Solvent and Temperature Effects on the Platinum-Catalyzed Oxidative Coupling of 1-Naphthols

Article abstract of DOI:10.1002/ejoc.201501280

Using H₂O₂ as the oxidant, 1-naphthols with electron-donating groups at the 2- and 4-positions couple oxidatively over a carbon-supported platinum catalyst to 3,3′-substituted 1,1′-binaphthalenylidene-4,4′-diones and 4,4′-substituted 2,2′-binaphthalenylidene-1,1′-diones, respectively. The binaphthalenyl diols are the intermediates. The selectivity to individual products is influenced by the reaction temperature (room temp. or reflux) and by the solvent used. Under reflux, complete conversions are obtained within 40 min. At room temp. high diol yields can be obtained, e.g. 96 % from 2-methyl-1-naphthol in MeOH. Under reflux the reaction proceeds always further to the diones (at least to some extent), and THF is a promising solvent for the selective one-pot two-step oxidation of 1-naphthols to the diones (e.g. 81 % from 4-methoxy-1-naphthol). In most other solvents (reflux) naphthoquinones are observed as byproducts. In an attempt to optimize the yield of menadione, 30.5 % was obtained in boiling MeNO₂.

Full text of DOI:10.1002/ejoc.201501280

DOI: 10.1002/ejoc.201501280
Full Paper
Oxidative Coupling
Solvent and Temperature Effects on the Platinum-Catalyzed
Oxidative Coupling of 1-Naphthols
Mabuatsela V. Maphoru,[a] Josef Heveling*[a,b] and Sreejarani Kesavan Pillai[c]
Abstract: Using H2O2 as the oxidant, 1-naphthols with elec- temp. high diol yields can be obtained, e.g. 96 % from 2-methyl-
tron-donating groups at the 2- and 4-positions couple oxida- 1-naphthol in MeOH. Under reflux the reaction proceeds always
tively over a carbon-supported platinum catalyst to 3,3′-substi- further to the diones (at least to some extent), and THF is a
tuted 1,1′-binaphthalenylidene-4,4′-diones and 4,4′-substituted promising solvent for the selective one-pot two-step oxidation
2,2′-binaphthalenylidene-1,1′-diones, respectively. The bi- of 1-naphthols to the diones (e.g. 81 % from 4-methoxy-1-
naphthalenyl diols are the intermediates. The selectivity to indi- naphthol). In most other solvents (reflux) naphthoquinones are
vidual products is influenced by the reaction temperature observed as byproducts. In an attempt to optimize the yield of
(room temp. or reflux) and by the solvent used. Under reflux, menadione, 30.5 % was obtained in boiling MeNO2.
complete conversions are obtained within 40 min. At room
Introduction
Hydrogen peroxide, which is a green oxidant,[7] is used as the
oxidizing agent.
Aryl–aryl coupling and cross-coupling reactions[1] are efficient
and reliable methods for the formation of new carbon–carbon
bonds in organic synthesis. These reactions lead to the forma-
tion of coupled aromatic rings, and the biaryl or triaryl frame-
works formed are central building blocks for a large number of
natural products.[2] They are also useful for many pharmaceuti-
cal applications, as organic dyes and as ligands in enantioselect-
ive catalysis.[1,3] The following reaction patterns are commonly
encountered: (i) oxidative coupling of various arenes, including
metal phenolates by chemical, catalytic or electrochemical
methods, (ii) coupling between aryl halides and aryl-metal spe-
cies, and (iii) electrophilic arylation of quinone derivatives with
hydroarenes in the presence of acids or bases.[2,4]
In order to limit the formation of by-products catalytic meth-
ods should gradually replace many of the stoichiometric proce-
dures currently employed.[5] Recently we reported the use of
bismuth-promoted platinum supported on activated carbon
(5 %Pt-5 %Bi/AC) as a heterogeneous catalyst for the oxidative
coupling of substituted 1-naphthols that contain electron-do-
nating groups para or ortho to the hydroxyl group.[6] Depend-
ing on the substrate and the reaction conditions, the principal
final products are the binaphthalenylidenediones (Scheme 1).
Scheme 1. Oxidative coupling of 2- or 4-substituted 1-naphthols to the corre-
sponding binaphthalenylidenediones (binaphthones) over promoted acti-
vated-carbon-supported platinum.
Oxidation and oxidative coupling reactions of naphthols are
favored by an increase of the electron density on the aromatic
system.[1,8] Therefore, charge transfer from external sources,
such as solvents or catalysts, could play an important role in
activating the substrate. Solvents are known to play a crucial
role in liquid-phase catalysis; this is referred to as the “solvent
effect”.[9] Solvents can stabilize intermediates and thus influ-
ence the kinetics of a reaction. They are also able to modify the
chemisorption of reactants and intermediates on the surface
of heterogeneous catalysts.[9] In this way they are expected to
influence the mechanism of a reaction. A decisive effect of sol-
vents on the selectivity and performance of oxidation and oxi-
dative coupling reactions has often been observed.[5,8,10] For
example, in the oxidation of 2-methyl-1-naphthol to menadione
with Ti-MMM-2 (mesoporous titanium-silicate) as the catalyst
[a] Department of Chemistry, Tshwane University of Technology,
Private Bag X680, Pretoria 0001, South Africa
E-mail: hevelingj@tut.ac.za
www.tut.ac.za
[b] Institute for NanoEngineering Research, Tshwane University of Technology,
Private Bag X680, Pretoria 0001, South Africa
[c] National Centre for Nano-Structured Materials, Council for Scientific and
Industrial Research,
P. O. Box 395, Pretoria 0001, South Africa
Supporting information and ORCID(s) from the author(s) for this article
are available on the WWW under http://dx.doi.org/10.1002/
ejoc.201501280.
Eur. J. Org. Chem. 2016, 331–337
331
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
and H2O2 as the oxidant, different solvents offer different select- thalenyl-4,4′-diol (2), 3,3′-dialkyl-1,1′-binaphthalenylidene-4,4′-
ivities:[10b] at 80 °C the yield of menadione obtained is 75 % in dione (3) and 2-alkyl-1,4-naphthoquinone (4).
MeCN, 38 % in EtOAc, 27 % in AcOH and 17 % in MeOH. Menad-
ione (vitamin K3) is a blood coagulating agent and is also used
as an intermediate for the preparation of other group K vita-
mins.[10b,11] The preparation of menadione from 2-methylnaph-
thalene by stoichiometric oxidation with CrO3/H2SO4 is often
used as an example for an industrial process that should be
The selectivity is influenced by the substituent on the sub-
strate, the solvent used and the reaction temperature. The re-
sults obtained are summarized in Table 2. Total conversions are
achieved in all cases in which the reaction is carried out under
reflux. Even at room temp. the less bulky methyl-substituted
starting material (SM) is totally converted in MeNO2 and MeOH,
while conversions between 49 and 75 % are achieved in the
other three solvents used (MeCN, Me2CO and THF). The real
replaced by
a
more environmentally benign catalytic
method.[7b,10b]
In this paper we report on the solvent effects observed in issue of interest is therefore the selectivity of the reaction. In
the platinum-catalyzed oxidative coupling of substituted 1- this regard, some common trends are observed for both sub-
naphthols. The substrates chosen are 2-methyl-1-naphthol, 2- strates. At room temp. the binaphthols 2 are the major pro-
ethyl-1-naphthol, 4-methoxy-1-naphthol and 1,4-dihydroxy- ducts. Independent of the conversion they are always formed
naphthalene. CH2Cl2, MeNO2, MeCN, Me2CO, MeOH and THF are with selectivities of 96–99.5 % (Table 2, entries 3, 5, 7, 10, 12
used as solvents. Some of their properties are summarized in and 15) and the quinones 4 are not observed. However, apart
Table 1. The donor number (DN) is a measure of the solvent from MeOH, in which traces of 4a are found (entry 7), the only
basicity (the ability to donate electrons).[12] The basicity in- important exception is MeNO2; MeNO2 is unique in that it pro-
creases with increasing DN. Solvents with a high DN are ex- motes the formation of considerable amounts of 4a even at
pected to increase the electron density of the solute, i.e. in our room temp. (22.5 % at 100 % conv.; Table 2, entry 1). Possibly
case they should render the naphthol substrate more suscepti- for steric reasons, only trace amounts of 4b are formed from
ble to oxidation. The Lewis acidity of a solvent (its ability to the more bulky 2-ethyl compound (entry 12).
accept electrons) increases with increasing acceptor number
Table 2. Oxidative coupling of 2-alkyl-1-naphthols (1) and oxidation of bin-
aphthols (2) over 5 % Pt–5 % Bi/AC in various solvents at room temp. or
(AN).[12] A solvent with a high AN is able to withdraw electrons
from a substrate, thus making it more susceptible to reduction.
Surely, other factors like the solvent polarity, the dielectric con-
stant and the solubility of the reactants will also affect the out-
come of a reaction,[13] and it cannot normally be expected that
the reactivity correlates with any single solvent parameter.
under reflux.
Entry SM Solvent
T
[°C]
Conv.
[%]
Selectivity [mol-%]
TOF (SM)
2
3
4
[min–1
]
1
2[a]
3
1a MeNO2 r.t.
1a MeNO2 96
1a
1a
100
100
49.2
100
73.0
100
62.1 trace
51.2 trace
99.2 trace
25.1 63.0
99.5 trace
15.1 60.7
22.5
30.5
–
11.8
–
12.8
trace
16.2
11.7
–
13.3
12.8
7.7
MeCN
MeCN
r.t.
75
r.t.
52
r.t.
60
60
r.t.
66
Table 1. Some properties of the selected solvents.[13]
4
15.7
11.4
13.9
15.1
15.6
16.3
11.7
12.5
11.4
13.1
13.3
5.8
5
1a Me2CO
1a Me2CO
Solvent
B.p.
[°C]
Dielectric
constant
Polarity
DN
AN
6[a]
7
[kcal mol–1
]
1a
1a
1a
1a
1a
MeOH
MeOH
MeOH
THF
≤ 100 96.0 trace
CH2Cl2
MeNO2
MeCN
Me2CO
MeOH
THF
40.2
101.2
82.0
56.2
64.6
66.0
7.0
36.6
35.7
20.7
32.7
7.6
polar aprotic
polar aprotic
polar aprotic
polar aprotic
polar protic
polar aprotic
1.0
2.7
14.1
17.0
19.0
20.0
20.4
20.5
18.9
12.5
41.5
8.0
8
100
100
75.4
100
73.2
–
1.4
83.0
86.4
9[b]
10
11[c]
12
13
14[c]
15
16[c]
17
18
98.7 trace
73.3 6.4
99.0 trace
THF
–
1b MeNO2 r.t.
1b MeNO2 96
trace
25.9
21.8
–
≤ 100 57.6 trace
≤ 100 35.3 27.9
37.7
100
100
100
1b
1b
1b
2a
2b
MeCN
MeOH
MeOH
MeOH
MeOH
75
r.t.
60
60
60
97.9 trace
–
–
–
61.1
98.2
97.2
5.7
–
–
10.5
15.4
15.3
Results and Discussion
[a] Black deposits together with other by-products were formed. [b] Recycled
catalyst containing 4.70 % Pt. [c] Two unknown compounds with similar re-
tention factors could not be separated by column chromatography.
Oxidation of 2-Substituted 1-Naphthols
Oxidation of 2-methyl-1-naphthol (1a) and 2-ethyl-1-naphthol
(1b) over 5 %Pt-5 %Bi/AC with H2O2 as the oxidant gives mainly
Under reflux, the selectivities shift from the binaphthols 2
three types of compounds (Scheme 2): 3,3′-dialkyl-1,1′-binaph- to the binaphthones 3 as the major products. The bi-
Scheme 2. Oxidation of 2-alkyl-1-naphthols over a bismuth-promoted platinum catalyst.
Eur. J. Org. Chem. 2016, 331–337
www.eurjoc.org
332
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
naphthols are regarded as the intermediates to the bi-
naphthones.[6] (Entries 17 and 18 in Table 2 confirm that the
binaphthols convert readily and with high selectivity into the
binaphthones.) For 1a, at complete conversions, the selectivities
to the binaphthone 3a are 63, 61 and 83 % in boiling MeCN,
Me2CO and MeOH, respectively (entries 4, 6 and 8). In these
reactions, apart from the intermediates 2, the quinones 4 are
the major by-products for both 1a and 1b. However, in MeOH
no residual binaphthol is found (entries 8 and 16).
THF is exceptional in that both at room temp. and under
reflux the binaphthol is formed with high selectivity and the
quinone is not observed (entries 10 and 11). Similarly, MeNO2
does not stimulate the formation of 3 (entries 2 and 13), but
under reflux increased amounts of the corresponding quinones
are obtained instead (31 and 26 % of 4a and 4b, respectively).
Figure 1 is a graphical illustration of the yields of 2a, 3a and
4a obtained from 2-methyl-1-naphthol.
Figure 2. Yields of binaphthol 2, binaphthone 3 and quinone 4 obtained from
2-methyl- (1a) and 2-ethyl-1-naphthol (1b) in three different solvents at room
temp. and under reflux.
Formation of Menadione
As is evident from Figure 1, the highest yields of menadione
(4a) are obtained in MeNO2 as the solvent. Performing the reac-
tion under dilute conditions should lower the concentration of
the chemisorbed SM on the catalyst surface and suppress the
formation of coupled products in favor of the quinone. Table 3
(entries 1–3) contains experiments performed at different con-
centrations of 1a in MeNO2 (conc. range: 0.507 to 0.085 mol/L).
Starting with the highest concentration (entry 1), the selectivity
to 4a increases initially from 9.4 % to 30.5 % (entry 2), but then
decreases again to 10.4 % (entry 3). In entry 1 appreciable
amounts of 2a are oxidized further to 3a, whereas with declin-
ing concentration more and more 2a desorbs from the catalyst
unconverted. The formation of 3a is a consecutive reaction,
while the formation of 4a competes with the synthesis of 2a,
but requires four times the amount of H2O2 per mol of SM
(Scheme 3). It is therefore not surprising that at high dilution
(entry 3) the limited availability of H2O2 favors the formation of
Figure 1. Yields of binaphthol 2a, binaphthone 3a and quinone 4a obtained
in various solvents from 2-methyl-1-naphthol (1a) at room temp. and under
reflux.
As is obvious from Figure 2, 1a and 1b follow the same reac-
tion patterns, but, possibly for steric reasons, the reactivity is
somewhat lower for 1b.
Entry 9 in Table 2 indicates that the catalyst can be recycled
without any significant decline in activity. Due to a small loss
of platinum during the first run (entry 8) the turn-over fre-
quency (TOF) increases from 15.6 to 16.3 min–1. In a control test
no catalytic activity of leached metal was found.
Scheme 3. Schematic illustration of the oxidation and oxidative coupling re-
actions of naphthols using H2O2. The stoichiometric factor 2 (in brackets)
applies only to the horizontal reaction, not to the vertical reaction.
Table 3. Oxidation of 2-methyl-1-naphthol (6.34 mmol) in MeNO2 under reflux (96 °C) using different substrate concentrations, varying amounts of H2O2 and
varying amounts of catalyst.
Entry
Catalyst
[g]
MeNO2
[mL]
H2O2
[mmol]
Conversion
[%]
Selectivity [mol-%]
TOF (SM)
TOF (H2O2)
2a
3a
4a
[min–1
]
[min–1
]
1
2[a]
3
4
5
0.04
0.04
0.04
0.04
0.04
0.02
12.5
25
75
100
25
25
22
22
22
88
66
22
100
100
100
100
100
84.7
45.1
51.2
89.3
68.9
2.6
43.8
–
–
–
80.6
–
9.4
15.4
12.8
15.7
15.5
14.1
25.4
13.4
13.6
10.3
14.7
14.9
12.7
30.5
10.4
29.6
6.5
6
95.6
trace
[a] Standard conditions: see entry 2 in Table 2.
Eur. J. Org. Chem. 2016, 331–337 www.eurjoc.org
333
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
2a rather than 4a. This argument is in agreement with the total
Remarkably, the formation of 4a decreases with increasing
absence of 3a in entries 2 and 3, as the conversion of 2a to donor number of the solvent (Figure 3). The same trend was
3a likewise calls for additional H2O2. Further, similar results are extracted from the data provided by Kholdeeva et al.[10b] It
obtained in reactions carried out at different SM concentrations, seems therefore that charge transfer from the solvent to the
but keeping the H2O2 concentration constant (entries 2 and 4). substrate is not involved in the transition state of this reaction.
In another experiment (entry 5), the amount of H2O2 added
to the solution (in comparison to entry 2) was increased by a
factor of three. Under these conditions almost all 2a is oxidized
further to 3a (sel. 81 %), and little 4a (6.5 %) is formed. Finally,
when cutting the amount of catalyst used in half (entry 6), the
reaction does not go to completion, 2a is produced with high
selectivity (96 %), and in comparison to entry 2 the reaction
rate for the formation of 4a is negligible.
It should be noted that all reaction steps in Scheme 3 pro-
duce water as the by-product. Therefore, they should be re-
garded as irreversible. Nevertheless, after all the SM has been
converted, the product selectivity can still change, as any bi-
naphthol can consume an equimolar amount of H2O2 and oxi-
dize further to the binaphthone. Consequently, the productive
usage of H2O2 would be a better measure of the reaction
Figure 3. Selectivity to menadione at complete conversion of 2-methyl-1-
naphthol plotted against the DN of the solvents used (reflux or 80 °C[10b]).
progress than the conversion of the SM. For this reason, TOFs
based on H2O2 are included in Table 3. Identical values of the
H2O2-based TOF correspond to the same reaction progress in
terms of mol of H2O2 consumed for the formation of products.
Oxidation of 4-Substituted 1-Naphthols
At a constant H2O2 uptake (entries 1 and 2) the selectivity to
4a increases with dilution of the system. At even higher dilution
(entry 3), the TOF of H2O2 decreases and mainly 2a is formed,
that is, a product requiring only 0.5 mol of H2O2 per mol of SM
(Scheme 3). Similarly, at high H2O2 concentrations and identical
H2O2 usage (entries 4 and 5), 4a (29.6 %) is formed at low con-
centrations of the SM (entry 4), while mainly 3a (80.6 %) is
formed at high concentrations (entry 5).
Substrate 5 (4-methoxy-1-naphthol) is the SM for a variety of
different oxidation products (Scheme 4), namely 4,4′-di-
methoxy-2,2′-binaphthalenyl-1,1′-diol (binaphthol 6), 4,4′-di-
methoxy-2,2′-binaphthalenylidene-1,1′-dione (Russig's blue or
binaphthone 7), 1′-hydroxy-4′-methoxy-2,2′-binaphthalenyl-1,4-
dione (coupled naphthoquinone 8), 1,4-naphthoquinone (9),
and 4-methoxy-1,2-naphthoquinone (10). The products ob-
served under our reaction conditions in different solvents at
Attention is drawn to the fact that quinone formation from room temp. or under reflux are listed in Table 4, together with
1 requires the transfer of one oxygen atom from H2O2 to the the corresponding conversions, selectivities and TOFs. Figure 4
substrate, while oxidative coupling is basically a dehydrogena- is a graphical presentation of the yields obtained. As for 3a and
tion reaction, and the role of the oxidant is limited to scaveng- 3b, the binaphthol must be regarded as the intermediate to
ing chemisorbed hydrogen from the surface of the metal cata- the binaphthone.[6] Entry 11 in Table 4 shows that under reflux
lyst.[6]
6 can indeed be converted with a yield of 99.8 % to 7.
Scheme 4. Oxidative coupling of 4-methoxy- (5) and 4-hydroxy-1-naphthol (11) over a bismuth-promoted platinum catalyst.
Eur. J. Org. Chem. 2016, 331–337 www.eurjoc.org © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
334

Full Paper
Table 4. Oxidation of 4-substituted 1-naphthols (5, 11) and oxidation of binaphthol 6 over 5 % Pt–5 % Bi/AC in various solvents at room temp. or under reflux.
Entry
SM
Solvent
T
[°C]
Conv.
[%]
Selectivity [mol-%]
TOF (SM)
6
7
8
9
10
[min–1
]
1[a]
2[a]
3
5
5
5
5
5
5
5
5
5
5
6
11
11
11
MeNO2
MeNO2
Me2CO
Me2CO
CH2Cl2
CH2Cl2
MeOH
MeOH
THF
THF
THF
MeOH
MeOH
MeOH
r.t.
96
r.t.
52
r.t.
38
r.t.
60
r.t.
66
66
r.t.
60
60
100
100
69.8
100
100
100
100
100
26.1
100
100
100
100
58.0
7.2
–
98.0
–
63.9
trace
–
55.5
67.1
–
85.4
35.5
67.0
62.0
–
97.3
81.0
99.8
–
–
–
–
–
–
16.9
20.7
–
10.2
–
–
–
–
–
–
–
92.4
95.2
41.9
trace
1.3
trace
trace
–
–
3.7
–
–
–
12.5
14.0
10.7
15.0
15.6
13.3
10.3
12.7
4.0
14.9
15.7
14.5
15.0
–
4
5
6[a]
7
17.4
trace
81.0
–
–
–
–
–
–
8
9
–
trace
14.0
–
–
–
10
11
12
13
14[b]
–
–
–
–
–
–
–
[a] A violet-blue compound that appears to be a polymer was detected (13C NMR) together with other by-products. [b] Blank reaction without catalyst.
The naphthoquinone 8 is another type of coupling product
that can be formed from 5, but not from 1.[6] It is observed as
the major product (81 %) in refluxing MeOH (entry 8) and also
as a by-product (17.4 %) in refluxing dichloromethane (entry 6).
Like in THF, the naphthoquinones 9 and 10 are not formed in
CH2Cl2.
A direct relationship between reactivity data and the solvent
donor number could not be established. Takeya et al. noticed
that for SnCl4-mediated oxidative coupling reactions of 1-naph-
thol solvents with a low DN were particularly favorable.[10a] In
our case a negative correlation between the selectivity to the
coupled products 6 and 7 and the AN was found (Figure 5).
Figure 4. Product yields obtained from 4-methoxy-1-naphthol (5) in various
solvents at room temp. and under reflux.
In contrast to substrates 1, 4-methoxy-1-naphthol forms the
binaphthone 7 even at room temp., most likely because the
methoxy group is more electron-donating than alkyl groups. In
fact, in THF (Table 4, entry 9) the intermediate binaphthol is
completely converted into the binaphthone, but in this case the
conversion of 5 is particularly low. In solution the binaphthol 6
is not entirely stable and slowly converts further to 7, even in
Figure 5. Selectivity to 6 + 7 at complete conversion of 4-methoxy-1-naphthol
(from Table 4) plotted against the AN of the solvents used (reflux).
the absence of a catalyst. (UV/Vis spectroscopy of 6 in CH2Cl2
showed the characteristic absorption of 7 at λmax = 630 nm
after the solution was allowed to stand for 2 h.) As an exception,
Me2CO appears to stabilize 6, and in this solvent 6 is the only
product observed at room temp. (entry 3). Hence, in Me2CO
substrates 1 and 5 follow the same selectivity pattern. Some
other observations are also similar to those found for the 2-
alkyl-substituted 1-naphthol substrates:
· Under reflux complete conversions are obtained in all sol-
vents tested.
· In THF no quinones are formed, even under reflux (entry
10).
The reaction of 1,4-dihydroxynaphthalene (11) in MeOH
(Scheme 4) gives 9 with selectivities of 92.4 and 95.2 % at room
temp. and 60 °C, respectively (Table 4, entries 12 and 13). No
coupling products are observed. This reaction also proceeds
without a catalyst (entry 14), but with low conversion (58 %)
and in an unselective manner (42 % of 9, corresponding to a
yield of 24 %). Over a platinum catalyst the reaction becomes
a simple and selective dehydrogenation process [Equation (1)],
similar to the conversion of the binaphthols to binaphthones
(compare Scheme 3; Table 2, entries 17, 18; Table 4, entry 11).
· In MeNO2 high amounts of naphthoquinones are formed at
room temp. and under reflux (entries 1 and 2).
HO–Ar–OH + H2O2 → O=Ar=O + 2 H2O
(1)
Eur. J. Org. Chem. 2016, 331–337
www.eurjoc.org
335 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
Conclusions
be obtained with yields of up to 96 %. The system is truly cata-
lytic and heterogeneous and has the potential to replace less
environmentally benign stoichiometric methods.
In the presence of hydrogen peroxide, 1-naphthols with elec-
tron-donating groups at the 2- and 4-positions couple oxida-
tively over a platinum catalyst to binaphthols. These can oxidize
further to the corresponding binaphthones. The direct oxid-
ation of the 1-naphthols to naphthoquinones competes with
the coupling reaction. Apart from the substituent and the posi-
tion of the substituent, the selectivity to the targeted products
is influenced by the solvent and by the reaction temperature
(room temp. or reflux). Among the SMs tested the reactivity
follows the order 1b < 1a < 5. Reactivity appears to increase
with the electron-donating capacity of the substituents and to
decrease with their steric demand. At room temp. the binaph-
thols (2) are obtained from 1a and 1b with high selectivity (96
to 99.5 %) in all solvents tested, except for MeNO2. At room
temp. binaphthone formation is not observed for 1, but in case
of the more reactive substrate 5, the binaphthol (6) reacts fur-
ther to the binaphthone (7). Under reflux conversions are al-
ways complete, the coupling reaction proceeds further to the
binaphthones, and naphthoquinones are formed as by-pro-
ducts. In this regard, THF is exceptional in that it does not pro-
mote the formation of quinones. THF is therefore a promising
solvent for the selective one-pot two-step oxidation of 1-
naphthols to binaphthones (e.g. 1 to 3 or 5 to 7). In striking
contrast, MeNO2 is the solvent of choice for quinone formation.
MeNO2 is the only solvent in which quinones form already at
room temp. In refluxing MeNO2 menadione can be obtained
with a yield of 30.5 %.
In the case of substrate 5 two different final coupling pro-
ducts can form: the binaphthone (7) and the coupled naphtho-
quinone 8. High yields of 8 are only obtained in boiling MeOH
(≤ 81 %).
In separate reactions, the binaphthols (2a, 2b and 6) oxidize
to the corresponding binaphthones (3a, 3b and 7). Under reflux
yields of 97.2–99.8 % are readily obtained. This confirms that
the binaphthols are the intermediates in the formation of the
binaphthones. Likewise, 1,4-dihydroxynaphthalene (11) oxidizes
to 1,4-naphthoquinone (yield: 95.2 %); no coupling products are
observed in this case.
Experimental Section
Materials and Catalyst: Activated carbon (Darko KB-G 174), bis-
muth(III) nitrate pentahydrate (98 %), hydrazine monohydrate
(98 %), 1,4-dihydroxynaphthalene (99 %), 4-methoxy-1-naphthol
(97 %), nitromethane (95 %) and silica gel for column chromatogra-
phy (pore size 60 Å, 70–230 mesh, 63–200 μm) were purchased
from Sigma–Aldrich. Hexachloroplatinic acid (metal content
40.31 %) was bought from SA Precious Metals. Hydrogen peroxide
(30 %), acetone (99 %), acetonitrile (99.5 %), dichloromethane
(99.5 %), ethyl acetate (99 %), methanol (99.5 %), petroleum ether
(90 %) and tetrahydrofuran (99.5 %) were obtained from SMM In-
struments, and Triton X-100 (98–102 %) from BDH. 2-Methyl-1-
naphthol and 2-ethyl-1-naphthol were prepared as described previ-
ously.[6]
The 5 %Pt-5 %Bi/AC catalyst was prepared by dissolving
H2PtCl6·6H2O (0.1328 g, 0.2564 mmol Pt) and Bi(NO3)3·5H2O
(0.1162 g, 0.2396 mmol Bi) in 50 % H2O/50 % MeOH (v/v, 100 mL).
Triton X-100 (3 mL) was added. The mixture was placed in an ultra-
sonic bath for 2 h to dissolve the bismuth nitrate. The AC support
(0.9005 g) was added and the mixture was stirred using a magnetic
stirrer. After 10 min a 98 % aqueous solution of N2H4·H2O (0.2 mL,
4.041 mmol N2H4) was added dropwise with a syringe to reduce
the metal ions. Stirring continued for 28 h. After this the catalyst
was filtered and washed with deionized water to pH 7. Finally it
was washed with MeOH and dried at 140 °C for 24 h. The amounts
of platinum and bismuth loaded onto the support were 4.91 and
4.89 %, respectively. Pt-Bi/AC catalysts are commercially available
and these can be used to the same effect.[6]
Oxidation Reactions and Product Separation: Coupling reactions
at room temp. or under reflux were carried out by placing the sub-
strate (6.34 mmol) and the catalyst (0.0401 g) into a 100 mL round-
bottomed flask, equipped with a condenser. A solvent (25 mL) was
added. The mixture was stirred with a magnetic stirring bar. While
stirring, 30 % aqueous H2O2 (3.1 mL, 22.1 mmol H2O2) was added
over 40 min directly into the solution using a peristaltic pump. After
the reaction the catalyst was separated by vacuum filtration,
washed with CH2Cl2 and MeOH, and left to air-dry for possible reuse
The catalyst can be recycled and the TOF for the reused cata- or analytical investigation. Column chromatography was used to
lyst remains above 15 min–1. Any metal leached from the cata-
separate the remaining reaction mixture (details given under prod-
uct characterization). Product selectivities were calculated as mol
percentages based on the mol of SM incorporated into each prod-
uct (unconverted SM excluded). Isolated yields are reported as per-
centages of the theoretical yield. Turn-over frequencies (TOFs) are
calculated as mol of SM incorporated into all identified products
per mol of platinum present on the catalyst over the total reaction
time (min–1). In addition (Table 3), a TOF was calculated based on
the theoretical quantity of mol of H2O2 required for total product
formation.
lyst into solution does not contribute to the outcome of the
reaction.
A positive correlation between reactivity data and the sol-
vent donor number (DN), as might be expected for an oxidation
reaction, was not found. However, the selectivity to 6 + 7 corre-
lates negatively with the acceptor number. For the formation of
menadione from 1a a negative relationship between DN and
the selectivity is indicated, not only for our own data, but also
for the results provided by others. No other correlations be-
tween solvent properties and reaction data could be estab-
lished. In particular, a relationship between the reaction tem-
perature (determined by the boiling points of the solvents) and
either conversion, TOFs or product selectivity was not found.
Our results show that solvent and temperature effects can be
used to control the selectivity of a reaction system consisting
Recycled catalysts were tested under conditions identical to those
used for fresh catalysts, that is, the molar ratios of all reaction ingre-
dients were kept constant in relation to the amount of the reused
catalyst.
Testing for catalytic activity of leached metal was done as follows:
a reaction (solvent: MeOH) was carried out as described above, but
no SM was added. After 40 min the catalyst was filtered off and the
of a consecutive and a parallel reaction. Individual products can coupling reaction was performed using only the solution (SM: 1a).
Eur. J. Org. Chem. 2016, 331–337
www.eurjoc.org
336 © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper
(Only trace amounts of 2a were found. The recovered catalyst con-
tained 4.78 % Pt and 4.64 % Bi.).
(C9), 133.6 (C5), 133.9 (C7), 153.1 (C2), 185.2 (C4), 185.3 ppm (C1). IR
(KBr): ν = 1662 (CO), 1618 cm–1
.
˜
4,4′-Dimethoxy-2,2′-binaphthalenyl-1,1′-diol (6): Light blue nee-
Analytical Methods: Elemental analyses (Pt, Bi) were performed on
a Spectro Arcos FHS ICP-OES instrument. FT-IR spectra were re-
corded from 4000 to 450 cm–1 on a Perkin–Elmer Spectrum Two
universal attenuated total reflectance Fourier transform infrared
spectrometer, equipped with KBr windows and a LiTaO3 detector.
The samples were mounted onto the diamond and a force gauge
between 130 and 150 N was applied. A 400/54/ASP Varian 400 MHz
premium-shielded NMR spectrometer was used to obtain 1H NMR
(100 MHz) and 13C NMR (400 MHz) spectra. Maximum absorption
wavelengths (λmax) over the range from 200 to 700 nm were deter-
mined using a single beam Pharmacia Biotech Ultrospec 3000 UV/
Visible spectrophotometer equipped with a tungsten filament lamp.
dles, Rf = 0.64 (n-hexane/ethyl acetate, 8:2); m.p. 218–220 °C. 1H
NMR (100 MHz, CDCl3, 25 °C): δ = 3.91 (s, 6 H, 2 OCH3), 6.88 (s, 2 H,
3
H
3,3′), 7.46–7.53 (m, 4 H, H7,7′,8,8′), 8.08 [d, J(H,H) = 8 Hz, 2 H, H9,9′],
8.21 [d, J(H,H) = 8 Hz, 2 H, H6,6′], 8.89 ppm (s, 2 H, 2 OH). 13C NMR
(400 MHz, CDCl3, 25 °C): δ = 56.09 (2 OCH3), 107.9 (C3,3′), 120.8
(C2,2′), 121.7 (C9,9′), 122.9 (C6,6′), 125.6 (C7,7′), 125.9 (C8,8′), 126.2
3
(C5,5′), 127.2 (C10,10′), 142.9 (C4,4′), 148.7 ppm (C1,1′). IR (KBr): ν =
˜
2583 cm–1 (The shift from 3400 cm–1 is due to the hydrogen bond
between –OH and the oxygen on the methoxy group.[14]).
1,4-Naphthoquinone (9): Light yellow needles, Rf = 0.75 (n-hex-
1
ane/ethyl acetate, 8:2); m.p. 120–122 °C. H NMR (100 MHz, CDCl3,
25 °C): δ = 6.95 (s, 2 H, H2,3), 7.72–7.74 (m, 2 H, H7,8), 8.04–8.06 ppm
(m, 2 H, H6,9). 13C NMR (400 MHz, CDCl3, 25 °C): δ = 126.3 (C6,9),
Characterization of the Organic Compounds: The characteriza-
tion of 2-methyl-1-naphthol (1a), 2-ethyl-1-naphthol (1b), 3,3′-di-
ethyl-1,1′-binaphthalenyl-4,4′-diol (2b), 3,3′-dimethyl-1,1′-binaph-
thalenylidene-4,4′-dione (3a), 4,4′-dimethoxy-2,2′-binaphthalenylid-
ene-1,1′-dione (7), 1′-hydroxy-4′-methoxy-2,2′-binaphthalenyl-1,4-
dione (8) and 4-methoxy-1,2-naphthoquinone (10) has been re-
ported previously.[6] The 1H NMR, 13C NMR and IR spectra of 2a, 4a,
3b, 4b, 6 and 9 are contained in the supporting information. These
compounds are described as follows.
131.8 (C5,10), 133.8 (C7,8), 138.6 (C2,3), 184.9 ppm (C1,4). IR (KBr): ν =
˜
1656 (CO), 1605 cm–1
.
Acknowledgments
M. V. Maphoru is grateful for financial support by the Deutscher
Akademischer Austauschdienst, DAAD-NRF bursary.
3,3′-Dimethyl-1,1′-binaphthalenyl-4,4′-diol (2a): Violet solid, Rf =
1
Keywords: C–C coupling · Oxidation · Platinum · Solvent
effects
0.92 (petroleum ether/ethyl acetate, 8:2); m.p. 247–248 °C. H NMR
3
(100 MHz, DMSO, 25 °C): δ = 2.37 (s, 6 H, 2 CH3), 7.08 [d, J(H,H) =
8.4 Hz, 2 H, H3,3′], 7.14 [t, 3J(H,H) = 7.6 Hz, 4 H, H7,7′,8,8′], 7.34 [t,
3J(H,H) = 7.6 Hz, 2 H, H6,6′], 8.23 [d, 3J(H,H) = 8.8 Hz, 2 H, H9,9′],
9.09 ppm (s, 2 H, 2 OH). 13C NMR (400 MHz, CDCl3, 25 °C): δ = 16.8
(CH3), 117.9 (C4,4′), 122.4 (C9,9′), 124.9 (C2,2′), 125.3 (C3,3′), 125.6
(C7,7′), 126.2 (C10,10′), 129.5 (C8,8′), 131.8 (C6,6′), 132.6 (C5,5′),
[1] J. Hassan, M. Sévignon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev. 2002,
102, 1359–1469.
[2] T. Takeya, H. Kondo, T. Otsuka, H. Doi, I. Okamoto, E. Kotani, Chem. Pharm.
Bull. 2005, 53, 199–206.
[3] T. Takeya, T. Otsuka, I. Okamoto, E. Kotani, Tetrahedron 2004, 60, 10681–
10693.
149.6 ppm (C1,1′). IR (KBr): ν = 3468 cm–1 (OH).
˜
2-Methyl-1,4-naphthoquinone (Menadione, 4a): Yellow needles,
Rf = 0.97 (petroleum ether/ethyl acetate, 8:2); m.p. 104–105 °C. 1H
NMR (100 MHz, CDCl3, 25 °C): δ = 2.17 (s, 3 H, CH3), 6.81 (s, 1 H,
H3), 7.69–7.71 (m, 2 H, H6,7), 8.02–8.09 ppm (m, 2 H, H5,8). 13C NMR
(400 MHz, CDCl3, 25 °C): δ = 16.44 (CH3), 126.1 (C3), 126.5 (C6), 132.1
(C10), 132.2 (C8), 133.5 (C9), 133.6 (C5), 135.6 (C7), 148.1 (C2), 184.9
[4] I. Okamoto, H. Doi, E. Kotani, T. Takeya, Tetrahedron Lett. 2001, 42, 2987–
2989.
[5] T. Takeya, H. Doi, T. Ogata, I. Okamoto, E. Kotani, Tetrahedron 2004, 60,
9049–9060.
[6] M. V. Maphoru, J. Heveling, S. K. Pillai, ChemPlusChem 2014, 79, 99–106.
[7] a) F. Menegazzo, M. Signoretto, G. Frison, F. Pinna, G. Strukul, M. Manzoli,
F. Boccuzzi, J. Catal. 2012, 290, 143–150; b) O. A. Kholdeeva, O. V. Zalo-
maeva, A. B. Sorokin, I. D. Ivanchikova, C. D. Pina, M. Rossi, Catal. Today
2007, 121, 58–64.
(C4), 185.5 ppm (C1). IR (KBr): ν = 1659 (CO), 1621 cm–1
.
˜
3,3′-Diethyl-1,1′-binaphthalenylidene-4,4′-dione (3b): Solid with
green metallic luster, Rf = 0.53 (petroleum ether/ethyl acetate, 9:1);
m.p. 156–157 °C. 1H NMR (100 MHz, CDCl3, 25 °C): δ = 1.13 [t,
[8] Y. Tanoue, K. Sakata, M. Hashimoto, S. I. Morishita, M. Hamada, N. Kai, T.
Nagai, Tetrahedron 2002, 58, 99–104.
3
3J(H,H) = 7.2 Hz, 6 H, 2 CH3], 2.58 [q, J(H,H) = 7.2 Hz, 4 H, 2 CH2],
[9] T. Mallat, A. Baiker, Catal. Sci. Technol. 2011, 1, 1572–1583.
[10] a) T. Takeya, H. Doi, T. Ogata, T. Otsuka, L. Okamoto, E. Kotani, Tetrahe-
dron 2004, 60, 6295–6310; b) O. A. Kholdeeva, O. V. Zalomaeva, A. N.
Shmakov, M. S. Melgunov, A. B. Sorokin, J. Catal. 2005, 236, 62–68.
[11] P. K. Khatri, S. L. Jain, Catal. Lett. 2012, 142, 1020–1025.
[12] a) V. Gutmann, Electrochim. Acta 1976, 21, 661–670; b) V. Gutmann, The
Donor-Acceptor Approach to Molecular Interaction, Plenum-Press, New
York, 1978.
7.63 (s, 4 H, H7,7′,8,8′), 7.85–7.89 (m, 4 H, H3,3′,6,6′), 8.32–8.34 ppm (m,
2 H, H9,9′). 13C NMR (400 MHz, CDCl3, 25 °C): δ = 13.01 (CH3), 23.48
(CH2), 127.8 (C4,4′), 130.1 (C6,6′), 130.6 (C10,10′), 131.0 (C8,8′), 132.3
(C9,9′), 134.1 (C5,5′), 137.5 (C7,7′), 140.4 (C2,2′), 141.6 (C3,3′), 183.8 ppm
(C1,1′). IR (KBr): ν = 1614 cm–1 (CO).
˜
2-Ethyl-1,4-naphthoquinone (4b): Yellow needles, Rf = 0.48 (petro-
leum ether/ethyl acetate, 8:2); m.p. 86–87 °C. 1H NMR (100 MHz,
CDCl3, 25 °C): δ = 1.18 [t, 3J(H,H) = 7.6 Hz, 3 H, CH3], 2.58 [q, 3J(H,H) =
7.2 Hz, 2 H, CH2], 6.78 (s, 1 H, H3), 7.70–7.72 (m, 2 H, H7,8), 8.04–
8.10 ppm (m, 2 H, H6,9). 13C NMR (400 MHz, CDCl3, 25 °C): δ = 11.83
(CH3), 22.62 (CH2), 126.0 (C3), 126.5 (C6), 132.0 (C10), 132.3 (C8), 133.6
[13] Y. Marcus, Chem. Soc. Rev. 1993, 22, 409–416.
[14] M. Hesse, H. Meier, B. Zeeh, Spectroskopische Methoden in der Or-
ganischen Chemie, Thieme, Stuttgart, Germany, 1979, p. 64–65.
Received: October 5, 2015
Published Online: November 20, 2015
Eur. J. Org. Chem. 2016, 331–337
www.eurjoc.org
337
© 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim