Cyanation of aryl chlorides using a microwave-assisted, copper-catalyzed concurrent tandem catalysis methodology

Article abstract of DOI:10.1021/om4004253

A microwave-assisted, copper-catalyzed concurrent tandem catalytic (CTC) methodology has been developed for the cyanation of aryl chlorides, where the aryl chloride is first converted to an aryl iodide via halogen exchange and the aryl iodide is subsequently transformed to the aryl nitrile. A variety of aryl chlorides were converted to aryl nitriles in 44-97% yield using 20 mol % of CuI, 40 mol % of N,N'-cyclohexane-1,2-diamine, and 1.2 equiv of KCN in acetonitrile at 200 C after 1-2 h. The same copper/ligand system served as a multifunctional catalyst for both steps of the concurrent catalytic process. Unlike our previously reported CTC hydrodehalogenation of aryl chlorides, CTC cyanation was catalytic in iodide. Kinetic simulations of the proposed CTC mechanism were consistent with experimental results and stipulate the relative reaction rates of the two catalytic cycles necessary to achieve reasonable yields of product. This article not subject to U.S. Copyright. Published 2013 by the American Chemical Society.

Full text of DOI:10.1021/om4004253

Article
pubs.acs.org/Organometallics
Cyanation of Aryl Chlorides Using a Microwave-Assisted,
Copper-Catalyzed Concurrent Tandem Catalysis Methodology
Mary M. Coughlin, Colin K. Kelly, Shirley Lin,* and Amy H. Roy MacArthur*
Department of Chemistry, United States Naval Academy, 572 Holloway Road, Annapolis, Maryland 21401, United States
*
S Supporting Information
ABSTRACT: A microwave-assisted, copper-catalyzed concurrent
tandem catalytic (CTC) methodology has been developed for the
cyanation of aryl chlorides, where the aryl chloride is first converted
to an aryl iodide via halogen exchange and the aryl iodide is
subsequently transformed to the aryl nitrile. A variety of aryl
chlorides were converted to aryl nitriles in 44−97% yield using
2
1
0 mol % of CuI, 40 mol % of N,N’-cyclohexane-1,2-diamine, and
.2 equiv of KCN in acetonitrile at 200 °C after 1−2 h. The same
copper/ligand system served as a multifunctional catalyst for both
steps of the concurrent catalytic process. Unlike our previously
reported CTC hydrodehalogenation of aryl chlorides, CTC cyana-
tion was catalytic in iodide. Kinetic simulations of the proposed CTC mechanism were consistent with experimental results and
stipulate the relative reaction rates of the two catalytic cycles necessary to achieve reasonable yields of product.
INTRODUCTION
powerful carbon−carbon bond-forming reactions involving an
aryl halide substrate, specifically the cyanation of aryl chlorides to
produce aryl nitriles. Aryl nitriles have important applications a6s
natural products, pharmaceuticals, agrochemicals, and dyes.
Additionally, they function as synthetic intermediates for a
■
(
1
As described by Baker and Bazan, concurrent tandem catalysis
CTC) is defined as a transformation in which two or more catalytic
cycles operate in a cooperative manner in a single reactor (Scheme 1).
a
multitude of other functional groups. Palladium-catalyzed cyana-
Scheme 1. A Concurrent Tandem Catalytic (CTC) Cycle
7−13
tions of aryl chlorides are known,
but there are relatively
fewer examples of copper-catalyzed reactions. Previous reports of
copper-catalyzed cyanation of aryl halides focused on aryl iodide
4
,14
and bromide substrates or included only highly activated aryl
15−18
chloride substrates.
Buchwald has reported a domino
(
CTC) halogen exchange−cyanation method for the copper-
a
Adapted from ref 1.
catalyzed cyanation of aryl bromides in the presence of 10 mol %
of CuI, 20 mol % of KI, 1 equiv of trans-N,N′-dimethylethyle-
nediamine, and 1.2 equiv of NaCN in toluene at 110−130 °C for
One advantage of CTC over one-pot, sequential tandem catalytic
processes is that “efficient catalysts may allow the coupling of
equilibrium-limited reactions with subsequent exothermic
5
24 h. The reaction tolerated a variety of functional groups and
heteroatoms and produced good to excellent yields of the
corresponding aryl nitriles after 24 h with conventional heating
methods. However, no mention of the use of aryl chloride
substrates was made. Given Buchwald’s work and our previous
success with CTC hydrodehalogenation, we were subsequently
able to develop a microwave-assisted, copper-catalyzed CTC
cyanation of aryl chlorides.
1
ones.” Recently we developed such a microwave-assisted CTC
methodology in which aryl chlorides or aryl bromides, in equi-
librium with aryl iodides, were converted to arenes through a
hydrodehalogenation reaction mediated by the same metal−
2
ligand system that performed the halogen exchange (Scheme 2).
This methodology allowed the exploitation of the greater reactivity
of C−I bonds over C−Cl and C−Br bonds despite the presence
of an equilibrium-limited first step in the case of aryl chloride to
3
RESULTS AND DISCUSSION
4
,5
■
aryl iodide halogen exchange. In our reported CTC hydro-
dehalogenation, the aryl chloride-aryl iodide halogen exchange
equilibrium was shifted to the right by the consumption of the
aryl iodide produced in the first catalytic reaction.
Having established the feasibility of a CTC cycle involving
halogen exchange followed by hydrodehalogenation, we now
demonstrate our ability to generalize this methodology by
varying the second catalytic step. We chose one of the synthetically
Reaction conditions were screened to optimize the micro-
wave-assisted CTC cyanation method using 4-chlorotoluene as
the substrate. The initial conditions examined were predicated
upon both Buchwald’s reported conditions for CTC cyanation
Received: May 14, 2013
This article not subject to U.S. Copyright.
Published XXXX by the American Chemical
Society
A
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Article
5
of aryl bromides (vide supra) and our experience with CTC
cyanation of aryl bromides, CTC cyanation of aryl chlorides
occurred in the presence of only a catalytic amount of iodide
(0.2 equiv present from 20 mol % of CuI); addition of NaI to increase
the total amount of iodide to 2 equiv led to a decrease in yield of
2
hydrodehalogenation of aryl chlorides and aryl bromides, where
a multifunctional copper/diamine species was the catalyst for
both cycles. The highest conversion of aryl chloride (86%) and
highest yield of 4-toluinitrile (78%) were obtained using
0 mol % of CuI, 40 mol % of N,N’-cyclohexane-1,2-diamine
ligand 1), and 1.2 equiv of KCN in acetonitrile at 200 °C after
4-toluinitrile (entry 8) due to the incomplete conversion of aryl iodide
2
(
and the formation of toluene via hydrodehalogenation.
The optimized reaction conditions for CTC cyanation were
applied to a variety of aryl chlorides (Table 2). For most sub-
strates, yields of aryl nitriles ranged between 44 and 97%. The
reaction is sensitive to steric crowding, as in the case of
heating for 1 h (Table 1, entry 1). The same reaction conducted
at 150 °C (entry 2), the temperature at which aryl bromides
could be converted to aryl nitriles after 1 h under microwave
heating using ligand 2, yielded only 10% of the desired product.
Performing the reaction with heating to 200 °C in a high-
temperature fluid bath instead of the microwave reactor resulted
in only a trace of product being formed (entry 3). Attempting the
reaction with ligand 2 at 200 °C produced only 6% product
2
1
-chlorotoluene (entry 2, trace amount of product formed), but
-chloronaphthalene was converted to the corresponding
product in 44% yield. Fortunately, unlike CTC hydrodehaloge-
nation, the presence of heteroatoms was well-tolerated by the
cyanation reaction. Cyanation occurred in the presence of func-
tional groups such as an alcohol (entry 4), a ketone (entry 5), an
ether (entry 6), and both nitrogen- and sulfur-containing hetero-
cycles (entries 7 and 8). By maintaining a catalyst loading of
20 mol % per C−Cl bond, 1,4-dichlorobenzene was converted to
(
entry 4). Decreasing catalyst/ligand loading by half (entry 5) led
to only a slightly lower yield of product, but given the high
temperatures required for CTC cyanation of aryl chlorides and
concerns over catalyst stability, 20 mol % of copper iodide and
0 mol % of ligand 1 was deemed optimal. Lower conversions
and yields were obtained when the cyanide source was NaCN
4
1
,4-dicyanobenzene in 27% yield (entry 9). A complex reaction
19
(
entry 6) or K Fe(CN) (entry 7). Similar to Buchwald’s CTC
3 6
mixture was observed in this reaction, which included products
Scheme 2. CTC Methodology for the Hydrodehalogenation
of Aryl Chlorides and Aryl Bromides
Scheme 3. CTC Methodology for the Cyanation of Aryl
Chlorides
Table 1. Screening Conditions for Cyanation of 4-Chlorotoluene
−
a
b
entry amt of CuI (mol %) ligand amt of ligand (mol %)
CN source
amt of NaI (equiv) T (°C) conversn of ArCl (%) yield of ArCN (%)
1
2
3
4
5
6
7
8
9
1
20
20
20
20
10
20
20
20
1
1
1
2
1
1
1
1
1
1
40
40
40
40
20
40
40
40
40
40
KCN
KCN
KCN
KCN
KCN
NaCN
0
0
0
0
0
0
0
2
0
0
200
150
86
11
1
78
10
trace
6
c
200
200
200
200
200
200
200
200
9
76
67
53
77
26
21
72
66
53
29
0
K Fe(CN)6
3
KCN
none
20
d
e
0
20
NaCN
2
a
b
c
d
Determined by GC. GC yield versus standard (n-decane). Conducted using thermal heating (oil bath) instead of microwave heating. CuCN
e
instead of CuI. Conducted with NaCN because the reaction with KCN could not reach the desired reaction temperature.
B
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Table 2. Microwave-Assisted Copper-Catalyzed Cyanation of
Aryl Chlorides To Form Aryl Nitriles via Concurrent Tandem
Catalysis
In the case of CTC hydrodehalogenation, the observation of
up to 40 mol % of aryl iodide being formed and then consumed
during the course of the reaction of 4-chlorotoluene to produce
toluene was consistent with the proposed CTC mechanism
(
Figure 1a). When this study was repeated for CTC cyanation
a
b
Isolated yields (average of two runs). ≥90% purity as determined by
1
H NMR spectroscopy and GCMS; the impurity is due to the
c
presence of a small amount of dimerization of the indole. GC yield
due to volatility of the product. Reaction performed with 40 mol %
d
CuI, 40 mol % ligand, and 4 equiv of KCN.
Figure 1. (a) Copper-catalyzed hydrodehalogenation of 4-chloroto-
luene, performed using 20 mol % of CuI, 1.5 equiv of ligand 1, 2 equiv of
NaI, and acetonitrile as solvent at 200 °C using microwave radiation. (b)
Copper-catalyzed cyanation of 4-chloropropiophenone, performed
using 20 mol % of CuI, 40 mol % of ligand 1, 1.2 equiv of KCN, and
acetonitrile as solvent at 200 °C using microwave radiation.
derived from various combinations of cyanation and hydro-
dehalogenation reactions.
Evidence that the cyanation reaction was taking place via the
proposed CTC cycle shown in Scheme 3 came from experiments
similar to those conducted for CTC hydrodehalogenation. First,
the cyanide source was confirmed to be KCN (versus the
2
0,21
acetonitrile solvent
chlorotoluene in the absence of the cyanide salt (Table 1, entry
), which resulted in no formation of 4-toluinitrile. In order to
) by conducting the cyanation of 4-
using 4-chloropropiophenone as the substrate, 4-iodopropio-
phenone was not detected by GC-MS at any point in the reaction
(Figure 1b). However, the observation of an aryl iodide
intermediate for a given CTC reaction should be dependent
upon the relative rates of the two catalytic cycles (Scheme 3). If
cyanation of aryl iodide were fast relative to the halogen exchange
step to convert aryl chloride to aryl iodide, any aryl iodide formed
would be quickly converted to aryl nitrile and would not form in
detectable quantities in the experimental data shown in Figure 1.
Kinetic modeling of the CTC cycle outlined in Scheme 3
confirms this hypothesis. The simplified reaction shown in eq 1
9
establish that aryl iodide was an important intermediate to the
transformation, the cyanation of 4-chlorotoluene was performed
in the complete absence of iodide by replacing CuI with CuCN
(
Table 1, entry 10). The result was only a 2% yield of 4-
toluinitrile and 17% consumption of 4-chlorotoluene, indicating
that the direct cyanation of the aryl chloride was approximately
3
0 times slower than the corresponding reaction in the presence
of 20 mol % of iodide (Table 1, entry 6).
C
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Figure 2. Kinetic simulations of the consumption of ArCl and ArI and the production of ArI and ArY as a function of reaction time according to the
proposed mechanism in eq 1 and Scheme 3 (Y = CN), where the rate of formation of ArY from ArI (k ) is (a) 10 times slower, (b) as fast, (c) 10 times
2
faster, and (d) 100 times faster than the rate of formation of ArI from ArCl (k1).
2
2
was modeled in GEPASI, where k , k , and k are pseudo rate
reaction and becomes unobservable. This prediction is consistent
with the absence of ArI in the plot of species concentration versus
time for CTC cyanation (Figure 1b) and indicates that the rate of
cyanation of aryl iodide is at least 100 times faster than halogen
exchange to form aryl iodide from aryl chloride.
The ability to simulate the kinetics of a CTC reaction also gave
us the opportunity to explore the potential limitations of using an
irreversible second catalytic step to drive the first catalytic step
that is equilibrium-limited where the equilibrium favors ArCl.
1
−1
2
constants for the reactions in the two catalytic cycles: Y = H, CN
in the case of hydrodehalogenation and cyanation, respectively.
k1
k
2
ArCl HooI Arl → ArY
k
(1)
−1
The initial conditions of the simulation were set where
ArCl] = 1.0 mM and [ArI] = [ArY] = 0 mM. All rate laws
involving ArCl and ArI were assumed to be first order in sub-
strate. From our previous study on halogen exchange between
aryl chlorides and aryl iodides under our CTC conditions, we
[
In principle, a CTC reaction with a very small K value would be
eq
still be capable of generating 100% yield of product at very long
reaction times, with the second catalytic step pushing the
equilibrium of the first step toward formation of ArI. However, in
practice, catalyst lifetime and substrate and product stability are
real-world constraints. We have observed experimentally a wide
variety of product yields ranging from trace amounts to 97% after
reaction times of 1−2 h at 200 °C under conditions of CTC
hydrodehalogenation and CTC cyanation. The kinetic simu-
lation program was used to determine how much final product
ArY can be generated at practical reaction times for different
magnitude of rate constants k , k , and k .
2
observed K (or k /k ) ≈ 1 for 4-iodotoluene/4-chlorotoluene
eq
1
−1
and K (or k /k ) ≈ 0.37 for 4-iodopropiophenone/
eq
1
−1
propiophenone in acetonitrile at 200 °C in the presence of
equiv of NaI. In order to observe the effect of changing rate
constant k relative to k , the values of k and k were held
2
2
1
1
−1
constant at 1.0 M/s while k was varied from 0.1 to 100 in
logarithmic steps. The results of the simulation are shown in
2
Figure 2. For all cases, as k increases relative to k , the amount of
2
1
ArI present at any given time in the reaction decreases. In Figure
a−c, it is apparent that when the magnitude of k is less than,
similar to, or 1 order of magnitude greater than k , one can expect
to observe ArI as an intermediate in the catalytic reaction.
Figure 2b, where k /k = 1, resembles the data in Figure 1a, the
CTC hydrodehalogenation reaction. However, as the magnitude
of k increases and becomes 100 times larger than k (Figure 2d),
2
1
−1
2
2
As before, the initial conditions of the simulation were
ArCl] = 1.0 mM and [ArI] = [ArY] = 0 mM and rate laws involv-
1
[
ing ArCl and ArI were assumed to be first order in substrate.
The equilibrium constant of the first step, Keq, was varied in two
2
1
different ways, either by changing k while holding k constant
2
1
1
−1
the concentration of ArI remains low during the course of the
or by varying k−1 while holding k constant. First, we conducted
1
D
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Figure 3. Kinetic simulations of the production of ArY according to the proposed mechanism in eq 1 and Scheme 3 (Y = CN), where the rate constant
for formation of ArY from ArI (k ) is (a) 1, (b) 10, and (c) 1000 times faster than the rate of formation of ArI from ArCl (k ).
2
1
the simulation by holding k constant at 1.0 M/s while vary-
sufficiently fast rate for the second catalytic cycle to compensate
if useful amounts of ArY are to be generated from ArCl. These
simulations indicate that, during future screenings for other CTC
methodologies involving halogen exchange, an effective strategy
to achieve good yields of the final products is to choose the most
active catalysts and coupling partners for the conversion of aryl
iodide in the second catalytic cycle. This insight suggests that
employing two different catalysts, one for halide exchange and
the other specifically optimized to conduct the second reaction
involving aryl iodide, may be advantageous over using a single,
multifunctional catalyst. We hope to find such an approach
particularly helpful for devising CTC processes starting with aryl
chlorides that take place at lower temperatures.
−1
−3
ing k from 10 to 1.0 M/s in logarithmic steps at different
1
values of k ranging from 1.0 to 100 M/s in logarithmic steps.
2
The evolution of product ArY as a function of time is shown for
k = 1 (Figure 3a), k = 10 (Figure 3b), and k = 100 (Figure 3c).
2
2
2
When k = k = 1.0 M/s (Figure 3a), the effect of decreasing K
1
2
eq
by each order of magnitude is to take the reaction from being
capable of reaching 100% yield in a reasonable reaction time to
less than 80% yield in the same amount of time. As expected, the
rates of formation for ArY in Figure 3b,c are identical, since
formation of ArI from ArCl is the rate-determining step in the
CTC reaction when k ≥ k and the magnitude of k has no
2
1
2
influence on the rate of formation of ArY. When k is 1 or 2
2
orders of magnitude larger than k , useful amounts of product
In conclusion, we have described here a microwave-assisted
CTC methodology for the cyanation of aryl chlorides where the
first step of the tandem catalytic cycle is halogen exchange to form
aryl iodides. Yields of up to 97% aryl nitrile were obtained. Kinetic
simulations assisted in our understanding of the mechanism of CTC
cyanation and define some key parameters in designing a successful
CTC methodology. In the future, we look forward to expanding the
repertoire of CTC reactions starting with aryl chlorides and
bromides to include C−N and C−O bond formation as well as
further examples of carbon−carbon coupling reactions. We also
hope to perform mechanistic studies on CTC hydrodehalogenation
and CTC cyanation in order to further expand the scope and utility
of these methodologies.
1
(
>80%) can still be achieved at reasonably short reaction times
only if K ≥ 0.1.
eq
An interesting situation arises when the rate constant k was
2
changed relative to K by holding k constant at 1.0 M/s at values
eq
1
of k ranging from 1 to 100 M/s while k was varied from 1 to
2
−1
1
1
000 M/s in logarithmic steps, effectively varying K from 1 to
0 . The results of the simulations are shown in Figure 4.
Figure 4 demonstrates that in order for useful amounts of ArY
eq
−3
(>80% yield to be worth isolating from a mixture) to be
generated, the ratio k /K must be no smaller than 0.1. That is, a
2
eq
successful CTC methodology with a small K value governing
eq
the first catalytic cycle due to a large k−1 value must have a
E
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Article
Figure 4. Kinetic simulations of the production of ArY according to the proposed mechanism in eq 1 and Scheme 3 (Y = CN), where the rate constant
for formation of ArCl from Ar I (k ) is 1−1000 times faster than the rate of formation of ArY from ArI (k ) for (a) k = 1, (b) k = 10, and (c) k = 100.
−1
2
2
2
2
1
0 mL of ethyl acetate. The combined organic layers were dried over
EXPERIMENTAL SECTION
■
magnesium sulfate, and the solvent was evaporated under vacuum. The
resulting brown oil was purified by column chromatography on silica.
General Considerations. All manipulations were carried out using
standard nitrogen drybox techniques. Anhydrous acetonitrile and
copper(I) iodide were purchased from Acros and used as received.
trans-N,N′-Dimethylcyclohexane-1,2-diamine was purchased from
Aldrich and used as received. Microwave reactions were performed in
a CEM Discover microwave reactor. GC/MS analysis was performed on
a Shimadzu GCMS-QP5050A instrument with a Restek Rxi-5 ms
capillary column (30 m, 0.25 mm i.d., 0.25 μm film thickness, 5%
diphenyl/95% dimethylpolysiloxane). H and C NMR spectra were
recorded on a JEOL ECX-400 spectrometer and were referenced to
residual protio solvent peaks. Melting points were measured on an
Electrothermal Mel-temp melting point device.
ASSOCIATED CONTENT
■
*
S
Supporting Information
Text and figures giving characterization data for all isolated
products. This material is available free of charge via the Internet
at http://pubs.acs.org.
1
13
AUTHOR INFORMATION
■
General Procedure for the Copper-Catalyzed Conversion of
Aryl Chlorides into Aryl Nitriles. In a nitrogen-filled glovebox, CuI
38 mg, 0.20 mmol, 20 mol %), KCN (78 mg, 1.2 mmol), and an aryl
Corresponding Author
*
E-mail:: lin@usna.edu (S.L.); macarthu@usna.edu (A.H.R.M.).
(
chloride (1.0 mmol) were weighed into an oven-dried microwave tube
containing a small stir bar. Acetonitrile (0.5 mL) was added by syringe
and then trans-N,N′-dimethylcyclohexane-1,2-diamine (63.8 μL, 0.40
mmol, 40 mol %) was added by positive displacement pipet, the sample
was stirred, and a septum-lined cap was added. The initial reaction
mixture appeared as a clear or pale yellow liquid over a white solid. After
the tube was removed from the box, the reaction was performed in a
CEM Discover microwave reactor for 60−120 min at 200 °C and 250 W
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
C.K.K. and M.M.C. gratefully acknowledge the Office of Naval
Research for partial support of this work on funding document
NN00014110WX30241. S.L. and A.H.R.M. thank the Naval
Academy Research Council and the Office of Naval Research for
partial support of this work on funding document
N0001409WR40059. We thank Prof. Lyle Isaacs at the
University of Maryland College Park for assistance with GEPASI.
(
with power adjustments to maintain temperature), with a 2 min ramp
time. After it was cooled, the reaction mixture contained a dark brown
liquid over a solid (the color was difficult to determine due to the dark
color of the liquid). The product mixture was quenched with 4 mL of a
30% aqueous ammonia solution, and the product was extracted with 4 ×
F
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