Oxidative Cross-Coupling of Two Different Phenols: An Efficient Route to Unsymmetrical Biphenols

Article abstract of DOI:10.1021/acs.orglett.5b01324

An efficient synthesis of unsymmetrical biphenols via the oxidative cross-coupling of two different phenols in the presence of K₂S₂O₈ and Bu₄N⁺·HSO₃^- (10 mol %) in CF₃COOH at ambient conditions is described. 1:1 Cross-coupling of substituted phenols with naphthols and 1:2 cross-coupling of naphthols with phenol are also disclosed. By using Bu₄N⁺·HSO₃^-, the homocoupling of phenols or naphthols was controlled. In these reactions, the ortho C-H bond of two different phenols and the ortho and para C-H bond of phenols were coupled together.

Full text of DOI:10.1021/acs.orglett.5b01324

Letter
pubs.acs.org/OrgLett
Oxidative Cross-Coupling of Two Different Phenols: An Efficient
Route to Unsymmetrical Biphenols
Nagnath Yadav More and Masilamani Jeganmohan*
Department of Chemistry, Indian Institute of Science Education and Research, Pune 411021, India
*
S Supporting Information
ABSTRACT: An efficient synthesis of unsymmetrical biphenols via
the oxidative cross-coupling of two different phenols in the presence
+
−
of K S O and Bu N ·HSO (10 mol %) in CF COOH at ambient
2
2
8
4
3
3
conditions is described. 1:1 Cross-coupling of substituted phenols
with naphthols and 1:2 cross-coupling of naphthols with phenol are
+
−
also disclosed. By using Bu N ·HSO , the homocoupling of phenols
4
3
or naphthols was controlled. In these reactions, the ortho C−H bond
of two different phenols and the ortho and para C−H bond of
phenols were coupled together.
he development of a highly efficient, easily accessible, and
reaction. The metal oxy radical was generated by the chromium
catalyst, and hydroxyl or alkoxyl radicals were generated by BDP
electrode.
T
environmentally friendly method for synthesizing biphenol
molecules under mild reaction conditions in a highly atom
economical manner is highly important in organic synthesis.
Biphenol units are present in various natural products, drug
A sulfate anion radical (SO − ) is efficiently used as a potential
•
4
9a
oxidant for the degradation of environmental pollutant. Due to
1
−•
molecules, and functional materials. In addition, biphenol
the strong electron-accepting ability of SO
4
, it has been
molecules are efficiently used as ligands in various organic
transformations including enantioselective reactions. Generally,
efficiently used to destroy soil carbon pollutants. Although the
2
−•
oxidation potential of SO
is 2.6 V, the reactivity between
4
nature prefers to synthesize phenolic ligands, biomaterials,
isoquinoline alkaloids, and natural products via an oxidative
phenol coupling in the presence of oxidative enzymes such as
organic compounds is considered to be slow but very
9
b,c
selective.
K
2
S
O
, Na
S
2
O
4
, and (NH
. Interestingly, the sulfate anion
)
S
O
are commonly
2
8
2
8
4
2
2
8
−
•
used sources to generate SO
3
laccase, peroxidase, and cytochrome P450 (CYP) as catalysts.
radical can be generated very easily at rt without having any
external reagents or sources. Our continuous interest in the
Meanwhile, it is also believed that this coupling reaction is
involved in the formation of humic materials in soils via
−
•
SO
possibility of using SO
anion radical chemistry prompted us to explore the
4
4
−•
enzymatic and abiotic routes. Thus, to understand the reaction
4
as an oxidant for the cross-coupling of
1
0a,b
two different phenols.
Herein, we report an efficient
mechanism and to develop a practical route to synthesize
biphenols has been a long time interest among organic
synthesis of unsymmetrical biphenols via the oxidative cross-
coupling of two different phenols in the presence of K S O in
5
−7
chemists.
Symmetrical biphenols are prepared by the
2
2
8
CF COOH at ambient temperature under air. The cross-
oxidative homocoupling of phenols in the presence of a
stoichiometric amount of metal salts or metal catalysts along
with a stoichiometric amount of oxidants or organic reagents
such as hypervalent iodine(III) reagents, DDQ, NaOCl, and
3
coupling reaction was also successfully extended into 1:1 cross-
coupling of substituted phenols with naphthols and 1:2 cross-
coupling of naphthols with phenol.
To accomplish the cross-coupling reaction, initially, a
combination of phenol and 2-naphthol substrates such as 4-
methoxyphenol (1a) and 2-naphthol (2a) were selected
5
−7
molecular oxygen as a terminal oxidant catalyzed by NaNO2.
Although the oxidative coupling of phenols has been known in
the literature for several decades, the cross-coupling of two
different phenols is not well studied and very challenging due to
the formation of many competitive side products. While
designing this type of oxidative coupling reaction, control of
competitive side products such as homocoupling of phenols,
Pummerer’s ketones, C−O bond formation between two
phenols, quinones, polymers, and dehydrotrimers is very
(
Scheme 1). The cross-coupling reaction of 1a with 2a was
tested in the presence of K S O in CF COOH at rt for 18 h
2
2
8
3
under air. In the reaction, the homocoupling of 1a (product 3aa
in 23% yield), the homocoupling of 2a (product 4aa in 27%
yield), and the expected cross-coupling product 5aa in 21% yield
−•
were observed. In the reaction, K S O generates the SO
2
2
8
4
8
a
anion radical at ambient temperature. It is believed that the
important. Very recently, the cross-coupling of phenols has
been successfully achieved by using a chromium salen catalyst or
−•
SO4 anion radical was stabilized by CF COOH solvent via
3
8a−c
an electrochemical oxidation pathway.
In the reaction, metal
oxy or hydroxy or alkoxyl radicals are key intermediates, which
abstract hydrogen from phenol and initiates the coupling
Received: May 6, 2015
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.orglett.5b01324
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
+
−
Scheme 1. Cross-Coupling of Phenol and Naphthol
In the presence of Bu N ·HSO (10 mol %), 2-naphthol (2a)
4 3
underwent cross-coupling with 4-methyl-2-methoxy phenol (1b)
or 2,6-dimethoxy phenol (1c), providing products 5ba and 5ca in
6
1% and 82% yields, respectively (Table 1, entries 1 and 2). In
a
Table 1. Cross-Coupling of Phenols and Naphthols
hydrogen bonding. Later, the SO4−• anion radical abstracts
hydrogen from 1a or 2a in the presence of CF COOH, providing
3
a cationic phenol or naphthol radical intermediate 6 and nontoxic
HSO4− salt. To gain further insight into the radical formation, the
oxidation potential of 1a and 2a was tested by using cyclic
voltammetry (see Supporting Information). The oxidation
potential of 1a is SHE 0.66 V, and that of 2a is SHE 0.88 V. It
−•
is expected that the SO4 anion radical prefers to abstract
hydrogen from 1a, forming a cationic intermediate 6a compared
to 2a due to the lower oxidation potential value. To support the
formation of a cationic intermediate 6a, a UV−vis spectroscopy
study was done. An intense absorption band was observed in the
visible region between 400 to 500 nm during the reaction of
phenol 1a with K S O at rt. This study strongly supports that a
2
2
8
cationic intermediate is formed during the reaction as previously
10c−e
observed by Kochi’s group.
Later, the nucleophilic addition
of 2a into cationic intermediate 6a provides the cross-coupling
product 5aa.
It is clear that the coupling reaction proceeds via a cationic
radical intermediate. We expected that if the reaction is done in
the presence of an ionic salt, a negatively charged counterion
could coordinate with a cationic phenol radical intermediate 6a
and stabilize it. Thus, the possibility is there to increase the yield
of cross-coupling product 5aa. With this idea, the reaction was
+
tested with a catalytic amount of 10 mol % of salts such as Bu N ·
a
4
All reactions were carried out using 1a−c (1.0 equiv), 2a−c (1.5
+ −
−
+
−
+
−
+
−
+
−
I , Bu N ·Br , Bu N ·Cl , Bu N ·F , and Bu N ·HSO .
Bu N ·I provided homocoupling product 4aa in 33% yield
4
4
4
4
3
equiv), Bu N ·HSO (10 mol %), and K S O (2.0 equiv) in
4
3
2
2
8
+
−
b
4
CF COOH at ambient conditions for 18 h. Isolated yield.
3
and cross-coupling product 5aa in 23% yield, respectively. The
other homocoupling product 3aa was not observed (Scheme 1).
+
−
Surprisingly, Bu N ·HSO afforded cross-coupling product 5aa
the reaction of 1c with 2a, the para C−H bond of 1c was coupled
with the α C−H bond of 2-naphthol (2a). The oxidation
potential of 1b is 0.7 V, and that of 1c is 0.62 V. Thus, in these
reactions, only phenols 1b−c form the cationic radical species,
and 2a acts as a nucleophile. Similarly, naphthalene-2,3-diol (2b)
reacted with 1a or 1c, affording the cross-coupling products 5ab
and 5cb in 64% and 51% yields, respectively (entries 3 and 4).
Further, naphthalene-2,7-diol (2c) reacted with 1a or 1c,
providing coupling products 5ac and 5cc in 73% and 69%
yields, respectively (entries 5 and 6). The mass balance of all
these reactions was mentioned in the Supporting Information.
In the naphthols 2b and 2d, two OH groups are present on the
aromatic moiety. We tried to incorporate two phenol moieties at
the α C−H bond of the corresponding OH groups (Scheme 2).
Thus, treatment of 2b or 2d with an excess amount of 2,6-
dimethoxyphenol (1c) (2.2 equiv) gave polycyclic naphthol
derivatives 7a and 7b in 71% and 75% yields, respectively (for
mass balance, see Supporting Information).
4
3
in 61% yield and homocoupling product 4aa in a very minor 1%
yield (for mass balance, see Supporting Information). Other salts
were not active for the reaction. The same reaction was also done
under a nitrogen atmosphere. In the reaction, product 5aa was
observed in 60% yield as in the case of under air. This result
clearly reveals that the reaction is not air sensitive. K S O is
2
2
8
crucial for the reaction, and without that the reaction did not
proceed. Meanwhile, 2.0 equiv of K S O are also needed to raise
2
2
8
the yield of cross-coupling product 5aa. The reaction was also
examined with 1.0 and 1.5 equiv of K S O . In these reactions,
2
2
8
product 5aa was observed only in 40% and 57% yields,
respectively. The cross-coupling reaction was also examined
with various solvents such as AcOH, MeOH, ClCH Cl, tert-amyl
2
alcohol, tert-BuOH, and CF SO H. In acetic acid solvent,
3
3
product 5aa was observed in very low 5% yield and the remaining
solvents were not effective. The reaction was also tried with
Na S O and (NH ) S O . (NH ) S O was also equally
2
2
8
4
2
2
8
4
2
2
8
effective, providing product 5aa in 60% yield. But, Na S O
provided only product 5aa in 40% yield. It is expected that the
Encouraged by the 1:1 and 2:1 cross-coupling of phenols with
naphthols, the possibility of cross-coupling of two different
phenols was tested (Scheme 3). The cross-coupling of two
2
2
8
solubility of Na S O is less in organic solvent.
2
2
8
B
DOI: 10.1021/acs.orglett.5b01324
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
a
Scheme 2. bis-Arylation of Naphthols 2b and 2d
Table 2. Cross-Coupling of Two Different Phenols
Scheme 3. Cross-Coupling of Two Different Phenols
different phenols is difficult due to the similar oxidation potential
values. Initially, 2-methoxy-4-methylphenol (1b) with 3,4-
dimethylphenol (1d) were taken as model substrates for the
reaction. The oxidation potential of 1b is 0.7 V, and that of 1d is
0
.87 V. Thus, 1b prefers to form a cationic radical intermediate in
−•
the presence of SO4 . The oxidation potential difference
between 1b and 1d is very small; thus, homocoupling of these
phenols would be expected more compared with the cross-
coupling product. Treatment of 1b (1.0 equiv) with 1d (1.0
equiv) in the presence of Bu N ·HSO (10 mol %) in
CF COOH at ambient temperature gave the cross-coupling
+
−
4
3
3
product 8bd in 35% yield along with the homocoupling product
of 1b, product 3bb, in 8% yield. Surprisingly, the homocoupling
product of 1d was not observed, and the remaining starting
material was recovered. To increase the yield of the cross-
coupling product, the amount of 1d was increased to 2.5 equiv to
3
8
1
.0 equiv. Surprisingly, with 3.0 equiv, cross-coupling product
bd was observed in 53% yield, and no homocoupling product of
b was detected. In 2.5 equiv, product 8bd was observed in 49%
a
yield.
Next, the scope of the cross-coupling reaction was examined
All reactions were carried out using 1b (1.0 equiv), 1e−n (3.0
+ −
mmol), Bu
N ·HSO
(10 mol %), and K S O (2.0 equiv) in
2 2 8
b c
4
3
CF COOH (0.5 mL) at rt for 18 h. Isolated yield. Homocoupling of
1
with various substituted phenols 1e−n (Table 2). Trisubstituted
phenols such as 2,3-dihydro-1H-inden-5-ol (1e), 2,4-dimethyl-
phenol (1f), or 4-tert-butyl-2-methyl phenol (1g) reacted
efficiently with 1b, giving the corresponding biphenols 8be−bg
in 56%, 57%, and 43% yields (entries 1−3). In the reaction of 1b
with 1f, the homocoupling product of 1f was observed in 7%
yield. Next, the cross-coupling reaction was examined with
disubstituted phenols. Thus, treatment of 2-methyl phenol (1h)
with 1b gave a mixture of two regioisomeric products, 2,4′-
biphenol 8bh in 46% yield and 2,2′-biphenol 8bh′ in 8% yield (eq
3
d
f was observed in 7% yield. Other regioisomer was observed.
8
% and 6% yields, respectively (eq 1). 1,4-Disubstituted phenols
such as 4-methyl (1l) or 4-tert-butyl phenols (1m) were also
involved in the reaction, giving the corresponding cross-coupling
products 8bl and 8bm in 42% and 38% yields, respectively
(
entries 8 and 9). We have tried the cross-coupling of a less
reactive unsubstituted phenol (1n) with 1b. In the reaction,
interestingly, 2,2′-biphenol (8an) was observed in 33% yield and
the other regioisomeric 2,4′-biphenol was not observed (entry
1), respectively (entry 4). Interestingly, in the reaction of 2-
1
0). In entry 5, ortho-para product 8bi and, in entry 10, ortho-
ortho product 8bn were observed. The exact reason for the
selectivity is not clear; it is possible that the para carbon electron
density of 1i and the ortho carbon electron density of 1n could be
more and favorable for the coupling reaction. Treatment of 1b or
1d or 1f with 2,6-dimethoxy phenol (1c) under similar reaction
conditions gave ortho-para cross-coupling products 8cb−8cf in
48%, 59%, and 56% yields, respectively (Scheme 4). In most
phenol cross-coupling reactions, only good to moderate yields
were observed. It is important to note that, for the first time in the
literature, this type of cross-coupling with various new phenols
was demonstrated.
hydroxy phenol (1i) with 1b, only 2,4′-biphenol 8bi was
observed in 38% yield (entry 5). In the reaction of 1b with 1i, the
ortho C−H bond of 1b was coupled with the para C−H bond of
1
i. 1,3-Disubstituted phenols such as 3-methoxy (1j) or 3-methyl
(
1k) phenols reacted with 1b, affording 2,2′-biphenols 8bj and
bk in 26% and 49% yields, respectively (entries 6 and 7). In
addition, the other regioisomers 8bj′ and 8bk′ were observed in
8
C
DOI: 10.1021/acs.orglett.5b01324
Org. Lett. XXXX, XXX, XXX−XXX

Organic Letters
Letter
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(
(
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(
(
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−•
phenol up to 0.7 V satisfactorily reacts with SO4 , providing the
corresponding cationic phenolic radical intermediate. If the
oxidation potential is above 0.7 V, the substrate does not oxidize
and participate in the reaction. For example, 4-tert-butyl-2-
(f) Dewar, M. J. S.; Nakaya, T. J. Am. Chem. Soc. 1968, 90, 7134.
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ox
ox
methyl phenol (1g) (Ep = 0.81) or 2-naphthol (2a) (E
0
=
p
ox
.81) did not react with 3,4-dimethylphenol (1d) (Ep = 0.87).
8
1, 6335. (d) Jiang, Q.; Sheng, W.; Tian, M.; Tang, J.; Guo, C. Eur. J. Org.
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ox
ox
observed. However, 1a (E = 0.66 V), 1b (Ep = 0.70 V), and 1c
(
p
ox
Ep = 0.62 V) were nicely involved in the reaction.
In conclusion, we have demonstrated an efficient synthesis of
unsymmetrical biphenols via the oxidative cross-coupling of two
+
−
different phenols in the presence of K S O and Bu N ·HSO
2
2
8
4
3
3
83. (c) Dohi, T.; Maruyama, A.; Yoshimura, M.; Morimoto, K.; Tohma,
(
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coupling of substituted phenols with naphthols and 2:1 cross-
coupling of substituted phenols with naphthols were also
described. In these reactions, the ortho C−H bond of two
different phenols and the ortho and para C−H bond of phenols
3
46, 675. (f) Zhai, L.; Shukla, R.; Wadumethrige, S. H.; Rathore, R. J.
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Eur. J. Org. Chem. 2006, 4569.
+
−
were coupled together. By using Bu N ·HSO , the homocou-
4
3
pling of phenols or naphthols and also overoxidation of the
desired cross-coupling products were controlled.
(8) (a) Lee, Y. E.; Cao, T.; Torruellas, C.; Kozlowski, M. C. J. Am.
ASSOCIATED CONTENT
Supporting Information
Chem. Soc. 2014, 136, 6782. (b) Elsler, B.; Schollmeyer, B.; Dyballa, K.
M.; Franke, R.; Waldvogel, S. R. Angew. Chem., Int. Ed. 2014, 53, 5210.
■
*
S
(c) Kirste, A.; Elsler, B.; Schnakenburg, G.; Waldvogel, S. R. J. Am. Chem.
General experimental procedure and characterization details.
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.orglett.5b01324.
Soc. 2012, 134, 3571.
(9) (a) Ahmad, M. Persulfate activation by major soil minerals. Ph.D.
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AUTHOR INFORMATION
Corresponding Author
■
*
5
13. (c) Madhavan, V.; Zemel, H.; Fessenden, R.; Neta, P. J. Am. Chem.
Soc. 1977, 99, 163.
10) (a) More, N. Y.; Jeganmohan, M. Org. Lett. 2014, 16, 804.
E-mail: mjeganmohan@iiserpune.ac.in.
(
Notes
(b) More, N. Y.; Jeganmohan, M. Chem.Eur. J. 2015, 21, 1337.
(
(
c) Vasudeva, W. C. J. Chem. Soc., Perkin Trans. 2 1975, 697.
d) Sankararaman, S.; Haney, W. A.; Kochi, J. K. J. Am. Chem. Soc.
The authors declare no competing financial interest.
1
987, 109, 7824. (e) Kita, Y.; Tohma, H.; Hatanaka, K.; Takada, T.;
ACKNOWLEDGMENTS
■
Fujata, S.; Mitoh, S.; Sakurai, H.; Oka, S. J. Am. Chem. Soc. 1994, 116,
3684.
We thank the CSIR (02(0179)/14/EMR-II), India for the
support of this research. N.Y.M. thanks the CSIR for a fellowship.
We thank Dr. K. Krishnamoorthy (NCL Pune) for checking the
oxidation potential value of phenols.
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DOI: 10.1021/acs.orglett.5b01324
Org. Lett. XXXX, XXX, XXX−XXX