN)
6
]
(
2
3
(2 mmol), and various concentration of pallada-
a
Determined by GC
As shown in Table 3, full conversions were achieved in
all cases except for 4-chloroacetophenone and 2-bromo-
acetophenone; for these substrates, we were unable to
obtain quantitative yields even by prolonging the reaction
time (Table 3, entries 12 and 15). This may be due to the
lower reactivity of Cl as the leaving group in the former
As shown in Tables 3 and 4, the use of homogenous metal
catalysts in conjunction with microwave irradiation improved
the yields of the reactions and decreased the reaction times
in comparison with traditional heating methods.
Concerning the reaction mechanism, we consider that at
first Pd(II) changes to Pd(0). Then, the oxidative addition
4 6
Table 3 Cyanation of aryl halides with K [Fe(CN) ] using catalyst 3 under microwave irradiation
a
Yield (%)
Entry
Aryl halides
Products
Time (min)
1
2
3
4
5
6
7
8
9
Iodobenzene
Benzonitrile
6
7
95
95
94
92
91
56
94
93
68
92
87
40
91
93
30
90
93
89
90
Bromobenzene
Chlorobenzene
4-Iodoanisole
4-Bromoanisole
3-Bromoanisole
Benzonitrile
Benzonitrile
10
12
13
15
4
4-Methoxybenzonitrile
4-Methoxybenzonitrile
3-Methoxybenzonitrile
4-Chlorobenzonitrile
3-Chlorobenzonitrile
2-Chlorobenzonitrile
1,4-Dicyanobenzene
4-Formylbenzonitrile
4-Acetylbenzonitrile
4-Acetylbenzonitrile
4-Acetylbenzonitrile
2-Acetylbenzonitrile
4-Nitrobenzonitrile
1,4-Dicyanobenzene
1-Naphthonitrile
1-Bromo-4-chlorobenzene
1-Bromo-3-chlorobenzene
1-Bromo-2-chlorobenzene
4-Bromobenzonitrile
4-Bromobenzaldehyde
4-Chloroacetophenone
4-Bromoacetophenone
4-Iodoacetophenone
3
15
7
10
11
12
13
14
15
16
17
18
19
6
15
7
6
2-Bromoacetophenone
1-Iodo-4-nitrobenzene
1,4-Diiodobenzene
20
18
7
b
1-Bromonaphtalene
8
9-Bromophenantrene
Phenanthrene-9-carbonitrile
10
4 6 2 3
Reaction condition: aryl halide (1.0 mmol), K [Fe(CN) ] (0.22 mmol), K CO (2 mmol), palladacycle 3 (0.4 mol%), DMF (3 mL), temperature
1
a
30 °C
Isolated yield
b
4 6 2 3
Reaction condition: aryl halide (1.0 mmol), K [Fe(CN) ] (0.44 mmol), K CO (4 mmol), palladacycle 3 (0.8 mol%), DMF (3 mL), tem-
perature 130 °C
1
23

Transition Met Chem (2011) 36:725–730
729
Table 4 Cyanation of aryl halides with K
4
[Fe(CN)
6
] using catalyst 3 under conventional heating conditions using an oil bath
a
Yield (%)
Entry
Aryl halides
Products
Time (h:min)
1
2
3
4
5
6
7
8
Iodobenzene
Bromobenzene
4-Iodoanisole
Benzonitrile
1:15
1:45
2:30
1
93
92
90
92
91
90
84
87
Benzonitrile
4-Methoxybenzonitrile
4-Chlorobenzonitrile
3-Chlorobenzonitrile
1,4-Dicyanobenzene
4-Formylbenzonitrile
Phenanthrene-9-carbonitrile
1-Bromo-4-chlorobenzene
1-Bromo-3-chlorobenzene
4-Bromobenzonitrile
0:45
2:15
1:30
2
4-Bromobenzaldehyde
9-Bromophenantrene
Reaction condition: aryl halide (1.0 mmol), K
1
4 6 2 3
[Fe(CN) ] (0.22 mmol), K CO (2 mmol), palladacycle 3 (0.4 mol%), DMF (3 mL), temperature
30 °C
Isolated yield
a
of aryl halides to Pd(0) forms the aryl palladium(II) inter-
that 2-methoxyphenethylamine prevents the fast agglomera-
tion of Pd(0) species.
mediate 1. After reaction between 1 and K [Fe(CN) ], the
4
6
intermediate 2 is produced. To test the proposed mechanism,
the mercury drop test was carried out. In the presence of a
heterogeneous catalyst, mercury leads to amalgamation on
the surface. In contrast, Hg(0) cannot have a poisoning effect
on homogeneous palladium complexes, where the Pd(II)
metal centre remains tightly bound to the ligand. When a drop
of Hg(0) was added to the reaction mixture with iodobenzene
under the optimized conditions and heated, the complex 3
lost its catalytic activity (Scheme 3). The data obtained can
be rationalized in terms of a Pd(0):Pd(II) cycle. We suggest
All the aromatic nitriles in this paper have been reported
previously and characterized by comparison with their
reported data [30–32].
Conclusion
A novel palladium-based catalyst was successfully applied
for the cyanation of aryl halides using K [Fe(CN) ] as a
4
6
nontoxic cyanating agent under both microwave irradiation
and conventional heating. Catalytic amounts of this com-
plex converted different aryl bromides and iodides to the
corresponding arenenitriles in high yields and short reac-
tion times, but aryl chlorides were changed to benzonitriles
with moderate yields. The results showed that under
microwave irradiation conditions, the reaction times can be
reduced from hours to minutes.
Acknowledgments We gratefully acknowledge the funding support
received for this project from Isfahan University of Technology
(IUT), I.R. Iran, and the financial support from the Center of Excel-
lence in Sensor and Green Chemistry Research.
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