Ytterbium triflate-assisted catalytic oxidative cross-coupling of 2-naphthol derivatives

Article abstract of DOI:10.1016/j.tetlet.2007.10.064

The oxidative coupling of 2-naphthol and 3-hydroxy-2-naphthoate derivatives with a copper catalyst under an O₂ atmosphere was carried out. The reaction in the presence of a catalytic amount of the Lewis acid, Yb(OTf)₃, proceeded in a cross-coupling specific manner.

Full text of DOI:10.1016/j.tetlet.2007.10.064

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Tetrahedron Letters 48 (2007) 8595–8598
Ytterbium triflate-assisted catalytic oxidative cross-coupling
of 2-naphthol derivatives
Shigeki Habaue,* Tomohisa Temma, Yukihiro Sugiyama and Pei Yan
Department of Chemistry and Chemical Engineering, Graduate School of Science and Engineering, Yamagata University,
Yonezawa, Yamagata 992-8510, Japan
Received 12 September 2007; revised 9 October 2007; accepted 12 October 2007
Available online 14 October 2007
Abstract—The oxidative coupling of 2-naphthol and 3-hydroxy-2-naphthoate derivatives with a copper catalyst under an O2 atmo-
sphere was carried out. The reaction in the presence of a catalytic amount of the Lewis acid, Yb(OTf)3, proceeded in a cross-
coupling specific manner.
Ó 2007 Elsevier Ltd. All rights reserved.
The precise control of radical reactions has been an
attractive topic in the fields of synthetic organic and
polymer chemistry over the past decade. A Lewis acid
catalyst is often used as a powerful tool for controlling
their selectivities, etc.1,2 For instance, during the radical
polymerization of acrylamides and methacrylamides, a
catalytic amount of the Lewis acid, such as the trifluoro-
methanesulfonate salts of ytterbium and yttrium
[Yb(OTf)3 and Y(OTf)3], significantly increased the iso-
tacticity of the obtained polymer, and this method was
further combined with one of the living radical polymer-
ization processes, the reversible addition–fragmentation
chain transfer polymerization, to afford a polymer with
the simultaneously controlled molecular weight and
tacticity.3
affected by the substrate structure, as well as the cata-
lyst, and have not still been sufficiently controlled.
The oxidative coupling process involves a radical cou-
pling step.7,8 Therefore, the reaction between the 2-
naphthol and 3-hydroxy-2-naphthoate derivatives with
copper catalysts in the presence of a Lewis acid was
examined, and it was found that the cross-coupling spe-
cific reaction proceeds when a catalytic amount of
Yb(OTf)3 is used. The Lewis acid effect on the oxidative
coupling of the 2-naphthol derivatives has been scarcely
reported to the best of our knowledge.
The oxidative coupling reaction of a 1:1 mixture of 2-
naphthol (1a) and methyl 3-hydroxy-2-naphthoate (2a)
with the CuCl(OH)-TMEDA catalyst (Fig. 1) in THF
at room temperature under an O2 atmosphere in the
absence and presence of Yb(OTf)3 was carried out9
and the results are listed in Table 1. The reaction with-
out the Lewis acid gave a cross-coupling product in a
low yield with a cross-coupling selectivity (Y-selectivity)
of 88% (entry 1). In contrast, a catalytic amount of
The oxidative coupling of 2-naphthol derivatives is a
facile and effective preparation route to the 1,10-bi-2-
naphthol (BINOL). We recently reported the oxidative
cross-coupling between 2-naphthol and 3-hydroxy-2-
naphthoate derivatives using the CuCl-2,20-isopropylid-
enebis(4-phenyl-2-oxazoline) [CuCl-Phbox] catalyst
(Fig. 1), in which the unsymmetrical BINOL (Y) was
selectively produced (Scheme 1).4–6 However, the
cross-coupling and stereoselectivities were significantly
O
O
N
N
Me2N
NMe2
Keywords: Binaphthol; Cross-coupling; Lewis acid; Oxidative
coupling.
Ph
Ph
TMEDA
(S)Phbox
*
Corresponding author. Tel./fax: +81 238 26 3116; e-mail: habaue@
yz.yamagata-u.ac.jp
Figure 1. Ligands.
0040-4039/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2007.10.064

8596
S. Habaue et al. / Tetrahedron Letters 48 (2007) 8595–8598
OH
R
HO OH
OH CO2R'
catalyst
O2
+
+
+
HO
CO2R'
R'O2C HO OH CO2R'
R
HO
R
R
X
Y
Z
Scheme 1.
Table 1. Oxidative cross-coupling of 1a and 2a
OH
HO OH CO2Me
OH
CuCl(OH)-TMEDA
Yb(OTf)3
+
CO2Me
THF, O2, r.t.
1a
(1 equiv.)
2a
(1 equiv.)
Entry
CuCl(OH)-TMEDA (equiv)
Yb(OTf)3 (equiv)
Time (h)
Y-selectivitya (%)
Yieldb (%)
1
2
3
4
5
6e
7
0.2
0.2
0.1
0.2
0.1
0.1
—
—
48
36
48
48
24
62
48
88c
>99
>99
>99
98d
79f
47
91
78
91
81
22
0
0.2
0.2
0.1
0.1
0.1
0.2
—
a Cross-coupling selectivity.
b Isolated yield of cross-coupling product Y.
c X:Y:Z = 12:88:0.
d X:Y:Z = 2:98:0.
e Solvent = CH2Cl2.
f X:Y:Z = 21:79:0.
Yb(OTf)3 in THF significantly increased both the yield
and Y-selectivity, and the reaction proceeded in a
cross-coupling specific manner (entries 2–5). The solvent
also influenced on the yield and Y-selectivity (entry 6).
The reaction without the copper catalyst did not give
any coupling product (entry 7). Accordingly, the Lewis
acid effectively promotes the oxidative cross-coupling
reaction when using the copper catalyst.
in a cross-coupling specific manner, the system did not
work as a catalytic process (entry 2).
A plausible mechanism for the catalytic oxidative cross-
coupling reaction has been suggested in previous
reports.4 During the reaction, the b-naphthol with elec-
tron-withdrawing group works as an acceptor for the
radical species generated by the one-electron oxidation
of the other substrate, 2-naphthol. The Lewis acid cata-
lyst, such as Yb(OTf)3 and Y(OTf)3, should effectively
activate the former acceptor substrate as shown in
Figure 2, in which the structure of the ester group can
also significantly affect the coupling stereochemistry.
The asymmetric oxidative cross-coupling reaction of
various 2-naphthol derivatives with the CuCl-Phbox
catalyst was examined (Table 2). The cross-coupling
specific reaction again proceeded when a catalytic
amount of Yb(OTf)3 was used as the co-catalyst. The
cross-coupling product obtained by the reaction
between 1a and 2a hardly showed an ee value (entry
1), whereas the reaction of the substrate having a phenyl
ester 2b resulted in a higher stereoselectivity than that
observed for the reaction in the absence of the Lewis
acid. For example, the reaction between 3-benzyloxy-
2-naphthol (1b) and 2b at 0 °C afforded a product with
a Y-selectivity of >99% and 88% ee (entry 6). The Lewis
acid catalyst significantly improves the stereoselectivity,
as well as the cross-coupling value. Y(OTf)3 was also
effective as an additive and showed a catalyst effect
almost equal to that of Yb(OTf)3 (entry 3). Although
the reaction in the presence of Sc(OTf)3 also proceeded
In conclusion, a novel catalyst system for the oxidative
coupling reaction of 2-naphthol derivatives, the binary
catalyst of the copper complex and lanthanide triflate,
simultaneously controls both cross-coupling and stereo-
selectivities. Further investigation and extension of this
system are now in progress.
Acknowledgement
This work was partially supported by Grants-in-Aid for
Scientific Research (No. 18039003) from the Ministry of
Education, Science, Sports, and Culture of Japan.

S. Habaue et al. / Tetrahedron Letters 48 (2007) 8595–8598
Table 2. Asymmetric oxidative cross-coupling of 1 and 2
8597
R3
R2
OH
OH
R1 HO OH CO2R'
CuCl-(S)Phbox (0.2 equiv.)
Yb(OTf)3 (0.1 equiv.)
+
R1
CO2R'
THF, O2
r.t., 48 h
R1, R2, R3 = H: 1a
R' = Me: 2a
R' = Ph: 2b
R1 = OBn, R2, R3 = H: 1b
R1, R3 = H, R2 = OMe: 1c
R1, R3 = H, R2 = OAc: 1d
R3
(1 equiv.)
R2
(1 equiv.)
Entry
1
2
Y-selectivitya (%)
Yieldb (%)
eec (%)
1d
2e
3f
4
1a
1a
1a
1a
1b
1b
1c
1d
2a
2a
2a
2b
2b
2b
2b
2b
>99 (97)
>99
>99
>99 (86)
97g (85)
>99 (87)
>99 (97)
>99 (97)
98
21
93
98
93
70
99
85
ꢀ0 (8)
12, S
5, S
59, R (55)
86, R (65)
88, S (70)
45, S (3)
5
6h
7i
8
55, R (46)
a Cross-coupling selectivity. In parentheses, values for the reaction without Lewis acid are given.4a,b
b Isolated yield of cross-coupling product.
c Determined by HPLC (Chiralpak AD-H or AS-H). In parentheses, values for the reaction without Lewis acid are given.4a,b
d (R)Phbox and Yb(OTf)3 (0.2 equiv) were used.
e Sc(OTf)3 (0.2 equiv) as a Lewis acid was used.
f Y(OTf)3 (0.2 equiv) was used.
g X:Y:Z = 3:97:0.
h (R)Phbox was used (temp. = 0 °C, time = 168 h).
i (R)Phbox was used.
N
N
N
N
Cu
Cu
Cl
Cl
O
O
HO
Yb
OR'
HO
CO2R'
O
H2O
OH
(OTf)3
Yb(OTf)3
N
N
Cu
cross-coupling
product
1/2 (1/2 O2 + H2O)
Cl
OH
Figure 2. Plausible mechanism.
4. (a) Temma, T.; Habaue, S. Tetrahedron Lett. 2005, 46,
5655–5657; (b) Temma, T.; Hatano, B.; Habaue, S.
Tetrahedron 2006, 62, 8559–8563; (c) Habaue, S.; Taka-
hashi, Y.; Temma, T. Tetrahedron Lett. 2007, 48, 7301–
7304.
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8598
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9. Typical procedure for the catalytic cross-coupling reaction
in the presence of Yb(OTf)3: To a THF solution of 1, 2, and
Yb(OTf)3 (Aldrich), a mixture of CuCl(OH)-TMEDA
(TCI) and THF was added. The reaction mixture
([1]0 = 0.17 M) was then stirred at room temperature under
an O2 atmosphere, diluted with CHCl3, and washed with
1 N HCl and brine. The organic layer was dried over
MgSO4. After filtration and concentration to obtain the
crude products, the cross-coupling compound was isolated
by silica gel column chromatography.
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