Highly selective oxidative cross-coupling of 2-naphthol derivatives with chiral copper(I)-bisoxazoline catalysts

Article abstract of DOI:10.1016/j.tetlet.2005.06.098

The asymmetric oxidative coupling reaction of 3-hydroxy-2-naphthoate and 2-naphthol derivatives with the CuCl-(S)-(-)-2,2′-isopropylidenebis(4- phenyl-2-oxazoline) catalyst under an O₂ atmosphere was carried out. The reaction proceeded in a highly cross-coupling selective manner (≤99.7%) with a moderate enantioselectivity of up to 65%.

Full text of DOI:10.1016/j.tetlet.2005.06.098

Tetrahedron
Letters
Tetrahedron Letters 46 (2005) 5655–5657
Highly selective oxidative cross-coupling of 2-naphthol
derivatives with chiral copper(I)–bisoxazoline catalysts
Tomohisa Temma and Shigeki Habaue*
Department of Chemistry and Chemical Engineering, Yamagata University, Yonezawa, Yamagata 992-8510, Japan
Received 10 May 2005; revised 16 June 2005; accepted 20 June 2005
Available online 6 July 2005
Abstract—The asymmetric oxidative coupling reaction of 3-hydroxy-2-naphthoate and 2-naphthol derivatives with the CuCl–(S)-
(ꢀ)-2,20-isopropylidenebis(4-phenyl-2-oxazoline) catalyst under an O2 atmosphere was carried out. The reaction proceeded in a
highly cross-coupling selective manner (699.7%) with a moderate enantioselectivity of up to 65%.
Ó 2005 Elsevier Ltd. All rights reserved.
Axially dissymmetric 1,10-bi-2-naphthol derivatives are
very important and versatile chiral auxiliaries, and have
been extensively utilized in asymmetric syntheses, cata-
lyzes, and resolutions.1 From this point of view, there
has been an intense interest in the preparation of the
optically active binaphthols. The catalytic asymmetric
oxidative coupling reaction of 2-naphthols with chiral
metal catalysts, such as Cu(I), Ru(II), and V(IV), is
one of the most facile methods for their synthesis,2–4
and the homo- or self-coupling of various 2-naphthols
affording a symmetrical binaphthol skeleton has been
mainly studied. In contrast, there are few reports on
the catalytic cross-coupling reaction leading to a
binaphthol having an unsymmetrical structure, although
several reactions using a stoichiometric amount of a me-
tal complex were reported.5 Only the report by Kozlow-
ski and co-workers, on the catalytic asymmetric
oxidative cross-coupling reaction between 2-naphthol
1a and methyl 3-hydroxy-2-naphthoate 2a with the
CuBF4–(S,S)-1,5-diaza-cis-decalin catalyst has recently
appeared,6 in which the cross-coupling product was
obtained in a poor yield (8%) with an enantioselectivity
of 72% ee (R).
2,3-dihydroxynaphthalene, with a novel copper(I)
catalyst system, CuCl–(S)-(ꢀ)-2,20-isopropylidenebis(4-
phenyl-2-oxazoline) [(S)Phbox], at room temperature
under an O2 atmosphere, although the diamines, such
as (+)-1-(2-pyrrolidinylmethyl)pyrrolidine [(+)-PMP]
and (ꢀ)-sparteine [(ꢀ)Sp] (Fig. 1), which are well known
as the conventional ligand for the copper catalyst of the
asymmetric oxidative coupling, hardly showed a poly-
mer productivity.7
During the course of our study, we found that this
catalyst system is also effective for the cross-coupling
reaction between 2-naphthol derivatives (1) and 3-
hydroxy-2-naphthoates (2) (Scheme 1).
The results of the reaction between 1a and the methyl
ester 2a using various copper(I) catalysts are listed in
Table 1. During the reaction with (+)-PMP, the
homo-coupling product of 2a (Z) was predominantly
produced (entry 1), whereas CuCl–(ꢀ)Sp preferentially
afforded a cross-coupling compound (Y) with a high
N
We reported the first synthesis of the poly(1,10-bi-2-
naphthol) derivative, poly(2,3-dihydroxy-1,4-naphthyl-
ene), through the asymmetric oxidative coupling
polymerization of the commercially available monomer,
N
NH
N
(+)PMP
(—)Sp
O
O
R = Ph : Phbox
R = CH2Ph : Bnbox
N
N
Keywords: Binaphthol; Cross-coupling; Asymmetric oxidative coupling.
R
R
*
Corresponding author. Tel./fax: +81 238 26 3116; e-mail: habaue@
yz.yamagata-u.ac.jp
Figure 1. Chiral ligands.
0040-4039/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2005.06.098

5656
T. Temma, S. Habaue / Tetrahedron Letters 46 (2005) 5655–5657
R2
R3
R1
COOR4
of the cross-coupling compound 3 was not able to be
catalyst / solvent
r.t.
+
determined by a high performance liquid chromato-
graphic (HPLC) analysis due to its mostly insoluble nat-
ure in EtOH and isopropanol, the small specific rotation
value ([a]D +1.0) and the CD intensity suggest that there
is almost no enantioselectivity (entry 1). Both the cross-
coupling selectivity and the enantioselectivity were sig-
nificantly influenced by the structures of the substrates,
1 and 2. The methyl and benzyl esters, 2a and 2c, affor-
ded coupling products with a high Y-ratio, whereas the
coupling reaction of the phenyl ester 2b with 1a,b, and
1c showed lower cross-coupling selectivities (71.5–
86.4%) (entries 3–5). The enantioselectivity during the
latter cross-coupling reaction using 2b, however, was
much higher than that observed for the former reac-
tions. For example, the coupling product 7 obtained
from 1c and 2b exhibited a cross-coupling selectivity of
85.2% and an enantioselectivity of 65% ee (R).
OH
OH
1
2
R2
R1
R2
R3
R1
COOR4
R3
OH
OH
OH
OH
R3
OH
OH
COOR4
R2
R1
COOR4
X
Y
Z
2a R4 = Me
2b R4 = Ph
2c R4 = Bn
1a R1 = H, R2 = H, R3 = H
1b R1 = OMe, R2 = H, R3 = H
1c R1 = OBn, R2 = H, R3 = H
1d R1 = H, R2 = OMe, R3 = H
1e R1 = H, R2 = H, R3 = OMe
1f R1 = H, R2 = OBn, R3 = H
1g R1 = H, R2 = H, R3 = OBn
Scheme 1.
Since the reaction of 2b demonstrated a higher stereo-
selectivity for Y, the effect of the substituent position
on the 2-naphthol was investigated (entries 7–10). The
6-substituted 2-naphthols, such as 1d and 1f, gave a
cross-coupling product with the high selectivities of
97.3% and 98.7%, respectively, but with a poor ee value
of 3% (S). The 2-naphthols having a substituent at the
7-position, 1e and 1g, in contrast, showed a much higher
enantioselectivity (51% and 58% ee (R)), although they
are still lower than that of the products obtained from
the 3-substituted 2-naphthols, 1b and 1c. The position
of the substituent on the 2-naphthol significantly
affected the enantioselectivity, as well as the cross-
coupling selectivity.
selectivity (93.8%) but in a poor yield (14%) (entry 2).
These complexes did not show any catalyst activity. As
is the case with the (S)Phbox ligand, in contrast, the
cross-coupling product was obtained in a good yield
with the high cross-coupling selectivity of 95.8% (entry
3), although the product showed a lower enantioselectiv-
ity, 10% ee (S), than that of the cross-coupling product
obtained using the (ꢀ)Sp ligand (76% ee (S)). The reac-
tion at the low temperature of ꢀ20 °C resulted in a
lower cross-coupling yield with an opposite and poor
enantioselectivity, while it showed the highest cross-cou-
pling selectivity of 99.7% (entry 4). The solvent CH2Cl2
and the structure of the bisoxazoline ligand, such as
(R)Bnbox, also significantly affected the catalyst activity
and stereoselectivity, which were lower than those ob-
served for the reaction with (S)Phbox in THF, although
high cross-coupling ratios were again observed (entries 5
and 6).
During the coupling reaction with CuCl–(S)Phbox, the
catalytic cross-coupling reaction selectively proceeded.
The following is one of the plausible mechanisms: the
naphthols bearing the ester moiety may act as an
acceptor molecule in the coupling reaction with the cop-
per(I)–bisoxazoline catalyst due to its electron-deficient
character, while the one-electron oxidation should be
predominantly promoted on the other substrate, the 2-
naphthols, to give a radical intermediate, and then the
cross-coupling reaction between these two naphthyls
may selectively occur.5a,e
Table 2 shows the results of the cross-coupling reaction
between various 2-naphthol derivatives and 3-hydroxy-
2-naphthoates using the CuCl–(S)Phbox catalyst at
room temperature for 3 h. The absolute configuration
of the cross-coupling compounds 4–12 was determined
by CD measurement.5b,c Although the optical purity
Table 1. Asymmetric oxidative cross-coupling reaction between 1a and 2aa
Entry
Catalyst
Time (h)
Coupling ratio X:Y:Zb
Cross-coupling product Y
Yieldc (%)
eed (%)
1
2
3
4e
5f
6
CuCl–(+)-PMP
CuCl–(ꢀ)Sp
48
5
3
5
3
1.8:37.8:60.4
6.0:93.8:0.2
0.3:95.8:3.9
0:99.7:0.3
0.4:94.4:5.2
0.6:95.3:4.1
9
14
87
45
54
68
2 (S)
74 (S)
10 (S)
3 (R)
2 (R)
5 (R)
CuCl–(S)Phbox
CuCl–(S)Phbox
CuCl–(S)Phbox
CuCl–(R)Bnbox
5
a Conditions: [CuCl]/[ligand]/[1a]/[2a] = 0.20:0.25:0.5:0.5 [1a] = 0.065 M, temperature = rt, O2 atmosphere.
b Determined by 1H NMR analysis and isolated yields.
c Isolated yield.
d Determined by HPLC analysis (Chiralpak AD).
e Reaction temperature = ꢀ20 °C.
f Solvent = CH2Cl2.

T. Temma, S. Habaue / Tetrahedron Letters 46 (2005) 5655–5657
Table 2. Asymmetric oxidative cross-coupling reaction with CuCl–(S)Phboxa
5657
Entry
1
2
Coupling ratio X:Y:Zb
Cross-coupling product Y
Y
Yieldc (%)
eed (%)
1
2
3
4
5
6
7
8
9
1b
1c
1a
1b
1c
1a
1d
1e
1f
2a
2a
2b
2b
2b
2c
2b
2b
2b
2b
6.8:93.2:0
5.5:91.8:2.7
5.0:86.4:8.6
26.6:71.5:1.9
12.9:85.2:1.9
0:96.4:3.6
0:97.3:2.7
3.9:90.4:5.7
0:98.7:1.3
3
75
86
72
55
73
84
81
71
76
63
ND
4
5
28 (R)e
55 (R)
64 (R)
65 (R)
9 (S)
6
7
8
9
3 (S)
51 (R)
3 (S)
10
11
12
10
1g
1.3:93.0:5.7
58 (R)
a Conditions: [CuCl]/[(S)Phbox]/[1]/[2] = 0.20:0.25:0.5:0.5, [1] = 0.065 M, solvent = THF, O2 atmosphere, temperature = rt, time = 3 h.
b Ratio of isolated yields.
c Isolated yield.
d Determined by HPLC analysis (Chiralpak AS or AD).
e Enantioselectivity of the homo-coupling products of 1c and 2a was 41% ee (S) and 37% ee (R), respectively.
3. Irie, R.; Masutani, K.; Katsuki, T. Synlett 2000, 1433–
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In conclusion, the asymmetric oxidative cross-coupling
reaction between two differently substituted 2-naphthol
derivatives with the CuCl–(S)Phbox catalyst was accom-
plished. The cross-coupling and enantioselectivities were
significantly affected by the structures of the 2-naphthol.
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Acknowledgements
This work was partially supported by Tokuyama
Science Foundation and Grants-in-Aid for Scientific
Research (No. 15350066) from the Ministry of Educa-
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