ORIGINAL PAPER

Development of Methodologies for Copper-Catalyzed C–O BondFormation and Direct Cyanation of Aryl Iodides

Arshad Mehmood • William G. Devine • Nicholas E. Leadbeater

Published online: 29 May 2010

� Springer Science+Business Media, LLC 2010

Abstract We present a new methodology using copper(I)

oxide as a catalyst in conjunction with cesium carbonate as

a base for the preparation of diaryl and aryl–alkyl ethers. A

range of aryl iodides, phenols and primary alcohols can be

used as substrates and reactions are performed using

microwave heating. We also present the use of Cu2Fe(CN)6

as both catalyst and cyanating agent for the direct con-

version of aryl iodides to nitriles.

Keywords Copper catalysis � Ether formation �Cyanation � Water � Microwave heating

1 Introduction

Historically, the copper-mediated Ullman reaction has

formed the basis for C–C, C–O, C–N and C–S bond

forming reactions [1]. The products from these couplings

prove useful industrially [2]. However, the original con-

ditions required extended reaction times at elevated tem-

peratures. As a result, there has been significant motivation

for finding alternatives. Palladium catalysis has proven

valuable to this end; a wide range of palladium complexes

as well as simple palladium salts being used with suc-

cess [3, 4]. Interest continues for the development of

copper catalysis for these transformations, this being the

topic of a number of recent reviews [5–8]. Within the

last 10–15 years, the field has opened up considerably

following reports presenting conditions for performing the

coupling reactions at lower reaction temperatures and with

catalytic quantities of copper complexes (for selected early

examples see [9–15]). Progress in the field has now reached

a point where copper catalysis can be considered a method

of choice as opposed to a last resort.

An interest of our research group is in the use of

microwave heating as an enabling technology for metal-

mediated coupling reactions and, more recently, scale-up

of these reactions [16–18]. Using microwave heating, it is

often possible to perform these reactions quickly and effi-

ciently (for recent reviews see [19, 20]). In the field

of palladium catalysis we have developed protocols for

Suzuki [21–25] and Heck [26–28] coupling reactions using

very low loadings of palladium salts. In addition, we have

developed routes for formation of diarylmethanes [29] and

the carbonylation of aryl halides using near-stoichiometric

quantities of carbon monoxide [30]. For C–O bond forming

reactions we have had more success with copper catalysis

as opposed to palladium. We have developed a route for

direct conversion of aryl halides to phenols via a C–O bond

forming reaction using water as the nucleophilic coupling

partner [31]. We use copper(I) iodide as a catalyst, a dia-

mine ligand, tripotassium phosphate as base and water as

the solvent. Heating at 180 �C for 30 min allows for the

conversion of a range of aryl bromides and iodides to the

corresponding phenols. This methodology was built on

previous work in our group using high-temperature or near-

critical water as the solvent in conjunction with a copper

catalyst and a mineral base [32]. It also compliments the

recent reports by Yu [33] and Taillefer [34] both of which

use a 1:1 water/DMSO solvent mix, copper iodide as a

catalyst and either 1,10-phenanthroline or a diketone as a

ligand. A range of aryl iodides and bromides can be used as

substrates and reactions are complete within 20–24 h at

100–130 �C.

A. Mehmood � W. G. Devine � N. E. Leadbeater (&)

Department of Chemistry, University of Connecticut, 55 North

Eagleville Road, Storrs, CT 06269-3060, USA

e-mail: nicholas.leadbeater@uconn.edu

123

Top Catal (2010) 53:1073–1080

DOI 10.1007/s11244-010-9535-3

We wanted to develop a copper-catalyzed approach to

the synthesis of diaryl and aryl–alkyl ethers. There is sig-

nificant literature precedent for this coupling (see for

example [35–42]). Interestingly though, there have been

relatively few examples which do not require addition of a

ligand or other additives to facilitate the reaction (see for

example [43, 44]). Recently, CuO nanoparticles have been

used with success for a range of C–N, C–O and C–S cross-

coupling reactions, these opening avenues for more simple,

general and ligand-free procedures [45, 46]. However, the

synthesis of the nanoparticles is often not trivial and issues

can arise with reproducibility. In addition, reaction times

are still long.

Alongside this work we also have an interest in the

development of copper catalysis for cyanation of aryl

halides. This is another important transformation, the nitrile

motif being found in a range of pharmaceuticals, dyes,

agrochemicals, and natural products [47, 48]. A number of

synthetic methods for the preparation of aryl nitriles have

been developed. The majority require the use of toxic inor-

ganic or organic cyanide sources, such as alkali-metal cya-

nides, trimethylsilyl cyanide [49], or acetone cyanohydrin

(for a review see [50], [51]). In 2004, Beller and co-workers

reported the use of potassium hexacyanoferrate(II),

K4[Fe(CN)6], as a cyanide source [52, 53]. This has greatly

reduced toxicity as compared to alkali metal cyanides such as

KCN. In addition, all six of the CN moieties of K4[Fe(CN)6]

can be used for cyanation of aryl halides, harnessing the full

potential of the reagent. Most procedures involve the use of

palladium complexes as catalysts for the reaction (for

selected recent examples, see [54–59]). In our laboratory we

have developed both palladium-catalyzed [60] and copper-

catalyzed [61] methodologies for this transformation based

around the use of water as a solvent in conjunction with

microwave heating and K4[Fe(CN)6] as the cyanide source.

Reactions are complete within 20 min microwave heating at

150–175 �C and use either 5 mol% Pd(OAc)2 or 15 mol%

CuI as catalysts. However, we were eager to develop our

copper-mediated process further with the objective of

increasing its versatility.

In this article we report developments in C–O bond

forming and direct cyanation chemistry using copper

catalysis and report new methodologies for both classes of

reaction.

2 Results and Discussion

2.1 C–O Bond Forming Reactions

As a starting point for our development of a route to the

synthesis of diaryl and aryl–alkyl ethers using copper

catalysis, we chose to use the conditions we had developed

previously for direct formation of phenols. Clearly we

could not use water as the reaction medium so instead

focused attention on polar aprotic organic solvents. How-

ever, when using copper(I) iodide as a catalyst, DMEDA as

a ligand and tripotassium phosphate as base, we were

unsuccessful in coupling 4-bromoanisole or 4-iodoanisole

with phenol. We screened DMF, NMP and 1,4-dioxane as

potential solvents. We therefore decided to turn our

attention to the use of copper oxides as catalyst precursors.

The reports by Punniyamurthy [45] and Kidwai [46]

showing the use of Cu or Cu2O nanoparticles as catalysts

for ligand-free C–O bond formation, prompted us to screen

copper(I) oxide as a possible catalyst candidate. Selecting

NMP as the solvent and using tripotassium phosphate as

base, we studied the coupling of 4-iodoanisole and phenol

using commercially available copper(I) oxide powder.

However, no product was obtained (Table 1, entry 1).

Literature suggests that cesium carbonate can serve as a

base for C–O bond forming reactions [62, 63]. Changing

from K3PO4 to Cs2CO3 we were able to obtain a 97% yield

of the desired product when performing the reaction at

170 �C for 30 min using 20 mol% Cu2O and a 1:2 ratio of

aryl iodide to phenol (Table 1, entry 2). The catalyst

loading could be decreased to 5 mol% and the ratio of

4-iodoanisole to phenol reduced to 1:1.5 without a signif-

icant effect on the reaction (Table 1, entries 3 and 4).

Changing the solvent to dioxane or toluene has a detri-

mental effect on the reaction (Table 1, entries 5 and 6). To

determine whether our conditions would prove effective

for aryl–alkyl coupling reactions we screened ethanol as a

coupling partner in place of phenol. We decided to perform

the reaction using the alcohol as the solvent for the reac-

tion. Heating the reaction mixture (4-iodoanisole, 5 mol%

Cu2O, Cs2CO3) to 170 �C and holding at this temperature

for 30 min gave a 87% yield of the desired 4-methoxy-

phenyl ethyl ether (Table 1, entry 7). We then tried

reducing the catalyst loading further but only a 35% yield

was obtained when using 2.5 mol% Cu2O (Table 1, entry

8). Thus, our optimal conditions were: 5 mol% Cu2O,

Cs2CO3 (2 equiv based on aryl halide), 170 �C for 30 min

using either alcohol or NMP as solvent.

With conditions in hand, we next turned attention to

screening substrates. Attention focused initially on aryl–

aryl coupling reactions. The couplings of a representative

range of phenols and aryl iodides was successful, the

desired products being obtained in good to excellent yields

(Table 2). The system is tolerant of electron-donating and

withdrawing functionalities on the phenol coupling partner.

In case of 4-hydroxypyridine, no desired product was

observed but the reaction mixture contained unidentified

side products (Table 2, entry 5). Interestingly though, the

coupling of 2-iodopyridine with phenol could be achieved

in 91% yield (Table 2, entry 6). Using hydroquinone as a

1074 Top Catal (2010) 53:1073–1080

123

Table 1 Optimization of conditions for C–O bond forming reactionsa

OMe OMe

I RO

MW+ ROH

Entry Conditionsb Yieldc

1 Cu2O (20 mol%), K3PO4 (2 mmol), phenol (2 mmol), NMP (1 mL), 170 �C for 30 min 0

2 Cu2O (20 mol%), Cs2CO3 (2 mmol), phenol (2 mmol), NMP (1 mL), 170 �C for 30 min 97

3 Cu2O (10 mol%), Cs2CO3 (2 mmol), phenol (1.5 mmol), NMP (1 mL), 170 �C for 30 min 95

4 Cu2O (5 mol%), Cs2CO3 (2 mmol), phenol (1.5 mmol), NMP (1 mL), 170 �C for 30 min 94

5 Cu2O (5 mol%), Cs2CO3 (2 mmol), phenol (1.5 mmol), dioxane (1 mL), 170 �C for 30 min Trace

6 Cu2O (5 mol%), Cs2CO3 (2 mmol), phenol (1.5 mmol), toluene (1 mL), 170 �C for 30 min 40

7 Cu2O (5 mol%), Cs2CO3 (2 mmol), ethanol (1.5 mL), 170 �C for 30 min 87

8 Cu2O (2.5 mol%), Cs2CO3 (2 mmol), ethanol (1.5 mL), 170 �C for 30 min 35

a All reactions performed using 4-iodoanisole as aryl halide substrate. Reaction mixture heated to target temperature and held for allotted timeb Conditions changed from entry 1 are highlighted in boldc Yield determined by NMR using an internal standard

Table 2 Preparation of diaryl ethersa

hal+

R

OH

R'

O

R R'5 mol% Cu2O, 2 equiv Cs2CO3

NMP

MW170 oC, 30 min

Entry Aryl halide Phenol Yieldb

1OMe

I

OH94

2OMe

I

OH

F83

3OMe

I

OH

O2N96

4OMe

I

OH

MeO92

5OMe

I N

OH0

6N I

OH

MeO91

7OMe

I

O

O57

8OMe

Br

OHtrace

a Conditions: 1.0 mmol (hetero)aryl halide, 1.5 mmol phenol, Cu2O (5 mol%), Cs2CO3 (2 mmol), 1 mL NMP. Reaction mixture heated to

170 �C and held at this temperature for 30 minb Yield determined by NMR using an internal standard

Top Catal (2010) 53:1073–1080 1075

123

substrate resulted in the formation of only one C–O bond

selectively (Table 2, entry 7). There was no indication of

the presence of the di-etherification product even though 2

equiv of hydroquinone was present. Using aryl bromides as

substrates was not successful, thus limiting the methodol-

ogy to couplings involving aryl iodides (Table 2, entry 8).

When using ethanol as a substrate for aryl–alkyl cou-

pling, it was again possible to perform the reaction in good

to excellent yields (Table 3, entries 1–7). Methanol, 1-

propanol and 2-methoxyethanol also served as a good

coupling partners (Table 3, entries 8–10). Using 2-butanol,

t-butanol or benzyl alcohol as substrates was not successful

with no ether formation being observed (Table 3, entries

11–13), and again, aryl bromides prove unreactive

(Table 3, entry 14). This shows that the methodology is

limited to primary alcohol and aryl iodide substrates. To

probe the reaction with thiols to affect a C–S bond forming

reaction, we screened the coupling of 4-bromoanisole and

1-octanethiol (Table 3, entry 15). We obtained a 73% yield

of the desired product.

While not as effective as some ligated copper complexes,

our methodology compares favorably with that using Cu or

Cu2O nanoparticles with the advantages of shorter reaction

times and the fact that commercially available copper(I)

oxide powder can be used as the catalyst.

2.2 Cyanation of Aryl Halides

As a starting point for our attempts to improve upon our CuI-

catalyzed cyanation of aryl iodides using K4[Fe(CN)6] as the

cyanating agent, we screened a range of other copper salts as

catalyst candidates. We obtained comparable yields in the

cyanation of 4-iodoanisole when using copper acetate, the

reaction being performed in water as a solvent using tetra-

butyl ammonium bromide (TBAB) as a phase transfer agent

and heated to 175 �C for 20 min (Table 4, entry 1). When

using CuO, we found we could perform the reaction effec-

tively without the need for a phase transfer agent, albeit

needing to extend the reaction time to 1 h (Table 4, entry 2).

We also noticed that after its initial use, the copper catalyst

changed from black to purple in color. This observation

interested us. Analysis showed that we had formed

Cu2[Fe(CN)6] during the course of the reaction. It occurred

to us that if we could pre-form Cu2[Fe(CN)6] we could

possibly use this both as a catalyst and as a cyanating agent,

thereby simplifying the protocol. The preparation of

Cu2[Fe(CN)6] was trivial. It could be prepared by the addi-

tion of K4[Fe(CN)6] to an aqueous solution of a soluble

cupric salt [64, 65]. It precipitated out of water and was easily

collected. We then tested this complex as both a catalyst and

cyanating agent, again using 4-iodoanisole as a substrate. We

obtained a 77% yield of 4-methoxybenzonitrile when using

0.3 equiv of Cu2[Fe(CN)6] validating our hypothesis. We

then undertook a preliminary screen of a range of aryl

iodides, finding that a number could be used as substrates

(Table 4, entries 4–8). In the case of 4-iodotoluene, a par-

ticularly low yield of the desired product was obtained

(Table 4, entry 6). We attributed this to the hydrophobic

nature of the substrate and thought that use of a phase-

transfer agent may be beneficial. We repeated the reaction

using 1 eq tetrabutylammonium bromide as an additive and

obtained a 75% yield, this confirming our assertion.

3 Conclusions

In summary, we report here extensions to our work in the

fields of copper catalyzed C–O bond formation and cya-

nation. In the case of the former, we present a methodology

using copper(I) oxide as a catalyst in conjunction with

cesium carbonate as a base for the preparation of diaryl and

aryl–alkyl ethers. A range of aryl iodides, phenols and

primary alcohols can be used as substrates. In the case of

the cyanation of aryl iodides, we present the use of

Cu2[Fe(CN)6] as a catalyst and cyanating agent. This

builds on our previous work and constitutes a simpler, user-

friendly methodology. Work is now underway to extend

further these two areas of chemistry as well as continue to

develop new palladium-catalyzed coupling strategies for

key bond-forming reactions.

4 Experimental Section

4.1 General Experimental

All materials were obtained from commercial suppliers and

used without further purification. All reactions were carried

out in air. NMR spectra were recorded at 293 K on a 300 or

400 MHz spectrometer. All products are known and were

characterized by comparison of NMR data with that in the

literature. A commercially available monomode micro-

wave unit (CEM Discover) was used. The machine consists

of a continuous focused microwave power delivery system

with operator selectable power output from 0 to 300 W.

Reactions were performed in 10 mL capacity glass vessels

sealed with a septum. The pressure was controlled by a

load cell connected directly to the vessel and the temper-

ature of the contents of the vessel was monitored using a

calibrated IR sensor located outside the reaction vessel.

The contents of the vessel were stirred by means of a

rotating magnetic plate located below the floor of the

microwave cavity and a Teflon-coated magnetic stir bar in

the vessel. Temperature, pressure and power profiles were

monitored using commercially available software provided

by the microwave manufacturer.

1076 Top Catal (2010) 53:1073–1080

123

Table 3 Preparation of aryl–alkyl ethersa

hal+

R

OR

R

5 mol% Cu2O, 2 equiv Cs2CO3

NMP

MW170 oC, 30 min

ROH

Entry Aryl halide Alcohol Yield b

1OMe

IEtOH 87

2OMe

IEtOH 80

3OMeI

EtOH 95

4I

EtOH 77

5I

EtOH 75

6Br

IEtOH 89c

7I

EtOH 97

8OMe

IMeOH 93d

9OMe

I

OH 90

10OMe

IMeO

OH 87

11OMe

I

OH0

12OMe

I

OH0

13OMe

I

OH0

14OMe

BrEtOH trace

15OMe

ISH

73

a Conditions: 1.0 mmol aryl halide, 1.5 mL alcohol, Cu2O (5 mol%), Cs2CO3 (2 mmol), reaction mixture heated to 170 �C and held at this

temperature for 30 minb Yield determined by NMR using an internal standardc Product: 4-bromophenyl ethyl etherd Reaction mixture heated to 160 �C

Top Catal (2010) 53:1073–1080 1077

123

4.2 General Procedure for the Cu-Catalyzed Synthesis

of Diaryl Ethers

A 10 mL capacity glass tube was charged with Cu2O

(7.2 mg, 0.05 mmol), aryl iodide (1.0 mmol), phenol

(1.5 mmol), Cs2CO3 (650 mg, 2 mmol) and 1.0–1.2 mL of

NMP. The vessel was sealed with a septum and placed into

the microwave cavity. An initial microwave power of

50 W was used, the temperature being ramped from r.t. to

the desired temperature of 170 �C where it was held for

30 min. During this time, the power was modulated auto-

matically to hold the reaction mixture at the set tempera-

ture. The mixture was stirred continuously during the

reaction. The mixture was allowed to cool to room tem-

perature, quenched with water and extracted with diethyl

ether (3 9 10 mL). The combined ether layers were

washed with water (3 9 10 mL), dried over anhydrous

magnesium sulfate, filtered and the solvent evaporated

under reduced pressure. The crude product was

characterized by comparison of NMR spectra with those in

the literature. Yield data was obtained by integration of

signals in the 1H-NMR and comparison with an internal

standard (1,2,4,5-tetramethyl benzene).

4.3 General Procedure for the Cu-Catalyzed Synthesis

of Aryl–Alkyl Ethers

A 10 mL capacity glass tube was charged with Cu2O

(7.2 mg, 0.05 mmol), aryl iodide (1.0 mmol), Cs2CO3

(650 mg, 2 mmol) and alcohol (1.5 mL). The vessel was

sealed with a septum and placed into the microwave cavity.

An initial microwave power of 50 W was used, the tem-

perature being ramped from r.t. to the desired temperature of

170 �C (160 �C for methanol) where it was held for 30 min.

During this time, the power was modulated automatically to

hold the reaction mixture at the set temperature. The mixture

was stirred continuously during the reaction. Work-up and

analysis was as in the case of diaryl ethers.

Table 4 Cyanation of aryl iodides

I+

R

CN

R

MnFe(CN)6H2O

MW

Entry Aryl halide Conditions Yield b

1OMe

I

0.3 eq K4Fe(CN)6, 20 mol% Cu(OAc)2,1 eq. TBAB, 175 °C, 30 min

71

2OMe

I

0.3 eq K4Fe(CN)6, 20 mol% CuO, 175 °C, 1 h

74

3OMe

I0.3 eq Cu2Fe(CN)6, 175 °C, 1 h 77

4COMe

I0.3 eq Cu2Fe(CN)6, 175 °C, 1 h 78

5OH

I0.3 eq Cu2Fe(CN)6, 175 °C, 1 h 73

6I

0.3 eq Cu2Fe(CN)6, 175 °C, 1 h 35 (75)c

7I

0.3 eq Cu2Fe(CN)6, 175 °C, 1 h 41

8OMe

I0.3 eq Cu2Fe(CN)6, 175 °C, 1 h 43

a Conditions: 1.0 mmol aryl halide, reaction mixture heated to target temperature and held for allotted timeb Yield determined by NMR using an internal standardc Addition of 1 equiv tetrabutylammonium bromide as a phase-transfer agent

1078 Top Catal (2010) 53:1073–1080

123

4.4 Preparation of Cu2[Fe(CN)6]�6H2O

To a 50 mL round bottom flask were added copper(II) ace-

tate monohydrate (1.82 g, 10 mmol) and potassium hexa-

cyanoferrate(II) trihydrate (2.11 g, 10 mmol) and dissolved

in water (10 mL). The flask was equipped with a magnetic

stir bar and placed over a magnetic stirrer. The reaction

mixture was stirred for 20 min at room temperature then

transferred to a 100 mL round bottom flask and an additional

40 mL of water was added. The flask was swirled to suspend

the Cu2[Fe(CN)6] and dissolve the potassium acetate by-

product. The contents of the flask was then allowed to settle

for 30 min and then the aqueous layer decanted off. This

process was repeated a total of three times and the remaining

Cu2[Fe(CN)6] was transferred to a watch glass and placed an

oven to dry (mCN: 2101 cm-1).

4.5 General Procedure for the Conversion of Aryl

Iodides to Nitriles

A 10 mL capacity glass tube was charged with

Cu2[Fe(CN)6]�6H2O (134.1 mg, 0.3 mmol), aryl iodide

(1.0 mmol), and water (2 mL). The vessel was sealed with a

septum and placed into the microwave cavity. An initial

microwave power of 100 W was used, the temperature being

ramped from r.t. to the desired temperature of 175 �C were it

was held for 1 h. During this time, the power was modulated

automatically to hold the reaction mixture at the set tem-

perature. The mixture was stirred continuously during the

reaction. The mixture was allowed to cool to room temper-

ature and then transferred to a separatory funnel using 10 mL

of water and 10 mL of diethyl ether. The reaction mixture

was then extracted with diethyl ether (3 9 20 mL). The

combined ether layers were washed once with brine

(15 mL), dried over anhydrous magnesium sulfate, filtered

and the solvent evaporated under reduced pressure. The

crude product was characterized by comparison of NMR

spectra with those in the literature. Yield data was obtained

by integration of signals in the 1H-NMR and comparison

with an internal standard (1,2,4,5-tetramethyl benzene).

Acknowledgements This work was funded by the National Science

Foundation (CAREER award CHE-0847262) and the University of

Connecticut. The authors thank CEM Corp. for microwave equipment

support.

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