Direct conversion of aryl halides to phenols using high-temperature or near-critical water and microwave heating

Article abstract of DOI:10.1016/j.tet.2006.01.103

The direct conversion of aryl halides to the corresponding phenols has been achieved using microwave heating. High-temperature or near-critical water is used as the solvent in conjunction with a copper catalyst and a mineral base.

Full text of DOI:10.1016/j.tet.2006.01.103

Tetrahedron 62 (2006) 4728–4732
Direct conversion of aryl halides to phenols using high-
temperature or near-critical water and microwave heating
*
Chad M. Kormos and Nicholas E. Leadbeater
Department of Chemistry, University of Connecticut, 55 North Eagleville Road, Storrs, CT 06269-3060, USA
Received 10 December 2005; revised 3 January 2006; accepted 4 January 2006
Available online 30 March 2006
Abstract—The direct conversion of aryl halides to the corresponding phenols has been achieved using microwave heating. High-temperature
or near-critical water is used as the solvent in conjunction with a copper catalyst and a mineral base.
Ó 2006 Elsevier Ltd. All rights reserved.
1. Introduction
218 atm. At these temperatures the water approaches proper-
ties more like polar organic solvents. Using conventional
heating, a range of chemistries can be performed in super-
heated water and it has been found that acid or base-
catalyzed reactions require less catalyst than normal, if any.
Strauss and co-workers showed in 1997 that by using micro-
wave heating, it is possible to open up new avenues for syn-
The concept of efficient and selective synthesis in water has
been exemplified as the rates, yields, and selectivities ob-
served for many reactions in water have begun to match,
or in many cases, surpass those in organic solvents.1 In
contrast to many other solvents, water not only provides
a medium for solution chemistry but also often participates
in elementary chemical events on a molecular scale. Water
also offers practical advantages over organic solvents. It is
cheap, readily available, non-toxic, and non-flammable.
Another area of current growing research interest is the use of
microwave energy as a heating source. This is evidenced
by the number of papers and recent reviews appearing in
the literature.2–4 As well as being energy efficient, micro-
wave heating can also enhance the rate of reactions and in
many cases improve product yields. With the advent of sci-
entific focused microwave systems, it is possible to control
the temperature, pressure, microwave power, and reaction
times very easily and with a high degree of reproducibility.
Bringing these two areas (chemistry in water and microwave
heating) together offers a clean, easy, and efficient method
for organic synthesis. An example of this is in the Suzuki
coupling reaction where it is possible to use parts per million
levels of palladium catalysts when using water as a solvent in
conjunction with microwave heating.5
9
ꢀ
thesis by working with water above 200 C. For example,
they have performed etherifications and multi-component
reactions that were not otherwise possible.10 This work
was performed in a specially designed microwave apparatus.
The majority of chemistry undertaken using water as a sol-
vent using commercially available single-mode apparatus
ꢀ
has been undertaken at temperatures at or below 200 C.
This is because they have a pressure limit of 20–30 bar
thereby limiting the temperature to which the water can be
heated. However, using a dedicated multi-mode reactor it
is possible to perform reactions in heavy-walled quartz reac-
tion vessels with operating limits of 80 bar tꢀhus allowing
water to be heated to temperatures close to 300 C.11 The use
of this apparatus for some organic transformations in near-
critical water has recently appeared in the literature.12 We
too have become interested in developing microwave method-
ologies for performing synthetic transformations using
ꢀ
ꢀ
high-temperature (w200 C) and near-critical (w300 C) water.
One particular transformation of interest to us was the direct
conversion of aryl halides to the corresponding phenols. We
present the results of our initial studies here.
In addition to using water for chemistry at ambient pressure
in open vessels there has been a growth of interest in the use
of high-temperature, near-critical, and supercritical water.6–8
High-temperature water is broadly defined as liquid water
2. Results and discussion
ꢀ
abovꢀe 200 C, near-critical water as that between 200 and
ꢀ
300 C, and supercritical water as that above 374 C and
The conversion of aryl halides to the corresponding phenols
in one step generally involves long reaction times and often
harsh conditions are required. In addition, most of the work
has been focused on chloro-, bromo- or iodo-benzene or on
* Corresponding author. Tel.: +1 860 486 5076; fax: +1 860 486 2981;
e-mail: nicholas.leadbeater@uconn.edu
0040–4020/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tet.2006.01.103

C. M. Kormos, N. E. Leadbeater / Tetrahedron 62 (2006) 4728–4732
4729
Table 2. Direct transformation of aryl halides to phenols in water using
electron-deficient aryl fluorides. Examples include perform-
ing the reaction in a steel bomb for 1½ days using CuI
as a catalyst,13 radical14–16 and photochemical17 reactions,
and fluoride displacement from electron-deficient sub-
strates.18–20 To circumvent these methods, there have been
numerous multi-step approaches reported.21 A theme run-
ning through most of the direct methods is the use of water
as a solvent, copper salt as catalysts, elevated temperatures,
and lengthy reaction times. We were keen to see if, using
microwave heating, it was possible to perform the reaction
efficiently and rapidly using high-temperature or near-criti-
cal water as a solvent. We started ouꢀr investigations working
at the lower temperature of 200 C using a monomode
microwave apparatus and working on a 1 mmol scale. We
screened a range of copper sources as catalysts in con-
junction with mineral bases (3.1 M in water) for the direct
conversion of 4-bromoacetophenone to 4-hydroxyacetophe-
copper powder as a catalyst and sodium hydroxide as basea
Br
OH
µw
Cu, NaOH, water
R
R
Entry
1
Aryl halide
Yield (%)
COMe
65
70
Br
NO2
2
Br
NO2
Br
3
30
0
ꢀ
none. The reaction mixtures were heated to 200 C and then
held at this temperature for 20 min before being allowed to
cool. Our results are summarized in Table 1. Of those
screened, copper dust was found to be the best catalyst for
the reaction, the use of Cu (I) and Cu (II) sources leads to
lower product yields (Table 1, entries 2, 4, and 5) and a cata-
lyst loading of 10 mol % was found to be optimal (Table 1,
entries 1–3). Using 3.1 equiv sodium hydroxide gave the
best results, with less leading to lower yields because of
incomplete conversion and more leading to product decom-
position (Table 1, entries 6 and 7). Sodium carbonate and
sodium acetate proved less successful as potassium hydroxide
(Table 1, entries 8–10). Running the reaction for a shorter
time led to lower conversion (Table 1, entry 11). We next
decided to screen a range of aryl halide substrates using
our conditions of 10 mol % copper powder, 3.1 equiv
4
Br
OMe
5b
17
Br
I
COMe
6
7
8
74
12
31
I
OMe
I
ꢀ
NaOH for a reaction time of 20 min at 200 C. The results
COMe
are shown in Table 2. Using activated aryl bromides and
iodides, good yields of the corresponding phenols were
obtained but with de-activated substrates product yields were
significantly lower. In the case of 4-chloroacetophenone,
no product was formed.
9
0
Cl
a Reactions were run in a sealed tube using 1 mmol aryl halide, 10 mol % Cu
powder and 1.0 mL of a 3.1 M NaOH solution. An initial microwave irra-
diation power of 75 W was used and the temperature being ramped from rt
to 200 ꢀC where it was then held for 20 min.
b Reaction mixture held at 200 ꢀC for 30 min.
Table 1. Catalyst and base screening for the direct conversion of 4-bromo-
acetophenone to 4-hydroxyacetophenonea
Ma and co-workers have recently reported the use of L-
proline as a promoter for the CuI-catalyzed coupling reaction
of aryl halides with amines.22,23 We were interested to see if
this catalytic system could also be applicable for our direct
synthesis of phenols. Again working with 4-bromoacetophe-
none as a test substrate, we set out to probe this. Our results
are summarized in Table 3. We found that using a 1:2 ratio
of CuI (10 mol %) to L-proline as reported for the amination
reactions led to a 60% yield of the desired phenol after heating
O
O
µw
[Cu], base, water
HO
Br
Entry
Copper source
Base
Yield (%)
1
2
3
4
5
6
7
8
Cu dust (5 mol %)
NaOH (3.1 equiv)
60
65
58
40
52
32
53
19
25
11
37
Cu dust (10 mol %)
Cu dust (20 mol %)
CuI (10 mol %)
Cu(OAc)2 (10 mol %)
Cu dust (10 mol %)
Cu dust (10 mol %)
Cu dust (10 mol %)
Cu dust (10 mol %)
Cu dust (10 mol %)
Cu dust (10 mol %)
NaOH (3.1 equiv)
NaOH (3.1 equiv)
NaOH (3.1 equiv)
NaOH (3.1 equiv)
NaOH (2.0 equiv)
NaOH (5.0 equiv)
KOH (3.1 equiv)
Na2CO3 (3.1 equiv)
NaOAc (3.1 equiv)
NaOH (3.1 equiv)
ꢀ
at 200 C for 20 min with NaOH as base (Table 3, entry 1).
Reduction of this ratio to 1:0.5 led to an increase in product
ꢀ
yield to 70% after heatingꢀat 200 C for 20 min and to 82%
if the reaction time at 200 C was extended to 30 min (Table
3, entries 2 and 3). With these conditions in hand we screened
representative substrates and the results being shown in
Table 4. In the case of aryl iodides, yields of product were
generally higher than those obtained using copper powder
as the catalyst (Table 4, entries 1–3). Aryl bromides other
than 4-bromoacetophenone proved less successful (Table 4,
entries 4–7) and, in the case of 4-chloroacetophenone, some
product was formed (Table 4, entry 8).
9
10
11b
a Reactions were run in a sealed tube using 1 mmol 4-bromoacetophenone
and base dissolved in water to give a 1 M solution. An initial microwave
irradiation power of 75 W was used and the temperature being ramped
from rt to 200 ꢀC where it was then held for 20 min.
b Reaction mixture held at 200 ꢀC for 10 min.

4730
C. M. Kormos, N. E. Leadbeater / Tetrahedron 62 (2006) 4728–4732
Table 3. Screening for the direct conversion of 4-bromoacetophenone to
Table 5. Direct transformation of aryl halides to phenols in water at 300 ꢀC
using CuI as a catalyst, L-proline as an additive, and sodium hydroxide as
basea
4-hydroxyacetophenone using CuI as a catalyst and L-proline as an additivea
O
O
Br
OH
µw, CuI, L-proline
µw, CuI, L-proline
NaOH, water
NaOH, water
HO
Br
R
R
Entry
Catalyst mixture
Reaction time
Yield (%)
Entry
1
Aryl halide
Reaction time
Yield (%)
100
1
2
3
CuI/L-proline (1:1)
CuI/L-proline (1:0.5)
CuI/L-proline (1:0.5)
20 min at 200 ꢀC
20 min at 200 ꢀC
30 min at 200 ꢀC
60
70
82
COMe
Heat to 300 ꢀC and stop
I
a Reactions were run in a sealed tube using 1 mmol aryl halide, 10 mol %
CuI, and 3.1 mL of a 1 M NaOH solution. An initial microwave irradiation
power of 35 W was used and the temperature being ramped from rt to
200 ꢀC where it was then held for the allotted time.
2
3
Heat to 300 ꢀC and stop
Heat to 300 ꢀC and stop
39
65
I
OMe
In a further probe of the chemistry, we decided to use the
same catalytic system (10 mol % CuI,ꢀ 5 mol % L-proline,
and NaOH as base) but work at 300 C in the dedicated
multi-mode reactor to see if the higher reaction temperature
had a beneficial effect on product yield, especially in the case
of aryl bromides. As shown in Table 5, we find that, with aryl
iodides, product yields can be further improved by working at
I
OMe
4
5
Heat to 300 ꢀC and stop
37
94
I
COMe
Heat to 300 ꢀC and hold
for 30 min
Br
6
7
Heat to 300 ꢀC and hold
for 30 min
30
43
Table 4. Direct transformation of aryl halides to phenols in water at 200 ꢀC
using CuI as a catalyst, L-proline as an additive, and sodium hydroxide as
basea
Br
OMe
Heat to 300 ꢀC and hold
for 30 min
Br
OH
µw, CuI, L-proline
Br
NaOH, water
R
R
COMe
8
Heat to 300 ꢀC and hold
for 30 min
44
Entry
1
Aryl halide
Yield (%)
Cl
COMe
a Reactions were run in a sealed tube using 5 mmol aryl halide, 10 mol %
CuI, 5 mol % L-proline, and 10 mL of a 1.5 M NaOH solution. An initial
microwave irradiation power of 1400 W was used and the temperature
being ramped from rt to 300 ꢀC then cooled to 50 ꢀC.
99
I
2
3
42
53
I
the higher temperature (Table 5, entries 1–4). The reaction
mixtures are heated to 300 C and then allowed to cool.
ꢀ
Holding the reaction at 300 C for 30 min is necessary
OMe
COMe
NO2
ꢀ
I
when working with aryl bromides (Table 5, entries 5–7);
shorter times lead to lower product yield. Using the same
conditions, with 4-chloroacetophenone as a substrate a
44% yield of product was obtained (Table 5, entry 8).
4
82
Br
5
6
7
10
0
3. Conclusion
Br
In conclusion, we have shown that it is possible to convert
the aryl halides directly to the corresponding phenols using
high-temperature or near-critical water as a solvent. The best
base for the reaction is sodium hydroxide. Whilst simple
copper powder is found to be a good catalyst (10 mol %)
for the reaction, the optimum catalyst system is 10 mol %
CuI together with 5 mol % L-proline as an additive. For
aryl bromide substrates, best results are obtained by heating
Br
OMe
0
Br
COMe
8
14
ꢀ
the reaction mixture to 300 C and holding at this tempera-
Cl
ture for 30 min. The same was found to be true for an aryl
chloride example. For aryl iodides, besꢀt results are obtained
by heating the reaction mixture to 300 C and then allowing
the reaction mixture to cool. It is also possible to perform the
a Reactions were run in a sealed tube, using 1 mmol aryl halide, 10 mol %
CuI, 5 mol % L-proline, and 1.0 mL of a 3.1 M NaOH solution. An initial
microwave irradiation power of 35 W was used and the temperature being
ramped from rt to 200 ꢀC where it was then held for 30 min.

C. M. Kormos, N. E. Leadbeater / Tetrahedron 62 (2006) 4728–4732
4731
ꢀ
reaction at a lower temperature of 200 C but to the slight
detriment of product yield.
Initial microwave irradiation of 75 W was used and the tem-
perature being ramped from rt to the desired temperature of
200 C. Once this was reached, the reaction mixture was
ꢀ
held at this temperature for 20 min. The reaction mixture
was stirred continuously during the reaction. After allowing
the mixture to cool to rt, the reaction vessel was opened and
the contents were acidified with 2 M hydrochloric acid to pH
5–7. The aqueous layer was extracted with ethyl acetate
(3Â15 mL). The organic washings were combined, dried
over MgSO4, and then ethyl acetate was removed in vacuo.
This left the crude product, which was isolated and character-
ized by comparison of NMR data with that in the literature.
4. Experimental
4.1. General
All materials were obtained from commercial suppliers and
used without further purification. Standard distilled water
was used throughout the study. 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.
4.4. General procedure for conversion of aryl halides to
ꢀ
phenols at 200 C using copper iodide as a catalyst and
L-proline as an additive
4.2. Description of the microwave apparatus
In a 10 mL glass tube 4-bromoacetophenone (199 mg,
1.0 mmol), copper iodide (19 mg, 0.1 mmol), L-proline
(5 mg, 0.05 mmol), and 1.0 mL of a 3.1 M NaOH solution
was placed. The vessel was then sealed with a septum and
placed into the microwave cavity. Initial microwave irradia-
tion of 35 W was used and the temperature being ramped
ꢀ
For reactions performed at 200 C, a commercially available
monomode microwave unit (CEM Discover) was used. The
machine consists of a continuous focused microwave power
delivery system with operator selectable power output from
0–300 W. Reactions were performed in 10 mL capacity ves-
sels sealed with a septum. The pressure was controlled by
a load cell connected directly to the vessel and the tempera-
ture of the contents of the vessel was monitored using a cali-
brated 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 micro-
ꢀ
from rt to the desired temperature of 200 C. Once this was
reached, the reaction mixture was held at this temperature
for 30 min. The reaction mixture was stirred continuously
during the reaction. After allowing the mixture to cool to rt,
the reaction vessel was opened and the contents were acidi-
fied with 2 M hydrochloric acid to pH 5–7. The aqueous layer
was extracted with ethyl acetate (3Â15 mL). The organic
washings were combined, dried over MgSO4, and then ethyl
acetate was removed in vacuo. This left the crude product,
which was isolated and characterized by comparison of
NMR data with that in the literature.
ꢀ
wave manufacturer. For reactions performed at 300 C,
a commercially available multimode microwave unit (Anton
Paar Synthos 3000) was used. The instrument is equipped
with two magnetrons, with combined continuous microwave
output power from 0 to 1400 W. Heavy-walled quartz reaction
vessels (80 mL capacity, up to 60 mL working volume)
were used. These vessels are dedicated for reactions at high
pressure (up to 80 bar) and temperatures. The quartz vessels
were capped with special seals with a protective PEEK cap
and then were placed inside protecting air cooling jackets
made of PEEK. The seals comprise of a release valve that
could be manually operated. The individual vessels were
placed in an eight-position rotor and fixed in place by screw-
ing down the upper rotor plate, and the rotor was finally
closed with a protective hood. The temperature was moni-
tored using an internal gas balloon thermometer placed in
one reference vessel and additionally by exterior IR ther-
mography. Pressure was monitored by a simultaneous hydrau-
lic pressure-sensing device for all vessels, with recording of
the highest pressure level and pressure increase. Reaction
vessels 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.
4.5. General procedure for conversion of aryl halides to
ꢀ
phenols at 300 C using copper iodide as a catalyst and
L-proline as an additive
In an 80 mL quartz tube 4-bromoacetophenone (996 mg,
5.0 mmol), copper iodide (95 mg, 0.5 mmol), L-proline
(25 mg, 0.25 mmol), and 10 mL of a 1.55 M NaOH solution
was placed. The vessel was sealed and loaded onto the rotor.
Three other vessels were prepared similarly. The loaded
rotor was subjected to a maximum of 1400 W microwave
ꢀ
power in a ramp to 300 C (limited by a maximum pressure
of 80.0 bar) over a period of 10 min and then held at this tem-
ꢀ
perature for 30 min before being allowed to cool to 50 C,
this takes around 30 min. The reaction mixture was stirred
continuously during the reaction. The vessel was vented, re-
moved from the rotor, and the contents were acidified with
2 M hydrochloric acid to pH 5–7. The aqueous layer was ex-
tracted with ethyl acetate (3Â15 mL). The organic washings
were combined, dried over MgSO4, and then ethyl acetate
was removed in vacuo. This left the crude product, which
was isolated and characterized by comparison of NMR
data with that in the literature.
4.3. General procedure for conversion of aryl halides to
phenols using copper dust as a catalyst
Acknowledgments
In a 10 mL glass tube 4-bromoacetophenone (199 mg,
1.0 mmol), copper powder (6 mg, 0.1 mmol), and 1.0 mL
of a 3.1 M NaOH solution was placed. The vessel was then
sealed with a septum and placed into the microwave cavity.
The University of Connecticut is thanked for funding. Anton
Paar GmbH and CEM Microwave Technology are thanked
for equipment support.

4732
C. M. Kormos, N. E. Leadbeater / Tetrahedron 62 (2006) 4728–4732
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