Oxidative coupling of 1-naphthols over noble and base metal catalysts

Article abstract of DOI:10.1002/cplu.201300307

Bismuth-promoted platinum catalysts were tested for the oxidative coupling of 2- and 4-substituted 1-naphthols at different temperatures and ambient pressure. The principal final products are the 3,3'-substituted 1,1'-binaphthalenylidene-4,4'- diones and the 4,4'-substituted 2,2'-binaphthalenylidene-1,1'- diones, respectively. Hydrogen peroxide was used as the oxidant. Only naphthols with electron-donating substituents reacted. The corresponding binaphthalenyl diols can be considered as reaction intermediates. Yields of up to 99% were obtained from 2-methyl-1-naphthol as the starting material within 20 minutes. Probably for steric reasons, the diol is the final product obtained from 2-ethyl-1-naphthol. For 4-methoxy- 1-naphthol the outcome is determined by the reaction temperature. At 25 8C the expected 1,1'-dione is the major product, whereas at 60 8C 1'-hydroxy-4'-methoxy-2,2'-binaphthalenyl- 1,4-dione is formed; the loss of one methoxy unit and the preservation of the hydroxy group can be explained by the competitive cleavage of one of the two O-Me bonds at higher temperature. Unpromoted platinum and a range of other metallic catalysts, including gold and Raney nickel, were also found to be active. The products obtained are brightly colored solids that could be used as dyes. The method described is truly catalytic and environmentally benign. The potential of the technique justifies further research to expand on the applicability of this novel method.

Full text of DOI:10.1002/cplu.201300307

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DOI: 10.1002/cplu.201300307
Oxidative Coupling of 1-Naphthols over Noble and Base
Metal Catalysts
[a]
[a]
[b]
Mabuatsela V. Maphoru, Josef Heveling,* and Sreejarani K. Pillai
Bismuth-promoted platinum catalysts were tested for the oxi-
dative coupling of 2- and 4-substituted 1-naphthols at different
temperatures and ambient pressure. The principal final prod-
ucts are the 3,3’-substituted 1,1’-binaphthalenylidene-4,4’-
diones and the 4,4’-substituted 2,2’-binaphthalenylidene-1,1’-
diones, respectively. Hydrogen peroxide was used as the oxi-
dant. Only naphthols with electron-donating substituents re-
acted. The corresponding binaphthalenyl diols can be consid-
ered as reaction intermediates. Yields of up to 99% were ob-
tained from 2-methyl-1-naphthol as the starting material
within 20 minutes. Probably for steric reasons, the diol is the
final product obtained from 2-ethyl-1-naphthol. For 4-me-
thoxy-1-naphthol the outcome is determined by the reaction
temperature. At 258C the expected 1,1’-dione is the major
product, whereas at 608C 1’-hydroxy-4’-methoxy-2,2’-binaph-
thalenyl-1,4-dione is formed; the loss of one methoxy unit and
the preservation of the hydroxy group can be explained by the
competitive cleavage of one of the two OÀMe bonds at higher
temperature. Unpromoted platinum and a range of other met-
allic catalysts, including gold and Raney nickel, were also
found to be active. The products obtained are brightly colored
solids that could be used as dyes. The method described is
truly catalytic and environmentally benign. The potential of
the technique justifies further research to expand on the ap-
plicability of this novel method.
Introduction
[
3]
Aryl–aryl bond formation reactions are of great importance in
synthetic organic chemistry. These reactions lead to the forma-
tion of coupled aromatic rings that can be used as organic
the formation of by-products. Another problem is the dispos-
al of the reduced (toxic) oxidants and of the undesired side
[9]
products. For the sake of sustainable chemistry, these stoi-
chiometric reactions should be replaced with environmentally
[
1,2]
dyes and for many other applications.
The oxidative cou-
[2,3]
pling of naphthol compounds is an important reaction princi-
ple that can be used for this purpose. With hydroxyarene de-
rivatives, such as phenols and naphthols, the reaction usually
involves an oxidative dehydrogenation step and the subse-
friendly catalytic methods.
In a European patent Hauck et al. describe the catalytic cou-
pling of 2-methyl-1-naphthol (1) to 3,3‘-dimethyl-1,1’-binaph-
thalenylidene-4-4’-dione (2) with a yield of 87% over a catalyst
containing 5% platinum and 5% bismuth supported on acti-
[
3]
quent CÀC or CÀO coupling of the resulting arenyl radicals.
[10]
This reaction pattern is also used in hair dyeing. For example,
naphthols are coupled with a p-aminophenol derivative in the
presence of oxidizers to make the keratin fiber (hair) bright
vated carbon (5%Pt–5%Bi/AC). The reaction is performed at
608C and ambient pressure using hydrogen peroxide as the
oxidant (Scheme 1). Compound 2 is a deep red solid. In the
patent only one example is described, and since then no fur-
ther applications for this new coupling method have been re-
ported. Herein, we explore the scope and the limitations of
this reaction by changing the substitution pattern of the naph-
thol substrate and by using different metals as catalysts. This
investigation is also expected to lead to a better understand-
ing of the interaction between the substrate and the catalyst.
[
4,5]
red.
This avoids the use of oxidizing dyes, which have the
problem of insufficient performance in terms of saturation or
[
6]
vividness of color, dyeing capability, and speed. Naphthols
such as 1-naphthol, 2-methyl-1-naphthol, 1,3-, 2,7- and 1,7-di-
[7,8]
hydroxynaphthalene are frequently used in hair dyeing.
Many stoichiometric oxidants can be used for the coupling
IV
III
III
of naphthols in the liquid phase. Examples are Cr , Mn , Fe ,
II
I
IV
[1,3]
Cu , Ag , and Pb compounds.
The use of stoichiometric re-
agents often leads to poor coupling selectivity, low yield, and
[a] M. V. Maphoru, Prof. Dr. J. Heveling
Department of Chemistry
Tshwane University of Technology
Private Bag X680, Pretoria 0001 (South Africa)
E-mail: hevelingj@tut.ac.za
[b] Dr. S. K. Pillai
National Centre for Nano-Structured Materials
Council for Scientific and Industrial Research
PO Box 395, Pretoria 0001 (South Africa)
Scheme 1. Oxidative coupling of 2-methyl-1-naphthol (1) to 3,3’-dimethyl-
1,1’-binaphthalenylidene-4,4’-dione (2).
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Results and Discussion
A variety of metals (see Figure 1) were tested as sup-
ported or unsupported catalysts for the oxidative
coupling of 2-methyl-1-naphthol (1) according to
Scheme 1. Commercially available catalysts that were
used are listed in the Experimental Section. Other
catalysts (including 5%Pt–5%Bi/AC) were prepared
according to the procedures given in the Experimen-
Scheme 2. Oxidation of 2-methyl-1-naphthol (1) to menadione (4) over titanium–silicate
catalysts; 3 and 2 are observed as by-products.
[
12]
Previously, Kral and Laatsch obtained 2 with a yield
of 72% by the coupling of 1 with stoichiometric
[
11]
amounts of AgO as the oxidant. Interestingly, com-
pounds 2 and 3 (Scheme 2) were observed by Zalo-
maeva et al. as by-products in the catalytic oxidation
of 1 to menadione (4), using Ti–silicate as the catalyst
[
12]
and hydrogen peroxide as the oxidizing agent. Me-
nadione is a precursor to vitamin K . Formation of 2
3
and 3 also competes with the formation of 4 in the
uncatalyzed oxidation of 1 with oxygen at low pres-
[
13]
sure (100 kPa) in the presence of sun light. In both
cases radical mechanisms were proposed for the for-
[
12,13]
mation of the dimers 2 and 3.
Takeya et al. found that 4-methoxy-1-naphthol (5)
couples in the presence of oxygen and semiconduc-
tors, such as SnO and ZrO , to give binaphthols and
2
2
binaphthoquinones, which are of importance for the
[
2,3]
pharmaceutical industry (Scheme 3).
These com-
pounds are also useful intermediates for the prepara-
tion of naturally occurring substances, such as diosin-
[
2]
digo B, biramentaceone, and violet-quinone.
Scheme 4. Naphthol substrates tested for the oxidative coupling over a 5%Pt–5%Bi/AC
catalyst at 608C.
The substrates employed in our study include
compounds 1, 5, and other readily accessible ortho-
or para-substituted 1-naphthols (see Scheme 4). The
substituents used were both electron donating (Me, Et, OMe)
and electron withdrawing (COMe, COOH, Cl). Unsubstituted 1-
naphthol and 1,5-dihydroxynaphthalene were also tested.
tal Section. Aqueous H O (30%) was used as the oxidizing
2 2
agent. Substrate 1 was found to couple both at 608C and at
room temperature over 5%Pt–5%Bi/AC and gave 2 (red leaf-
lets with a green metallic luster), and yields of up to 99% were
obtained (Table 1). Unpromoted (bismuth free) 5%Pt/AC and
10%Pd/AC show similar activities (at 608C). In Table 1 the re-
sults obtained over Pt and Pd catalysts are summarized by re-
cording the yields and the turnover frequencies (TOF). No con-
version took place over the support material (AC) on its own,
or in the absence of a catalyst (Table 1, entries 5 and 6).
The catalysts 5%Ag/AC, 5%Au/AC, 5%Ru/AC, 5%Ru/Al O ,
2
3
5
%Rh/Al O , 30%Ir/Al O , and Raney Ni were also found to be
2 3 2 3
active when qualitatively tested in a test tube according to the
procedure given in the Experimental Section. In contrast, Fe
powder, Cu powder, and 5%Ni/Al O –SiO were inactive under
2
3
2
these conditions. Figure 1 summarizes the effectiveness of the
metals tested.
2-Ethyl-1-naphthol (9, Scheme 5) couples over the in-house
prepared 5%Pt–5%Bi/AC catalyst at room temperature to give
a purple/red compound that was identified as 3,3’-diethyl-1,1’-
binaphthalenyl-4,4’-diol (15; Table 1, entry 9). A yield of 37%
Scheme 3. Oxidation of 4-methoxy-1-naphthol (5) in the presence of semi-
[
2,3]
À1
conductors, such as SnO
2
.
was obtained (TOF=2.8 min ). The balance was unconverted
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Figure 1. Effectiveness of metals for the oxidative coupling of 2-methyl-1-
naphthol (1) to the dione 2, according to Scheme 1.
Scheme 5. Oxidative coupling of 2-ethyl-1-naphthol (9) over the in-house
prepared 5%Pt–5%Bi/AC catalyst.
starting material. The formation of the diol 15 showed that 9,
in contrast to 1, dimerized but further oxidation to the
dione 16 did not take place.
4-Methoxy-1-naphthol (5, Scheme 6) was found to couple at
room temperature over the in-house prepared 5%Pt–5%Bi/AC
catalyst and gave the deep blue 4,4’-dimethoxy-2,2’-binaphtha-
lenylidene-1,1’-dione (7, Russig’s Blue) as the major product
(62%), along with trace amounts of the deep violet 1’-hydroxy-
4‘-methoxy-2,2’-binaphthalenyl-1,4-dione (8; Table 1, entry 10).
The yellow 4-methoxy-1,2-naphthoquinone (17) was formed as
a minor by-product (3.7%). Interestingly, at 608C 8 became the
major product (81%); 7 and 17 were not formed under these
conditions (Table 1, entry 11).
Apart from 2-methyl-, 2-ethyl-, and 4-methoxy-1-naphthol (1,
Scheme 6. Oxidative coupling of 4-methoxy-1-naphthol (5) over the in-
9, and 5), all the other naphthols shown in Scheme 4 did not
house prepared 5%Pt–5%Bi/AC catalyst.
react when tested over the in-house prepared 5%Pt–5%Bi/AC
catalyst at 608C. A possible rationale for the difference in sub-
strate reactivity with respect to dependence on the substitu-
tion pattern is discussed below.
found to be active, surely because they remain in the metallic
state because of their high electrode potentials.
According to Table 1, almost complete conversion of 1 into
2
was achieved over all promoted and unpromoted Pt and Pd
Activity of the catalysts
catalysts when the reaction was carried out at 608C. However,
at room temperature and with the amount of catalyst de-
creased by half, the yields were lower and thin layer chroma-
tography (TLC) indicated that some of the starting material
was still present.
Transition metals with relatively low standard electrode poten-
tials (Cu, Fe, and Ni were tested) are inactive for the coupling
reaction (compare Figure 1). These metals are readily oxidized,
which renders them inactive. This outcome is confirmed by the
fact that Raney nickel (Raney Ni contains nickel in the metallic
state.) is active, whereas 5%Ni/SiO –Al O is inactive. On the
Under both conditions (RT and 608C) a commercial 5%Pt–
5%Bi/AC catalyst supplied by Evonik was also tested (see
Table 1, entries 1 and 7). The results obtained for this catalyst
are similar to those for the in-house prepared 5%Pt–5%Bi/AC
2
2
3
surface of the latter material (prior to a prereduction step)
nickel is present in the oxidized form. All precious metals were
Table 1. Yields and TOFs obtained for the oxidative coupling of 1-naphthols over various Pt and Pd catalysts, according to Schemes 1, 5, and 6.
[
b]
À1 [c]
Entry
Substrate
Catalyst
Ratio
t [min]
T [8C]
Yield [%] (TOF/min )
2
15
7
8
17
[
a]
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
9
5
5
5%Pt–5%Bi/AC
5%Pt–5%Bi/AC
5%Pt/AC
10%Pd/AC
AC
0.08
0.08
0.08
0.08
0.08
0.08
0.04
0.04
0.04
0.04
0.04
20
20
20
20
20
20
40
40
40
40
40
60
60
60
60
60
60
RT
RT
RT
RT
60
99.4 (7.7)
99.4 (7.7)
99.0 (7.6)
99.0 (2.1)
0
0
97.2 (7.5)
96.6 (7.4)
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
[
a]
no catalyst
[
a]
5%Pt–5%Bi/AC
5%Pt–5%Bi/AC
5%Pt–5%Bi/AC
5%Pt–5%Bi/AC
5%Pt–5%Bi/AC
37.0 (2.9)
–
–
–
–
–
10
62.0 (4.8)
0
trace
81.0 (6.2)
3.7 (0.60)
0
11
precious metalÀ1 À1
t . AC=activated carbon.
[a] Commercial catalyst. [b] Mass of catalyst over mass of substrate. [c] TOF in molproductmol
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[
20]
sample (Table 1, entries 2 and 8). The absence of a significant
difference seems to indicate that the reaction is not very struc-
ture sensitive with respect to the catalyst; these two materials
were surely not made by the same method, nor can they be
expected to be based on the same activated carbon.
FeCl ·6H O as catalysts. Just like 13, the 2,7-derivative did,
3 2
however, not react over our metallic-state catalysts.
The TOFs indicate that the metal utilization of a 10%Pd/AC
Proposed reaction mechanisms
À1
catalyst (Table 1, entry 4, TOF=2.1 min ) is significantly lower
À1
Most mechanisms proposed in the literature for both oxide-
mediated and uncatalyzed oxidative coupling reactions of 2-
than that of the catalysts containing 5%Pt (TOF>7.6 min ). It
has to be kept in mind that these numbers were calculated
based on the total amount of precious metal present on each
catalyst. Higher metal loadings can be expected to lead to
lower metal dispersions, that is, to more unutilized subsurface
metal that does not directly contribute to catalysis, thus result-
ing in lower turnover frequencies. However, the results could
be a better indication of the intrinsic relative activities of Pd
and Pt if the calculation of the TOFs was based on the exposed
metal surface areas, but these were not measured.
[2,3,12,13]
and 4-substituted naphthols involve radicals.
The reac-
tion sequence proposed (Scheme 7) for the metal catalyzed re-
action of 1 is related to this group of mechanisms, but the
“
radical” is stabilized by bonding to the metal surface (e.g. Pt).
In the suggested mechanism, Pt abstracts the hydrogen atom
from the hydroxy group of the naphthol substrate, thus lead-
ing to chemisorbed aryloxide and hydrogen. By resonance the
“
aryloxy” species can be bonded through the oxygen atom
and also through the carbon in ortho and para position (com-
pare Scheme 7) without disturbing the aromaticity of the
second ring. Coupling at the para position (reductive elimina-
tion from the Pt surface) leads to a dimer. Hydrogen shift gives
the diol (3), which can be considered as a physisorbed inter-
mediate. Chemisorption of 3 leads to another resonance stabi-
lized aryl species bonded to the Pt surface. Oxidation (abstrac-
tion of the second hydroxy hydrogen atom) leads to the final
product, which is the fully conjugated quinoid dione (2).
The use of a 5%Pt–5%Bi/AC catalyst was predetermined by
[
10]
the procedure given in the prior-art patent. Our results do,
however, not point to a significant influence of bismuth on the
performance of the catalysts (Table 1). In addition, bismuth
plays no role in the mechanisms proposed below. Neverthe-
less, platinum–bismuth catalysts are of considerable commer-
cial importance. Although the exact role of bismuth is still con-
troversial, bismuth appears to enhance the efficiency of the
active metal (Pt) by protecting it from poisoning, sintering,
[
14–17]
leaching, and overoxidation.
This, at least, is the role at-
tributed to bismuth promoters added to platinum and palladi-
um catalysts used for the selective oxidation of alcohols into
[
14,17]
ketones, aldehydes, and carboxylic acids.
A positive influ-
ence of bismuth on the oxidative coupling reactions described
here can therefore only be established by a combination of
catalyst recycling and characterization experiments. Such ex-
periments, together with the use of other catalyst supports,
are currently in progress and will be the subject of a forthcom-
ing contribution.
Influence of the substituents on the substrate reactivity
Of the 1-naphthol derivatives shown in Scheme 4 all unsubsti-
tuted 1-naphthols (13, 14), and those with -COCH , -COOH,
3
and -Cl substituents in 2- or 4-positions (10, 11, and 12) did
not react. This observation suggests that an electron-donating
group in the 2- or 4-position is required for the reaction to
occur. This finding is in agreement with the study conducted
by Tanoue et al. on the oxidative coupling of 4-substituted 1-
[18]
naphthols using a tenfold excess of AgO–40%HNO reagent.
3
They found that electron-withdrawing substituents deactivate
the aromatic ring and inhibit the coupling process. The pres-
ence and the type of functional groups determine, therefore,
whether the coupling reaction takes place or not.
The literature does not provide information with respect to
the oxidative coupling of 1,5-dihydroxynaphthalene (13).
Nonetheless, 2,7-dihydroxynaphthalene is known to couple
readily to give 7,7’-dihydroxy-1,1’-binaphthalene-2,2’-diol. Cata-
Scheme 7. Proposed reaction sequence for the oxidative coupling of 2-
methyl-1-naphthol on a platinum surface, using H
2 2
O as the oxidant. (The
[19]
lytic procedures include the use of CuSO /Al O
3
and
wavy bonds represent bonds to the platinum surface.)
4
2
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As shown in the insert of
Scheme 7, the chemisorbed hy-
drogen is converted into water
by chemisorbed oxygen, which
originates from the catalytic de-
composition of hydrogen perox-
ide (the oxidant used for the
overall reaction). Water is the
only by-product.
Coupling of 2-substituted 1-
naphthols takes place selectively
at the para position. Coupling at
the ortho position can neither
lead to a dione nor to a binaph-
thol (diol), because no hydrogen
atom is available in that position.
Apart from this, coupling could
be sterically hindered as a result
of the ortho substituent. Howev-
er, as seen in Scheme 8, coupling
occurs ortho to the OH group
for 4-substituted 1-naphthols,
such as 4-methoxy-1-naphthol
(
5). Otherwise, the coupling of 4-
substituted naphthols follows
Scheme 8. Proposed reaction sequence for the oxidative coupling of 4-methoxy-1-naphthol on a platinum sur-
the mechanistic principle pro- face, using H O as the oxidant. (The wavy bonds represent bonds to the platinum surface.)
2
2
posed for 2-substituted naph-
thols.
Under identical conditions (RT and ambient pressure) there
is a major difference in the reactivity of the methyl- (1) and the
ethyl-substituted substrate (9). Compound 1 couples readily
and the intermediate binaphthol 3 is not observed, but oxi-
dized further and gave the dione 2 as the final product
At 608C 8 becomes the principal reaction product. According
to Takeya et al., 8 is formed from 7 in the presence of Brønsted
[2]
acids. These can be present as acidic sites on the surface of
semiconductor oxides used as reaction mediators (compare
Scheme 3), or added as HCl. Activated carbon was found to be
inactive for this purpose. Under our reaction conditions (at RT
or 608C) compound 7 did not convert into compound 8. Con-
sequently, for the reaction catalyzed by Pt, 7 cannot be regard-
ed as an intermediate to 8. We therefore propose that in our
case the binaphthol 6 (Scheme 8) is the intermediate for both
compound 7 and 8. As seen in Scheme 8, Route 1 leads to
dione 7, and the reaction sequence follows the principle
shown in Scheme 7 for the formation of 2. Route 2 leads to
the formation of 8. The difference between Routes 1 and 2 is
straight forward. In the last step of Route 2 one of the two OÀ
Me bonds is cleaved rather than the remaining OÀH bond.
This leads directly to 8. In place of water, methanol is formed
as the by-product. (PtÀOH stems from the homolytic cleavage
(Scheme 7). In contrast, compound 9 forms only the “inter-
mediate” diol (15) with a comparatively low yield (Scheme 5).
It appears therefore that 1 is more reactive than 9. The ethyl
group is more electron-donating, but sterically more demand-
ing than the methyl group. At this stage, the difference in reac-
tivity between the two compounds can only be explained in
terms of a trade-off between electron-donation and steric fac-
tors. According to the first step of the reaction sequence
shown in Scheme 7, a hydrogen atom is abstracted from a hy-
droxy group ortho to the alkyl group. On a metal surface this
step might be more difficult if the alkyl group is more bulky.
Although some of compound 9 is able to react and form the
binaphthol 15 with a relatively low yield of 37%, the even
more sterically demanding binaphthol cannot react further to
form the corresponding dione (16). Interestingly, Kral and
Laatsch obtained both, 2 with a yield of 72% and 16 with
a yield of 75%, by coupling of the corresponding 2-alkyl-1-
naphthols using stoichiometric amounts of AgO as the oxi-
[21]
of the oxidant H O on the metal surface.
)
2
2
The formation of by-product 17 (Scheme 6) can be ex-
plained by the scavenging of an ortho-bonded aryl intermedi-
ate by perhydroxide, as shown in Scheme 9. Adsorbed per-
hydroxide (PtÀOOH) is one of the expected intermediates in
[
11]
[22]
dant. Their results seem to confirm that in our case steric
hindrance on the catalyst surface retards the reaction of 9. Ob-
viously, steric factors would then also contribute to the non-
reactivity of compounds 10 and 11 (Scheme 4).
the decomposition of H O on platinum surfaces.
2
2
Conclusion
When 4-methoxy-1-naphthol (5) is the substrate (Scheme 6),
the expected dione 7 is only observed at room temperature.
The oxidative coupling of 2-methyl-1-naphthol (1) at 608C and
ambient pressure over bismuth-promoted 5%Pt–5%Bi/AC cat-
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system and for expanding the applicability of this promising
new method.
Experimental Section
Materials
A commercial 5%Pt–5%Bi/AC catalyst (CF105RA/W) was obtained
from Evonik (Germany). Activated carbon (Darko KB-G 174),
1
0%Pd/AC, 5%Rh/Al O , 5%Ru/Al O , 5%Ru/AC, 5%Ni/SiO –Al O ,
2 3 2 3 2 2 3
Scheme 9. Proposed mechanism for the formation of the ortho-naphthoqui-
none 17 during the oxidative coupling of 4-methoxy-1-naphthol (5) on
Raney nickel, copper powder (99%, mesh size 325), bismuth(III) ni-
trate pentahydrate (98%), hydrogen tetrachloroaurate(III) trihydrate
a platinum surface, using H
2 2
O as the oxidant. (The wavy bonds represent
(
(
99.9%), 4-chloro-1-naphthol (97%), 1,5-dihydroxynaphthalene
99%), 2,7-dihydroxynaphthalene (98%), 2-acetyl-1-naphthol (99%),
bonds to the platinum surface.)
4
-methoxy-1-naphthol (97%), 2-methyl-1-naphthol (98%, used as
alysts gave 3,3’-dimethyl-1,1’-binaphthalylidene-4,4’-dione (2)
with yields of >99% within 20 minutes (TOF=7.7 min ).
reference material), 1-naphthol (98%), and hydrazine monohydrate
(98%) were purchased from Sigma–Aldrich. Hexachloroplatinic acid
À1
(
metal content 40.31%) was purchased from SA Precious Metals,
Aqueous H O (30%) was used as the oxidant. Two unpromot-
2
2
and Fe powder (99%) from Glassworld. 30%Ir/Al O is a proprietary
2
3
ed AC-supported Pt and Pd catalysts also gave very high yields
99%). Other supported noble metal catalysts (Ru, Rh, Ir, Ag,
catalyst. 1-Hydroxy-2-naphthoic acid (98%) and 2-methyl-1-naph-
thyl acetate (98%) were purchased from Tokyo Chemical Industry.
Hydrogen peroxide (30%), silver nitrate (99.9%), and methanol
(
and Au) were also found to be active when qualitatively tested
in a test tube at room temperature. Raney nickel (Ni in the
metallic state) was the only base–metal catalyst that showed
activity. It is concluded that the reaction proceeds only over
catalysts that contain the active metal in the oxidation state
zero.
(99.5%) were purchased from SMM Instruments. Triton X-100 (98–
1
02%) was purchased from BDH.
Substituents at the 2- or 4-position have a decisive influence
on the reactivity of the substrate. All unsubstituted 1-naph-
thols and those containing electron-withdrawing groups
Analytical methods
The amount of metal that remained in solution during catalyst
preparation was determined with a Spectro Arcos FHS ICP-OES in-
strument in order to indirectly determine the amount of metal
loaded onto the support. The FTIR spectra were recorded from
(
-COCH , -COOH, and -Cl were tested) did not react. 2-Ethyl-1-
3
naphthol (9) was found to couple at room temperature and
gave 3,3’-diethyl-1,1’-binaphthalenyl-4,4’-diol (15) with a rela-
tively low yield of 37%; in contrast with 1 as substrate the cor-
responding dione (16) was not observed. The binaphthalenyl
diols are regarded as intermediates for the formation of the bi-
naphthalenylidene diones. It is therefore assumed that the
more bulky ethyl group ortho to the hydroxy group sterically
hinders the further reaction of the diol on the catalyst surface.
For 4-methoxy-1-naphthol as the substrate the outcome of
the coupling reaction is determined by the reaction tempera-
ture: at room temperature the expected 4,4’-dimethoxy-2,2’-bi-
naphthalenylidene-1,1’-dione (7) was the major product (62%),
whereas at 608C 1’-hydroxy-4‘-methoxy-2,2’-binaphthalenyl-
À1
4000 to 450 cm on a PerkinElmer Spectrum Two spectrometer
equipped with KBr windows and a LiTaO detector. The samples
3
were mounted onto the diamond and a force gauge between 130
and 150 N was applied. A 400/54/ASP Varian 400 MHz premium-
1
shielded NMR spectrometer was used to obtain H NMR (100 MHz)
13
and C NMR (400 MHz) spectra. Maximum absorption wavelengths
lmax) over the range from 200 to 700 nm were determined using
(
a single-beam Pharmacia Biotech Ultrospec 3000-UV/Visible spec-
trophotometer with a tungsten filament lamp.
Preparation of catalysts
1
,4-dione (8) was formed (81%). The loss of one methoxy
All in-house made catalysts were synthesized using an electroless
deposition method. The 5%Pt–5%Bi/AC catalyst was prepared by
dissolving H PtCl ·6H O (0.1328 g, 0.2564 mmol) and Bi(NO ) ·5H O
group and the preservation of the hydroxy group can be ex-
plained by the competitive cleavage of an OÀMe bond (rather
than the second OÀH bond) on the platinum surface at elevat-
ed temperature.
2
6
2
3 3
2
(
0.1162 g, 0.2396 mmol) in a 50% H O/50% CH OH mixture
2
3
(100 mL) to give a cloudy primrose/yellow suspension. Triton X-100
(3 mL) was added and the mixture was placed in an ultrasonic
bath for 45 min to dissolve bismuth nitrate. After the salts were
dissolved, activated carbon (0.9005 g) was added and the mixture
was agitated using a magnetic stirrer. After 10 min, N H ·H O
The products obtained are brightly colored solids that could
be used as organic dyes and as intermediates for the fine
chemicals industry.
2
4
2
In conclusion, heterogeneous noble metal catalysts can ef-
fectively replace conventional stoichiometric oxidants used for
the oxidative coupling of naphthols. This method will contrib-
ute to the development of more ecological and sustainable
chemical processes. The results and discussions presented here
form the basis for ongoing improvements of the catalytic
(
0.2 mL, 4.041 mmol N H ) was added to reduce the metal ions.
2 4
Stirring continued for 48 h and the catalyst was filtered, washed
with deionized water to pH 7, and dried under vacuum. 5%Pt/AC,
5
%Ag/AC, and 5%Au/AC catalysts were prepared in the same way.
The metal composition of the catalysts was found by ICP-OES to
be close to the expected values of 5 mass% (4.7–5.3%).
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Preparation of naphthol substrates
material as the reference and 90% petroleum ether/10% ethyl ace-
tate as the mobile phase. The products were purified by column
chromatography on silica gel.
2
-Methyl-1-naphthol (1) was prepared by reducing 2-methyl-1-
naphthyl acetate (18) with LiAlH . Lithium aluminum hydride
4
(
1.89 g, 49.8 mmol) was transferred into a three-necked round-bot-
At 608C: The naphthol substrate (31.60 mmol), CH
the catalyst (0.40 g) were heated at reflux, and within 15 min 30%
aqueous H (6.8 mL, 48.5 mmol H ) was dropped directly into
the mixture while stirring. Refluxing and stirring continued for an-
other 5 min. The work-up process was carried out as described
above.
3
OH (64 mL), and
tomed flask (250 mL). The flask was placed in an ice bath and
equipped with a condenser and a dropping funnel. The air was re-
placed with nitrogen, and distilled THF (30 mL) was added to the
LiAlH4 powder by syringe through a septum on the dropping
funnel, while agitating with a magnetic stirrer. Compound 18
O
2
O
2 2
2
(
4.50 g, 22.5 mmol) was dissolved in THF (20 mL), the solution was
For all reactions with 4-methoxy-1-naphthol (5) as the substrate
only 1.00 g (5.74 mmol) starting material was used. The stoichio-
metric ratio of all ingredients was kept the same as for the other
substrates.
injected into the dropping funnel through the septum, and added
dropwise to the LiAlH slurry. The ice bath was replaced with an oil
4
bath, and the reaction mixture was refluxed at 668C. After 4 h 50%
THF/50% water was added dropwise (to protonate the intermedi-
ate) until the grey color disappeared and a greenish/yellow color
emerged. The mixture was filtered through a celite layer on a fritted
glass funnel. The filtrate was partitioned in ethyl acetate/water. The
The same procedures as described above were followed in the at-
tempts to convert 7 into 8 (both at RT and at 608C).
Test tube reactions: 1 (tip of a spatula) and a “pinch” of the catalyst
were mixed in a test tube that contained methanol (ca. 3 mL). A
few drops of 30% aqueous H O were added while swirling the
organic phase was dried with MgSO , filtered, and the solvent was
4
removed on a rotary evaporator. The product purity was checked
on a UV-active TLC plate using 5% ethyl acetate/95% petroleum
ether as the mobile phase. Commercial samples of 1 and 18 were
used as reference materials. The product was recrystallized from
CH Cl / n-hexane (10 mL/30 mL) and orange crystals formed. Yield:
2
2
tube. Appearance of a red color indicated that the catalyst was
active. The red solution was tested on a TLC plate against the ex-
pected product (2) as the reference.
2
2
8
8% (3.13 g). Characterization results are summarized further
below.
Characterization of organic compounds
2
-Ethyl-1-naphthol (9) was prepared by reducing the carbonyl
group in 2-acetyl-1-naphthol (10). KOH pellets (5.06 g, 90.2 mmol)
were transferred into a two-necked round-bottomed flask (250 mL)
containing diethylene glycol (100 mL). The flask was placed in an
oil bath and equipped with a condenser. The mixture was heated
to 1408C to dissolve the KOH. The solution was allowed to cool to
[23]
2
-Methyl-1-naphthol (1):
R =0.20 (petroleum ether/ethyl ace-
f
1
tate 9.5:0.5); m.p. 64–668C; H NMR (100 MHz, CDCl , 258C): d=
3
3
3
2
7
.31 (s, 3H, CH ), 5.00 (bs, 1H, OH), 7.13 (d, J(H,H)=8 Hz, 1H, H ),
3
4,6,7
5
.28–7.38 (m, 3H, H ), 7.67–7.69 (m, 1H, H ), 8.02 ppm (dd,
8
13
J(H,H)=8.4–1.2 Hz, 1H, H ); C NMR (400 MHz, CDCl , 258C): d=
3
9
08C and 10 (5.03 g, 27.0 mmol) was added together with
4
9
2
3
7
1
5.64 (CH ), 116.3 (C ), 120.2 (C ), 120.9 (C ), 124.3 (C ), 125.4 (C ),
125.4 (C ), 127.6 (C ), 128.9 (C ), 133.4 (C ), and 148.5 (C ); IR (KBr):
n=3334 (OH), 3051, 2951, 2930, 1599, 1573 cm
3
N H ·H O (15 mL, 303 mmol N H ), followed by heating at reflux
10
8
6
5
1
2
4
2
2
4
(
1308C). After 1 h excess hydrazine and water were distilled off
À1
.
until the temperature of the solution reached 2058C. The remain-
ing solution was heated at reflux (2058C) for 3 h and, after cooling
to 1008C, poured into an Erlenmeyer flask containing deionized
water (90 mL). The mixture was acidified to pH 2 by slow addition
of HCl (6 molL ) and allowed to cool down to room temperature.
The organic compounds were extracted with diethyl ether and the
2
-Ethyl-1-naphthol (9): R =0.54 (petroleum ether/ethyl acetate 9:1);
f
1
3
m.p. 66–688C; H NMR (100 MHz, CDCl , 258C): d=1.30 (t, J(H,H)=
3
3
7
.6 Hz, 3H, CH ), 2.76 (q, J(H,H)=7.6 Hz, 2H, CH ), 5.18 (bs, 1H,
3
2
3
3
4,6,7
À1
OH), 7.27 (d, J(H,H)=8.4 Hz, 1H, H ), 7.42–7.50 (m, 3H, H ), 7.78
3
5
3
8
(
d, J(H,H)=7.2 Hz, 1H, H ), 8.12 ppm (d, J(H,H)=8 Hz, 1H, H );
1
3
C NMR (400 MHz, CDCl , 258C): d=14.39 (CH ), 23.00 (CH ), 120.4
organic phase was washed with water, dried with MgSO , and fil-
3
2
3
2
10
4
4
9
3
7
(C ), 120.9 (C ), 122.6 (C ), 124.4 (C ), 125.3 (C ), 125.4(C ), 127.4
tered. The volume of the filtrate was reduced (rotary evaporator).
The purity of the residue was checked on a UV-active TLC plate
using 90% petroleum ether/10% ethyl acetate as the mobile
phase. The product was purified by column chromatography on
silica gel. The fractions containing 9 were combined and the
volume reduced. The residue was allowed to stand and dark
orange crystals formed. Yield: 45.3% (2.10 g). Characterization re-
sults are summarized further below.
8
6
5
1
(C ), 127.7 (C ),133.3 (C ), 147.9 ppm (C ); IR (KBr): n=3378 (OH),
À1
3
054, 2959, 2930, 2867, 1602, 1573 cm .
[11]
3
,3’-Dimethyl-1,1’-binaphthalenylidene-4,4’-dione (2):
Rf =0.27
1
(petroleum ether/ethyl acetate 8:2); m.p. 2508C; H NMR (100 MHz,
6,6’,7,7’
CDCl , 258C): d=2.38 (s, 6H, 2CH ), 7.58 (m, 4H, H
), 7.80 (d,
3
3
3
5,5’
3,3’
8,8’
J(H,H)=6.4 Hz, 2H, H ), 7.87 (s, 2H, H ), 8.27 ppm (m, 2H, H );
13
4,4’
C NMR (400 MHz, CDCl , 258C): d=17.16 (CH ), 127.7 (C ), 130.2
3
3
6,6’
10,10’
8,8’
9,9’
5,5’
(
(
C
C
), 130.8 (C
), 131.1 (C ), 132.1 (C ), 134.3 (C ), 136.5
7,7’
2,2’
3,3’
1,1’
), 138.9 (C ), 140.2 (C ), 184.2 ppm (C ); IR (KBr): n=2921,
À1
Oxidative coupling reactions
2851, 1615 (CO), 1580 cm ; UV/Vis (CH Cl): l =490 nm.
3
max
[
24]
At room temperature: the reactions were carried out by adding
the naphthol substrate (18.96 mmol) and 5%Pt–5%Bi/AC (0.12 g)
to a three-necked round-bottomed flask (100 mL), equipped with
a dropping funnel. Methanol (40 mL) was added. While stirring,
3,3’-Diethyl-1,1’-binaphthalenyl-4,4’-diol (15): Rf =0.33 (petroleum
1
ether/ethyl acetate 9:1); m.p. 166–1688C; H NMR (100 MHz, CDCl ,
258C): d=1.33 (t, J(H,H)=7.8 Hz, 6H, 2CH ), 2.84 (q, J(H,H)=
7.6 Hz, 4H, 2CH ), 5.24 (bs, 2H, 2OH), 7.25–7.28 (m, 4H, H
7.34 (d, J(H,H)=8 Hz, 2H, H ), 7.44 (t, J(H,H)=7.2 Hz, 2H, H ),
3
3
3
3
6
,6’,7,7’
),
2
3
5,5’
3
3,3’
3
0% aqueous H O (4.1 mL, 29.2 mmol H O ) was added dropwise
2
2
2
2
3
8,8’
13
directly into the solution within 20 min. Stirring was continued for
another 20 min. The reaction mixture was filtered through a celite
coated fritted glass funnel, and the filter cake was washed with
8.21 ppm (d, J(H,H)=8.4 Hz, 2H, H ); C NMR (400 MHz, CDCl ,
3
4,4’
9,9’
258C): d=14.41 (CH ), 23.02 (CH ), 121.0 (C ), 122.0 (C ), 124.3
3
2
2
,2’
3,3’
7,7’
10,10’
8,8’
(C ), 125.1 (C ), 125.4 (C ), 126.6 (C
), 129.5 (C ), 131.0
6,6’
5,5’
1,1’
CHCl . The solvent was evaporated (rotary evaporator). The residual
(C ), 132.7 (C ), 147.5 ppm (C ); IR (KBr): n=3261 (OH), 2965,
2927, 2870, 1576, 1506 cm ; UV/Vis (CH Cl): l =320 nm.
3
À1
liquid was analyzed on an UV-active TLC plate using the starting
3
max
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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FULL PAPERS
www.chempluschem.org
[
25]
4
(
(
7
,4’-Dimethoxy-2,2’-binaphthalenylidene-1,1’-dione (7):
R =0.75
[1] J. Hassan, M. Sꢁvginon, C. Gozzi, E. Schulz, M. Lemaire, Chem. Rev. 2002,
f
1
1
02, 1359–1469.
petroleum ether/ethyl acetate 8:2); m.p. 259–2608C; H NMR
3
[2] T. Takeya, T. Otsuka, I. Okamoto, E. Kotani, Tetrahedron 2004, 60, 10681–
10693.
[3] T. Otsuka, I. Okamoto, E. Kotani, T. Takeya, Tetrahedron Lett. 2004, 45,
100 MHz, CDCl , 258C): d=4.06 (s, 6H, 2OCH ), 7.47 (t, J(H,H)=
3
3
5
,5’
3
5,5’
.2–7.6 Hz, 2H, H ), 7.60 (t, J(H,H)=7.6 Hz, 2H, H ), 7.78 (d,
3
5,5’
3
5,5’
J(H,H)=7.6 Hz, 2H,
H ), 8.15 (d, J(H,H)=7.6 Hz, 2H, H ),
2643–2647.
8
,8’
13
8
.39 ppm (s, 2H, H ); C NMR (400 MHz, CDCl , 258C): d=55.85
3
3
[
4] N. M. Fakhouri, US Pat. 4169703, 1979.
,3’
6,6’
10,10’
8,8’
(
(
(
OCH ), 103.1 (C ), 121.9 (C ), 127.6 (C
C
C
), 128.9 (C ), 131.4
3
[5] M.-I. Lim, L. Stasaitis, Y.-G. Pan, A. Chan, US Pat. US5420362, 1993.
[6] T. Mano, J. Kawese, D. Misu, M. Obayashi, Eu. Pat. Appl. 0345728A2,
1989.
9
1
,9’
,1’
5,5’
7,7’
2,2’
4,4’
), 131.7 (C ), 131.8 (C ), 133.1 (C ), 157.3 (C ), 189.3 ppm
); IR (KBr): n=3007, 2969, 2937, 2838, 1602 (CO), 1583,
À1
1
557 cm ; UV/Vis (CH Cl): l =630 nm.
[7] F. Brody, S. Pohl, US Pat. 3970423, 1976.
3
max
[
8] J. Wood, D. Pratt, A. Aechtner, C. Olsson, I. Takeshi, Int. Pat. Appl.
WO2011/113452A1, 2011.
[
26]
1
(
(
(
(
2
1
(
(
’-Hydroxy-4’-methoxy-2,2’-binaphthalenyl-1,4-dione (8): R =0.64
f
1
petroleum ether/ethyl acetate 8:2); m.p. 187–1888C; H NMR
100 MHz, CDCl , 258C): d=3.97 (s, 3H, OMe), 6.54 (s, 1H, H ), 7.12
s, 1H, H ), 7.55–7.58 (m, 2H, ArH), 7.78–8.25 (m, 5H, ArH), 8.37–8.40
m, 1H, ArH), 8.49 ppm (s, 1H, OH); C NMR (400 MHz, CDCl ,
3
[9] L. Chen, J. B. Lan, Z. H. Mao, X. Q. Yu, R. G. Xie, Chin. Chem. Lett. 2004,
15, 903–906.
[10] M. Hauck, J. Heveling, A. Wellig, Eur. Pat. EP1263705, 2003.
3
’
3
3
13
[
11] A. Kral, H. Laatsch, Z. Naturforsch. B 1993, 48, 1401–1407.
3
’
9’
6’
2’
[12] O. V. Zalomaeva, N. N. Trukhan, I. D. Ivanchikova, A. A. Panchenko, E. Ro-
duner, E. P. Talsi, A. B. Sorokin, V. A. Rogov, O. A. Kholdeeva, J. Mol. Catal.
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Skobelev, E. P. Talsi, J. Phys. Chem. B 2011, 115, 11971–11983.
58C): d=55.79 (OMe), 104.6 (C ), 114.4 (C ), 121.7 (C ), 123.7 (C ),
7‘ 5‘ 6 9
10’
8’
26.3 (C ), 126.7 (C ), 127.3 (C ), 127.5 (C ), 127.7 (C or C ), 127.9
6
9
5
10
5
10
7
8
C or C ), 131.7 (C or C ), 132.5 (C or C ) 134.0 (C or C ), 134.8
C or C ), 138.9 (C ), 145.5 (C ), 149.7 (C ), 149.9 (C ), 184.5 (C ),
88.5 ppm (C ); IR (KBr): n=3328 (OH), 3070, 2952, 2924, 2854,
[
7
8
3
2
4’
1’
1
4
1
1
[
14] T. Mallat, Z. Bodnar, P. Hug, A. Baiker, J. Catal. 1995, 153, 131–143.
À1
656 (CO), 1589 cm ; UV/Vis (CH Cl): l =570 nm.
[15] M. Besson, P. Gallezot, Catal. Today 2000, 57, 127–141.
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3069–3074.
3
max
[
[26]
4
-Methoxy-1,2-naphthoquinone (17): Rf =0.09 (petroleum ether/
ethyl acetate 8:2); m.p. 190–1918C; H NMR (100 MHz, CDCl3,
3
1
3
2
1
7
1
(
(
58C): d=4.00 (s, 6H, OCH ), 5.96 (s, 1H, H ), 7.57 (dt, J(H,H)=7.6,
3
7 9 7
9’
.2 Hz, 2H, H or H ), 7.68 (dt, J(H,H)=7.6, 1.2 Hz, 1H, H or H ),
3
6
.84 (d, J(H,H)=8 Hz, 1H, H ), 8.10 ppm (dd, J(H,H)=7.6, 1.2 Hz,
H, H ); C NMR (400 MHz, CDCl , 258C): d=56.83 (OCH ), 103.1
C ), 124.7 (C ), 129.1 (C ), 130.3 (C ), 131.5 (C ), 131.9 (C ), 134.9
C ), 168.7(C ), 179.4 (C ), 179.5 ppm (C ); IR (KBr): n=3060, 2956,
9
13
3
3
5
3
6
10
8
9
7
4
2
1
À1
[21] P. B. Balbuena, S. R. Calvo, E. J. lamas, P. F. Salazar, J. M. Seminario, J.
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2
924, 2851, 1701 1641 (CO), 1608, 1561 cm
.
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Acknowledgements
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23] H. Taguchi, S. Kita, Y. Tani, Biosci. Biotechnol. Biochem. 1995, 59, 2001–
2
003.
[24] W. Brackman, E. Havinga, Recl. Trav. Chim. Pays-Bas 2004, 74, 1021–
039.
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004, 60, 6295–6310.
This work was financially supported by the National Research
Foundation of South Africa under Grant Unique No. 63266. We
thank Makhosazana Gamedze for the NMR analyses and Evonik
1
[
[
2
(Germany) for a free catalyst sample. M. V. Maphoru is grateful
26] T. Ogata, I. Okamoto, E. Kotani, T. Takeya, Tetrahedron 2004, 60, 3941–
for financial support through a CSIR-TUT bursary.
3948.
Keywords: 1-naphthols
catalysis · oxidation · platinum
·
CÀC coupling
· heterogeneous
Received: September 2, 2013
Published online on November 6, 2013
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2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemPlusChem 2014, 79, 99 – 106 106