A copper metal-organic framework as an efficient and recyclable catalyst for the oxidative cross-dehydrogenative coupling of phenols and formamides

Article abstract of DOI:10.1002/cctc.201300400

A crystalline porous metal-organic framework Cu₂(BPDC)₂(BPY) (BPDC=4,4'-biphenyldicarboxylate, BPY=4,4'-bipyridine) was synthesized and characterized by several techniques including XRD, SEM, TEM, thermogravimetric analysis, FTIR, atomic absorption spectrophotometry, hydrogen temperature-programmed reduction, and nitrogen physisorption measurements. The Cu₂(BPDC)₂(BPY) could be employed as a heterogeneous catalyst for the copper-catalyzed cross-dehydrogenative coupling reaction of DMF with 2-substituted phenols to form organic carbamates through CH activation under oxidative conditions. The Cu₂(BPDC)₂(BPY) offered higher catalytic activity than common copper salts such as Cu(OAc)₂, CuCl, CuCl₂, CuI, and Cu(NO₃)₂ as well as other Cu-MOFs such as Cu₃(BTC)₂, Cu(BDC), and Cu(BPDC). The Cu₂(BPDC)₂(BPY) catalyst could be facilely separated from the reaction mixture, could be recovered and reused several times without significant degradation in catalytic activity.

Full text of DOI:10.1002/cctc.201300400

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DOI: 10.1002/cctc.201300400
A Copper Metal–Organic Framework as an Efficient and
Recyclable Catalyst for the Oxidative Cross-
Dehydrogenative Coupling of Phenols and Formamides
[
a]
Nam T. S. Phan,* Tung T. Nguyen, and Phuong H. L. Vu
A crystalline porous metal-organic framework Cu (BPDC) (BPY)
organic carbamates through CÀH activation under oxidative
2
2
(BPDC=4,4’-biphenyldicarboxylate, BPY=4,4’-bipyridine) was
conditions. The Cu (BPDC) (BPY) offered higher catalytic activi-
2
2
synthesized and characterized by several techniques including
XRD, SEM, TEM, thermogravimetric analysis, FTIR, atomic ab-
sorption spectrophotometry, hydrogen temperature-pro-
grammed reduction, and nitrogen physisorption measure-
ments. The Cu (BPDC) (BPY) could be employed as a heteroge-
ty than common copper salts such as Cu(OAc) , CuCl, CuCl ,
2 2
CuI, and Cu(NO ) as well as other Cu–MOFs such as Cu (BTC) ,
3
2
3
2
Cu(BDC), and Cu(BPDC). The Cu (BPDC) (BPY) catalyst could be
2
2
facilely separated from the reaction mixture, could be recov-
ered and reused several times without significant degradation
in catalytic activity.
2
2
neous catalyst for the copper-catalyzed cross-dehydrogenative
coupling reaction of DMF with 2-substituted phenols to form
Introduction
Organic carbamates have emerged as important intermediates
commonly employed in the synthesis of several agrochemicals
and pharmaceutical candidates as well as a variety of biologi-
vantages of the ease of handling, simple workup, recyclability,
[13]
and reusability. Furthermore, using solid catalysts would also
decrease contamination of the desired products with hazard-
[
1,2]
[14,15]
cally active natural products.
These structures are conven-
ous or harmful metals.
tionally synthesized from chloroformates or isocyanates with
phosgene substitutes as starting materials, thus suffering dis-
Metal-organic frameworks (MOFs) have attracted significant
attention as promising materials with potential applications in
gas separation and storage, sensors and luminescence, drug
storage and delivery, templated low-dimensional material prep-
[
3]
advantages in terms of health and safety issues. The transi-
tion-metal-catalyzed oxidative carbonylation reactions of
amines or the Hofmann and Curtius rearrangements were used
[16–21]
aration, and catalysis.
Combining some special properties
[
4–6]
as greener alternatives to the traditional approach.
Howev-
of both organic and inorganic porous materials, MOFs could
offer unexpected catalytic activity in organic synthesis. Al-
though using MOFs as catalysts or catalyst supports is a young
research area, several MOF-catalyzed organic transforma-
er, these methods require the preparation of prefunctionalized
starting materials, being a major concern from atom-economi-
[
7–9]
cal and environmental aspects.
coupling reaction by the direct activation of CÀH bonds would
offer shorter and more efficient synthetic schemes. DMF
could be employed as a versatile building block for various
The cross-dehydrogenative
[
22,23]
[24–31]
tions,
ranging from carbonÀcarbon
to carbonÀheter-
[
8,10]
[32–35]
oatom
forming reactions, have been investigated and re-
ported in the literature. As a variety of MOF structures contain
metal sites with potential coordinative unsaturation, it is ex-
pected that their application in the field of heterogeneous cat-
alysis would be an area that will attract further research in the
[
11]
units, including Me NCO, CO, Me N, and oxygen. Recently,
2
2
Reddy and co-workers reported the first example of the cou-
pling reaction between 2-carbonyl-substituted phenols and
[36–38]
DMF to form carbamates using Cu(OAc) as a homogeneous
near future.
In this work, we wish to report a copper-cata-
2
catalyst in the presence of tert-butyl hydroperoxide as the oxi-
lyzed cross-dehydrogenative coupling reaction of DMF with 2-
substituted phenols to form organic carbamates using Cu2-
[
7]
dant. Chang and co-workers then expanded the protocol and
demonstrated that CuCl exhibited high activity for the same
(BPDC) (BPY) (BPDC=4,4’-biphenyldicarboxylate, BPY=4,4’-bi-
2
[
12]
reaction. To improve the green credentials of the reaction,
heterogeneous catalysts should be employed to achieve ad-
pyridine) as an efficient heterogeneous catalyst. Although the
synthesis procedure and the crystal structure of the Cu2-
(
BPDC) (BPY) was previously investigated by James and co-
2
[39]
workers, the application of this Cu–MOF in the field of catal-
ysis was not reported. The catalyst could be recovered and
reused without a significant degradation in catalytic activity.
The Cu (BPDC) (BPY) also offered significantly higher catalytic
[
a] Dr. N. T. S. Phan, T. T. Nguyen, P. H. L. Vu
Department of Chemical Engineering
HCMC University of Technology, VNU-HCM
2
68 Ly Thuong Kiet, District 10, Ho Chi Minh City (Vietnam)
2
2
Fax: (+84)8-38637504
E-mail: ptsnam@hcmut.edu.vn
activity than other Cu–MOFs such as Cu (BTC) , Cu(BDC), and
3
2
Cu(BPDC) (BDC=1,4-benzenedicarboxylate; BTC=1,3,5-benze-
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/cctc.201300400.
netricarboxylate). To the best of our knowledge, the carbamate
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formation through the oxidative cross-dehydrogenative cou-
pling of phenols and formamides in the presence of a hetero-
geneous catalyst was not previously mentioned in the
literature.
oven for 24 h. After cooling the vials to RT, the solid product was
removed by decanting with mother liquor and washed in DMF (3ꢁ
1
0 mL) for 3 d. Solvent exchange was performed with methanol
(3ꢁ10 mL) at RT for 3 d. The material was then evacuated under
vacuum at 1408C for 6 h, yielding 0.121 g of Cu (BPDC) (BPY) in
2
2
the form of blue crystals (79.3% based on copper nitrate).
Experimental Section
Catalytic studies
Materials and instrumentation
In a typical experiment, a mixture of 2-hydroxybenzaldehyde
0.143 mL, 1 mmol) and n-hexadecane (0.1 mL, 0.88 mmol) as an in-
All reagents and starting materials were obtained commercially
from Sigma–Aldrich and Merck, and were used as received without
any further purification unless otherwise noted. Nitrogen physi-
sorption measurements were conducted using a Micromeritics
(
ternal standard in DMF (4 mL, 50 mmol) was added into a 25 mL
flask containing the predetermined amount of Cu (BPDC) (BPY) as
2
2
the catalyst and tert-butyl hydroperoxide (70 wt.% in water;
.218 mL, 1.5 mmol) as an oxidant. The catalyst concentration was
2
020 volumetric adsorption analyzer system. Samples were pre-
0
treated by heating under vacuum at 1508C for 3 h. A Netzsch Ther-
calculated with respect to the copper/2-hydroxybenzaldehyde
molar ratio. The reaction mixture was stirred at 1008C for 120 min.
Reaction conversion was monitored by withdrawing aliquots from
the reaction mixture at different time intervals, quenching with
water (1 mL). The organic components were then extracted into di-
ethyl ether (2 mL), dried over anhydrous Na SO , analyzed by GC
moanalyzer STA 409 was used for thermogravimetric analysis (TGA)
À1
with a heating rate of 108Cmin under a nitrogen atmosphere.
XRD patterns were recorded by using a CuKa radiation source on
a D8 Advance Bruker powder diffractometer. SEM studies were
conducted on a S4800 Scanning Electron Microscope. TEM studies
were performed using a JEOL JEM 1400 Transmission Electron Mi-
croscope at 100 kV. The Cu (BPDC) (BPY) sample was dispersed on
2
4
with reference to n-hexadecane. The product identity was further
2
2
confirmed by GC–MS. To investigate the recyclability of Cu (BPDC) -
2
2
holey carbon grids for TEM observation. Elemental analysis with
atomic absorption spectrophotometry (AAS) was performed on an
AA-6800 Shimadzu. FTIR spectra were obtained on a Nicolet 6700
instrument, with samples dispersed on potassium bromide disks.
The chemisorption experiments were studied in a Micromeritics
(BPY), the catalyst was separated from the reaction mixture by
simple centrifugation, washed with copious amounts of methanol,
dried under air, and reused if necessary. For the leaching test, a cat-
alytic reaction was stopped after 10 min, analyzed by GC, and cen-
trifuged to remove the solid catalyst. The reaction solution was
then stirred for a further 110 min. Reaction progress, if any, was
monitored by GC as previously described.
2
020 analyzer. For hydrogen temperature programmed reduction
(
H –TPR), the sample was outgassed at 1008C for 30 min with
2
À1
helium, then cooled down to RT, and exposed to 50 mLmin of
1
À1
0% H /Ar as the temperature ramped at 2.58Cmin to 6008C.
2
The amount of hydrogen consumption was determined from TCD
Results and Discussion
signal intensities, which were calibrated using an Ag O reference
sample.
2
Catalyst synthesis and characterization
Gas chromatographic (GC) analyses were performed by using a Shi-
madzu GC 2010-Plus equipped with a flame ionization detector
Herein, the Cu (BPDC) (BPY) was synthesized by a solvothermal
2
2
[39]
method, as developed by James and co-workers. The Cu–
MOF was previously reported to possess interpenetrated pil-
(
0
FID) and an SPB-5 column (length=30 m, inner diameter=
.25 mm, and film thickness=0.25 mm). The temperature program
II
for GC analysis held samples at 1008C for 0.5 min; heated them
lared-grid structure based on Cu paddle-wheels, 4,4’-biphenyl-
À1
[39]
from 100 to 1308C at 408Cmin ; heated them from 130 to 1808C
dicarboxylate, and 4,4’-bipyridine. The Cu (BPDC) (BPY) was
2
2
À1
at 408Cmin and held them at 1808C for 1 min; heated them
characterized by using a variety of different techniques, and
the result was in good agreement with the literature. Elemen-
À1
from 1808C to 2808C at 508Cmin and held them at 2808C for
2
min. Inlet and detector temperatures were set constant at 2808C.
À1
tal analysis by AAS indicated a copper loading of 2.5 mmolg .
n-Hexadecane was used as an internal standard to calculate reac-
tion conversions. GC–MS analyses were performed by using a Hew-
lett Packard GC–MS 5972 with a RTX-5MS column (length=30 m,
inner diameter=0.25 mm, and film thickness=0.5 mm). The tem-
The X-ray diffraction peaks of the Cu (BPDC) (BPY) (see the
2
2
Supporting Information, Figure S1) were consistent with the
theoretical patterns from the single crystal data previously re-
[39]
perature program for GC–MS analysis heated samples from 60 to
ported in the literature. As expected, the SEM micrograph of
the Cu (BPDC) (BPY) showed that a crystalline material was
À1
2
808C at 108Cmin and held them at 2808C for 2 min. Inlet tem-
2
2
perature was set constant at 2808C. MS spectra were compared
with the spectra gathered in the NIST library.
achieved (Figure S2). The TEM observation indicated that the
Cu (BPDC) (BPY) possessed a porous structure (Figure S3),
2
2
though the pore of the Cu-MOF appeared to be complex and
different from that of conventional microporous and mesopo-
rous materials. Nitrogen physisorption measurements con-
firmed that the Cu (BPDC) (BPY) would contain mainly micro-
Synthesis of the MOF Cu (BPDC) (BPY)
2
2
The Cu (BPDC) (BPY) was synthesized by following the procedure
2
2
2
2
[39]
previously reported by James and co-workers. In a typical prepa-
ration, a solid mixture of H BPDC (H BPDC=4,4’-biphenyldicarbox-
porous (diameter <20 ꢂ) pores, with a median pore width of
2
2
2
À1
7
.6 ꢂ (Figure S4). Langmuir surface areas of 1675 m g were
ylic acid; 0.1039 g, 0.4 mmol), BPY (0.033 g, 0.2 mmol), and Cu-
observed for the material, as calculated from nitrogen adsorp-
tion/desorption isotherm data (Figure S5). Although James and
co-workers previously reported the synthesis procedure and
the crystal structure of the Cu (BPDC) (BPY), surface areas and
(
(
NO ) ·3H O (0.105 g, 0.4 mmol) was dissolved in a mixture of DMF
3 2 2
30 mL), pyridine (0.3 mL), and methanol (3 mL). The resulting solu-
tion was stirred at 708C for 5 min, and then distributed to four
0 mL vials. The vials were then heated at 1208C in an isothermal
2
2
2
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[39]
pore size distribution of the Cu–MOF were not mentioned.
TGA results indicated that the Cu–MOF was stable up to over
758C (Figure S6). FTIR spectra of the Cu (BPDC) (BPY) exhibit-
2
2
2
ed a significant difference as compared to those of the
H BPDC and the BPY linkers (Figure S7). The H –TPR result ex-
2
2
hibited the nature of copper species within the Cu (BPDC) -
2
2
(
BPY). Two broad reduction peaks were observed at 3408C and
108C, attributed to the reduction of Cu and Cu ions,
2
+
+
[40,41]
4
respectively (Figure S8). However, further investigations would
be necessary to clarify the nature of copper sites in the Cu–
MOF structure.
Catalytic studies
The Cu (BPDC) (BPY) was used as a heterogeneous catalyst for
2
2
Figure 1. Effect of temperature on reaction conversion. 60 (~), 80 (^), 1008C
the cross-dehydrogenative coupling reaction of 2-hydroxyben-
zaldehyde with DMF to form 2-formylphenyl dimethylcarba-
mate as the principal product (Scheme 1). Initial studies ad-
&
( ).
and 1:25 (i.e., decreasing the volume of DMF) resulted in
a drop in the reaction rate, with 65% and 48% conversion ob-
served after 120 min, respectively (Figure 2).
Scheme 1. The cross-dehydrogenative coupling reaction of DMF with 2-sub-
stituted phenols to form organic carbamates using the Cu
catalyst.
2 2
(BPDC) (BPY)
dressed the effect of temperature on the reaction conversion,
having performed the coupling reaction in DMF at 5 mol%
Cu (BPDC) (BPY) catalyst, with the 2-hydroxybenzaldehyde/
2
2
DMF molar ratio of 1:50, in the presence of one equivalent of
tert-butyl hydroperoxide as an oxidant, at 608C, 808C, and
1008C, respectively. Aliquots were withdrawn from the reaction
Figure 2. Effect of 2-hydroxybenzaldehyde/DMF molar ratio on reaction
mixture at different time intervals and analyzed by GC, giving
kinetic data during the course of the reaction. It was found
that the reaction performed at 1008C could lead to 70% con-
version after 120 min. As expected, decreasing the reaction
temperature to 808C resulted in a significant drop in the reac-
tion rate, with 42% conversion observed after 120 min. The
coupling reaction proceeded with difficulty at 608C, affording
a conversion of only 12% after 120 min (Figure 1). In the first
example of the coupling between 2-carbonyl-substituted phe-
conversion. 1:25 (~), 1:37.5 (^), 1:50 (
).
&
Another factor that should be considered for the Cu2-
(BPDC) (BPY)-catalyzed coupling reaction of 2-hydroxybenzal-
2
dehyde with DMF to form 2-formylphenyl dimethylcarbamate
is the catalyst concentration. The coupling reaction was then
performed at 1008C in DMF, with the 2-hydroxybenzaldehyde/
DMF molar ratio of 1:50, in the presence of one equivalent of
tert-butyl hydroperoxide as the oxidant, at 1 mol%, 3 mol%,
and 5 mol% Cu (BPDC) (BPY) catalyst, respectively. It should be
nols and DMF to form carbamates using Cu(OAc) as a homo-
2
geneous catalyst, the reaction was performed successfully at
2
2
8
08C, though 1.5 equivalents of tert-butyl hydroperoxide as
noted that almost no coupling reaction was detected in the
absence of Cu (BPDC) (BPY), confirming the necessity of using
[
7]
the oxidant were required. Chang and co-workers then inves-
tigated the same transformation, and demonstrated that the
coupling reaction could proceed readily at 708C in the pres-
ence of CuCl as catalyst. However, the cross-dehydrogenative
coupling protocol employed up to 6 equivalents of tert-butyl
2
2
the Cu–MOF as catalyst for the cross-dehydrogenative cou-
pling reaction. It was observed that the reaction using 1 mol%
catalyst could afford 38% conversion after 120 min. As expect-
ed, if increasing the Cu–MOF concentration from 1 mol% to
3 mol%, the reaction rate was enhanced significantly, with
57% conversion obtained after 120 min. Furthermore, the reac-
[
12]
hydroperoxide as the oxidant.
Moreover, it was observed
that decreasing the reagent molar ratio from 1:50 to 1:37.5
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Figure 4. Effect of oxidant on reaction conversion. no oxidant (~), H
AgNO ), PhI(OAc) CO ).
(*), K
(~), Ag
(^), TBHP (
O
2
(^),
Figure 3. Effect of catalyst concentration on reaction conversion. 0 (~), 1 (^),
2
3
(
&
2
2
S
2
O
8
2
3
&
3
(
), 5 mol% (*).
&
[7]
[44]
Reddy, Chang
and co-workers previously employed tert-
tion rate could be improved by using 5 mol% Cu (BPDC) (BPY)
butyl hydroperoxide as the oxidant for the coupling reaction
between 2-carbonyl-substituted phenols and DMF to form car-
bamates. Moreover, experimental results revealed that the con-
centration of the tert-butyl hydroperoxide exhibited a profound
effect on the reaction rate, having performed the coupling re-
action in the presence of 1, 1.2, and 1.5 equivalents of tert-
butyl hydroperoxide as the oxidant, respectively. It was found
that increasing the concentration of tert-butyl hydroperoxide
led to a significant enhancement in the reaction rate. Conver-
sions of 88% and 97% were obtained after 120 min for the re-
action using 1.2 and 1.5 equivalents of tert-butyl hydroperox-
ide as the oxidant, respectively (Figure 5). To examine the mass
balance of the reaction, a control experiment was performed
2
2
catalyst, achieving 70% conversion after 120 min (Figure 3).
Indeed, Reddy and co-workers previously reported that the
coupling reaction between 2-carbonyl-substituted phenols and
DMF to form carbamates could proceed readily in the presence
[
7]
of 5 mol% Cu(OAc) catalyst. Chang and co-workers pointed
2
out that by using CuCl as the catalyst instead of Cu(OAc) for
2
the same reaction, the catalyst concentration could be reduced
[
12]
to 1 mol%. For other cross-dehydrogenative coupling reac-
tions through direct CÀH activation, the copper catalyst con-
centration could vary from 2 mol% to 20 mol%, depending on
the nature of the catalyst as well as of the substrate. These re-
actions included the coupling of N,N-dialkylformamides with b-
keto esters and 2-carbonyl-substituted phenols (5 mol% CuBr
2
[
7]
or 5 mol% Cu(OAc)2), the coupling of carboxylic acids with
[
42]
N,N-dialkylformamides (10 mol% Cu(ClO ) ·6H O),
and the
4
2
2
oxidative amidation of aldehydes with secondary amines (2–
[
43]
20 mol% CuI).
As for other oxidative cross-dehydrogenative coupling reac-
[
8]
tion by the direct activation of CÀH bonds, at least one
equivalent of an oxidant should be employed for the Cu -
2
(
BPDC) (BPY)-catalyzed coupling between 2-hydroxybenzalde-
2
hyde and DMF to form 2-formylphenyl dimethylcarbamate. It
was therefore decided to investigate the effect of different oxi-
dants on the reaction conversion. The coupling reaction was
performed at 1008C in DMF, using a 2-hydroxybenzaldehyde/
DMF molar ratio of 1:50, with 5 mol% Cu (BPDC) (BPY) catalyst,
2
2
in the presence of one equivalent of the oxidant, including
tert-butyl hydroperoxide, hydrogen peroxide, AgNO , Ag CO ,
3
2
3
PhI(OAc) , and K S O , respectively. It was observed that tert-
2
2
2
8
Figure 5. Effect of oxidant concentration on reaction conversion. 1 (~), 1.2
butyl hydroperoxide exhibited the best performance in the
Cu (BPDC) (BPY)-catalyzed reaction of 2-hydroxybenzaldehyde
(
^), 1.5 equiv (
).
&
2
2
with DMF, with 70% conversion achieved after 120 min. How-
ever, the reaction using hydrogen peroxide as the oxidant oc-
curred with significantly more difficulty, affording only 25%
conversion after 120 min. Similarly, AgNO , Ag CO , PhI(OAc) ,
to determine the weight of the product at 97% conversion of
2-hydroxybenzaldehyde. It was found that an isolated yield of
88% was obtained for the 2-formylphenyl dimethylcarbamate
product, indicating that approximately 9% product was lost
during the separation process. It should be noted that no by-
3
2
3
2
and K S O offered low reactivity and should not be used as
2
2
8
the oxidant for the coupling reaction (Figure 4). Indeed,
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product was detected by GC–MS, which confirmed the selec-
tivity of 100% to 2-formylphenyl dimethylcarbamate.
ever, further investigation would be necessary to elucidate the
reaction mechanism of the Cu (BPDC) (BPY)-catalyzed cross-de-
2
2
To verify the necessity of the oxidant in the cross-dehydro-
genative coupling reaction of 2-hydroxybenzaldehyde and
DMF to form 2-formylphenyl dimethylcarbamate, ascorbic acid
as the antioxidant was added to the reaction mixture after the
first 10 min. If the reaction conversion was not detected in the
presence of the antioxidant, this observation would imply that
the catalytically active copper sites on the Cu–MOF could be
poisoned by the ascorbic acid or the radicals produced in the
catalytic cycle could be decomposed. The coupling reaction
was then performed at 1008C in DMF, with the 2-hydroxyben-
zaldehyde/DMF molar ratio of 1:50, in the presence of
hydrogenative coupling reaction between 2-hydroxybenzalde-
hyde and DMF.
For organic transformations using solid catalysts, reaction
solvents could exhibit a significant effect on the reaction rate,
[45,46]
depending on the nature of the catalyst.
In the cross-dehy-
drogenative coupling reaction of 2-hydroxybenzaldehyde with
DMF to form 2-formylphenyl dimethylcarbamate, a large
excess of DMF should be used to act as the reaction medium.
The effect of different cosolvents on the coupling reaction
using the Cu (BPDC) (BPY) catalyst was then investigated. The
2
2
reaction was performed in the mixture of DMF and the cosol-
vent (1:1 by volume) at 1008C in DMF, in the presence of one
equivalent of tert-butyl hydroperoxide as the oxidant, at
5 mol% Cu (BPDC) (BPY) catalyst, with the 2-hydroxybenzalde-
1
5
0
.5 equivalents of tert-butyl hydroperoxide as the oxidant, with
mol% Cu (BPDC) (BPY) catalyst. After 10 min reaction time,
2
2
.1, 0.25, and 0.5 mol% of ascorbic acid as the antioxidant was
2
2
then added to the reaction mixture, respectively. The mixture
was then stirred for an additional 110 min at 1008C, and ali-
quots were sampled at different time intervals and analyzed by
GC. It was found that adding the antioxidant to the reaction
mixture suppressed the reaction. Indeed, the coupling reaction
was stopped immediately after adding 0.5 mol% ascorbic acid.
Furthermore, one more control experiment was also performed
by adding 0.5 mol% of the radical scavenger 2,2,6,6-tetrame-
thylpiperidine-1-oxyl (TEMPO) to the reaction mixture after
hyde/DMF molar ratio of 1:50. It was observed that the reac-
tion rate was decreased significantly in the presence of the co-
solvent, indicating that the coupling reaction should be per-
formed in DMF. Indeed, without using cosolvent, the reaction
could proceed to 97% conversion after 120 min. A conversion
of only 50% was observed after 120 min for the reaction using
n-butanol as the co-solvent. DMSO was also found to be un-
suitable as the cosolvent for the reaction, with 58% conversion
obtained after 120 min. The value could be improved to 85%
and 93% for the case of p-xylene and acetonitrile as the cosol-
vent. The reaction performed in the mixture of DMF and ethyl-
acetate also proceeded to 97% conversion after 120 min,
though lower rate was observed as compared to the case of
DMF (Figure 7).
1
0 min. It was observed that the presence of the TEMPO pre-
vented the cross-dehydrogenative coupling reaction to occur
Figure 6). As the total amount of the copper was 10 times
(
Figure 6. Adding antioxidant to the reaction mixture after 10 min. No antiox-
idant (~), 0.1 mol% ascorbic acid (^), 0.25 mol% ascorbic acid (
&
), 0.5 mol%
ascorbic acid (*), 0.5 mol% TEMPO (^).
Figure 7. Effect of cosolvent on reaction conversion. DMF (~), DMSO (^), p-
xylene (
), acetonitrile (*), n-butanol (~), ethylacetate (^).
&
higher than that of the ascorbic acid or the TEMPO, the ad-
sorption of the ascorbic acid or the TEMPO on catalytically
active copper sites on the Cu–MOF should not be the reason
of this behavior. It could be proposed that the cross-dehydro-
genative coupling reaction was significantly affected by the in-
teraction of the antioxidant with the radicals in the catalytic
cycle. It should be noted that almost no reaction was observed
in the absence of the oxidant, confirming the necessity of the
oxidant in the cross-dehydrogenative coupling reaction. How-
To emphasize the advantages of employing the Cu (BPDC) -
2
2
(BPY) as the catalyst for the cross-dehydrogenative coupling
reaction of 2-hydroxybenzaldehyde with DMF, the reaction was
also performed by using common copper salts as homogene-
ous catalysts, including Cu(OAc)2, CuCl, CuCl2, CuI, and
Cu(NO ) . In the first example of the coupling reaction between
3
2
2-carbonyl-substituted phenols and DMF to form carbamates,
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Reddy and co-workers investigated the activity of several
copper salts, and reported that Cu(OAc) was the best catalyst
2
[
7]
for the transformation. Chang and co-workers also employed
several copper salts as catalysts for the same reaction, and
[
12]
demonstrated that CuCl was the catalyst of choice. The cou-
pling reaction was then performed at 1008C in DMF, with the
2-hydroxybenzaldehyde/DMF molar ratio of 1:50, in the pres-
ence of 1.5 equivalents of tert-butyl hydroperoxide as the oxi-
dant, with 5 mol% copper catalyst. It was found that the
Cu(OAc) -catalyzed coupling reaction could afford 90% conver-
2
sion after 120 min. Cu(NO ) offered similar activity for the cou-
3
2
pling reaction, with 90% conversion achieved after 120 min. It
was also observed that CuI, CuCl, and CuCl exhibited lower ac-
2
tivity than Cu(OAc) , though conversions of 61%, 77%, and
2
8
5% were still obtained after 120 min. Notably, the Cu (BPDC) -
2 2
Figure 9. Effect of different Cu–MOFs as catalysts on reaction conversion.
(BPY)-catalyzed coupling reaction could proceed to 97% after
Cu
2
(BPDC)
2
(BPY) (^), Cu
3
(BTC)
2
(*), Cu (BPDC) (~), CuBDC (
).
&
120 min (Figure 8). Furthermore, the catalytic activity of the
thus preventing the formation of catalytically inactive compact
structure. Although the reaction was performed in DMF, the
presence of the BPYs in the Cu-MOF structure would make
more active sites accessible to the reactants. However, further
studies would be necessary to clarify the difference in catalytic
activity between these Cu–MOFs.
If a solid catalyst is employed for a liquid-phase organic
transformation, an important issue that should be taken into
account is the possibility that some of active sites could dis-
solve into the reaction solution during the course of the reac-
tion. In several cases, the reaction was partially or totally cata-
lyzed by these leached species, thus implying that the transfor-
mation would not occur under real heterogeneous catalysis
[46]
conditions. To determine if active copper species dissolved
from the solid Cu (BPDC) (BPY) catalyst contribute to the total
2
2
conversion of the cross-dehydrogenative coupling reaction of
-hydroxybenzaldehyde with DMF, a control experiment was
2
Figure 8. Effect of different copper salts as catalysts on reaction conversion.
Cu (BPDC)
(^), CuCl (^).
(BPY) (~), Cu(NO ), CuCl (*), CuI (~), Cu(OAc)
2
2
3
)
2
2
(
&
2
performed using a simple centrifugation during the course of
the reaction. After the solid Cu–MOF was separated from the
reaction mixture, if the catalytic reaction continued, this behav-
ior would indicate that the active species were from the solu-
tion rather than from the solid Cu (BPDC) (BPY) catalyst. The
Cu (BPDC) (BPY) in the coupling reaction of 2-hydroxybenzal-
2
2
dehyde with DMF was also compared with that of other Cu–
MOFs including Cu (BTC) , Cu(BDC), and Cu(BPDC), respectively.
2
2
coupling reaction was then performed at 1008C in DMF, with
the 2-hydroxybenzaldehyde/DMF molar ratio of 1:50, in the
presence of 1.5 equivalents of tert-butyl hydroperoxide as the
oxidant, with 5 mol% Cu (BPDC) (BPY) catalyst. After 10 min re-
3
2
These Cu–MOFs were synthesized by a solvothermal method,
[
47–50]
according to literature procedures.
Interestingly, it was ob-
served that the Cu (BPDC) (BPY) exhibited significantly higher
2
2
2
2
activity than other Cu–MOFs in the coupling reaction. Al-
though the well-known Cu (BTC) previously exhibited high ac-
action time with 65% conversion detected, the Cu (BPDC) -
2 2
(BPY) catalyst was separated from the reaction mixture by
simple centrifugation. The solution phase was then transferred
to a new reactor vessel, and stirred for an additional 110 min
at 1008C, with aliquots being sampled at different time inter-
vals, and analyzed by GC. Experimental results showed that
almost no further conversion was detected after the Cu2-
3
2
[
34,35,49,51–55]
tivity in several copper-catalyzed organic reactions,
the cross-dehydrogenative coupling reaction of 2-hydroxyben-
zaldehyde with DMF using this Cu–MOF proceeded to only
78% conversion after 120 min. Furthermore, low conversions
were also observed for the transformation using Cu(BDC) or
Cu(BPDC) as the catalysts (Figure 9). Corma and co-workers
previously proposed that the desolvated Cu(BDC) would pos-
sess a compact structure, and that that heating in DMF should
be a good treatment to regenerate the catalytically active
(BPDC) (BPY) catalyst was removed from the reaction mixture
2
(Figure 10). These observations confirmed that the coupling re-
action of 2-hydroxybenzaldehyde with DMF could only pro-
ceed in the presence of the solid Cu–MOF catalyst, and there
was no contribution from leached active copper species in the
liquid phase.
[
51]
phase of the Cu(BDC). In the Cu (BPDC) (BPY) structure, the
2
2
[39]
BPDCs act as grid-forming ligands and the BPYs as pillars,
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Figure 11. Catalyst recycling studies.
Figure 10. Leaching tests indicated no contribution from homogeneous
catalysis of active species leaching into the reaction solution. 5 mol% Cu -
2
(BPDC)
2
(BPY) (~), removing catalyst after 10 min (^),
Although high conversions were obtained for the
cross-dehydrogenative coupling reaction of 2-hydrox-
ybenzaldehyde with DMF using Cu(OAc) or Cu(NO )
2
3 2
as catalyst, it was apparent that the problem of cata-
lyst recycling and reuse would remain to be solved.
For the development of more environmentally
benign processes, heterogeneous catalysts should be
used to obtain advantages of the ease of handling,
simple workup, recyclability, and reusability. It is tar-
geted that the solid catalyst can be facilely separated
from the reaction mixture, and can be reused several
times before it eventually deactivates completely. In
this research, the Cu (BPDC) (BPY) catalyst was inves-
2
2
tigated for recoverability and reusability in the cross-
dehydrogenative coupling reaction of 2-hydroxyben-
zaldehyde with DMF over six successive runs, by re-
peatedly separating the Cu–MOF catalyst from the re-
action mixture, washing it and then reusing it. The
2 2
Figure 12. FTIR spectra of a) the fresh Cu (BPDC) (BPY) and b) after the 1st use.
coupling reaction was then performed at 1008C in DMF, with
the 2-hydroxybenzaldehyde/DMF molar ratio of 1:50, in the
presence of 1.5 equivalents of tert-butyl hydroperoxide as the
oxidant, with 5 mol% Cu (BPDC) (BPY) catalyst. After the cou-
Cu (BPDC) (BPY) showed that a slight difference in the overall
2 2
structure was observed for the reused Cu–MOF. It was found
that the sharp peak at 2q=68 remained unchanged even after
th
the 6 use. However, other peaks on the XRD result of the Cu–
2
2
pling reaction was complete, the Cu (BPDC) (BPY) catalyst was
MOF after the 6th use were found to be different to those of
the fresh catalyst, indicating the formation of a new phase in
the Cu (BPDC) (BPY) (Figure 13). Further investigations would
2
2
separated by simple centrifugation, then washed with copious
amounts of methanol to remove any physisorbed reagents,
and dried at room temperature under air. The recovered Cu2-
2
2
be needed to clarify the transformation of the Cu–MOF during
the course of the reaction.
(
BPDC) (BPY) was then reused in further reactions under identi-
2
cal conditions to those of the first run. It was found that the
Cu (BPDC) (BPY) catalyst could be recovered and reused sever-
The study was then extended to the cross-dehydrogenative
coupling reaction between DMF with different ortho-substitut-
ed phenols, including 2-hydroxybenzaldehyde, methylsalicy-
late, 2-methylphenol, 2’-hydroxyacetophenone, 2-nitrophenol,
and phenol, respectively. The coupling reaction was then per-
formed at 1008C in DMF, with the phenol/DMF molar ratio of
1:50, in the presence of 1.5 equivalents of tert-butyl hydroper-
oxide as the oxidant, with 5 mol% Cu (BPDC) (BPY) catalyst. It
2
2
al times for the coupling reaction without a significant degra-
dation in catalytic activity. Indeed, a conversion of 84% was
still achieved after 120 min in the 6th run (Figure 11). To con-
firm the recoverability and reusability of the Cu (BPDC) (BPY)
2
2
in the coupling reaction of 2-hydroxybenzaldehyde with DMF,
the recovered Cu–MOF catalyst was also characterized by FTIR
and XRD. The FTIR spectra of the reused Cu (BPDC) (BPY) indi-
2
2
was observed that the phenol structure exhibited a profound
influence on the Cu (BPDC) (BPY)-catalyzed cross-dehydrogen-
2
2
cated a similar absorption to that of the fresh Cu–MOF
Figure 12). Moreover, XRD results of the fresh and the reused
2
2
(
ative coupling reaction with DMF. Phenol was almost unreac-
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mide molar ratio of 1:50, in the presence of 1.5 equiv-
alents of tert-butyl hydroperoxide as the oxidant,
with 5 mol% Cu (BPDC) (BPY) catalyst. It was ob-
2
2
served that DEF and NFP exhibited lower reactivity
than DMF, with 63% and 48% conversions detected
after 120 min in the reaction with 2-hydroxybenzal-
dehyde. Similarly, the cross-dehydrogenative cou-
pling reaction between methylsalicylate with DEF or
NFP also proceeded with a lower rate than with DMF,
though 55% and 52% conversions were still ob-
tained after 120 min. As expected, 2’-hydroxyaceto-
phenone offered higher reactivity than 2-hydroxy-
benzaldehyde and methylsalicylate. Indeed, the Cu2-
(
BPDC) (BPY)-catalyzed coupling reaction of 2’-hy-
2
droxyacetophenone with DEF and NFP afforded 78%
and 81% conversions after 120 min (Figure 15).
Moreover, control experiments were performed to
assess if the reaction occurred on the external surface
2 2
Figure 13. X-ray powder diffractogram of a) the fresh Cu (BPDC) (BPY), b) after 1st use,
and c) after the 6th use.
tive in the coupling reaction, with only 8% conversion being
observed after 120 min. The presence of a substituent at the
ortho position led to an enhancement in the reaction rate. The
cross-dehydrogenative coupling reaction between 2’-hydroxya-
cetophenone and DMF could proceed to completion, with
quantitative conversion achieved after 60 min. 2-Hydroxyben-
zaldehyde and methylsalicylate were also found to be reactive
towards the Cu (BPDC) (BPY)-catalyzed cross-dehydrogenative
2
2
coupling reaction with DMF, affording 97% and 92% conver-
sions after 120 min, respectively. However, 2-methylphenol and
2
-nitrophenol exhibited low reactivity in the coupling reaction
with DMF, with 38% and 42% conversions detected after
20 min, respectively (Figure 14). The substrate scope investi-
1
gation was also expanded to the cross-dehydrogenative cou-
pling reaction between 2-hydroxybenzaldehyde, methylsalicy-
late, and 2’-hydroxyacetophenone with different formamides,
including N,N-diethylformamide (DEF), N,N-dimethylformamide
Figure 15. Effect of different formamides on reaction conversion. 2-hydroxy-
benzaldehyde+DMF (~), 2-hydroxybenzaldehyde+NFP (^), 2-hydroxyben-
zaldehyde+DEF (
&
), 2’-hydroxyacetophenone+DMF ( ), 2’-hydroxyaceto-
&
phenone+NFP (*), methylsalicylate+DEF (^), methylsalicylate+NFP (~).
(DMF), and N-formylpiperidine (NFP), respectively. The coupling
reaction was also performed at 1008C with the phenol/forma-
or in the internal pore structure of the Cu (BPDC) (BPY). Mat-
2
2
suoka and co-workers previously reported that the larger pore
size of M–MOFs (M=Mo or Cr) using biphenylene rings as or-
ganic linkers instead of phenylene rings could lead to higher
[56]
catalytic activity owing to the easier substance diffusion. Su
and co-workers also investigated whether the interior active
sites of the Zn-MOF could contribute to the catalysis by con-
ducting the carbonyl cyanosilylation of different aldehydes
[57]
with increasing dimension. It was therefore decided to inves-
tigate the effect of phenol size on reaction conversion. The CÀ
H oxidative coupling was then performed at 1008C in DMF,
with the phenol/DMF molar ratio of 1:50, in the presence of
6
5
equivalents of tert-butyl hydroperoxide as the oxidant, with
mol% Cu (BPDC) (BPY) catalyst, using phenol, 4-phenylphe-
2
2
nol, and 2-naphthol as the substrate, respectively. It should be
noted that the cross-dehydrogenative coupling reaction of
phenol, 4-phenylphenol, and 2-naphthol with DMF proceeded
with difficulty in the presence of 1.5 equivalents of tert-butyl
Figure 14. Effect of different ortho-substituted phenols on reaction conver-
sion. 2-hydroxybenzaldehyde (~), 2’-hydroxyacetophenone (
late (*), 2-nitrophenol (~), ^ 2-methylphenol, phenol (
).
&
), methylsalicy-
&
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hydroperoxide, an excess amount of the oxidant was required
to improve the reaction conversion. Indeed, Chang and co-
workers previously employed up to 6 equivalents of tert-butyl
hydroperoxide for the same reaction using CuCl as the cata-
the cross-dehydrogenative coupling reaction of DMF with 2-
substituted phenols could only proceed in the presence of the
solid Cu (BPDC) (BPY) catalyst, and there was no contribution
2
2
from leached copper sites, if any, in the reaction solution. Our
results here demonstrate the feasibility of applying Cu–MOFs
as heterogeneous catalysts for several cross-dehydrogenative
coupling reactions through CÀH activation under oxidative
conditions.
[
12]
lyst. It was found that 2-naphthol and 4-phenylphenol of-
fered relatively higher reactivity than that of phenol in the cou-
pling reaction with DMF (Figure 16). It should be noted that
Acknowledgements
The Vietnam National University-Ho Chi Minh City (VNU-HCM) is
acknowledged for financial support through project No. B2013-
20-05.
Keywords: aldehydes · CÀH activation · copper · metal-
organic frameworks · oxidation
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Received: May 24, 2013
Revised: June 27, 2013
Published online on && &&, 0000
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FULL PAPERS
A perfect match: The crystalline porous
N. T. S. Phan,* T. T. Nguyen, P. H. L. Vu
& – &&
metal–organic framework Cu (BPDC) -
2
2
&
(BPY) (BPDC=4,4’-biphenyldicarboxy-
late, BPY=4,4’-bipyridine) proves to be
an efficient and reusable catalyst for the
synthesis of organic carbamates
A Copper Metal–Organic Framework
as an Efficient and Recyclable Catalyst
for the Oxidative Cross-
through the copper-catalyzed cross-de-
hydrogenative coupling reaction of
DMF with 2-substituted phenols.
Dehydrogenative Coupling of Phenols
and Formamides
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ChemCatChem 0000, 00, 1 – 11
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ÞÞ
These are not the final page numbers!