Solvent-free oxidative coupling of 2-naphthols catalyzed by hydrotalcite-like compounds in aerobic conditions

Article abstract of DOI:10.2174/157017812800233705

Abstract: The hydrotalcite-like compound having ruthenium ions, cobalt ions, and iron ions in the Brucite layer and CO3 anions in the interlayer (Ru-Co-Fe-CO₃ HTLC) was found to be effective heterogeneous catalyst for the solvent-free oxidative coupling of 2-naphthols in aerobic conditions. The catalyst could be easily recovered and reused repeatedly for the reaction.

Full text of DOI:10.2174/157017812800233705

Letters in Organic Chemistry, 2012, 9, 263-266
263
Solvent-Free Oxidative Coupling of 2-Naphthols Catalyzed by
Hydrotalcite-Like Compounds in Aerobic Conditions
Kazuhiro Sugamoto*, Yoh-ichi Matsushita and Takanao Matsui
Faculty of Engineering, University of Miyazaki, Gakuen-Kibanadai, Miyazaki 889-2192, Japan
Received March 30, 2011: Revised April 27, 2011: Accepted January 31, 2012
Abstract: The hydrotalcite-like compound having ruthenium ions, cobalt ions, and iron ions in the Brucite layer and CO3
anions in the interlayer (Ru-Co-Fe-CO3 HTLC) was found to be effective heterogeneous catalyst for the solvent-free
oxidative coupling of 2-naphthols in aerobic conditions. The catalyst could be easily recovered and reused repeatedly for
the reaction.
Keywords; 1,1’-bi-2-naphthols, 2-naphthol, green chemistry, heterogeneous catalysis, oxidation, solid-phase synthesis.
INTRODUCTION
for 24 h in aerobic conditions (Table 2). After the reaction, 5
ml of EtOAc was added to the reaction mixture. The catalyst
was separated by filtration, and washed with 5 ml of EtOAc.
The separated catalyst was dried in vacuo before recycling.
The recovered catalyst was used for successive runs. The
activity of recovered catalyst decreased after the catalyst had
been used several times (Entry 3). Matsushita et al. reported
the recovered HTLC was washed with 10% sodium
carbonate aqueous solution and then water, which could be
reused as a catalyst without an appreciable loss of activity
for the oxidation of alcohol catalyzed by Ru-Co-Al-CO3
HTLC [15d]. Similarly, the activation of the catalyst was
recovered when the recovered catalyst was washed with
same manner (Entry 4).
1,1’-Bi-2-naphthols have been widely used as chiral
auxiliaries on various asymmetric reactions [1]. The
synthesis of 1,1’-bi-2-naphthols has been attempted via the
oxidative coupling reaction of 2-naphthols using various
homogeneous metal catalyst, such as Cu(II) amine complex
[2], VO(acac)2 [3], copper Schiff base complex [4],
methyloxorhenium [5], and V2O5 [6]. Recently, CuSO4/Al2O3
[7], Cu-exchanged montmorillonite [8], FeCl3/Al2O3 [9],
Cu/MCM-41 [10], V/MCM-41 [11], Ru(OH)x/Al2O3 [12], Fe
impregnated pillared montmorillonite K10 [13], and
biopolymer supported Cu(II) catalyst [14] have been used for
heterogeneous catalysts on the oxidative coupling of 2-
naphthols. Although these methods require solvent, the use
of heterogeneous catalysts offers several advantages over
homogeneous ones, such as ease of recovery and recycling
and atom utility. Hydrotalcite-like compounds (HTLCs) are
reported to be a turnable heterogeneous catalyst for various
organic reactions [15]. We report herein a solvent-free
oxidative coupling of 2-naphthols catalyzed by HTLCs in
aerobic conditions.
The present reaction was applied to preparation of
several binaphthols (Table 3). The solvent-free oxidative
coupling of 6-bromo-2-naphthol and 8-methoxy-2-naphthol
in aerobic conditions gave corresponding binaphthols in low
yields at 50 ºC for 24 h, respectively (Entries 1 and 4).
Increasing reaction temperature afforded binaphthols in 89%
yield at 100 ºC and 98% yield at 80 ºC, respectively (Entries
3 and 5). The oxidative coupling of 2,6-di-tert-butylphenol
and 2,4-di-tert-butylphenol at 50 ºC for 24 h gave 3,3’,5,5’-
tetra-tert-butyl-4,4’-diphenoquinone and 3,3’,5,5’-tetra-tert-
butyl-2,2’-biphenyldiol in 96% yields, respectively (Entries
6 and 7).
We first examined the effect of several types of HTLCs
(150 mg) on solvent-free oxidative coupling of 2-naphthol
(1, 1 mmol) at 50 ºC for 24h in aerobic conditions. The
results are summarized in Table 1. All HTLCs were prepared
according to a procedure in the literature [15d]. The reaction
of 1 using HTLC having cobalt ions and iron ions in the
Brucite layer and CO3 anions in the interlayer (Co-Fe-CO3
HTLC) and Cu-Fe-CO3 HTLC gave the 1,1’-bi-2-naphthol
(2) in low yield (Entries 1 and 2). The Ru-Co-Fe-CO3 HTLC
containing small amounts of ruthenium ions was the most
effective catalyst (Entry 3). On the other hand, substitution
of iron ions with aluminum ions (Ru-Co-Al-CO3 HTLC) and
ruthenium ions with rhodium ions (Rh-Co-Fe-CO3 HTLC) in
Brucite layer were not effective (Entries 4 and 5).
In conclusion, solvent-free oxidation coupling of 2-
naphthols catalyzed by Ru-Co-Fe-CO3 HTLC in aerobic
conditions gave binaphthols in good yields. From the
standpoint of green chemistry and technology, the present
reaction has the following significant advantages: (i) solvent-
free reaction, (ii) the use of air as solo oxidant, (iii)
heterogeneous catalyst, (iv) simple workup procedures, (v)
reusability of catalyst.
Next, we tried recovery and reuse of Ru-Co-Fe-CO3
HTLC on the solvent-free oxidative coupling of 1 at 50 ºC
EXPERIMENTAL
Typical Procedure
*Address correspondence to this author at the Faculty of Engineering,
University of Miyazaki, Gakuen-Kibanadai, Miyazaki 889-2192, Japan; Tel
+81-985-58-7390; Fax: +81-985-58-7323;
A mixture of 1 (144 mg, 1 mmol) and Ru-Co-Fe-CO3
HTLC (150 mg) was stirred by stirring bar at 50 ºC for 24 h.
After the reaction, 5 ml of EtOAc was added to the reaction
E-mail: sugamoto@cc.miyazaki-u.ac.jp
1875-6255/12 $58.00+.00
© 2012 Bentham Science Publishers

264 Letters in Organic Chemistry, 2012, Vol. 9, No. 4
Sugamoto et al.
Table 1. Solvent-Free Oxidative Coupling of 2-Naphthol Catalyzed by Various HTLCs in Aerobic Conditions
HTLC (150 mg)
Air (1 atm)
OH
OH
50 oC, 24 h
Solvent-free
OH
1
1 mmol
2
Entry
HTLC (Molar ratio)
Yield of 1/%a)
Recov. of 2/%a)
1
Co-Fe-CO3 HTLC (Co/Fe = 3.0/1.3)
Cu-Fe-CO3 HTLC (Cu/Fe =3.0/1.3)
21
16
86
37
8
72
69
0
2
3
Ru-Co-Fe-CO3 HTLC (Ru/Co/Fe = 0.1/ 3.0/1.0)
Ru-Co-Al-CO3 HTLC (Ru/Co/Al = 0.1/ 3.0/1.0)
Rh-Co-Fe-CO3 HTLC (Rh/Co/Fe = 0.1/ 3.0/1.0)
4
58
78
5
aIsolated yield.
Table 2. Repeated Use of Ru-Co-Fe-CO3 HTLC for the Solvent-Free Oxidative Coupling of 2-Naphthol
Ru-Co-Fe-CO3 HTLC (150 mg)
Air (1 atm)
OH
OH
50 oC, 24 h
OH
Solvent-free
1
1 mmol
2
Run
Yield of 2/%a)
Recov. of 1/%a)
Recov. of catalyst/%
1
2
90
80
46
94
79
0
8
102
94
3
47
0
95
4b)
95
5
16
95
aDetermined by GC.
bThe recovered catalyst from the reaction of run 3 was washed with 10% sodium carbonate aqueous solution and then water. The washed catalyst was dried at rt in vacuo and used on
the reaction of run 4.
Table 3. Solvent-Free Oxidative Coupling of Substituted 2-Naphthols and Di-Tert-Butylphenols Catalyzed Ru-Co-Fe-CO3 HTLC in
Aerobic Conditions for 24 h
Yield/%a)
Recov./%a)
Substrate
Product
Entry
Temp./ ºC
1
2
3
50
80
Br
Br
53
77
89
47
18
0
Br
OH
OH
100
OH

Solvent-Free Oxidative Coupling of 2-Naphthols
Letters in Organic Chemistry, 2012, Vol. 9, No. 4
265
(Table 3). Contd…..
Recov./%a)
Yield/%a)
Substrate
Entry
Product
Temp./ ºC
4
5
50
80
37
98
48
0
MeO
MeO
OH
OH
MeO
OH
6
50
96
0
OH
O
O
7
50
96
0
OH
OH
OH
aIsolated yield.
[5]
[6]
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mixture. The catalyst was separated by filtration, and washed
with 5 ml of EtOAc. The separated catalyst was dried in
vacuo before recycling. The filtrate was concentrated under
reduced pressure to afford the crude product. The product
was purified by silica gel column chromatography with
hexane- EtOAc to give 2 (123 mg) in 86% yield. Similarly
other compounds were oxidized using this procedure and
their conditions are given in the Table 3. The products were
identified by comparing their physical and spectral data with
those of authentic compounds reported in literature [7, 16].
[7]
[8]
[9]
CONFLICT OF INTEREST
Declared none.
[10]
[11]
ACKNOWLEDGEMENT
Declared none.
[12]
[13]
[14]
[15]
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Heterogeneously catalyzed aerobic oxidative biaryl coupling of 2-
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