Highly selective reductive cleavage of aromatic carbon-oxygen bonds catalyzed by a cobalt compound

Article abstract of DOI:10.1016/j.catcom.2014.03.036

A cobalt catalyst has been demonstrated, for the first time, to be effective for the reductive cleavage of inert aromatic CO bonds with high selectivity. Compared with previous Ni catalysts, the cobalt catalyst reported here is more commercially available and air-stable.

Full text of DOI:10.1016/j.catcom.2014.03.036

Catalysis Communications 52 (2014) 36–39
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Short Communication
Highly selective reductive cleavage of aromatic carbon–oxygen bonds
catalyzed by a cobalt compound
Yun-Lai Ren , Ming Tian , Xin-Zhe Tian , Qian Wang , Huantao Shang , Jianji Wang a,b,⁎, Z. Conrad Zhang c
a
a
a
a
a
a
School of Chemical Engineering & Pharmaceutics, Henan University of Science and Technology, Luoyang 471003, Henan, PR China
School of Chemical and Environmental Sciences, Henan Normal University, Xinxiang, Henan 453007, PR China
Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Zhongshan Road, Dalian, Liaoning 116023, PR China
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
A cobalt catalyst has been demonstrated, for the first time, to be effective for the reductive cleavage of inert
aromatic C\O bonds with high selectivity. Compared with previous Ni catalysts, the cobalt catalyst reported
here is more commercially available and air-stable.
Received 24 January 2014
Received in revised form 27 March 2014
Accepted 28 March 2014
Available online 13 April 2014
©
2014 Elsevier B.V. All rights reserved.
Keywords:
Reductive cleavage
Carbon–oxygen bond
Cobalt (II) acetylacetonate
Catalysis
1
. Introduction
2. Experimental
Transition-metal catalyzed selective activation of aromatic carbon–
2.1. Materials and instruments
oxygen bonds is an important process for the conversion of readily avail-
able phenolic derivatives into various organic chemicals [1–4]. This pro-
cess is challenging because the bond energy of the aryl carbon–oxygen
bonds is quite high [5,6]. It has been reported that selective activation
of aryl carbon–oxygen bonds can proceed smoothly in the presence of
catalysts such as Pd and Ni compounds [1–4]. However, there is no
example with Co species as the catalyst for this process. Thus our objec-
tive is set to develop a Co catalyst for the selective activation of aromatic
carbon–oxygen bonds.
The selective reductive cleavage of aromatic carbon–oxygen bonds in
aryl ethers is selected as an example to demonstrate our strategy. In liter-
atures, the reductive cleavage of aromatic carbon–oxygen bonds has been
reported to suffer from poor selectivity, and the competing cleavage of
the π-bonds in aromatic rings is also a troubling problem [7–9]. Until
the past few years, several effective homogeneous and heterogeneous
catalyst systems based on Ni compounds were developed for the reduc-
tive cleavage of aromatic C\O bonds [10–18]. Recently, Rh and Fe com-
pounds were demonstrated to be highly effective for the selectively
reductive cleavage [19,20]. In this work, we describe an air-stable Co
compound used to catalyze this transformation.
All substituted diphenyl ethers were synthesized by the reaction be-
tween the corresponding substituted iodobenzenes and substituted
phenols [18]. The other chemicals were obtained from commercial
vendors and used without further purification.
1
H-NMR spectra were recorded on a Bruker 400 MHz instrument
with chemical shifts reported in ppm relative to the internal standard
of tetramethylsilane. Gas chromatography analyses were performed
on a Varian CP-3800 instrument with a FID detector and a CP-WAX
5
7CB FS capillary chromatographic column (25 m × 0.32 mm). GC–MS
spectra were recorded on an Agilent 6890/5973N gas chromatography–
mass spectrometer, and data were reported in m/z, % relative intensity,
and possible fragment.
2.2. General experimental procedure for the reductive cleavage of aromatic
carbon–oxygen bonds
To a dried 40 mL tube equipped with a magnetic stirrer, 0.2 mmol
of substrate, 0.5 mmol of LiAlH , 0.03 mmol of Co(acac) , 0.5 mmol of
4 2
t-BuONa, and 2 or 1.5 mL of toluene were added under nitrogen at-
mosphere. The reaction tube was sealed and placed in a constant-
temperature oil bath set at 140 °C to perform the reaction for 24 h
(
note: the pressure tube was required because the pressure of the reac-
⁎
Corresponding author at: School of Chemical Engineering & Pharmaceutics, Henan
tion system was more than the standard atmospheric pressure). Once
the reaction time was reached, the mixture was cooled to room temper-
ature. The mixture was then acidified to pH 5–6 with 2 M hydrochloric
University of Science and Technology, 263, Kaiyuan Road, Luoyang, 471023, P. R. China.
Fax: 86 379 64210415.
E-mail address: Jwang@henannu.edu.cn (J. Wang).
http://dx.doi.org/10.1016/j.catcom.2014.03.036
1566-7367/© 2014 Elsevier B.V. All rights reserved.

Y.-L. Ren et al. / Catalysis Communications 52 (2014) 36–39
37
acid. GC analysis of the mixture provided the GC yields of the products
note: in order to minimize the analysis error, the mixture after the
bonds in various diphenyl ethers underwent the reductive cleavage
smoothly, while the competing cleavage of π-bonds in phenyl rings
was not observed in most of the experiments. Steric hindrance was
found to have an important effect on the reaction. For example,
the C\O bonds of 4,4′-dimethyldiphenyl ether was cleaved to afford
the products in high yields, whereas its more sterically hindered
o-constitutional isomer was less reactive (Table 2, entries 2 and 3).
Alkyl C\O bonds in alkyl aryl ethers were less reactive under our
conditions, and the reductive cleavage selectively occurred at the aro-
matic C\O bonds in the coexistence of aromatic C\O and alkyl C\O
bonds (Table 2, entries 4,5,8,9). However, methylene C\O bonds in
benzyl phenyl ethers were preferentially cleaved over the aromatic
C\O bonds (Table 2, entry 12). In addition, it was noted that the aro-
matic C\O bonds in phenols were not reactive. Similar phenomenon
was reported in previous investigations related to Ni and Rh-catalyzed
reductive cleavage of aromatic C\O bonds [13,18].
When 4,4′-dimethoxydiphenyl ether was employed as the substrate,
phenol was obtained in 64% yield, indicating that the aromatic C\O bond
at the side of the phenylalkyl ether moiety was also cleaved (Table 2, en-
tries 4 and 5). There was no equivalence in yields for the two products in
the case of methoxy-substituted diphenyl ethers (Table 2, entries 4 and
5), which seemed to result from the formation of the by-products from
the cleavage of the other aromatic C\O bonds besides the most reactive
(
reaction was not purified or concentrated). The reductive cleavage
products were identified by GC–MS data. Some products were purified
1
by column chromatography, and identified by H-NMR.
3
. Results and discussion
At the start of our investigations, the reductive cleavage of diphenyl
ether was chosen as a model reaction to demonstrate the efficiency
of various cobalt salts under the following conditions: 0.5 mmol of
4
diphenyl ether, 1.25 mmol of LiAlH , 0.075 mmol of Co salt, 1.25 mmol
of t-BuONa, 2 mL of toluene, reaction temperature of 140 °C and reaction
time of 24 h. It was found that no conversion took place in the absence of
Co salt even after prolonged reaction time (Table 1, entry 1). When CoCl
was used as the catalyst precursor, only a small amount of phenol
and benzene products were obtained. CoSO and Co(NO were also
ineffective as the catalyst precursors. However, the reaction with
Co(CH CO afforded phenol in 93% yield and benzene in 90% yield. Of
the screened cobalt salts, Co(acac) was optimal, and catalyzed the reac-
tion to give the desired phenol and benzene products in very high yields
Table 1, entry 6). The SIPr·HCl as the ligand was indispensable for the
homogeneous Ni-catalyzed reductive cleavage of aromatic C\O bonds
18], which prompted us to add the SIPr·HCl to increase the catalysis abil-
ity of the Co catalyst, but a similar result was obtained in case of combina-
tion of Co(acac) with SIPr·HCl. Change of SIPr·HCl to other ligands such
as DMEDA, 2,2′-bipyridine, 1,10-phenanthroline or PPh led to a decrease
2
4
3
)
2
3
2 2
)
2
(
[
Table 2
a
2
Co-catalyzed reductive cleavage of various aromatic C\O bonds.
3
in yields of phenol and benzene (Table 1, entries 8–11). It was noted that
no product could be found in the absence of sodium tert-butoxide,
suggesting that sodium tert-butoxide played a key role in the reaction.
Entry
1
Substrate
Product 1
GC yield
(%)
Product 2
GC yield
(%)
Although both sodium tert-butoxide and LiAlH
4
were sensitive to mois-
b
b
ture, the removal of trace water in the solvent did not lead to an increase
in the yields of the desired products. A high conversion of diphenyl ether
93
98c
13
92
was observed in the case of the use of the Co(acac)
exposed to air for 24 h, revealing that the catalyst is air-stable. It is clear
from economic perspectives that the use of LiAlH may limit the applica-
tions, thus inexpensive hydrogen gas was chosen to replace LiAlH as the
2
that had been
2
3
4
5
6
7
8
9
~100
20
89
44
92
95
–
4
4
hydrogen source. Unfortunately, such an attempt was unsuccessful. The
reductive cleavages of diphenyl ether using poly(methylhydrosiloxane)
or sodium borohydride as the reducing agent were also performed, but
no conversion of the substrate was observed in all the cases.
Based on this optimized procedure, a series of diphenyl ethers were
tested to evaluate the scope of this novel protocol for the reductive
cleavage of aromatic C\O bonds. As shown in Table 2, aromatic C\O
64
85
90
Table 1
Selective reductive cleavage of aromatic C–O bonds in diphenyl ether catalyzed by Co
90
a
catalysts.
49
–
–
–
–
–
–
Entry
Co salt
Ligand
Yield of
Yield of
Benzene (%)
62
–
phenol (%)b
b
1
2
3
4
5
6
7
8
9
–
–
–
–
–
–
–
0
8
trace
93
3
96
98
7
22
33
25
0
4
trace
90
5
95
93
8
19
31
57
1
1
0
1
80
–
CoCl
Co(NO
Co(CH
CoSO
Co(acac)
Co(acac)
Co(acac)
Co(acac)
Co(acac)
Co(acac)
2
3
)
2
3
2 2
CO )
93c
90
–
4
2
c
2
SIPr·HCl
DMEDA
2,2′-Bipyridine
1,10-Phenanthroline
c
12
–
2
2
10
1
2
13
Trace
–
1
2
PPh
3
a
a
Reaction conditions: diphenyl ether (0.5 mmol), LiAlH
4
(1.25 mmol), ligand
Reaction conditions: substrate (0.2 mmol), LiAlH
4
(0.5 mmol), Co(acac)
2
(0.03 mmol),
(
0.075 mmol), Co salt (0.075 mmol), toluene (2 mL), t-BuONa (1.25 mmol), 140 °C, 24 h.
toluene (2 or 1.5 mL), t-BuONa (0.5 mmol), 140 °C, 24 h.
b
b
Determined by GC.
Determined with an internal standard.
c
c
DMEDA: N,N′-dimethyl ethylenediamine; for the configuration of SIPr·HCl, see Fig. S1.
The isolated yields were respectively 88% (entry 2) and 81% (entry 11).

38
Y.-L. Ren et al. / Catalysis Communications 52 (2014) 36–39
LiAlH , Co(acac) , t-BuONa
The heterogeneous
system
4
2
O
o
Toluene, 140 C, 1 h
Filtration
The homogeneous
LiAlH , t-BuONa
4
system containing Co
o
HO
Trace
Toluene, 140 C, 20 h
O
Scheme 1. The investigation on the catalytic activity of homogeneous species.
LiAlH , Co(acac) , t-BuONa, N
2
4
2
-
O
OH +
2% yield
O +
o
Toluene, 140 C, 24 h
5
90% yield
+
H /H O
2
OH +
7% yield 89% yield
8
Scheme 2. The reductive cleavage of diphenyl ether.
C\O bond. Indeed, benzene and 4-methoxyphenol by-products were
observed (Table 2, entry 5). The aromatic C\O bond in phenyl methyl
ether without substitute group showed poor reactivity (Table 2, entry
assuming that a part of phenate is converted to phenol via the transfer
of the H from the residual H O in the solvent to phenate. It is worth not-
2
+
ing that the formation of the reductive cleavage products does exist in the
reaction medium before the hydrolysis (acidification) step based on the
experimental results that the benzene and phenol products were ob-
served in respectively 90% and 52% yield before the acidification
(Scheme 2).
1
3), and the introduction of methoxy or hydroxyl to the phenyl rings
increased the reactivity of phenyl methyl ether, but it is confusing that
the resulting experimental results from the reactions of methoxy or
hydroxyl-substituted phenyl methyl ether were less reproducible.
Next, several control experiments were carried out to gain insight
into the mechanism of the Co catalyst. It is possible that Co2 catalyst
precursor can be effectively reduced by the highly reducing agents to
low oxidation states [21], therefore the heterogeneous Co clusters or
+
4. Conclusion
In summary, it is demonstrated for the first time that the reduced
2
particles based on the reduction of Co(acac) catalyst precursor may be
cobalt is an effective catalyst for the reductive cleavage of inert aryl
C\O bonds under the conditions of the present study. The present ap-
proach allowed a series of aromatic C\O bonds in diphenyl ethers to
be reductively cleaved into the corresponding products in moderate to
high yields. It was suggested that the heterogeneous Co species from
the real catalytically active species. As expected, a certain amount of
heterogeneous black clusters or particles was observed in the reaction
system. After the heterogeneous species was filtered out when the reac-
tion proceeded for 1 h, the obtained homogeneous system had less cata-
lytic activity (see Scheme 1). This revealed that the heterogeneous Co
species seemed to be the real active species. But the obtained heteroge-
neous species from the filtration of the reaction mixture had less catalytic
activity. At the end of the reaction, 0.2 mmol diphenyl ether and 0.5 mmol
the reduction of Co(acac)
The Ni(COD) used in previous literatures is more expensive and easily
decomposed [10–18]. Compared with the Ni(COD) catalyst, the
present Co catalyst is commercially available and air-stable. However,
attempt to use inexpensive hydrogen gas to replace LiAlH as the hydro-
2
might be the real catalytically active species.
2
2
4
LiAlH were added into the mixture containing the catalyst to perform the
4
reaction for another 24 h, and only 38% substrate was converted. These
experimental results suggest that the catalytic species are easy to be
deactivated and have a poor recyclability. Upon the basis of previous liter-
atures [17,18,22] and our experimental results (see Scheme 2), we pro-
pose a plausible mechanistic pathway (Scheme 3), where the oxidative
addition and the reductive elimination successively occur to give benzene
and phenate products. Surprisingly, the phenol product was also observed
before the acidification (Scheme 2), which is possibly rationalized by
gen source was unsuccessful. Investigation on improving the present
method to be more economic is underway in our laboratory.
Acknowledgments
The financial supports from the National Basic Research Program of
China (973 Program, Grant No. 2011CB211702) and the National
Co(acac) + LiAlH + t-BuONa
2
4
Heterogeneous catalytic species
O-
O
+ LiAlH4
H +
H+
Residual H O in the solvent
2
H +
OH
O-
OH
H
Scheme 3. Considerations on the mechanistic pathway.

Y.-L. Ren et al. / Catalysis Communications 52 (2014) 36–39
39
[
8] J. Zakzeski, P.C.A. Bruijnincx, A.L. Jongerius, B.M. Weckhuysen, Chem. Rev. 110
2010) 3552–3599.
9] M.K. Huuska, Polyhedron 5 (1986) 233–236.
Natural Science Foundation of China (Grant No. 21002023) are greatly
acknowledged.
(
[
[
[
10] P. Alvarez-Bercedo, R. Martin, J. Am. Chem. Soc. 132 (2010) 17352–17353.
11] M. Tobisu, K. Yamakawa, T. Shimasaki, N. Chatani, Chem. Commun. 47 (2011)
Appendix A. Supplementary data
2946–2948.
[
12] T. Mesganaw, N.F.F. Nathel, N.K. Garg, Org. Lett. 14 (2012) 2918–2921.
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.03.036.
[13] P. Kelley, S. Lin, G. Edouard, M.W. Day, T. Agapie, J. Am. Chem. Soc. 134 (2012)
5480–5483.
[
[
14] J. Cornella, E. Gómez-Bengoa, R. Martin, J. Am. Chem. Soc. 135 (2013) 1997–2009.
15] A. Fedorov, A.A. Toutov, N.A. Swisher, R.H. Grubbs, Chem. Sci. 4 (2013) 1640–1645.
References
[16] A.G. Sergeev, J.D. Webb, J.F. Hartwig, J. Am. Chem. Soc. 134 (2012) 20226–20229.
[
[
[
17] J.Y. He, C. Zhao, J.A. Lercher, J. Am. Chem. Soc. 134 (2012) 20768–20775.
18] A.G. Sergeev, J.F. Hartwig, Science 332 (2011) 439–443.
19] M. Chatterjee, T. Ishizaka, A. Suzuki, H. Kawanami, Chem. Commun. 49 (2013)
[
[
[
[
1] T. Mesganaw, N.K. Garg, Org. Process Res. Dev. 17 (2013) 29–39.
2] F.S. Han, Chem. Soc. Rev. 42 (2013) 5270–5298.
3] R. Jana, T.P. Pathak, M.S. Sigman, Chem. Rev. 111 (2011) 1417–1492.
4] B.M. Rosen, K.W. Quasdorf, D.A. Wilson, N. Zhang, A.M. Resmerita, N.K. Garg, V.
Percec, Chem. Rev. 111 (2011) 1346–1416.
4
567–4569.
20] Y.L. Ren, M.J. Yan, J.J. Wang, Z.C. Zhang, K.S. Yao, Angew. Chem. Int. Ed. 52 (2013)
2674–12678.
[
1
[
5] J. Hassan, M. Sévignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev. 102 (2002)
[
[
21] G. Cahiez, A. Moyeux, Chem. Rev. 110 (2010) 1435–1462.
22] I.P. Beletskaya, A.V. Cheprakov, Coord. Chem. Rev. 248 (2004) 2337–2364.
1359–1470.
[
[
6] J.-P. Corbet, G. Mignani, Chem. Rev. 106 (2006) 2651–2710.
7] E. Furimsky, Appl. Catal. A Gen. 199 (2000) 147–190.