Dual activation in oxidative coupling of 2-naphthols catalyzed by chiral dinuclear vanadium complexes

Article abstract of DOI:10.1016/j.tet.2008.01.110

An efficient enantioselective oxidative coupling of 2-naphthol derivatives based on a concept of dual activation catalysis is realized. A chiral dinuclear vanadium(IV) complex (R_a,S,S)-1e possessing (S)-tert-leucine moieties at the 3,3′-positions of the (R)-binaphthyl skeleton was developed, which was found to promote the oxidative coupling of 2-naphthol to afford (S)-BINOL with 91% ee. To verify the dual activation mechanism, mononuclear vanadium(IV) complex (S)-8 was also prepared. Kinetic analysis revealed that the reaction rate of oxidative coupling of 2-naphthol promoted by (R_a,S,S)-1e is 48.3 times faster than that of (S)-8. The two vanadium metals in the chiral complex activate two molecules of 2-naphthol simultaneously in an intramolecular manner coupling reaction, achieving a high reaction rate with high enantiocontrol. Reaction mechanisms of the oxidative coupling reaction promoted by either vanadium(IV) or vanadium(V) complexes are also described.

Full text of DOI:10.1016/j.tet.2008.01.110

Available online at www.sciencedirect.com
Tetrahedron 64 (2008) 3361e3371
www.elsevier.com/locate/tet
Dual activation in oxidative coupling of 2-naphthols catalyzed
by chiral dinuclear vanadium complexes
Shinobu Takizawa, Tomomi Katayama, Hidenori Somei, Yasuaki Asano, Tomokazu Yoshida,
Chiaki Kameyama, Doss Rajesh, Kiyotaka Onitsuka, Takeyuki Suzuki, Masafumi Mikami,
y
*
Hiroshi Yamataka , Doss Jayaprakash, Hiroaki Sasai
The Institute of Scientific and Industrial Research (ISIR), Osaka University, Mihogaoka, Ibaraki-shi, Osaka 567-0047, Japan
Received 18 January 2008; accepted 30 January 2008
Available online 2 February 2008
Abstract
An efficient enantioselective oxidative coupling of 2-naphthol derivatives based on a concept of dual activation catalysis is realized. A chiral
dinuclear vanadium(IV) complex (Ra,S,S)-1e possessing (S)-tert-leucine moieties at the 3,30-positions of the (R)-binaphthyl skeleton was devel-
oped, which was found to promote the oxidative coupling of 2-naphthol to afford (S)-BINOL with 91% ee. To verify the dual activation mech-
anism, mononuclear vanadium(IV) complex (S)-8 was also prepared. Kinetic analysis revealed that the reaction rate of oxidative coupling of
2-naphthol promoted by (Ra,S,S)-1e is 48.3 times faster than that of (S)-8. The two vanadium metals in the chiral complex activate two molecules
of 2-naphthol simultaneously in an intramolecular manner coupling reaction, achieving a high reaction rate with high enantiocontrol. Reaction
mechanisms of the oxidative coupling reaction promoted by either vanadium(IV) or vanadium(V) complexes are also described.
Ó 2008 Published by Elsevier Ltd.
1. Introduction
a phosphate, which has both acidic and basic sites in one func-
tionality;3 and (3) dual activation by two catalytic sites in a sin-
gle catalyst.4 For efficient dual activation of the substrates,
a balance between these two functionalities in the catalyst is
required. For example, in an acidebase type catalyst the
self-quenching reaction of acidic and basic moieties on the
catalyst can lead to its inactivation.
We postulated that a chiral complex possessing two identi-
cal metal centers in a single molecule, which could activate
two substrates simultaneously in a homolytic coupling reac-
tion of 2-naphthol molecules, would enhance the reaction
rate with high enantioselectivity. To achieve the new dual
activation catalysis in an oxidative coupling reaction, we de-
signed the catalyst (Ra,S,S)-15 bearing two active sites attached
to a binaphthyl skeleton, taking advantage of the activation en-
tropy (Fig. 2). Activation entropy strongly contributes to intra-
molecular manner coupling after two molecules of 2-naphthol
are complexed with the dinuclear vanadium catalyst. In 2003
Gao reported promising results on oxidative coupling reaction
of 2-naphthol promoted by dinuclear copper catalysts, though
Catalytic asymmetric reactions are among the most power-
ful synthetic methods to obtain optically active compounds.1
Recent challenges focus on the development of asymmetric
catalysts for carbonecarbon bond-forming reactions with
high activity and broad substrate generality, which lead to
practical, efficient, and environmentally benign chemical syn-
theses. Dual activation systems for substrates such as nucleo-
philes and electrophiles lead to enhanced reaction rates and
more specific control of the transition structure with respect
to the catalyst’s asymmetric environment.2e4 Dual activation
catalyses can be classified into three categories (Fig. 1): (1)
dual activation using two different kinds of catalysts;2 (2) con-
jugated-type dual activation with a functional group such as
* Corresponding author. Tel.: þ81 6 6780 8465; fax: þ81 6 6879 8469.
E-mail address: sasai@sanken.osaka-u.ac.jp (H. Sasai).
y
Present address: Department of Chemistry, Rikkyo University, 3-34-1
Nishi-Ikebukuro, Toshima-ku, Tokyo 171-8501, Japan.
0040-4020/$ - see front matter Ó 2008 Published by Elsevier Ltd.
doi:10.1016/j.tet.2008.01.110

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S. Takizawa et al. / Tetrahedron 64 (2008) 3361e3371
Figure 1. Schematic drawing of dual activation systems in catalytic asymmetric reactions.
Figure 2. Design of dinuclear vanadim complex (Ra,S,S)-1 for the oxidative coupling.
seven days are required for the complete attainment (85%
yield, 88% ee).6a In this manuscript, the design and synthesis
of dinuclear vanadium complexes that promote enantioselec-
tive oxidative coupling of 2-naphthol derivatives through an
intramolecular dual activation mechanism6 are described.
The synthesis of optically pure BINOL derivatives are of
importance because they are used as chiral ligands and auxilia-
ries for a wide range of organic functional transformations.7
Catalytic asymmetric preparation of BINOL systems has con-
tinued to attract the attention of many researchers.8 The asym-
metric oxidative coupling of 2-naphthol molecules is one of
the most useful methods for the synthesis of optically pure
BINOL derivatives. Vanadium-mediated couplings, which occur
via a favorable one-electron phenolic oxidation, proceed under
mild reaction conditions and tolerate many functional groups,
with the further advantage that only water is formed as a side
product.9e13 In 2001, Chen et al.9a and Uang et al.10a indepen-
dently reported the first efficient asymmetric vanadium(IV)
catalysts for oxidative coupling of 2-naphthol molecules
prepared from vanadyl sulfate, aldehyde derivatives, and (S)-
amino acids. Gong et al. reported dinuclear vanadium cata-
lysts, which possess a VeOeV linkage with a maximum
enantioselectivity of 98%.12 These vanadium catalysts showed
good enantioselectivity for the coupling of 2-naphthol deriva-
tives, although the catalytic activity was moderate. Uang et al.
found that catalytic activity could be improved by the addition
of Lewis10a or Brønsted acids,10b resulting in the production of
BINOL with moderate to good enantioselectivity. Chen et al.
also developed the vanadium(IV) catalysts9b derived from
vanadyl sulfate, (þ)-ketopinic acid, and a-amino acids. To
improve the coupling yields, the effect of oxygen pressure
was examined. Although the coupling rate of 2-naphthol was
greatly enhanced by more than six times under an oxygen
pressure of 8.8 atm, a significant drop in ee for BINOL was
observed. Development of active vanadium complexes with
high enantiocontrol remains a challenge.
2. Results and discussion
2.1. Preparation of dinuclear vanadium(IV) complexes
Scheme 1 shows the synthetic procedure for preparing
catalysts 1aee.14 Since NMR spectroscopy is not applicable
for paramagnetic vanadium(IV) species,15 FAB-HRMS and
FTIR analyses were used to characterize the complex 1. The
oxidative coupling reactions of 2-naphthol by catalysts 1
were carried out in CH2Cl2 at 30 ꢀC for 24 h under oxygen.
The catalytic activities of (Ra)-1a and (Ra,S,S)-1b were low
and racemic BINOL was obtained (Table 1, entries 1 and 2).
(Ra,S,S)-1c, possessing benzyl groups, promoted the coupling
reaction in 30% yield with low enantioselectivity (8% ee) (en-
try 3). However, (Ra,S,S)-1d bearing iso-propyl groups pro-
moted the oxidative coupling reaction to afford (S)-BINOL
in 52% yield with 66% ee (entry 4). (Ra,S,S)-1e, possessing

S. Takizawa et al. / Tetrahedron 64 (2008) 3361e3371
3363
R
N
O
O
1) (S)-amino acid, NaOAc
V
O
THF/H2O, reflux, 0.5 h
OH
O
(Ra)-1a: R = H
O
O
HO
O
2) VOSO4
THF/H2O, rt, 2 h
(Ra,S,S)-1b: R = Ph
(Ra,S,S)-1c: R = Bn
(Ra,S,S)-1d: R = i-Pr
(Ra,S,S)-1e: R = t-Bu
O
V
N
O
O
R
(R)-2
Scheme 1. Preparation of dinuclear vanadium(IV) complexes 1.
Table 1
Enantioselective coupling of 2-naphthol catalyzed by chiral dinuclear vanadium-
(IV) catalysts
Table 2
Activity of the catalyst derived from various amounts of VOSO4, (R)-2 and
(S)-tert-leucine
Entry VOSO4 Atmosphere Catalyst (mol %) Time (h) Yieldb (%) eec (%)
a
Chiral Catalyst (5 mol %), O2 (1 atm)
CH2Cl2, 30 °C, 24 h
1
2
3
4
5
1
2
4
4
4
O2
O2
O2
Air
Air
5
5
5
5
1
24
24
15
64
83
76
99
13 (R)
78 (S)
83 (S)
91 (S)
90 (S)
OH
OH
OH
24
24
4a
3a
168
a
b
c
Numbers show molar equivalents to diformyl compounds.
Isolated yields.
Entry
Catalyst
Yielda (%)
eeb (%)
1
2
3
4
5
6
7
a
(Ra)-1a
6
21
30
52
83
9
rac
rac
Determined by HPLC (Chiralpak AS-H, hexane/i-PrOH¼9:1).
(Ra,S,S)-1b
(Ra,S,S)-1c
(Ra,S,S)-1d
(Ra,S,S)-1e
(Sa,S,S)-1e
(S,S)-5
Table 3
Effects of solvent on (Ra,S,S)-1e catalyzed coupling reaction of 2-naphthola
8 (R)
66 (S)
83 (S)
2 (R)
81 (S)
Entry
Solvent
Yieldb (%)
eec (%)
1
2
3
4
5
6
a
THF
MeCN
CCl4
Trace
Trace
42
d
d
36 (S)
63 (S)
91 (S)
89 (S)
57
Isolated yields.
Determined by HPLC (Chiralpak AS-H, hexane/i-PrOH¼9:1).
b
CHCl3
CH2Cl2
(CH2Cl)2
40
76
tert-butyl groups, made the reaction proceed faster than the
other complexes, maintaining the highest enantioselectivity
(83% ee) (entry 5). The diastereomeric complex (Sa,S,S)-1e
and the tropos16 complex (S,S)-5 were also prepared. The
diastereomeric complex (Sa,S,S)-1e was less active than
(Ra,S,S)-1e, giving (R)-BINOL in only 9% yield with 2% ee
(entry 6). The complex (S,S)-5 also showed a decreased reac-
tion rate (entry 7). (Ra,S,S)-1e would constitute a matched pair
and (Sa,S,S)-1e would be a mismatched pair,17 for the intramo-
lecular manner coupling of 2-naphthol as shown in Figure 2.
In the case of complex (S,S)-5 with free rotation of the biphe-
nyl unit, the equilibrium between matched and mismatched
conformations diminishes the reaction rate. These results re-
vealed that two amino derived active sites affect the enantio-
control of the coupling and the binaphthyl backbone assists
to align the two molecules of 2-naphthol with each other for
the oxidative coupling reaction.
99
At 30 ꢀC for 24 h under air.
Isolated yields.
b
c
Determined by HPLC (Chiralpak AS-H, hexane/i-PrOH¼9:1).
deliberately prepare complex 6 for comparison of its catalytic
activity with (Ra,S,S)-1e resulted in a switch of the major prod-
uct enantiomer to (R)-BINOL with 13% ee (Table 2, entry 1).
In order to avoid the formation of a mononuclear vanadium
catalyst, 4 equiv of vanadyl sulfate was used (entry 3). The ex-
cess vanadyl sulfate can be filtered off after complex forma-
tion. Although the reaction rate promoted by (Ra,S,S)-1e was
slightly decreased under air, (S)-BINOL was obtained with
higher enantioselectivity (91% ee) than that produced under
oxygen (entry 4). The effects of reaction solvents are shown
in Table 3. In polar solvents such as THF and MeCN, the coup-
ling reaction did not proceed (Table 3, entries 1 and 2). In
either CCl4 or CHCl3, the reaction mixture was heterogeneous
N
V
N
V
O
O
O
O
O
O
O
O
O
O
N
O
V
O
O
O
O
V
N
V
N
O
O
O
O
O
HO
N
(Sa,S,S)-1e
(S,S)-5
OH
An insufficient amount of vanadyl sulfate leads to the for-
mation of vanadium(IV) complex 6, which bears only one
vanadium metal in the catalyst molecule. An attempt to
O
6

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S. Takizawa et al. / Tetrahedron 64 (2008) 3361e3371
Table 4
Effects of temperature on (Ra,S,S)-1e catalyzed coupling reaction of
2.5
Catalyst (Ra,S,S)-1e (5 mol %)
Catalyst (S)-8 (10 mol %)
2-naphthola
2
1.5
1
Entry
Solvent
Temp (ꢀC)
Time (h)
Yieldb (%)
eec (%)
k(Ra,S,S)-1e= 0.1738 [M-1h-1]
r = 0.997
1
2
3
4
5
6
7
8
9
10
a
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
CH2Cl2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
(CH2Cl)2
ꢁ20
ꢁ10
0
168
144
72
Trace
67
d
76 (S)
87 (S)
90 (S)
91 (S)
90 (S)
92 (S)
87 (S)
71 (S)
79 (S)
Quant
59
76
k(S)-8 = 0.0036 [M-1h-1]
r = 0.999
20
30
24
24
0.5
0
Reflux
0
30
24
50
45
24
24
76
Quant
31
0
10
20
30
60
Reflux
24
24
t / h
Figure 3. Kinetic analysis of (Ra,S,S)-1e and (S)-8 catalyzed oxidative coupling
reactions of 2-naphthol; a¼initial concentration of 2-naphthol (0.2 M),
x¼concentration of BINOL.
Under air.
Isolated yields.
b
c
Determined by HPLC (Chiralpak AS-H, hexane/i-PrOH¼9:1).
of catalyst (Ra,S,S)-1e and (S)-8 were DE
¼
ðRa;S;SÞ-1e
and (S)-BINOL was obtained in low yields with moderate
enantioselectivity (entries 3 and 4). In (CH2Cl)2, the coupling
reaction proceeded smoothly to afford (S)-BINOL with similar
enantiocontrol to that mediated in CH2Cl2 (entries 5 and 6).
The effects of reaction temperature were then examined in
CH2Cl2 or (CH2Cl)2 (Table 4). Although higher reaction tem-
peratures resulted in higher catalytic activity in this system,
the catalytic activity was diminished at the reflux condition
due to decomposition of the complex (entries 6 and 10). The
reaction did not proceed below ꢁ20 ꢀC due to the low solubi-
lity of (Ra,S,S)-1e (entry 1).
0:74 kcal molꢁ1 and DE(S)-8¼23 kcal molꢁ1, and activation
entropy and enthalpy of catalyst (Ra,S,S)-1e were DSz
¼
ðRa;S;SÞ-1e
ꢁ7:8 ꢂ 10ꢁ2 kcal mol K and DHðzR ;S;SÞ-1e ¼ 0:16 kcal mol,
a
and those of (S)-8 were DSzðSÞ-8 ¼ ꢁ1:5 ꢂ 10ꢁ2 kcal mol K,
and DHðzSÞ-8 ¼ 21 kcal mol, respectively. The lower activation
entropy using (Ra,S,S)-1e than those of (S)-8 catalyzed reaction
are attributed to the dual activation of two 2-naphthol mole-
cules in one chiral complex to produce BINOL. The lower
activation enthalpy using (S)-8 than those of (Ra,S,S)-1e medi-
ated reaction contributes to the intermolecular radicaleradical
coupling of reaction intermediates after complexation of (S)-8
with 2-naphthol. A difference of activation enthalpy between
(Ra,S,S)-1e and (S)-8 suggests that (Ra,S,S)-1e mediated coup-
ling excludes a simple radicaleradical coupling mechanism.
2.2. Kinetic analysis of dinuclear and mononuclear
vanadium(IV) complexes in coupling of 2-naphthol
To confirm the dual activation in this reaction, mononuclear
vanadium(IV) complex (S)-8 was prepared from 3-formyl-2-
naphthol (7), (S)-tert-leucine, and vanadyl sulfate (Scheme
2), and was characterized by FAB-HRMS, FTIR, and ESR
spectroscopy.
2.3. Development of dinuclear vanadium(V) complex
(Ra,S,S)-9
Single crystals suitable for X-ray analysis were obtained by
recrystallization of (Ra,S,S)-1e from MeOH/Et2O/H2O in the
presence of NaOAc.19,20 The binding energy of V 2p3/2 for
the crystals is 516.9 eV by X-ray photoelectron spectroscopy
(XPS), which is attributed to V(V).21 The complex was oxi-
dized to a distorted octahedral vanadium(V) species with
one extra hydroxide to each vanadium center [V1eO3,
1) (S)-tert-leucine
NaOAc
THF/H2O,
O
reflux, 1.5 h
N
V
O
2) VOSO4
THF/H2O
rt, 2 h
O
OH
O
22
˚
O
1.747(5) A] (Fig. 4a). The hydroxide is anti to the imine
7
(S)-8
˚
nitrogen. The V]O [V1eO2, 1.602(8) A] bond is syn to the
Scheme 2. Preparation of mononuclear vanadium(IV) complex (S)-8.
tert-butyl groups in the template. The sodium cation is coordi-
nated by the oxygen atoms of the two carboxylic groups
˚
[Na1eO4, 2.393(5) A] on the (S)-tert-leucine regions. The
In the kinetic analysis, the amount of the mononuclear cata-
lyst (S)-8 used was twice to that of (Ra,S,S)-1e since (S)-8 has
only one active center (Fig. 3). The catalysis by (Ra,S,S)-1e
and (S)-8 obeyed second-order kinetics up to about 15 h. Cata-
lyst (S)-8 gave almost racemic BINOL. The calculated rate
coordination of oxygen to several sodium cations results in
a helical molecular aggregation within the crystal (Fig. 4b).
To determine the oxidative state of vanadium after complex
formation to give (Ra,S,S)-1e, superconducting quantum inter-
ference device (SQUID) analyses were performed on the
dinuclear vanadium complexes using VOSO4 and V2O5 as
standards for the vanadium(IV) and vanadium(V) samples, re-
spectively (Fig. 5).23 The magnetic susceptibility of (Ra,S,S)-
1e prepared under air was 5 emu molꢁ1 at 5 K in 5 T (curve
c). In contrast, the vanadium complex prepared under argon
constants for the coupling reactions at 20 ꢀC were k
¼
ðRa;S;SÞ-1e
0:1738 Mꢁ1 hꢁ1 and k
¼ 0:0036 Mꢁ1 hꢁ1
.
18 The coupling
ðSÞ-8
reaction in the presence of 5 mol % of (Ra,S,S)-1e was thus
shown to be 48.3 times faster than that using 10 mol % of
(S)-8 (Fig. 3). The calculations based on Arrhenius and Eyring
plots between 20 and 30 ꢀC indicated activation energies

S. Takizawa et al. / Tetrahedron 64 (2008) 3361e3371
3365
1) 2 eq. (S)-tert -leucine
N
V
O
MS 3A, EtOH
reflux, 2 h
N
V
O
O
O
O
O
OH
O
HO
O
O
(R)-2
O
O
O
2) 4 eq. VOCl3,
CH2Cl2, rt, 12 h
Na+
OH-
V
N
HOO H
O
O
O
O
V
N
(Ra,S,S)-9
Scheme 3. Preparation of dinuclear vanadium(V) complex (Ra,S,S)-9.
(a)
providing (S)-BINOL in 90% yield with 83% ee after
24 h.24 To compare the catalytic activity of the dinuclear vana-
dium(V) complex, (Ra,S,S)-9 was prepared from VOCl3, (R)-2,
and (S)-tert-leucine as shown in Scheme 3. The complex
1
(Ra,S,S)-9 was characterized by H, 13C, 51V NMR, FTIR,
and MS. The coupling of 2-naphthol was catalyzed by
(Ra,S,S)-9 with reaction rates enhanced 2.3 times faster than
that of (Ra,S,S)-1e.25 Gong also reported dinuclear vana-
dium(V) catalysts for oxidative coupling of 2-naphthol bearing
a VeOeV linkage. In contrast to our catalysts, Gong’s cata-
lyst affords the (R)-configuration of BINOL as a major product
(8 days, 63%, 71% ee (R)) although the absolute configura-
tions of the components of the catalysts are the same.12
(b)
Figure 4. (a) Structure of dinuclear vanadium(V) complex (Ra,S,S)-9$NaOH.
A counter anion OHꢁ, the hydrogen atom of which has not been determined,
is omitted for clarity and (b) a perspective view of molecular aggregation of
the vanadium (V) complex.
2.4. Dinuclear vanadium complex catalyzed oxidative
coupling reaction of 2-naphthol
gave a value of 28 emu molꢁ1 (curve b). The magnetic suscep-
tibility of the single crystals obtained by the recrystallization,
as shown in Figure 4, exhibited 0 emu molꢁ1, clearly indicat-
ing a vanadium(V) species (curve d). These results suggest
that the dinuclear vanadium(IV) complex is readily oxidized
to afford a vanadium(V) species during preparation in air.
When single crystals (Fig. 4) were used as the catalyst for
the oxidative coupling of 2-naphthol, no catalytic activity was
observed. This may be attributed to the low solubility in
CH2Cl2. However, the addition of 1 equiv of 5 N HCl (aq)
to the sodium in the crystal gave a homogeneous solution.
To the homogeneous solution containing 5 mol % of the dinu-
clear vanadium(V) complex was added 2-naphthol under air,
To elucidate the generality of the dual activation of the
dinuclear vanadium complexes in the oxidative coupling, the
C3, C4, C6 or C7 substituted 2-naphthol derivative was ex-
plored (Table 5). 2-Naphthol possessing electron donating or
withdrawing groups at the C4 (R2), C6 (R3), and C7 (R4) po-
sitions resulted in coupling products with high enantioselectiv-
ities (entries 6e28). 9-Phenanthrol was found to be an
adequate substrate (entries 29e31), although appending a sub-
stituent at the C3 position (R1¼CO2Me, OMe) led to the cor-
responding product with diminished yield and ee (entries 4, 5,
32, and 33). These C3 substituted 2-naphthols barely approach
vanadium on the catalyst due to the steric hindrance. The
dinuclear vanadium-mediated coupling of 5,6,7,8-tetrahydro-
2-naphthol afforded H8-BINOL26 for the first time but with
low chemical yields and low enantioselectivities (entries 34
and 35). In terms of enantioselectivity, the best outcome was
achieved by using dinuclear vanadium(V) complex (Ra,S,S)-
10, which was prepared from VOCl3, (R)-3,30-diformyl-
H8-BINOL, and (S)-tert-leucine. (S)-BINOL products were
obtained in 97% ee when using (Ra,S,S)-10 (entries 3 and
18). Gong also has reported the H8-BINOL-based dinuclear
vanadium(V) complex with high enantiocontrol.12c
2.5. Reaction mechanism for dinuclear complex catalyzed
oxidative coupling
Figure 5. SQUID magnetometer plot of magnetic susceptibility for vanadium
under a magnetic field at 5 K for (a) VOSO4; (b) (Ra,S,S)-1e (prepared under
Ar); (c) (Ra,S,S)-1e (prepared under air); (d) single crystals (Ra,S,S)-9$NaOH
obtained by recrystallization of (Ra,S,S)-1e from MeOHeEt2OeH2O contain-
ing NaOAc and (e) V2O5.
An induction period was observed when (Ra,S,S)-1e was
used for the coupling of 2-naphthol. The %ee of the product
increased during the course of the reaction when using

3366
S. Takizawa et al. / Tetrahedron 64 (2008) 3361e3371
Table 5
Coupling reaction of 2-naphthol derivatives catalyzed by dinuclear vanadium complexes
R2
S
R3
R1
N
V
O
O
R2
O
dinuclear vanadium complex
O
HO
O
R3
R4
R1
O
V
N
R4
R4
OH
OH
Ra
(5 mol%)
OH
O
air, CH2Cl2
O
OH
S
3
R3
R1
R2
(S)-4
dinuclear vanadium(V) complex
(Ra,S,S)-10
Entry
Vanadium complex
Substrate
Temp (ꢀC)
Time (h)
Yielda (%)
eeb (%)
1
(Ra,S,S)-1e
(Ra,S,S)-9
(Ra,S,S)-10
(Ra,S,S)-1e
(Ra,S,S)-9
(Ra,S,S)-1e
(Ra,S,S)-9
(Ra,S,S)-1e
(Ra,S,S)-9
(Ra,S,S)-10
(Ra,S,S)-1e
(Ra,S,S)-1e
(Ra,S,S)-1e
(Ra,S,S)-1e
(Ra,S,S)-9
(Ra,S,S)-1e
(Ra,S,S)-9
(Ra,S,S)-10
(Ra,S,S)-1e
(Ra,S,S)-9
(Ra,S,S)-10
(Ra,S,S)-1e
(Ra,S,S)-9
(Ra,S,S)-10
(Ra,S,S)-1e
(Ra,S,S)-1e
(Ra,S,S)-9
(Ra,S,S)-10
(Ra,S,S)-1e
(Ra,S,S)-9
(Ra,S,S)-10
(Ra,S,S)-1e
(Ra,S,S)-9
(Ra,S,S)-1e
(Ra,S,S)-9
R1¼R2¼R3¼R4¼H, 3a
30
0
24
72
72
240
240
24
4a, 76
4a, Quant
4a, 56
4b, Trace
4b, 10
4c, Quant
4c, 98
91
90
97
d
4
2
3a
3a
3
0
4
R1¼CO2Me, R2¼R3¼R4¼H, 3b
30
30
30
30
30
30
30
30
0
5
3b
R1¼R2¼H, R3¼OMe, R4¼H, 3c
6
86
89
68
81
78
86
92
89
89
80
93
90
97
88
86
93
86
86
93
92
87
87
87
77
84
77
45
48
13
14
7
3c
R1¼R2¼H, R3¼Br, R4¼H, 3d
24
48
8
4d, 43
4d, 83
4d, 31
4e, 35
9
3d
3d
48
48
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
a
R1¼H, R2¼Br, R3¼R4¼H, 3e
48
R1¼R2¼H, R3¼Me, R4¼H, 3f
72
72
24
4f, 62
R1¼R2¼H, R3¼OMOM, R4¼H, 3g
0
4g, 83
4h, 94
4h, 94
4i, 91
R1¼R2¼H, R3¼OBn, R4¼H, 3h
30
0
3h
R1¼R2¼H, R3¼Bn, R4¼H, 3i
72
72
72
72
24
0
3i
0
4i, 91
4i, 69
4j, 82
3i
0
R1¼R2¼H, R3¼Ph, R4¼H, 3j
30
30
30
30
30
0
3j
36
72
4j, Quant
4j, Quant
4k, 55
4k, 98
4k, 67
4l, 51
3j
R1¼R2¼R3¼H, R4¼OMe, 3k
24
24
3k
3k
72
72
24
R1¼R2¼R3¼H, R4¼OMOM, 3l
0
R1¼R2¼R3¼H, R4¼OCH2CHCH2, 3m
30
30
0
4m, 71
4m, Quant
4m, 45
4n, 84
4n, 94
4n, 84
4o, 35
4o, 35
4p, 12
4p, 26
3m
3m
24
72
24
24
24
240
240
120
120
9-Phenanthrol, 3n
3n
0
0
3n
R1¼OMe, R2¼R3¼R4¼H, 3o
0
30
30
30
30
3o
5,6,7,8-Tetrahydro-2-naphthol, 3p
3p
Isolated yields.
Determined by HPLC (4a, 4f, 4g, 4k, 4l, Chiralpak AS-H, hexane/i-PrOH¼9:1; 4b, 4e, 4o, Chiralpak AS, hexane/i-PrOH¼1:1; 4c, Chiralpak AS, hexane/
b
i-PrOH¼4:1; 4d, Chiralpak AD, hexane/i-PrOH¼9:1; 4h, 4j, Chiralpak IA, hexane/i-PrOH¼17:3; 4i, Chiralcel OD, hexane/i-PrOH¼17:3; 4m, Chiralpak IA, hex-
ane/i-PrOH¼19:1; 4n, Chiralpak AD-H, hexane/i-PrOH¼17:3; 4p, Chiralpak AS, hexane/i-PrOH¼19:1).
(Ra,S,S)-1e, from 38% ee at 4% conversion to 86% ee at 14%
conversion, while the %ee of the product remained nearly con-
stant (83e85% ee) when using (Ra,S,S)-9. In the coupling of
2-naphthol using 5 mol % of (Ra,S,S)-9 under Ar, BINOL was
formed with only 9% yield after 48 h due to no re-oxidation
of the vanadium(IV) complex in the absence of air.
After introducing air to the reaction vessel, the catalytic
cycle sufficiently resumed to produce BINOL in 99% yield
(Fig. 6).
An attempt at promoting cross-couplings of 3awith 3b and 3c
with 3d catalyzed by (Ra,S,S)-9 afforded only homolytic coup-
ling products 4 and no cross-coupling product was observed
in any case (Scheme 4). The (Ra,S,S)-9 mediated coupling in-
cludes neither a radicaleanion nor a radicaleradical coupling,
in contrast to Gong’s dinuclear vanadium(V) catalyzed coupling
for which radicaleradical coupling is observed.12c Dinuclear va-
nadium complex-mediated reaction mechanisms have been con-
troversial, although all of these results are in agreement with an

S. Takizawa et al. / Tetrahedron 64 (2008) 3361e3371
3367
also the enantioselectivity of (S)-11 were significantly higher
than that of the mononuclear vanadium(IV) complex (S)-8
(Scheme 2). These results suggest that intermolecular manner
coupling would not proceed readily when using the dinu-
clear vanadium(IV) complex in comparison to the dinuclear
vanadium(V) complex.
3. Conclusion
We have developed dual activation catalysts for the homo-
lytic coupling of 2-naphthol promoted by dinuclear vanadium
complexes. Chiral dinuclear vanadium(IV) complex (Ra,S,S)-
1e and vanadium(V) complex (Ra,S,S)-9 possessing (S)-
tert-leucine moieties at the 3,30-positions of a (R)-binaphthyl
skeleton smoothly promoted the oxidative coupling of 2-naph-
thol with high reaction rates to afford (S)-BINOL products in
high yield and high enantioselectivity. The structural and ki-
netic analyses of the mononuclear vanadium complexes and
dinuclear vanadium complexes revealed that two vanadium
metals in one chiral complex activate two molecules of
2-naphthol simultaneously in a homolytic coupling reaction,
achieving a high reaction rate with high enantiocontrol. Appli-
cations of the dinuclear vanadium complexes on a large scale
as well as their extension to other asymmetric reactions are in
progress.
Figure 6. Time course in homolytic coupling reaction of 2-naphthol catalyzed
by 5 mol % of (Ra,S,S)-9 at 30 ꢀC under Ar and air.
intramolecular manner coupling as shown in Scheme 5. In this
mechanism, the dinuclear vanadium(V) complex reacts with
two molecules of 2-naphthol resulting in Ia. The C1 positions
of 2-naphthol molecules approach each other by the rotation of
the binaphthyl axis yielding Ib, and Ib is then intramolecularly
coupled after a single electron transfer to a vanadium(V) species.
(S)-BINOL was released from II and vanadium(V) species was
regenerated after isomerization of the coupling product and
oxidation of the vanadium(IV) species.27
Table 6 shows the results of 2-naphthol coupling by vana-
dium(V) complexes under various reaction conditions. No
by-product was observed in any reaction. Both mononuclear
and dinuclear vanadium(V) complexes promote couplings un-
der air6b with higher enantioselectivity than that under oxygen
(entries 1, 2, 4, and 5). Recently, Gong also reported that
higher enantioselectivity was seen by a dinuclear vanadium(V)
complex bearing a VeOeV linkage under air.12c Lowering the
temperature to 0 ꢀC led to an increase in the enantioselectivity
(entries 3 and 6). Since the coupling of 2-naphthol was cata-
lyzed by (Ra,S,S)-9 with reaction rates enhanced 4.7 times
faster than that of mononuclear vanadium(V) complex
(S)-11,28 intermolecular manner couplings proceeded as a
minor pathway. Interestingly, not only the reaction rate but
4. Experimental section
4.1. General remarks
1H and 13C NMR spectra were recorded with a JEOL JNM-
EX270 FT NMR spectrometer (1H NMR 270 MHz, 13C NMR
67.7 MHz). All signals are expressed as parts per million
downfield from tetramethylsilane used as an internal standard.
51V NMR spectra were recorded on a JEOL JNM-LA600
(158 MHz) spectrometer in CD3OD or CD2Cl2 with VOCl3
as an external standard (0 ppm). FTIR spectra were recorded
CO2Me
CO2Me
CO2Me
OH
OH
5 mol % (Ra,S,S)-9
CH2Cl2, air, 30 °C, 24 h
OH
OH
4a
+
3a
+
+
83% ee (S)
OH
3b
CO2Me
4b
4ab
4a : 4ab : 4b= 100 : 0 : 0
MeO
Br
MeO
MeO
Br
5 mol % (Ra,S,S)-9
CH2Cl2, air, 30 °C, 24 h
OH
OH
OH
OH
OH
+
+
+
OH
OH
OH
3c
3d
Br
Br
MeO
4c
86% ee (S)
4cd
4d
68% ee (S)
4c : 4cd : 4d = 4.7 : 0 : 1
Scheme 4. Cross-coupling of 3a with 3b and 3c with 3d in the presence of (Ra,S,S)-9.

3368
S. Takizawa et al. / Tetrahedron 64 (2008) 3361e3371
(Ra,S,S)-9
2
OH
2 H2O
N
O
V
O
O
O O
O
O
O
O
(S)-BINOL
V
N
O
.
Ia
2
OH
O
O
N
N
N
O
O
V
V
O
O
V
O
O
O
O
O
O
O
O
O
O
V
O
O
N
O
O
II
Ib
intramolecular
manner
coupling
oxidation
isomerization
H2O
1/2 O2
Scheme 5. Proposed reaction mechanism for (Ra,S,S)-9 catalyzed oxidative coupling reaction.
on a PerkineElmer system 2000 FT-IR. Optical rotations were
measured with a JASCO P-1030 polarimeter. HPLC analyses
were performed on a JASCO HPLC system (JASCO PU-980
pump and UV-975 UV/Vis detector) using a mixture of hexane
and i-PrOH as an eluent. X-ray crystallographic analyses were
carried out with a RIGAKU AFC-7R, and all calculations were
performed using the crystal structure determination package of
Molecular Structure Corporation. Fast atom bombardment
(FAB) mass spectra were recorded on a JEOL JMS-600H.
High resolution mass spectra (HRMS) were recorded on
a JEOL JMS-D 300. ESI-TOF mass spectra were recorded
on a JEOL JMS-T100LC. Elemental analysis was performed
on a PerkineElmer 2400. Analytical TLC was performed on
Merck silica gel plates with 60 F254 indicator. Visualization
was accomplished with UV light and phosphomolybdic acid.
Column chromatography on SiO2 was performed with Kanto
Silica Gel 60 (40e100 mm). Commercially available organic
and inorganic compounds were used without further purifica-
tion, except for the solvent, which was distilled by a known
method before use.
Table 6
Coupling reaction of 2-naphthol catalyzed by vanadium(V) complexes
Catalyst
CH2Cl2
N
V
3a
4a
O
O
O
O
OH
Catalyst (S)-11
Entry Catalyst
(mol %)
Atmosphere Temp Time Yielda eeb (%)
(1 atm)
(ꢀC)
(h)
(%)
1
2
3
4
5
6
(Ra,S,S)-9 (5) O2
(Ra,S,S)-9 (5) Air
(Ra,S,S)-9 (5) Air
30
30
0
24
24
72
24
24
24
Quant
96
71
85
90
61
81
85
Quant
46
37
(S)-11 (10)
(S)-11 (10)
(S)-11 (10)
O2
30
30
0
4.2. Chiral dinuclear vanadium complexes 1
Air
Air
20
a
A round-bottomed flask (100 mL) was charged with the
corresponding amino acid (3.3 mmol), anhydrous NaOAc
Isolated yields.
Determined by HPLC (Chiralpak AS-H, hexane/i-PrOH¼9:1).
b

S. Takizawa et al. / Tetrahedron 64 (2008) 3361e3371
3369
(500 mg, 6.0 mmol), and water (6 mL). After stirring at 70 ꢀC
for 10 min to generate a clear solution, a solution of (R)-2,14
(1.5 mmol) in THF (22 mL) was added to the reaction mix-
ture. The reaction mixture was refluxed for 1.5 h to generate
a clear red solution, and the consumption of (R)-2 was moni-
tored by TLC (acetone/hexane¼1:3). The resulting solution
was gradually cooled to room temperature for 2 h. A solution
ofvanadylsulfatehydrate (978 mg, 6.0 mmol)inH2O(5 mL)was
added to the reaction mixture and the solution was stirred for 2 h
under Ar. THF was evaporated and the corresponding vanadium
complex was collected by filtration, washed sequentially with wa-
ter (ca. 60 mL) and ether (ca. 60 mL) under air, and dried under
vacuum to give 1 as a dark green powder (71e85%). (Ra)-1a:
IR (cmꢁ1): 1647 (C]N), 1625 (C]O), 931 (V]O); FAB-
HRMS: found: m/z 586.9864; calcd for C26H17N2O8V2:
[MþH]þ, 586.9822. (Ra,S,S)-1b: IR (cmꢁ1): 1650 (C]N),
1610 (C]O), 981 (V]O); FAB-HRMS: found: m/z 739.0490;
calcd for C38H25N2O8V2: [MþH]þ, 739.0498. (Ra,S,S)-1c: IR
(cmꢁ1): 1654 (C]N), 1614 (C]O), 975 (V]O); FAB-HRMS:
found: m/z 767.0807; calcd for C40H29N2O8V2: [MþH]þ,
767.0803. (Ra,S,S)-1d: IR (cmꢁ1): 1652 (C]N), 1612 (C]O),
977 (V]O); FAB-HRMS: found: m/z 671.0778; calcd for
C32H29N2O8V2: [MþH]þ, 671.0803. (Ra,S,S)-1e: IR (cmꢁ1):
1661 (C]N), 1614 (C]O), 987 (V]O); FAB-HRMS: found:
m/z 699.1111; calcd for C34H33N2O8V2: [MþH]þ, 699.1116;
ESI-TOF-LRMS (MeOH): found: m/z 783 [Mþ2MeOþNa]þ.
(Sa,S,S)-1e: IR (cmꢁ1): 1655 (C]N), 1600 (C]O), 989
(V]O); FAB-HRMS: found: m/z 699.1074; calcd for
C34H33N2O8V2: [MþH]þ, 699.1116.
The reaction mixture was refluxed for 2 h to generate a clear
red solution, and the consumption of 7 was monitored by
TLC (acetone/hexane¼1:3). The resulting solution was gradu-
ally cooled to 30 ꢀC for 2 h. A solution of vanadyl sulfate hy-
drate (277 mg, 1.7 mmol) in water (0.5 mL) was added to the
reaction mixture and the solution was stirred for 2 h under Ar.
THF was evaporated and the vanadium complex was collected
by filtration, washed sequentially with water and ether under
air, and dried under vacuum to give (S)-8 as a dark green pow-
der (248 mg, 71%). FABMS: found: m/z 351 (MþH)þ. IR
(cmꢁ1): 1661 (C]N), 1621 (C]O), 985 (V]O); FAB-
HRMS: found: m/z 351.0670; calcd for C17H18NO4V:
[MþH]þ, 351.0675.
4.5. (Ra,S,S)-9
A round-bottomed flask (100 mL) was charged with (R)-2
(342 mg, 1.0 mmol), (S)-tert-leucine (289 mg, 2.2 mmol),
˚
MS 3 A (1.0 g), and EtOH (30 mL). The reaction mixture
was refluxed at 80 ꢀC for 2 h to generate a red-orange suspen-
sion and the consumption of (R)-2 was monitored by TLC (ac-
etone/hexane¼1:3). After evaporation of EtOH, the residue
was suspended in CH2Cl2 (20 mL) and then VOCl3
(0.38 mL, 4.0 mmol) was added under Ar. The reaction mix-
ture was stirred for 12 h, and filtered by Celite to remove
˚
MS 3 A. The filtrate was evaporated and the resulting black
solid was dissolved in MeOH (ca. 20 mL) and the solvent
was evaporated again. The residue was dissolved in CH2Cl2
(ca. 50 mL) and washed with H2O (4ꢂca. 40 mL), and the
aqueous layer was extracted with CH2Cl2 (4ꢂca. 40 mL).
The combined extract was dried over anhydrous Na2SO4 and
concentrated under vacuum to give (Ra,S,S)-9 (461 mg, 63%)
as a black powder. Complex (Ra,S,S)-9 (952 mg, 1.3 mmol)
and NaOH (57 mg, 1.4 mmol) were dissolved in MeOH
(44 mL) and the resulting dark-brown solution was filtered
through a membrane filter. Ether (ca. 350 mL) was added to the
filtrate, and the solution was allowed to stand at room temper-
ature for 3e7 days. Dark-brown needles (266 mg, 27%) were
obtained as (Ra,S,S)-9$NaOH salt. The needles (296 mg,
0.38 mmol) were suspended in CH2Cl2 and neutralized with
5 N HCl (aq) (77 mL, 0.38 mmol). The reaction mixture was
diluted with water and extracted with CH2Cl2 two times.
The extract was dried over Na2SO4 and then filtered. The or-
ganic solvent was removed by evaporation. After drying under
vacuum, an extra pure (Ra,S,S)-9 (194 mg, 69%) resulted as
a black powder. (Ra,S,S)-9: 1H NMR (270 MHz, CD3OD):
d 8.96 (s, 2H, CH]N), 8.48 (s, 2H, AreH), 8.06 (t,
J¼4.7 Hz, 2H, AreH), 7.70 (t, J¼4.9 Hz, 2H, AreH),
7.44e7.38 (m, 4H, AreH), 4.23 (s, 2H, CHet-Bu), 1.25 (s,
18H, t-Bu); 13C NMR (67.7 MHz, CD3OD): d 167.9, 138.7,
137.6, 130.6, 130.5, 129.6, 127.4, 125.3, 124.1, 83.8, 38.8,
28.2; 51V NMR (CD3OD): d ꢁ557.2; IR (cmꢁ1): 3436 (OeH),
1683 (C]N), 1608 (C]O), 996 (V]O); ESI-TOF-LRMS
(MeOH): found: m/z 783 [Mꢁ2(OH)þ2(OMe)þNa]þ; CSI-
TOF-LRMS (CH2Cl2): found: m/z 1511 [2(MþNaþH)]þ.
Anal. Calcd for C34H36N2O11V2: C, 54.41; H, 4.83; N, 3.73.
Found: C, 53.98; H, 4.60; N, 3.56.
4.3. (S,S)-5
A round-bottomed flask (50 mL) was charged with (S)-tert-
leucine (1.0 mmol), anhydrous NaOAc (167 mg, 2.0 mmol),
and water (10 mL). After stirring at 70 ꢀC for 10 min to gen-
erate a clear homogeneous solution, the reaction mixture was
treated with a solution of 3,30-diformyl-2,20-dihydroxy-1,10-bi-
phenyl14 (0.5 mmol) in THF (5 mL). The reaction mixture was
refluxed for 30 min to generate a red homogeneous solution,
and then was gradually cooled to 30 ꢀC for 2 h. To the Schiff
base solution was added a solution of vanadyl sulfate
(4 mmol) in water (4 mL). The reaction mixture was stirred
for 2 h under Ar and then evaporated to remove the residual
THF. The vanadium complex was collected by filtration and
washed sequentially with water (3ꢂ10 mL) and CCl4
(3ꢂ10 mL) under air, and then dried under vacuum to provide
(S,S)-5 as a dark blue powder (226 mg, 76%). IR (cmꢁ1): 1651
(C]N), 1614 (C]O), 976 (V]O); FAB-HRMS: found: m/z
599.0805; calcd for C26H29N2O8V2: (MþH)þ, 599.0803.
4.4. (S)-8
A round-bottomed flask (100 mL) was charged with (S)-
tert-leucine (144 mg, 1.1 mmol), anhydrous NaOAc (164 mg,
2.0 mmol), and water (4 mL). After stirring at 70 ꢀC for
10 min to generate a clear solution, a solution of 3-formyl-2-
naphthol (7) (173 mg, 1.0 mmol) in THF (6 mL) was added.

3370
S. Takizawa et al. / Tetrahedron 64 (2008) 3361e3371
4.6. (Ra,S,S)-10
reaction mixture was purified by silica gel column chromatog-
raphy (acetone/hexane¼1:4) to give the coupled product.
A round-bottomed flask (50 mL) was charged with (R)-
5,50,6,60,7,708,80-octahydro-3,30-diformyl-BINOL26
(70 mg,
˚
Acknowledgements
0.2 mmol), (S)-tert-leucine (58 mg, 0.44 mmol), MS 3 A (ca.
500 mg), and EtOH (7 mL). The reaction mixture was refluxed
at 80 ꢀC for 30 min to generate a yellow suspension and the
consumption of (R)-5,50,6,60,7,708,80-octahydro-3,30-diformyl-
BINOL was monitored by TLC (acetone/hexane¼1:3). After
evaporation of EtOH, the residue was suspended in CH2Cl2
(10 mL), and then VOCl3 (75 mL, 0.8 mmol) was added to give
adarkgreen solution. Thereactionmixturewasstirredfor6 hun-
This work was supported by a Grant-in-Aid for Scientific
Research from the Ministry of Education, Culture, Sports, Sci-
ence and Technology, Japan. We thank Dr. Takeshi Yanagida
for SQUID analyses, Dr. Masao Takahashi for XPS analyses,
Professor Shunichi Fukuzumi and Dr. Tomoyoshi Suenobu
for ESR analyses, Dr. Shuichi Emura for XANES analyses,
and the technical staff of the Materials Analysis Center at
Osaka University.
˚
der Ar, and filtered with Celite to remove MS 3 A. The filtrate
was evaporated to afford a dark green solid. The resulting solid
was dissolved in MeOH (15 mL) and the solvent was evaporated
again. The residue was washed with water (ca. 50 mL) and Et2O
(ca. 50 mL). Drying under vacuum gave (Ra,S,S)-10 (84 mg,
References and notes
1. (a) Comprehensive Asymmetric Catalysis; Jacobsen, E. N., Pfaltz, A.,
Yamamoto, H., Eds.; Springer: Berlin, 1999; Catalytic Asymmetric Synthe-
sis, 2nd ed.; Ojima, I., Ed.; Wiley-VCH: Weinheim, 2000.
1
56%) as a dark green powder. H NMR (270 MHz, CD3OD):
d 8.59 (s, 2H, CH]N), 7.41 (s, 2H, AreH3), 4.12 (s, 2H,
CHet-Bu), 2.89 (br, 4H, CH2), 2.34 (br, 4H, CH2), 1.82 (br,
8H, CH2), 1.19 (s, 18H, t-Bu); 13C NMR (67.7 MHz, CD3OD):
d 168.1ꢂ2, 148.3, 135.6, 134.8ꢂ2, 131.9, 102.0, 84.8, 39.4,
31.6, 31.2, 29.1, 29.0, 25.1; IR (cmꢁ1): 3449 (O-H), 1686
(C]N), 1611 (C]O), 1001 (V]O); ESI-TOF HRMS: found:
m/z 791.1929 [Mꢁ2(OH)þ2(OMe)þNa]þ; calcd for
C36H46N2NaO10V2: 791.1929.
2. (a) Corey, E. J.; Wang, Z. Tetrahedron Lett. 1993, 34, 4001; (b) Amouri,
H. El.; Gruselle, M. Chem. Rev. 1996, 96, 1077; (c) Konsler, R. G.; Karl,
J.; Jacobsen, E. N. J. Am. Chem. Soc. 1998, 120, 10780; (d) Daikai, K.;
Kamaura, M.; Inanaga, J. Tetrahedron Lett. 1998, 39, 7321; (e) Sasai,
H.; Arai, T.; Watanabe, S.; Shibasaki, M. Catal. Today 2000, 62, 17; (f)
Molenveld, P.; Engbersen, J. F. J.; Reinhoudt, D. N. Chem. Soc. Rev.
2000, 29, 75; (g) Nishiwaki, N.; Knudsen, K. R.; Gothelf, K. V.; Jørgensen,
K. A. Angew. Chem., Int. Ed. 2001, 40, 2992; (h) France, S.; Wack, H.;
Hafez, A. M.; Taggi, A. E.; Witsil, D. R.; Lectka, T. Org. Lett. 2002, 4,
1603; (i) Taggi, A. E.; Hafez, A. M.; Wack, H.; Young, B.; Ferraris, D.;
Lectka, T. J. Am. Chem. Soc. 2002, 124, 6626; (j) Inanaga, J.; Furuno,
H.; Hayano, T. Chem. Rev. 2002, 102, 2211; (k) Taggi, A. E.; Hafez,
A. M.; Lectka, T. Acc. Chem. Res. 2003, 125, 4050; (l) Imbriglio, J. E.;
Vasbinder, M. M.; Miller, S. J. Org. Lett. 2003, 5, 3741; (m) Sammis,
G. M.; Danjo, H.; Jacobsen, E. N. J. Am. Chem. Soc. 2004, 126, 9928;
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(o) Sohtome, Y.; Hashimoto, Y.; Nagasawa, K. Adv. Synth. Catal. 2005,
347, 1643.
4.7. (S)-11
A round-bottomed flask (50 mL) was charged with 7 (86 mg,
˚
0.5 mmol), (S)-tert-leucine (58 mg, 0.5 mmol), MS 3 A, and
EtOH (7 mL). The reaction mixture was refluxed at 80 ꢀC for
4 h to generate a red suspension and the consumption of 7
was monitored by TLC (acetone/hexane¼1:3). After evapora-
tion of EtOH, the residue was suspended in CH2Cl2 (ca.
8 mL) and then VOCl3 (0.09 mL, 1 mmol) was added. The re-
action mixture was stirred for 15 h under Ar and filtered by Cel-
3. (a) Akiyama, T.; Itoh, J.; Yokota, K.; Fuchibe, K. Angew. Chem., Int. Ed.
2004, 43, 1566; (b) Uraguchi, D.; Terada, M. J. Am. Chem. Soc. 2004,
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˚
ite to remove MS 3 A. The filtrate was evaporated to afford
a dark green solid. The resulting solid was dissolved in
MeOH and the solvent was evaporated again. The residue was
washed sequentially with water and ether. Drying under vacuum
gave (S)-11 as a dark green powder (69 mg, 38%). FAB-LRMS:
found: m/z 368 (MþH)þ; IR (cmꢁ1): 3439 (OeH), 1661
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1
(C]N), 1615 (C]O), 992 (V]O); H NMR (269.6 MHz,
CD3OD): d 8.95 (br, 2H, CH]N), 4.24 (br, 2H, CHet-Bu),
1.25 (s, 9H, t-Bu); ESI-TOF HRMS: Found: m/z 404.0679
[MꢁOHþOMeþNa]þ; calcd for C18H20NNaO5V: 404.0679.
4.8. Representative procedure for oxidative coupling
of 2-naphthol
A round-bottomed flask (20 mL) was charged with
a CH2Cl2 solution (1 mL) of 2-naphthol (0.2 mmol). The vana-
dium catalyst (0.01 mmol) was added to the solution as a pow-
der, and the reaction mixture was stirred. The consumption of
2-naphthol was monitored by TLC (acetone/hexane¼1:3). The

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18. The calculated rate constants for the coupling reactions at 30 ꢀC were
´
´
ꢀ
´
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k
¼ 0:1813 Mꢁ1 hꢁ1 and k
¼ 0:0116 Mꢁ1 hꢁ1; a¼initial
ðRa;S;SÞ-1e
ðSÞ-8
concentration of 2-naphthol (0.2 M), x¼concentration of BINOL.
2.5
Catalyst (R ,S,S)-1e (5 mol %)
a
Catalyst (S)-8 (10 mol %)
2
k(Ra,S,S)-1e = 0.1813 [M-1h-1]
1.5
r = 0.998
1
k(S)-8 = 0.0116 [M-1h-1]
0.5
r = 0.952
0
0
10
20
30
t / h
19. The vanadium(IV) complexes are readily oxidized to afford a vanadium(V)
species during recrystallization in air. Pawar, V. D.; Bettigeri, S.; Weng,
S.-S.; Kao, J.-Q.; Chen, C.-T. J. Am. Chem. Soc. 2006, 128, 6308, also
see Ref. 9a.
20. Crystallographic data for structural analysis have been deposited with the
Cambridge Crystallographic Data Centre, CCDC no. 666356. Copies of
this information may be obtained free of charge from The Director, CCDC,
12 Union Road, Cambridge, CB2 1EZ UK (fax: þ44 1223 336033; email:
deposit@ccdc.cam.ac.uk. or www: http://www.ccdc.cam.ac.uk).
21. Sawatzky, G. A.; Post, D. Phys. Rev. B 1979, 20, 1546.
22. The pre-edge peak position (5468.0 eV) of the crystal by X-ray absorption
near edge structure (XANES) analysis is comparable as that of dinuclear
vanadium(V) complex (Ra,S,S)-9 (5468.0 eV) and mononuclear vana-
dium(V) complex (S)-11 (5468.0 eV).
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53.
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23. The magnetic susceptibilities of VOSO4 and V2O5 as vanadium(IV) and
(V) standards are 38 and 0 emu molꢁ1, respectively.
24. The 1H NMR chart of the vanadium(V) species causing form the addition
of 1 equiv of 5 N HCl aq to the sodium cation on the crystal was same as
that of (Ra,S,S)-9.
25. The calculated rate constant for (Ra,S,S)-9 catalyzed coupling reaction at
20 ꢀC was k
¼ 0:4000 Mꢁ1 hꢁ1
ðRa;S;SÞ-9
26. Cram, D. J.; Helgeson, R. C.; Peacock, S. C.; Kaplan, L. J.; Domeier,
L. A.; Moreau, P.; Koga, K.; Mayer, J. M.; Chao, Y.; Siegel, M. G.;
Hoffman, D. H.; Sogah, G. D. Y. J. Org. Chem. 1978, 43, 1930.
27. The iodimetry of the reaction solvent after the coupling of 2-naphthol
indicated no generation of H2O2.
12. (a) Luo, Z.; Liu, Q.; Gong, L.; Cui, X.; Mi, A.; Jiang, Y. Chem. Commun.
2002, 914; (b) Luo, Z.; Liu, Q.; Gong, L.; Cui, X.; Mi, A.; Jiang, Y.
Angew. Chem., Int. Ed. 2002, 41, 4532; (c) Guo, Q.-X.; Wu, Z.-J.; Luo,
28. The calculated rate constant for (S)-11 catalyzed coupling reaction at
20 ꢀC was k
¼ 0:0850 Mꢁ1 hꢁ1
.
ðSÞ-11