Chiral dinuclear vanadium(v) catalysts for oxidative coupling of 2-naphthols

Article abstract of DOI:10.1039/b717068h

Preparation and structural analysis of chiral dinuclear vanadium(v) catalysts with high catalytic activity for the oxidative coupling of 2-naphthols are described. The Royal Society of Chemistry.

Full text of DOI:10.1039/b717068h

View Article Online / Journal Homepage / Table of Contents for this issue
COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Chiral dinuclear vanadium(V) catalysts for oxidative coupling of
2-naphtholswz
Shinobu Takizawa, Tomomi Katayama, Chiaki Kameyama, Kiyotaka Onitsuka,
Takeyuki Suzuki, Takeshi Yanagida, Tomoji Kawai and Hiroaki Sasai*
Received (in Cambridge, UK) 5th November 2007, Accepted 21st January 2008
First published as an Advance Article on the web 14th February 2008
DOI: 10.1039/b717068h
Preparation and structural analysis of chiral dinuclear vana-
dium(V) catalysts with high catalytic activity for the oxidative
coupling of 2-naphthols are described.
troscopy revealed that (Ra,S,S)-1 was a paramagnetic vana-
dium(IV) complex. During further investigation, single crystals
suitable for X-ray analysis were obtained as an adduct of
NaOH by recrystallization of (Ra,S,S)-1 from MeOH–
Et2O–H2O in the presence of NaOAc. The binding energy of
V 2p3/2 for the crystals was determined to be 516.9 eV by X-ray
photoelectron spectroscopy (XPS), which is attributed to
V(V).5a The X-ray crystal structure (ESIw) suggested that the
complex had been oxidized to a distorted octahedral vana-
dium(V) species with one extra hydroxide for each vanadium
center (V1–O3, 1.747(5) A) (Fig. 2).5b,6 The hydroxide is anti
to the imine nitrogen. The VQO (V1–O2, 1.602(8) A) bond is
syn to the tert-butyl groups in the template. The (R)-BINOL
derivative lies about a twofold axis. The sodium cation is
coordinated by the oxygen atoms of two carboxylic groups
(Na1–O4, 2.393(5) A) on the (S)-tert-leucine regions. The
coordination of oxygen to several sodium cations results in
helical molecular aggregation within the crystal (see ESIz).
To determine the oxidation state of the vanadium after
preparation from (Ra,S,S)-1, superconducting quantum inter-
ference device (SQUID) analyses were performed on the di-
nuclear vanadium complexes using VOSO4 and V2O5 as
standards for the vanadium(IV) and vanadium(V) samples,
respectively (Fig. 3).7 The magnetic susceptibility of (Ra,S,S)-
1 prepared under air was 5 emu molꢀ1 at 5 K and 5 T (curve c).
In contrast, the vanadium complex prepared under Ar gave a
value of 28 emu molꢀ1 (curve b). The magnetic susceptibility of
the single crystals obtained by recrystallization, as shown in
Fig. 2, was 0 emu molꢀ1, clearly indicating a vanadium(V)
species (curve d). These results suggest the dinuclear
vanadium(IV) complex is readily oxidized to afford the
vanadium(V) species during preparation under air.
Optically pure BINOL and its derivatives are used as chiral
auxiliaries and ligands for a wide range of asymmetric synth-
eses.1 In light of the important applications of these com-
pounds, catalytic asymmetric preparation of BINOLs has
continued to attract the attention of many researchers.2 Asym-
metric oxidative coupling of 2-naphthols using a chiral vana-
dium catalyst is one of the most useful methods for the
synthesis of optically pure BINOL derivatives.3 The vana-
dium-mediated couplings, which occur via a favorable one-
electron phenolic oxidation, generally proceed under mild
reaction conditions and tolerate many functional groups, with
the further advantage that only water is formed as a side
product. Uang and co-workers found that the activity of
catalysts could be improved by the addition of a Lewis or
Brønsted acid,3a,b resulting in the production of BINOL with
moderate to good enantioselectivity. Barhate and Chen found
that a high oxygen pressure dramatically enhanced the reac-
tion rate but led to a significant drop in ee for the coupling
product.3c Although chiral vanadium catalysts give good
enantioselectivity in the coupling, 2 to 15 days are required
for complete reaction due to the low catalyst activities. Devel-
opment of active vanadium complexes with high enantiocon-
trol remains a challenge. In this paper, we present chiral
dinuclear vanadium(V) complexes, (Ra,S,S)-2, as highly active
catalysts for asymmetric coupling of 2-naphthols (Fig. 1).
Our work has been focused on developing bifunctional
enantioselective catalysts which can promote carbon–carbon
bond forming reactions via a dual activation mechanism.4 We
have previously reported on the chiral dinuclear vanadium(IV)
complex (Ra,S,S)-1 for oxidative coupling of 2-naphthols.4a
Simultaneous activation of two molecules of 2-naphthol by
two vanadium metals in the chiral template led to smooth
enantioselective homocoupling. Characterization using
HRMS-FAB, FT-IR and electron spin resonance (ESR) spec-
When the single crystals (Fig. 2) were used as the catalyst for
the oxidative coupling of 2-naphthol, no catalytic activity was
The Institute of Scientific and Industrial Research (ISIR), Osaka
University Mihogaoka, Ibaraki, Osaka 567-0047, Japan. E-mail:
sasai@sanken.osaka-u.ac.jp; Fax: +81-6-6879-8469; Tel: +81-6-
6879-8465
w CCDC 666356. For crystallographic data in CIF or other electronic
format see DOI: 10.1039/b717068h
Fig. 1 Dinuclear vanadium complexes for the oxidative coupling of
z Electronic supplementary information (ESI) available: Experimental
section and crystallography. See DOI: 10.1039/b717068h
2-naphthols.
ꢁc
This journal is The Royal Society of Chemistry 2008
1810 | Chem. Commun., 2008, 1810–1812

View Article Online
Scheme 1 Preparation of dinuclear vanadium(V) complex (Ra,S,S)-2a.
Table 1 Coupling reaction of 2-naphthol catalyzed by vanadium(V)
complexes
Fig. 2 Structure of the dinuclear vanadium(V) complex (Ra,S,S)-2aꢂ
NaOH. A counter anion OHꢀ, the hydrogen atom of which has not
been determined, is omitted for clarity.
Catalyst
(5 mol%)
Yielda Eeb
(%) (%)
Entry
Atmosphere
T/1C t/h
1
2
3
4c
a
(Ra,S,S)-2a O2
(Ra,S,S)-2a Air
(Ra,S,S)-2a Air
30
30
0
24
24
72
24
Quant. 71
96 85
Quant. 90
46 61
(S)-3
O2
30
b
c
Isolated yield. Determined by chiral HPLC. 10 mol% of (S)-3
was used.
homogeneous solution, containing 5 mol% of the dinuclear
vanadium(V) complex, was added 2-naphthol under air, pro-
viding (S)-BINOL in 90% yield with 83% ee after 24 h.
To determine the exact catalytic activity of the dinuclear
vanadium(V) catalyst, (Ra,S,S)-2a was prepared from VOCl3,
(R)-3,30-diformyl-2,20-dihydroxy-1,10-binaphthyl8 and (S)-
tert-leucine as shown in Scheme 1. (Ra,S,S)-2a was character-
ized by 1H, 13C, 51V NMR, FT-IR, MS, and elemental
analysis. Since the couplings were catalyzed by (Ra,S,S)-2a
with reaction rates 2.3 times those of (Ra,S,S)-1,9a the vana-
dium(IV) complex is likely a precatalyst.9b,c Next, (Ra,S,S)-2a
was applied to the coupling of 2-naphthol under various
reaction conditions (Table 1). In all reactions, no byproducts
Fig. 3 SQUID magnetometer plot of magnetic susceptibility for
vanadium under a magnetic field at 5 K for (a) VOSO4, (b)
(Ra,S,S)-1 (prepared under Ar), (c) (Ra,S,S)-1 (prepared under air),
(d) single crystals (Ra,S,S)-2aꢂNaOH obtained by recrystallization of
(Ra,S,S)-1 from MeOH–Et2O–H2O containing NaOAc, and (e) V2O5.
observed due to low solubility in CH2Cl2. However, the
addition of 1 equiv. of 5 M HCl aq. to the sodium cation on
the crystal gave a homogeneous solution. To the above
Table 2 Coupling reaction of 2-naphthols catalyzed by dinuclear vanadium(V) complexes (Ra,S,S)-2
Entry
Catalyst
Substrate
T/1C
t/h
72
72
36
48
24
24
24
240
240
72
72
72
48
72
72
24
Yielda (%)
Eeb (%)
1
2
3
4
5
6
7
8
(Ra,S,S)-2a
(Ra,S,S)-2a
(Ra,S,S)-2a
(Ra,S,S)-2a
(Ra,S,S)-2a
(Ra,S,S)-2a
(Ra,S,S)-2a
(Ra,S,S)-2a
(Ra,S,S)-2a
(Ra,S,S)-2b
(Ra,S,S)-2b
(Ra,S,S)-2b
(Ra,S,S)-2b
(Ra,S,S)-2b
(Ra,S,S)-2b
(Ra,S,S)-2b
b
4a, R1 = R2 = R3 = H
0
0
5a, Quant.
5b, 91
5c, Quant.
5d, 83
5e, 98
5f, Quant.
5g, 94
5h, 35
5i, 10
5a, 56
5b, 69
5c, Quant.
5d, 31
5e, 67
5f, 45
5g, 84
90
90
86
81
89
87
84
48
4
97
97
93
78
93
87
77
4b, R1 = R3 = H, R2 = Bn
4c, R1 = R3 = H, R2 = Ph
30
30
30
30
0
30
30
0
4d, R1 = R3 = H, R2 = Br
4e, R1 = R2 = H, R3 = OMe
4f, R1 = R2 = H, R3 = OCH2CHCH2
4g, 9-phenanthrol
4h, R1 = OMe, R2 = R3 = H
9
4i, R1 = CO2Me, R2 = R3 = H
10
11
12
13
14
15
16
a
4a
4b
4c
4d
4e
4f
0
30
30
0
0
0
4g
Isolated yield. Determined by chiral HPLC.
ꢁc
This journal is The Royal Society of Chemistry 2008
Chem. Commun., 2008, 1810–1812 | 1811

View Article Online
Synlett, 2000, 1433; (e) M. Sridhar, S. K. Vadivel and U. T.
Bhalerao, Tetrahedron Lett., 1997, 38, 5695; (f) M. Takemoto, Y.
Suzuki and K. Tanaka, Tetrahedron Lett., 2002, 43, 8499; (g) T.
Temma and S. Habaue, Tetrahedron Lett., 2005, 46, 5655; (h) K.
H. Kim, D.-W. Lee, Y.-S. Lee, D.-H. Ko and D.-C. Ha, Tetra-
hedron, 2004, 60, 9037; (i) H. Aoyama, M. Tokunaga, J. Kiyosu, T.
Iwasawa, Y. Obora and Y. Tsuji, J. Am. Chem. Soc., 2005, 127,
10474; (j) T. Temma, B. Hatano and S. Habaue, Tetrahedron, 2006,
62, 8559; (k) X. Xie, P.-W. Phuan and M. C. Kozlowski, Angew.
Chem., Int. Ed., 2003, 42, 2168; (l) J. Gao, H. Reibenspies and A.
E. Martell, Angew. Chem., Int. Ed., 2003, 42, 6008; (m) X. Li, J. B.
Hewgley, C. A. Mulrooney, J. Yang and M. S. Kozlowski, J. Org.
Chem., 2003, 68, 5500; (n) M. C. Kozlowski, X. Li, P. J. Carroll
and Z. Xu, Organometallics, 2002, 21, 4513; (o) C. A. Mulrooney,
X. Li, E. S. DiVirgilio and M. C. Kozlowski, J. Am. Chem. Soc.,
2003, 125, 6856; (p) C. A. Mulrooney, X. Li, E. S. DiVirgilio and
M. C. Kozlowski, J. Am. Chem. Soc., 2003, 125, 6856.
3 (a) C.-Y. Chu, D.-R. Hwang, S.-K. Wang and B.-J. Uang, Chem.
Commun., 2001, 980; (b) C.-Y. Chu and B.-J. Uang, Tetrahedron:
Asymmetry, 2003, 14, 53; (c) N. B. Barhate and C.-T. Chen, Org.
Lett., 2002, 4, 2529; (d) 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; (e) M. Tada, N. Kojima, Y. Izumi, T. Taniike and Y.
Iwasawa, J. Phys. Chem. B, 2005, 109, 9905; (f) M. Tada, T.
Taniike, L. K. Kantam and Y. Iwasawa, Chem. Commun., 2004,
2542; (g) S. Habaue, S. Murakami and H. Higashimura, J. Polym.
Sci. Part A: Polym. Chem., 2005, 43, 5872; (h) Z. Luo, Q. Liu, L.
Gong, X. Cui, A. Mi and Y. Jiang, Chem. Commun., 2002, 914; (i)
Z. Luo, Q. Liu, L. Gong, X. Cui, A. Mi and Y. Jiang, Angew.
Chem., Int. Ed., 2002, 41, 4532.
4 (a) H. Somei, Y. Asano, T. Yoshida, S. Takizawa, H. Yamataka
and H. Sasai, Tetrahedron Lett., 2004, 45, 1841; (b) K. Matsui, S.
Takizawa and H. Sasai, Tetrahedron Lett., 2005, 46, 1943; (c) D.
Jayaprakash, S. Takizawa, T. Arai and H. Sasai, J. Exp. Nanosci.,
2006, 1, 477; (d) K. Matsui, S. Takizawa and H. Sasai, J. Am.
Chem. Soc., 2005, 127, 3680; (e) K. Matsui, S. Takizawa and H.
Sasai, Synlett, 2006, 761; (f) T. Arai, T. Sekiguti, K. Otsuki, S.
Takizawa and H. Sasai, Angew. Chem., Int. Ed., 2003, 42, 2144; (g)
S. Takizawa, H. Somei. D. Jayaprakash and H. Sasai, Angew.
Chem., Int. Ed., 2003, 42, 5711.
5 (a) G. A. Sawatzky and D. Post, Phys. Rev. B, 1979, 20, 1546; (b)
V. D. Pawar, S. Bettigeri, S.-S. Weng, J.-Q. Kao and C.-T. Chen, J.
Am. Chem. Soc., 2006, 128, 6308.
6 The pre-edge peak position (5468.0 eV) of the crystal, as deter-
mined by X-ray absorption near edge structure (XANES) analysis,
is comparable to that of the dinuclear vanadium(V) complex
(Ra,S,S)-2a (5468.0 eV) and the mononuclear vanadium(V) complex
(S)-3 (5468.0 eV).
were observed. The reaction under O2 afforded (S)-BINOL in
quantitative yield with 71% ee (entry 1). Although the reaction
rate was slightly decreased under air, (S)-BINOL was obtained
in 96% yield with higher enantioselectivity (85% ee) than that
produced under O2 (entry 2). Lowering the temperature to
0 1C (entry 3) led to an increase in the selectivity to 90% ee
(entry 3). The reaction rate of mononuclear vanadium(V)
catalyst (S)-3 was quite low in comparison with that of
(Ra,S,S)-2a when using 10 mol% of (S)-3 (entry 4). The higher
reaction rate and enantioselectivity obtained using the com-
plex (Ra,S,S)-2a as compared to those obtained using (S)-3
can be attributed to the simultaneous activation of two
molecules of 2-naphthol to form BINOL.9a
We then examined the coupling reaction of various
2-naphthol derivatives. These results are summarized in
Table 2. Electron donating and withdrawing substituents at
the C6, C7 positions of 2-naphthol resulted in coupling
products with good enantioselectivities. Although 9-phenan-
throl was found to be an adequate substrate (entry 7), append-
ing a substituent at the C3 position (R1 = OMe, CO2Me) led
to the corresponding products with diminished yield and ee
(entries 8 and 9). These C3 substituted 2-naphthols barely
approach vanadium on the catalyst due to steric hindrance.
The best outcome in terms of enantioselectivity was achieved
using the dinuclear vanadium(V) complex (Ra,S,S)-2b bearing
a H8-BINOL backbone (Fig. 1).10 (S)-BINOLs were obtained
in 97% ee when using (Ra,S,S)-2b (entries 14 and 15). In
conclusion, we have developed chiral dinuclear vanadium(V)
catalysts with high catalytic activity for the oxidative coupling
of 2-naphthols. Large-scale application of the dinuclear vana-
dium(V) catalysts is currently in progress. After preparation of
this manuscript, a paper by Gong and co-workers was pub-
lished, in which dinuclear vanadium(V) catalysts bearing a
V–O–V linkage were employed for the synthesis of BINOLs.
These catalysts were found to promote oxidative coupling of
2-naphthols with high enantioselectivity.11
7 The magnetic susceptibilities of VOSO4 and V2O5 as vanadium(IV)
and (V) standards are 38 and 0 emu molꢀ1, respectively.
8 H.-C. Zhang, W.-S. Huang and L. Pu, J. Org. Chem., 2001, 66, 481.
9 (a) Catalysis of 2-naphthol coupling by complexes (Ra,S,S)-1,
(Ra,S,S)-2a and (S)-3 obeyed second-order kinetics for up to about
15 h. The calculated rate constants for the coupling reactions using
(Ra,S,S)-1, (Ra,S,S)-2a and (S)-3 as catalysts were k(R ,S,S)-1
=
a
0.1738 Mꢀ1 hꢀ1, k(R ,S,S)-2a = 0.4000 Mꢀ1 hꢀ1, and k(S)-3 = 0.0850
a
M
ꢀ1 hꢀ1, respectively; (b) an induction period was observed when
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports,
Science and Technology, Japan. We thank Dr Masao Taka-
hashi for XPS analyses, Dr Shuichi Emura for XANES
analyses, and the technical staff of the ISIR Materials Analysis
Center at Osaka University.
(Ra,S,S)-1 was used for the coupling of 2-naphthols. The % ee of
the product increases during the course of the reaction when using
(Ra,S,S)-1, from 38% ee at 4% conversion to 86% ee at 14%
conversion, while the % ee of the product stays nearly constant
(83–85% ee) when using (Ra,S,S)-2a; (c) in the coupling of 2-
naphthol using 5 mol% of (Ra,S,S)-2a under Ar, BINOL was
formed in only 9% yield after 48 h due to prevention of regenera-
tion of the active vanadium(V) complex from the inactive
vanadium(IV) complex in the absence of air. Purging the above
solution with air led to the reaction occuring smoothly, producing
BINOL in a total yield of 99%.
Notes and references
1 (a) J. M. Brunel, Chem. Rev., 2005, 105, 857; (b) B. E. Love, Curr.
Org. Synth., 2006, 3, 169.
10 D. J. Cram, R. C. Helgeson, S. C. Peacock, L. J. Kaplan, L. A.
Domeier, P. Moreau, K. Koga, J. M. Mayer, Y. Chao, M. G.
Siegel, D. H. Hoffman and G. D. Y. Sogah, J. Org. Chem., 1978,
43, 1930.
11 Q.-X. Guo, Z.-J. Wu, Z.-B. Luo, Q.-Z. Liu, J.-L. Ye, S.-W.
Luo, L.-F. Cun and L.-Z. Gong, J. Am. Chem. Soc., 2007, 129,
13927.
2 (a) M. Smrcina, J. Polakova, S. Vyskocil and P. Kocovsky,
J. Org. Chem., 1993, 58, 4534; (b) M. Nakajima, I. Miyoshi, K.
Kanayama, S. Hashimoto, M. Noji and K. Koga, J. Org.
Chem., 1999, 64, 2264; (c) X. Li, J. Yang and M. C. Kozlowski,
Org. Lett., 2001, 3, 1137; (d) R. Irie, K. Matsutani and T. Katsuki,
ꢁc
This journal is The Royal Society of Chemistry 2008
1812 | Chem. Commun., 2008, 1810–1812