Solvent-controlled selective synthesis of biphenols and quinones: Via oxidative coupling of phenols

Article abstract of DOI:10.1039/c7cc04829g

A regioselective synthesis of unsymmetrical and symmetrical biphenols and binaphthols via oxidative coupling of phenols or naphthols in the presence of K₂S₂O₈ in CF₃COOH under ambient conditions is described. Interestingly, the 1:1 ratio of H₂O and CH₃CN solvent mixtures at 80 °C instead of CF₃COOH provided substituted unsymmetrical quinones. A gram-scale synthesis of biphenols and binaphthols was demonstrated.

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Solvent-controlled selective synthesis of biphenols and quinones via
oxidative coupling of phenols
Nagnath Yadav Morea and Masilamani Jeganmohan*a,b
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
A regioselective synthesis of unsymmetrical and symmetrical
biphenols and binaphthols via oxidative coupling of phenols
or naphthols in the presence of K2S2O8 in CF3COOH at
ambient conditions is described. Interestingly, 1:1 ratio of
10 H2O and CH3CN solvent mixtures at 80 °C instead of
CF3COOH provided substituted unsymmetrical quinones. A
gram scale synthesis of biphenols and binaphthols was
demonstrated.
Biphenols and binaphthols are highly valuable organic
15 molecules and these structural units are present in various
biologically active molecules, natural products and functional
materials.1 In addition, these molecules are used as ligands in
various organic transformations and enantioselective reactions.1
The oxidative coupling of phenols or naphthols mediated by
20 metal salts or oxidizing agents is a powerful and easily affordable
method to synthesize biphenol and binaphthol molecules in a
highly regioselective manner. It is important to note that the
nature prefers to synthesize natural products and biomaterials via
the oxidative phenol coupling by using oxidative enzymes such
25 as laccase, peroxidase and cytochrome P450 (CYP) as catalysts.2
Traditionally, biphenols and binaphthols are prepared by the
coupling of phenols or naphthols by using a stoichiometric
amount of metal salts.3 Later, the oxidative coupling reaction was
carried out with metal catalysts and stoichiometric amount of
30 oxidants.4 In the meantime, a similar type of coupling reaction
has been done by employing organic reagents such as hypervalent
iodine(III) reagents, peroxides, DDQ, NaOCl and NaNO2.5
Subsequently, the oxidative coupling reaction can be achieved via
an electrochemical oxidation pathway using boron-doped
35 diamond electrode (eq 1).6
Inspiration by this work and our continuous interest on
persulfate chemistry prompted us to explore the possibility of
using persulfate reagent for the oxidative coupling of phenols and
50 naphthols.7 Herein, we report the oxidative dehydrogenative
homocoupling of phenols providing unsymmetrical and
symmetrical biphenols in the presence of K2S2O8 in CF3COOH at
ambient conditions. When 1:1 ratio of H2O and CH3CN solvent
mixtures was used instead of CF3COOH, para-para coupling
55 biphenols and quinone derivatives were obtained. It is believed
that the coupling reaction proceeds via the radical intermediate
and the corresponding radical intermediate allows the incoming
nucleophiles to attack at the various sites of phenols selectively.
Thus, in the reaction, ortho-ortho, ortho-meta, meta-para and
60 para-para coupling products were observed from the
homocoupling of phenols.
The direct oxidative coupling of substituted Guaiacols and
phenols to symmetrical biphenols (ortho-ortho coupling reaction)
by the metal catalysts or organic reagents has been well
documented in the literature. For the first time, by applying BDD
40 anode, Waldvogel’s group synthesized the unsymmetrical biaryl
in the homocoupling reaction of Guaiacol derivative via the
radical mechanism.6a Very recently, Pappo’s group has prepared
unsymmetrical biaryl molecule by the homocoupling of Guaiacol
derivative in the presence of iron catalyst via radical-anion
45 pathway (eq 1).4k
Scheme 1 Cross-coupling of phenols 1a-b
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DOI: 10.1039/C7CC04P82a9gGe 2 of 4
Initialy, the oxidative homocoupling of 2,6-dimethoxylphenol 50 the reaction, providing biphenols 2f and 2g in 55% and 49%
(1a) was carried out with K2S2O8 in CF3COOH at room
temperature for h. Interestingly, in this reaction, only
unsymmetrical biphenol 2a was observed in 71% yield via the
meta, para C-H coupling corresponding with the hydroxy group
yields, respectively. It is noteworthy that the sensitive iodo and
bromo groups were compatible for the reaction. Interestingly, 4-
phenylphenol (1h) and hydroxyl substituted 4-phenylphenol 1i
afforded the corresponding biphenols 2h and 2i in 69% and 79%
4
5
of phenol 1a (Scheme 1). No other competitive symmetrical 55 yields, respectively. Next, the scope of the reaction was examined
ortho-ortho coupled biphenol was observed in the reaction. The
coupling reaction did not proceed without K2S2O8. Apart from
K2S2O8, the reaction was also examined with Na2S2O8,
with naphthols 1j-m. 2-Naphthol (1j) underwent homocoupling
efficiently at room temperature giving binaphthol derivative 2j in
80% yield.
naphthalene-2,7-diol (1l) and naphthalene-2,6-diol (1m) provided
was as effective as K2S2O8, giving 2a in 65% yield. Na2S2O8 and 60 the corresponding binaphthols 2k-m in 70%, 72% and 78%
A
highly useful naphthalene-2,3-diol (1k),
10 (NH4)2S2O8, PhI(OAc)2 and oxone. Among them, (NH4)2S2O8
oxone were partially effective, affording 2a in 40% and 49%
yields, respectively. PhI(OAc)2 inefficient for the reaction.
CF3COOH is also crucial for the reaction. AcOH and iso-PrOH
15 solvents were less effective, providing product 2a in 31% and
yields, respectively. It is important to note that in 1i and 1l-m,
two hydroxy groups are present on the aromatic ring.
Chemoselectively, only one of the hydroxy groups was involved
in the coupling reaction. The coupling reaction was not
23% yields, respectively. Other solvents MeOH, DCE, H2O and 65 compatible with phenols having electron-withdrawing
CF3SO3H were not effective. The coupling reaction was
examined with various amount of K2S2O8. Using 1.0 equiv and
1.5 equiv of K2S2O8, product 2a was observed only in 55% and
20 66% yields, respectively. With 2.0 equiv of K2S2O8, product 2a
was observed in 71% yield (Scheme 1). In this transformation,
the naturally occurring Syringol undergoes SET mechanism
giving phenol cationic radical intermediate A with K2S2O8.
Further, intermediate A was converted into para-phenoxyl radical
25 B under the reaction conditions (see ESI, page S4-S7). The
stability of phenoxyl and phenol cationic radical intermediates
was well studied in the literature.8 Later, intermediate B
undergoes radical-anion coupling with another molecule 2a to
produce exclusively unsymmetrical biphenol 2a. Next, the
30 oxidative coupling reaction was examined with 4-methyl-2-
methoxy phenol (1b) (Scheme 1). In the reaction, interestingly
49% yield of ortho-meta coupling unsymmetrical biphenol 2b
along with ortho-ortho coupling biphenol 2b’ in 13% yield were
observed. Due to the substitution at the para position of 1b could
35 cause of the dropping the yield of ortho-meta coupling product
2b. It is important to note that the 2,4-dimethylphenol (1c)
provided symmetrical ortho, ortho coupling product 3a in 73%
yield with K2S2O8 in CF3COOH under ambient conditions
(Scheme 2). No other competitive side product was observed in
40 the reaction.
substituents such as CO2Me, COMe, F, Cl, NO2 and Br at the
para position.
Scheme 3 Scope of phenols and naphthols
Scheme 2 Homocoupling of 2,4-dimethylphenol (1c)
70
The scope of oxidative coupling reaction was examined with
45 various substituted phenols 1d-i (Scheme 2). The homocoupling
of 2-bromo-4-methylphenol (1d) and naturally occurring sesamol
(1e) proceeded well with K2S2O8 giving symmetrical biphenols
2d and 2e in 58% and 80% yields, respectively. 4-Methoxy
phenol (1f) and 4-iodophenol (1g) also efficiently participated in
Scheme 4 para-para Coupling of phenols
A similar type of oxidative coupling reaction was examined
with 2,6-dimethyl phenol (1n) under the optimized reaction
conditions. However, no coupling product was observed. Later,
75 the solvent and temperature optimization was done. When
biphasic solvent system that is 1:1 ratio of H2O and CH3CN
2
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solvent mixtures was used instead of CF3COOH at 80 °C, the
oxidative coupling reaction yielded symmetrical para-para
similar reaction conditions providing quinoid coupling products
6g and 6h in 80% and 73% yields, respectively (Scheme 6).
coupling biphenol 4a in 62% yield. Similarly, in the 1:1 ratio of 35 Product 6g undergoes condensation with benzene-1,2-diamine (7)
H2O and CH3CN solvent mixtures, 2,6-dimethoxyphenol (1a)
provided para-para coupling biphenol 4b in 81% yield. It is
intereting to note that the 2,6-dimethoxyphenol (1a) provides
unsymmetrical meta, para C-H coupling product 2a in
CF3COOH solvent. Whereas, in 1:1 ratio of H2O and CH3CN
solvent mixtures, the same substrate provided the symmetrical
10 para, para coupling product 4b.
It is surprising to note the different type of reactivity and
coupling product was observed in the oxidative coupling of ortho
providing biologicaly active phenazine derivative 8 in 69%
yield.9b It is noteworthy to mention that the 1,4-naphthquinone
(6h) was observed in 1-naphthol (1u) substrate under similar
recation conditions.
A gram scale synthesis of biphenols and binaphthols was
demonstrated in Scheme 7. The homocoupling of 500 mg of
sesamol (1e), 1.0 gm of 4-methoxy phenol (1f) and 1.0 gm of 2-
naphthol (1j) with K2S2O8 under ambient conditions provided the
expected products 2e, 2f and 2j in 70%, 55% and 60% yields,
5
40
substituted phenols with K2S2O8 at 80 °C in 1:1 mixture of water 45 respectively. Similarly, 1.0 gm of 2-methylphenol (1o) reacted
and CH3CN solvents. When 2-methylphenol (1o) was treated
15 with K2S2O8 in a 1:1 mixture of water and CH3CN solvents,
quinoid type coupling product 6a was observed in 78% yield
(Scheme 5).9a In a similar fashion, 2-isopropylphenol (1p), 2-
phenylphenol (1q) and 2-benzylphenol (1r) provided quinoid
coupling products 6b-d in 75%, 60% and 51% yields,
20 respectively. The reaction was further examined with meta
substituted phenols such as meta Me and OMe substituted
phenols. These substrates were not compatible for the reaction
and no expected coupling product was observed. However, 2,5-
dimethylphenol (1s) and 2-methyl-4-hydroxy phenol (1t)
25 provided quinone derivatives 6e and 6f in good yields.
with K2S2O8 in a 1:1 mixture of water and CH3CN solvents
providing product 6a in 55% yield.
Scheme 7 A gram scale synthesis of biphenols
50
In conclusion, we have demonstrated an efficient synthesis of
symmetrical and unsymmetrical biphenols and binaphthols by
using green and easily available K2S2O8 as an oxidant. In the
reaction, ortho-ortho, ortho-meta, meta-para and para-para
coupling products were observed in the homocoupling of phenols
55 with different solvents. Meanwhile, the preparation of
unsymmetrical quinoid from 2-substituted phenols and 2-
naphthols was described. A gram scale synthesis of biphenols and
binaphthols was also demonstrated.
Scheme 5 Synthesis of substituted benzoquinones
We thank the DST-SERB (EMR/2014/000978), India for the
60 support of this research. R. M. and G. S. thank the CSIR for a
fellowship.
Notes and references
a
Department of Chemistry, Indian Institute of Science Education and
b
Research, Pune 411021, India;
Department of Chemistry, Indian
65 Institute of Technology Madras, Chennai 600036, Tamil Nadu, India.
E-mail: mjeganmohan@iiserpune.ac.in and mjeganmohan@iitm.ac.in
† Electronic Supplementary Information (ESI) available: Detailed
experimental
procedures
and
spectroscopic
data.
See
DOI: 10.1039/b000000x/
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