Efficient oxidative dimerization of 1-naphthols to 2,2′-binaphthyls with dioxygen mediated by semiconductors

Article abstract of DOI:10.1016/j.tetlet.2004.01.133

A novel and efficient oxidative dimerization of 1-naphthols 1 with dioxygen in the presence of several semiconductors including SnO₂, ZrO₂, and activated charcoal as catalytic mediators took place selectively to give the corresponding 2,2′-binaphthols 2 or 2,2′-binaphthyl-1,1′-quinones 3 in excellent yields without light irradiation. Among these semiconductors, the catalytic activity of SnO ₂ could be fully restored by appropriate reactivation treatment after oxidation. The products 2 and 3 should be useful as synthetic intermediates for natural binaphthyls.

Full text of DOI:10.1016/j.tetlet.2004.01.133

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 2643–2647
Efficient oxidative dimerization of 1-naphthols to 2,20-binaphthyls
with dioxygen mediated by semiconductors
Tsuyoshi Otsuka, Iwao Okamoto, Eiichi Kotani and Tetsuya Takeya*
Showa Pharmaceutical University, 3-3165 Higashi-tamagawagakuen, Machida, Tokyo 194-8543, Japan
Received 22 December 2003; revised 22 January 2004; accepted 23 January 2004
Abstract—A novel and efficient oxidative dimerization of 1-naphthols 1 with dioxygen in the presence of several semiconductors
including SnO2, ZrO2, and activated charcoal as catalytic mediators took place selectively to give the corresponding 2,20-binaphthols
2 or 2,20-binaphthyl-1,10-quinones 3 in excellent yields without light irradiation. Among these semiconductors, the catalytic activity
of SnO2 could be fully restored by appropriate reactivation treatment after oxidation. The products 2 and 3 should be useful as
synthetic intermediates for natural binaphthyls.
Ó 2004 Elsevier Ltd. All rights reserved.
So-called oxidative dimerization and oxidative coupling
of hydroxyarenes commonly involve oxidative dehydro-
genation and subsequent C–C or C–O coupling of the
resulting arenyl radical. The oxidative dimerization of
hydroxyarenes such as naphthols and phenols is one of
the most important tools of modern organic synthesis
and is often used in biomimetic or chemical synthesis of
natural products and binaphthols used as chirality
inducers.1 For instance, a number of methods have been
developed for the preparation of binaphthols and their
derivatives by the oxidative coupling of naphthols. This
has been achieved by using a variety of oxidants, such as
metal salts, Ag(I), Pb(IV), Fe(III), Mn(III), Cu(II), etc.,
in homogeneous solution.2 In most cases, however, the
coupling reactions are not catalytic but require more
than stoichiometric amounts of oxidants. There are a
few exceptions employing such catalyst systems as
CuCl–(OH)ÆTMEDA complex or VO(acac)2 under
dioxygen (O2) as coupling reagents.2a;3 In the oxidation
of naphthols, the use of homogeneous oxidants often
leads to poor selectivity and low yield of the desired
products, accompanied with side reactions, so that the
reactions are difficult to control. Recently, solid Lewis
acids,4 have been used, such as FeCl3 as heterogeneous
catalysts, either alone or in binary combinations such as
CuSO4/Al2O3 and FeCl3/montmorillonite, with aerial
oxygen or dioxygen for oxidative coupling of 2-naph-
thols.
The replacement of current stoichiometric oxidations
for the production of fine chemicals with environmen-
tally benign catalytic oxidations is one of the major
tasks in green chemistry.
Dioxygen seems to be an ideal oxidant for such pur-
poses, as well as in biomimetic synthesis of natural
products, because the supply is ample, and the molecule
environmentally friendly and nontoxic. In addition,
dioxygen participates in metabolic processes in mam-
malians and plants.5 Further, various semiconductors
(SC),6a;b particularly titanium dioxide (TiO2),6c are
extensively used as sensitizers in heterogeneous photo-
catalytic oxidations. They are particularly useful from
an environmental viewpoint, because they are almost
nontoxic, stable, and inexpensive. Several such reactions
have been reported, including oxygenations and oxida-
tive cleavage.6
From the practical and environmental viewpoints, SC
should be advantageous as heterogeneous catalysts, and
it is expected that dioxygen can be effectively used for the
facile regeneration of such catalysts in a catalytic cycle.
As far as we know, there have been no reports to date on
oxidative dimerization of 1-naphthols (NPOH) with
dioxygen using SC in the absence of light irradiation.
Keywords: Binaphthol; Diphenoquinone; Semiconductor; Oxidation;
Tin and compounds; Dioxygen.
In this paper, we report a novel and efficient oxidative
dimerization of NPOH 1 to 2,20-binaphthyl derivatives 2
* Corresponding author. Tel./fax: +81-42-721-1579; e-mail: takeya@
ac.shoyaku.ac.jp
0040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.01.133

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T. Otsuka et al. / Tetrahedron Letters 45 (2004) 2643–2647
or 3 with dioxygen as the oxidant, mediated by several
SC.
followed by drying at 130 °C for 12 h under reduced
pressure before use. The recovered SnO2 showed almost
unchanged catalytic reactivity, since reaction of 1a using
the recovered SnO2 in MeCN under identical conditions
gave 2a (64%) and 3a (26%). However, recovered ZrO2
and Act-C did not acquire full catalytic activity after the
above reactivation treatment.
Preliminary experiments on the oxidative dimerization
were done with NPOH 1a as a substrate, without light
irradiation. Several SC6 and solvents saturated with
dioxygen [namely, TiO2, Nb2O5, SnO2, ZrO2, activated
charcoal (Act-C),7 etc., and CH2Cl2, MeCN, MeNO2,
etc.] were investigated, and the results are shown in
Table 1. The best result was obtained with Act-C in
MeCN (entry 1). That is, the reaction of 1a using the
Act-C/O2 reagent system in the presence of O2 in MeCN
afforded selectively the so-called Russigꢀs Blue,8a 2,20-
binaphthyl-1,10-quinone (3a; BNPQ), in excellent yield.
A similar result was obtained in the dark.
Similar reactions were explored using 1b or 1c,9 which
can be precursors for the synthesis of binaphthyl natural
products (Table 1).8
In the case of 1b, the best result was obtained with the
ZrO2/O2 system in MeCN for 0.75 h, affording 3b
selectively in excellent yield (entry 8). The reaction with
the SnO2/O2 system under similar conditions gave a
complex mixture containing a substantial amount of 2b.
In order to isolate 2b, we performed column chroma-
tography of the reaction mixtures under various condi-
tions. However, all the attempts were unsuccessful,
producing mainly solid mixtures of 2b and 3b. There-
fore, the mixture of 2b and 3b was treated with benzyl
bromide/K2CO3 to give 6 (81%) as a major product
together with nonreacted 3b (2%) (entry 9).
In the case of using the ZrO2/O2 system in CH2Cl2,
BNPQ 3a was obtained as a major product along with
the ortho-naphthoquinone 5a as a by-product (entry 2).
However, good results were not obtained with the
Nb2O5/O2 or the TiO2/O2 system (entries 3 and 4).
Furthermore, with the SnO2/O2 system in CH2Cl2, 2,20-
binaphthol (2a; BNPOH) and 10-hydroxy-2,20-binaph-
thyl-1,4-quinone (4a; HBNPQ) were obtained in yields
of 67% and 28%, respectively (entry 5). A solvent effect
of MeCN was only observed in the reaction with the
SnO2/O2 system. When MeCN was used in place of
CH2Cl2 under similar conditions, 4a was not obtained at
all, and in this case 2a and 3a were obtained in yields of
64% and 28%, respectively (entry 6). The reason for this
is discussed below. The catalyst (SnO2) could be easily
recovered by simple filtration and washing with CH2Cl2,
In the case of 1c, the best result was obtained with the
SnO2/O2 system in MeCN. This oxidative coupling was
very sluggish, but selectively afforded 2c in good yield
(entry 11). In all experiments with 1c using various
semiconductors, a longer period of reaction was
required for completion of the reaction, in comparison
with the cases of 1a–b. This may be owing to the influ-
ence of the hydrogen-bond formation10 between the
H
OH
OH
OMe
Me
O
O
Me
8
1
MeO
OMe
1b
OMe
OMe
1a
OMe
1c
1d
oxidative dimerization semiconductor/ O2
OMe R3
R1 OH
R1
R3
R2
O
R2
R2
R1
O
2
R2
2
1'
R2
1
4
OH R1
R3
2a-c
OMe
OMe
R3
O
3a-c
R1
4a-c
OBn
O
Me
O
a: R1=R2=R3=H
2
b: R1=H, R2=Me, R3=OMe
c: R1=OMe, R2=R3=H
R3
OMe
5a-c
MeO
OMe
6
Scheme 1.

T. Otsuka et al. / Tetrahedron Letters 45 (2004) 2643–2647
2645
Table 1. Semiconductor-mediated oxidative dimerization of naphthol 1 in the presence of dioxygen (O2)a
Entry
NPOH
Reagent
Solvent
Temperature (°C) Time (h)
Product (isolated yield, %)
1
1a
1a
1a
1a
1a
1a
1a
1b
1b
1c
1c
1d
1b
2a
1a
Act-Cb
ZrO2
Nb2O5
TiO2
MeCN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
MeCN
MeNO2
MeCN
CH2Cl2
MeCN
MeCN
CH2Cl2
CHCl3
MeCN
CH2Cl2
70
23
23
23
23
70
23
70
23
70
70
23
23
70
23
16
1.5
3a (95)
2
3a (75)
3a (51)
2a (4)
5a (17)
5a (43)
3a (15)
4a (28)
3a (28)
3a (6)
3
3.5
0.5
4
4a (18)g
5a (20)
5
SnO2
SnO2
SnO2
ZrO2
SnO2
ZrO2
SnO2
SnO2
Ag2O
SnO2
ZrO2
72
2a (67)
2a (64)
2a (81)
3b (96)
6 (81)c
6
29
72
7
8
0.75
9d
10d
11d
12
13e
14
15f
24
70
3b (2)
3c (14)
4b (3)g
4c (trace)g
2c (68)
2c (86)
136
72
No reaction
6 (30)c
0.5
3b (34)
5b (8)
0.75
1.5
3a (93)
No reaction
a General procedure: a slurry of semiconductor powder [Act-C (1 g)b, SnO2 (5 g), ZrO2 (5 g), Nb2O5 (5 g), or TiO2 (1 g) ] and 1a (0.25 mmol) in a
dioxygen-saturated solvent (MeCN, CH2Cl2, or MeNO2; 10 mL) was vigorously stirred at an appropriate temperature (23 or 70 °C) under normal
laboratory light. Similar results were obtained in the dark. These semiconductors are commercially available (Wako Pure Chemical Industries, Ltd,
Japan).
b With activated charcoal.
c Yield from 1b.
d Unreacted NPOHs 1 were also recovered: entry 9, 1b (14%); entry 10, 1c (8%); entry 11, 1c (12%).
e With Ag2O (1.5 equiv) under air.
f This reaction was carried out under air. CH2Cl2 as a solvent was used without any treatment.
g Structures 4 were elucidated by analyses of IR, 1H NMR, 13C NMR spectra, with the aid of 2D NMR spectral analyses, and transformation to the
corresponding 2,20-binaphthoquinones by the reaction with 69% HNO3. The 1H NMR spectra of compounds 4 showed the presence of a naphtholic
OH at the C-10 position (d 8.51 in 4a; d 8.60 in 4b; d 9.41 in 4c). The 13C NMR spectra of compounds 4 displayed the presence of two carbonyl
carbons at the C-1 and C-4 positions (d 184.5, 188.5 in 4a; d 183.2, 185.3 in 4b; d 183.6, 188.3 in 4c).
hydroxyl proton (at C1) and the methoxyl group (at C8)
in 1c, as shown in Scheme 1, though this remains to be
confirmed.
The proposed mechanism for the SC-mediated oxidative
dimerization of NPOH 1 using O2 is illustrated in
Scheme 2. This is analogous to that proposed for the
oxidative reaction of 1-naphthols with the SnCl4/O2
system reported previously by us.11 This reaction is
initiated by the formation of the electron donor–accep-
tor (EDA) complex A between 1 and a Lewis acid site on
the SC surface. Subsequent processes proceed on the SC
surface. The SC (semiconductor), such as SnO2, ZrO2 or
Act-C, plays an important role in the present oxidation.
That is, it acts not only as a characteristic Lewis acid
catalyst, but also as a mediator for electron transfer.
Alternatively, O2 may act as a one-electron acceptor
from the anion-radical species (SCꢀꢁ) and a one-proton
acceptor from the cation-radical species (NPOHþꢁ)
within complex B. Finally, the resulting hydroperoxy
radicals act as a two-hydrogen acceptor from 2.
On the other hand, in the case of the naphthol-ether 1d
with the SnO2/O2 system, the reaction did not proceed
under similar conditions (entry 12). Finally, the reaction
of 1b with the well-known oxidant Ag2O in chloroform
was examined, but gave only a mixture of the coupling
product and its oxidized quinone (entry 13).
The noteworthy features of the above reactions were as
follows. The hydroxyl group in 1a–c was required for
the formation of the binaphthyl derivatives, based on
the result of entry 12. The SC-mediated oxidation of 1a–
c in the presence of O2 made it possible to control the
synthesis in the direction of either 3 or 2. For example,
the reaction of 1a or 1b with the Act-C/O2 or the ZrO2/
O2 system afforded the corresponding 3a or 3b in
excellent yield, respectively, while the reaction of 1c with
the SnO2/O2 system gave 2c in high yield. The SC were
essential for the catalysis of the present oxidation, since
it did not proceed in their absence. The resulting prod-
ucts 2, 3, and 4 would be useful synthetic intermediates
for naturally occurring diosindigo B, biramentaceone
and violet-quinone.8b;c
Next, to throw light on whether HBNPQ 4 is formed by
acid-induced demethylation or oxidative demethylation
of the enol-ethers 3 in the reaction of 1 in CH2Cl2
(entries, 4, 5, and 9 in Table 1), the reactions of 3a–c with
various reagents in the absence of O2 in CH2Cl2 or
MeCN were examined (Scheme 3). The results are sum-
marized in Table 2. With SnO2 in CH2Cl2, selective
demethylation of 3a–c proceeded efficiently to give 4a–c
under mild conditions in high yield (entries 1, 3, and 4).
Similar results were obtained in the dark. These results
indicate that the formation of 4 from 3 does not require
dioxygen and light, but requires SnO2. Alternatively, the
formation of the hydroxyl group (at C-10) in 4 from 3
requires a proton (Hþ) or a hydrogen source. This sug-
gests the participation of a Brønsted acid (Sn–OH) site
In order to confirm the formation mechanism of 3 in the
present oxidation, for example, the reaction of 2a with
the SnO2/O2 system was carried out to give 3a in high
yield(entry 14 in Table 1). No reaction occurred in the
absence of O2 under similar conditions. The above results
indicate that 3 is produced via 2 by the oxidation of 1.

2646
T. Otsuka et al. / Tetrahedron Letters 45 (2004) 2643–2647
O2
SC
2 X
NP-O
H
SC
EDA complex A
NPOH
2 X
2 X
Lewis acid site
SC: Semiconductor
1
O
H
biaryl coupling
NP-O
SC
2 X
2 X
O
H
O
OMe
radical C
2 x SC + 2 X HOO
2 X HOO
EDA complex B
OMe
OMe
O
O
H
2
H
O
O
2 x H2O2
OMe
OMe
7
3
Scheme 2.
OMe R3
R1
O
R1
O
6'
4'
1'
R2
R2
demethylation
2
R2
1
4
OH R1
SnO2 in CH2Cl2
6
R3
OMe
4a-c
3a-c
R3
O
Scheme 3.
Table 2. Demethylation of the enol-ethers 3 to 4 with various reagents
a
in the absence of O2
In order to examine the reason why MeCN has the
solvent effect described above, the reaction of 3a using
MeCN in place of CH2Cl2 as a solvent was carried out
under similar conditions. Surprisingly, this reaction did
not proceed at all (entry 2). In the above case, as well as
in the case of entry 6 in Table 1, 4a was presumably not
obtained because the interaction between the Sn–OH
sites and the carbonyl group in 3a is blocked by the
MeCN molecules, which bind to the Sn–OH sites, based
on a report described below. Yates and co-workers12c
observed that the adsorption of MeCN on Brønsted acid
sites of a metal oxide surface, such as TiO2, by means of
IR spectroscopy. The reaction with ZrO2 under similar
conditions gave 4a in low yield, while no reaction
occurred with Act-C. These results indicate that the
formation of 4 by the reaction of 1 or 3 can be well
controlled when ZrO2 or Act-C as a catalyst and MeCN
are employed.
Entry Substrate Reagent Solvent
Time (h) Product (%)b
1
3a
3a
3b
3c
3a
3a
3a
SnO2
SnO2
SnO2
SnO2
ZrO2
Act-C
HCl
CH2Cl2
MeCN
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
19
72
2
4a (94)
2
No reaction
4b (95)
4c (93)
3
4
5c
2
72
72
8
4a (12)
No reaction
4a (91)
6
7d
a General procedure: a slurry of semiconductor powder [SnO2 (5 g),
ZrO2 (5 g) or Act-C (1 g)] and 3 (0.25 mmol) in an argon-saturated
solvent (CH2Cl2 or MeCN) was vigorously stirred at 23 °C under
normal laboratory light. Similar results were obtained in the dark.
b Isolated yield.
c Together with recovered 3a (40%).
d Using 10% HCl aqua.
on the SnO2 surface, because proton sources, such as
water (H2O) and acids were not used in the work-up after
the reaction according to general procedure described in
Table 2. Early infrared studies (IR spectroscopy) of the
adsorption of carbon monoxide (CO) on the SnO2 sur-
face explicitly showed the existence of Sn–OH sites on
the SnO2 surface.12b Indeed, the reaction of 3a with HCl
as a Brønsted acid in place of SnO2 gave 4a in good yield
(entry 7). Accordingly, 4 is concluded to be formed by
demethylation of 3 at Sn–OH sites on the SnO2 surface.
In this study, we found that heterogeneous SC, such as
SnO2, ZrO2 and Act-C, are efficient solid oxidation
catalysts for the oxidative dimerization of NPOH 1 to
2,20-binaphthyl derivatives 2 or 3 using dioxygen as an
oxidant.
In particular, SnO2 is a useful and convenient catalyst
for the selective synthesis of 2,20-binaphthols 2 and for
selective O-demethylation of the enol-ether 3 to 4. The
SnO2-mediated oxidations appear to be free from pos-

T. Otsuka et al. / Tetrahedron Letters 45 (2004) 2643–2647
2647
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