Palladium-catalyzed cyanation of aryl triflates

Article abstract of DOI:10.1080/00397910701831498

A simple catalyst system comprising Pd(OAc)2 and DPPF was proved to be efficient for the cyanation of aryl triflates with potassium hexacyanoferrate(II) as cyanide source. Copyright Taylor and Francis Group, LLC.

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Palladium-Catalyzed Cyanation
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a
a
Yi-Zhong Zhu & Chun Cai
a
Chemical Engineering College, Nanjing University
of Science and Technology, Nanjing, China
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1
Synthetic Communications , 38: 2753–2760, 2008
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print/1532-2432 online
DOI: 10.1080/00397910701831498
Palladium-Catalyzed Cyanation of Aryl Triflates
Yi-Zhong Zhu and Chun Cai
Chemical Engineering College, Nanjing University of Science and Technology,
Nanjing, China
2
Abstract: A simple catalyst system comprising Pd(OAc) and DPPF was proved
to be efficient for the cyanation of aryl triflates with potassium hexacyanoferra-
te(II) as cyanide source.
Keywords: Aryl nitriles, aryl triflates, cyanation, palladium, potassium
hexacyanoferrate(II)
INTRODUCTION
Aryl nitriles are of considerable interest as integral parts of dyes, herbi-
[1]
cides, natural products, and pharmaceuticals. In addition, nitriles play
a crucial role as they can be easily converted into a variety of functional
[
2]
groups such as acids, ketones, oximes, and amines.
Various methods for the synthesis of aryl nitriles have been reported.
In general, aryl nitriles are synthesized from aryl halides and stoichio-
[3]
metric amounts of copper(I) cyanide, from aniline by diazotization
[4]
[5]
and subsequent Sandmeyer reaction, or by ammoxidation. Neverthe-
less, the most convenient method is based on the transition-
metal-mediated displacement of aromatic halides and triflates by the
[
6,7]
cyanide ion. Until now, a number of successful palladium-
[
and
cyanations have been reported. However, most of
8,9]
nickel-catalyzed
the work has concentrated on the inconvenient traditional cyanide
sources, which have some severe drawbacks. To avoid these problems,
potassium hexacyanoferrate(II) was rediscovered as a nontoxic cyanide
Received October 24, 2007
Address correspondence to Chun Cai, Chemical Engineering College, Nanjing
University of Science and Technology, Nanjing, China. E-mail: c.cai@mail.
njust.edu.cn
2
753

2754
Y.-Z. Zhu and C. Cai
[10]
source by Beller in 2004. This procedure provided, for the first time, a
generally applicable and relatively nontoxic approach for the synthesis of
various aryl nitriles. Since then, several publications have reported on the
development of new catalytic systems for the cyanation using potassium
[11]
hexacyanoferrate(II) as cyanide source. A close look at the literature,
however, showed that all these methods explored aryl halides as the sub-
strates. The diversity of available phenols and the simple conversion of
phenols to aryl triflates make it clear that the cyanation of aryl triflates
as an alternative route to aryl nitriles will have significant synthetic value.
[12]
Following our interest in the cyanation, herein, we present our studies
toward the conversion of aryl triflates to the corresponding aryl nitriles
using potassium hexacyanoferrate(II) as cyanide source.
RESULTS AND DISCUSSION
As a starting point for the development of our cyanation system, the reac-
tion of phenyl triflate with K [Fe(CN) ] was investigated (Table 1).
4
6
K [Fe(CN) ]ꢀ3H O was ground to a fine powder and dried under vacuum
4
6
2
ꢁ
at 80 C overnight. First, phenyl triflate was submitted to the ligand-free
palladium-mediated cyanation according to literature procedures.
[
11a,11b]
Unfortunately, the yield of our target product, benzonitrile, was low with
recovery of significant amounts of unreacted phenyl triflate and the
by-product phenol (entry 1). Increasing the amount of Pd(OAc) and
2
adding PPh as ligand only led to a slight increase in product yield (entry
3
2
changed to 1,1 -bis(diphenylphosphino)ferrocene (DPPF) (entry 3).
). Fortunately, an excellent yield of 91% was obtained when PPh was
0
3
a
Table 1. Evaluation of various reaction conditionsa
b
c
Entry
Pd source
Pd(OAc)2
Ligand
—
Yield (%)
d
1
34
51
91
23
77
trace
2
3
4
Pd(OAc)
Pd(OAc)
2
2
PPh
3
DPPF
DPPF
DPPF
e
Pd(OAc)2
5
6
Pd
Pd
2
(dba)
(dba)
3
2
3
PPh
3
a
Reaction conditions: 1 mmol phenyl triflate, 20 mol% dry K [Fe(CN) ], 3 mL
4
6
ꢁ
DMAc, 5 mol% Pd source, 100 mol% Na CO , 140 C, 24 h.
3
2
b
Ligand–Pd source ¼ 2:1.
c
Yields were determined by GC with 1,3-dimethoxybenzene as the internal
standard.
d
0
.1 mol% Pd(OAc)
2
was used.
e
2
mol% Pd(OAc) was used.
2

Palladium-Catalyzed Cyanation of Aryl Triflates
2755
However, reducing the amount of palladium source to 2 mol% had a dra-
matic effect on the reaction, with the yield being lowered to only 23%
(
ne)dipalladium [Pd (dba) ] in the presence of DPPF as ligand led to a
entry 4). Replacing the palladium source with tris(dibenzylideneaceto-
2
3
moderate yield of 77% (entry 5). In contrast, almost no product was
obtained when combined with PPh (entry 6).
3
The influences of bases and solvents on the cyanation of phenyl tri-
flate were also investigated (Table 2). Except for the catalysts, the bases
also played very important roles in obtaining the corresponding product.
Note that 20% yield was obtained under the base-free condition (entry 1).
Na CO was proved to be the most efficient (entry 2). Interestingly, the
2
3
use of K CO3 and Cs CO , which are very efficient bases in many
3
2
2
cross-coupling reactions, resulted in little corresponding product
with only a large amount of by-product phenol being formed (entries
3
moderate yields (entries 5, 6). Other bases such as Bu N, potassium fluo-
, 4). Employing the bases, C H O KNaꢀ4H O and NaOAc, could give
4
4
6
2
3
ride (KF), C H ONa, KOH, NaOH, t-BuOK, HCOONaꢀ2H O, and
2
5
2
Table 2. Cyanation of phenyl triflate in the presence of different bases and
a
solvents
b
Entry
Base
—
Solvent
Yield (%)
1
2
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMAc
DMF
20
91
<3
8
72
61
19
24
25
5
36
2
0
30
90
12
Na
2
CO
CO
Cs CO
3
c
3
K
2
3
c
4
d
2
3
5
C
4
H
4
O
6
KNaꢀ4H
2
O
6
7
8
9
NaOAc
Bu N
KF
3
2 5
C H ONa
KOH
NaOH
10
11
12
13
14
15
16
t-BuOK
HCOONaꢀ2H O
2
K
3
PO
4
ꢀ3H
2
O
Na
Na
2
CO
CO
3
2
3
NMP
a
Reaction conditions: 1 mmol phenyl triflate, 20 mol% dry K [Fe(CN) ],
4
6
ꢁ
5
mol% Pd(OAc)
2
, 10 mol% DPPF, 3 mL solvent, 100 mol% base, 140 C, 24 h.
Yields were determined by GC with 1,3-dimethoxybenzene as the internal
standard.
b
c
A large amount of phenol was obtained.
O
d
C
H
4 4
6
2
KNaꢀ4H O–potassium sodium tartrate tetrahydrate.

2756
Y.-Z. Zhu and C. Cai
K PO ꢀ3H O were less effective (entries 7–14). Solvent other than
3
4
2
DMAc, such as N,N-dimethylformamide (DMF) (entry 15), gave
almost the same result, but N-methyl-2-pyrroolidone (NMP) afforded
benzonitrile only in a poor yield of 12% (entry 16). These results showed
that the selection of base and solvent was very important for obtaining
high activity.
To evaluate the scope and limitation of this procedure, the reactions
of a wide variety of aryl triflates were examined (Table 3). Very efficient
a
Table 3. Pd-catalyzed cyanation of various aryl triflates
b,c
Entry
1
Aryl triflates
Aryl nitriles
2a
Yield (%)
83
2
2b
85
3
4
2c
81
80
2d
5
6
2e
2f
88
71
7
2g
2h
66 (62)
68
8
a
Reaction conditions: 1 mmol aryl triflate, 20 mol% dry K [Fe(CN) ], 3 mL
4
6
ꢁ
2 2 3
DMAc, 5 mol% Pd(OAc) , 10 mol% DPPF, 100 mol% Na CO , 140 C, 24 h.
b
Yields were determined by GC with 1,3-dimethoxybenzene as the internal
standard. All prepared benzonitriles are commercially available.

Palladium-Catalyzed Cyanation of Aryl Triflates
2757
transformation of meta-substituted and para-substituted aryl triflates
entries 1–4) were observed under the reaction conditions. On the other
hand, 2-naphthyl triflate also afforded the desired product in good yield
entry 5). However, the hindered aryl triflates were converted into the
(
(
nitriles in moderate yields (entries 6–8). Because slow addition of the aryl
triflates could prevent cleavage of the triflates and generation of phe-
[
13]
nols, the yield however, of the desired product did not improve when
performing the reaction using that method.
To conclude, a simple catalyst system comprising Pd(OAc) and
2
DPPF was effective for the cyanation of various aryl triflates with
potassium hexacyanoferrate(II) as cyanide source. The diversity of
available phenols and the simple conversion of phenols to aryl triflates
and the notoxic cyanide source make this catalytic reaction attractive
as a synthetic method of aryl nitriles.
EXPERIMENTAL
Typical Procedure for the Cyanation of Aryl Triflates
After standard cycles of evacuation and filling with dry and pure nitro-
gen, an oven-dried tube was charged with 1 mmol of aryl triflate,
2
1
0 mol% of K [Fe(CN) ], 5 mol% of Pd(OAc) , 10 mol% of DPPF,
4 6 2
00 mol% of Na CO , and 3 mL of DMAc. The tube was evacuated
2
3
and filled with nitrogen. Then the tube was sealed, and the mixture
ꢁ
was stirred at 140 C for 24 h. After cooling to room temperature, the
mixture was diluted with ethyl acetate (30 mL) and filtered. Then 1,3-
dimethoxybenzene (130mL) was added as the internal standard. The
filtrate was washed with water (3 ꢂ 10 mL) and analyzed by gas chroma-
tography. The GC yields were determined by obtaining correction factors
using authentic samples of the expected products. For isolating products:
After the reaction was completed, the mixture was diluted with ethyl acet-
ate (30 mL) and filtered. The filtrate was washed with water (3 ꢂ 10 mL).
The organic phase was dried over Na SO , filtered, and concentrated in
2
4
vacuo. Finally, the product was isolated by flash chromatography on
silica gel with EtOAc–petroleum ether as the eluent. All prepared benzo-
nitriles are known compounds and identified by gas chromatography-
mass spectrometric (GC-MS).
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