Table 3 Solvent effect for enantioselective oxidative coupling of 2-naph-
thol
chloroform and 1,2-dichloroethane merely improved the en-
antioselectivity (Table 3).
The results obtained from the enantioselective oxidation of
other substituted 2-naphthols catalyzed by complex 4 are
summarized in Table 4. No variation in enantioselectivities was
observed, but reaction rate was increased with electron donating
capacity of the substituent.
In conclusion, oxovanadium complex has been used for the
first time10 in the enantioselective coupling of 2-naphthols. A
low concentration requirement of catalyst, mild reaction
conditions and high chemical yields render our method
attractive.
We are grateful to the National Science Council, Republic of
China, for support of this work.
Entry
Solvent
Yield (%)
Ee (%)a
1
2
3
4
5
6
CH2Cl2
CHCl3
(CH2Cl)2
CCl4
CH3CN
THF
73
82
38
12
46
24
48
51
46
13
8
Notes and references
0
a Determined by HPLC with Kromasil 100-5CHI-DMB column (iPrOH–
hexane = 5+95, 1 mL min21).
‡ Representative procedure for enantioselective oxidative coupling of
2-naphthols: to a stirred solution of complex (0.1 mmol) and TMSCl (13 mL,
0.1 mmol) in chloroform (10 mL) exposed to molecular oxygen at room
temperature was added 2-naphthol (5 mmol). After 24 h, the reaction
mixture was treated with
6 M HCl (10 mL) and extracted with
presumed that the electronic effect of substituents in the
aromatic ring could influence catalyzing oxidative coupling,
and investigated complexes containing 3,5-di-tert-butyl, 3-tert-
butyl, 5-methoxy and 5-nitro substituents, but no significant
enantioselectivity was observed with these complexes. How-
ever, we observed a marginal increase in enantioselectivity
when the concentration of the substrate was increased from 0.1
to 0.5 M (Table 1, entry 7). An interesting feature was that
enantioselectivity increased with decrease in concentration of
complexes from 10 to 2 mol%, more so, in the case of complex
4 (Table 1, entry 8), and (R)-binaphthol was obtained in 42%
ee.
To study the promoter effect, we used various additives. The
enantioselectivity was better in TMSCl than in TMSOTf, and
the chemical yield was a little lower (Table 2, entry 4). There
was no change in enantioselectivity with variation of silyl
groups in the additives, but it affected the reaction rate (Table 2,
entries 4–6). Polar chlorosolvents such as dichloromethane,
dichloromethane (3 3 20 mL). The combined organic extracts were dried
(Na2SO4), and concentrated. The residue was purified by silica gel column
chromatography eluting with ethyl acetate–hexane (1+5) to furnish the
coupling product.
1 (a) H. Vilter, Phytochemistry, 1984, 23, 1387; (b) J. M. Arber, E. de
Boer, C. D. Garner, S. S. Hasnain and R. Wever, Biochemistry, 1989,
28, 7968.
2 (a) B. J. Hales, E. E. Case, J. E. Moringstar, M. F. Dzeda and A.
Mautner, Biochemistry, 1986, 24, 7251; (b) R. L. Robson, R. R. Eady,
T. H. Richardson, R. W. Miller, M. Hawkins and J. R. Postgate, Nature,
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Cramer, J. Am. Chem. Soc., 1988, 110, 4057; (d) B. J. Hales, A. E. True
and B. M. Hoffman, J. Am. Chem. Soc., 1989, 111, 8519.
3 T. Hirao, Chem. Rev., 1997, 97, 2707.
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Jpn., 1990, 63, 2620; (b) K. Nakajima, M. Kojima, K. Toriumi, K. Saito
and J. Fujita, Bull. Chem. Soc. Jpn., 1989, 62, 760; (c) C. Bolm and F.
Bienewald, Angew. Chem., Int. Ed. Engl., 1995, 34, 2640.
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Int. Ed. Engl., 1995, 34, 1059; (c) C. Bolm, T. K. K. Luong and K.
Harms, Chem Ber./Recl., 1997, 130, 887; (d) C. Bolm and T. Kühn,
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Yamamoto, J. Am. Chem. Soc., 2000, 122, 10452.
Table 4 Enantioselective oxidative coupling of 2-naphthol derivatives
Entry
1
Naphthol
Time/h
24
Yield (%)
82
Ee (%)a
51
6 D.-R. Hwang, C.-P. Chen and B.-J. Uang, Chem. Commun., 1999,
1207.
7 (a) R. Irie, K. Masutani and T. Katsuki, Synlett, 2000, 1433; (b) M.
Nakajima, I. Miyoshi, K. Kanayama and S. Hashimoto, J. Org. Chem.,
1999, 64, 2264.
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1969, 31, 2841; (b) J. J. R. Frausto da Silva, R. Wootton and R. D.
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10 Chen and his co-workers have independently done similar work using
oxovanadium complexes and published their results concurrently. S.-W.
Hon, C.-H. Li, J.-H. Kuo, N. B. Barhate, Y.-H. Liu, Y. Wang and C.-T.
Chen, Org. Lett., 2001, 3, 869.
2
3
4
24
24
69
91
51
51
—
50
trace
a Determined by HPLC with Kromasil 100-5CHI-DMB column (iPrOH–
hexane = 5+95, 1 mL min21).
Chem. Commun., 2001, 980–981
981