Dual activation in a homolytic coupling reaction promoted by an enantioselective dinuclear vanadium(IV) catalyst

Article abstract of DOI:10.1016/j.tetlet.2004.01.007

A new concept of dual activation catalysis in homolytic coupling reaction of 2-naphthol derivatives is described. The dinuclear vanadium(IV) catalyst (R,S,S)-1a promotes the oxidative coupling reaction of 2-naphthol derivatives with high reactivity and enantioselectivity. This dual activation mechanism is supported by the fact that the reaction rate of oxidative coupling of 2-naphthol promoted by the (R,S,S)-1a is 48 times faster than that of the mononuclear complex (S)-3 and a lower catalyst loading of (R,S,S)-1a shows higher catalyst efficiency both in enantioselectivity and chemical yield.

Full text of DOI:10.1016/j.tetlet.2004.01.007

Tetrahedron
Letters
Tetrahedron Letters 45 (2004) 1841–1844
Dual activation in a homolytic coupling reaction promoted by an
enantioselective dinuclear vanadium(IV) catalyst
Hidenori Somei, Yasuaki Asano, Tomokazu Yoshida, Shinobu Takizawa,
Hiroshi Yamataka and Hiroaki Sasai*
The Institute of Scientific and Industrial Research (ISIR), Osaka University, Mihogaoka, Ibaraki, Osaka 567-0047, Japan
Received 17 December 2003; revised 26 December 2003; accepted 6 January 2004
Abstract—A new concept of dual activation catalysis in homolytic coupling reaction of 2-naphthol derivatives is described. The
dinuclear vanadium(IV) catalyst (R,S,S)-1a promotes the oxidative coupling reaction of 2-naphthol derivatives with high reactivity
and enantioselectivity. This dual activation mechanism is supported by the fact that the reaction rate of oxidative coupling of
2
(
-naphthol promoted by the (R,S,S)-1a is 48 times faster than that of the mononuclear complex (S)-3 and a lower catalyst loading of
R,S,S)-1a shows higher catalyst efficiency both in enantioselectivity and chemical yield.
Ó 2004 Elsevier Ltd. All rights reserved.
Co-operative activation of both of two reactants in
intermolecular reactions by a catalyst has opened up a
new field in the development of asymmetric catalysis.
the substrates, a balance between acidity and basicity of
the catalyst is required.
1
With suitable activation of the reactants, an enhance-
ment of enantioselectivity and an acceleration of the
reaction rate is expected. Heterobimetallic asymmetric
catalysts, developed by the present authors, are repre-
We postulated that a chiral complex possessing two
metal centers of the same kind in a single molecule,
which could activate two substrates simultaneously in a
homolytic coupling reaction, would enhance the reac-
tion rate with high enantioselectivity, as depicted in
Figure 1 (Type B). Herein we report the first design and
synthesis of dinuclear vanadium(IV) complexes that
promote enantioselective oxidative coupling of 2-naph-
thol derivatives through a Type B dual activation
mechanism.
2a;b;c
sentative examples providing this kind of activation.
In heterobimetallic catalysts consisting of two different
kinds of metals in a catalyst molecule, one metal works
as a Lewis acid and the other metal–ligand moiety works
as a base enabling a dual activation system as shown in
2
Figure 1 (Type A). However, for efficient activation of
For chiral BINOL derivatives, catalytic asymmetric
oxidative coupling of 2-naphthol derivatives is consid-
ered to be one of the most facile synthetic methodolo-
3
4
gies. Recently, Chen and co-workers and Uang and
5
co-workers independently reported new efficient
asymmetric vanadium(IV) catalysts for oxidative cou-
pling prepared from VOSO , aldehyde derivatives, and
4
6
7
(
S)-amino acids. Quite recently Gong and co-workers
also reported dinuclear vanadium catalysts, which pos-
sess V–O–V linkage. We designed the novel catalyst
Figure 1. Schematic drawing of two kinds of activation system in
catalytic asymmetric reactions.
8
(R,S,S)-1 bearing two active sites attached to a
binaphthyl skeleton to achieve Ôdual activationÕ in
an oxidative coupling reaction (Fig. 2). The catalyst
0
(
R,S,S)-1 was prepared from VOSO
0
4
, (R)-3,3 -diformyl-
Keywords: Enantioselective catalyst.
*
0
9
Corresponding author. Tel.: +81-6-6879-8465; fax: +81-6-6879-8469;
e-mail: sasai@sanken.osaka-u.ac.jp
2,2 - dihydroxy-1,1 -binaphthyl , and (S)-amino acids.
As expected, the catalyst (R,S,S)-1a smoothly promoted
0
040-4039/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2004.01.007

1842
H. Somei et al. / Tetrahedron Letters 45 (2004) 1841–1844
Table 1. Comparison of catalyst activities in the oxidative coupling reaction of 2-naphthol
Catalyst, O2
OH
OH
Solvent, Temp
OH
a
b
Entry
Chiral catalyst (mol %)
Temperature (°C) Solvent
Time (h)
Yield (%)
Ee (%)
1
2
(R,S,S)-1a (5)
(S,S,S)-1a (5)
30
30
0
CH
CH
2
Cl
Cl
2
2
24
24
83
9
83 (S)
2
2
(R)
c
3
GongÕs catalyst (10)
(R,S,S)-1a (10)
CCl
CCl
4
192
192
63
16
71 (R)
8 (R)
d
4
0
4
a
b
c
Isolated yield.
Determined by HPLC (DAICEL Chiralpak AS).
7
0
0
0
GongÕs result using the catalyst prepared from (R)-3,3 -diformyl-2,2 -dihydroxy-1,1 -binaphthyl (1 mol equiv), (S)-tert-leucine (2 mol equiv), and
VOSO (2 mol equiv).
Reaction mixture was heterogeneous.
4
d
the oxidative coupling of 2-naphthol to afford (S)-BI-
NOL in 83% yield with 83% ee at ambient temperature
10
N
V
O
(
Table 1, entry 1). In contrast to the result obtained
O
O
with (R,S,S)-1a, the diastereomeric catalyst (S,S,S)-1a
was less active, giving (R)-BINOL in only 9% yield with
O
HO
OH
11
O
2% ee (Table 1, entry 2). Among the catalysts we
prepared (R,S,S)-1a gave the best results.
N
12
2
Although the preparation procedure for (R,S,S)-1a is
7
similar to that of GongÕs catalyst, the absolute config-
urations of the major products were opposite to each
other (Table 1, entries 1 and 3). Since Gong and
Figure 3. Plausible structure of mononuclear vanadium complex 2
derived from 1 equiv of VOSO
4
.
4
co-workers carried out the coupling reaction in CCl , we
examined the (R,S,S)-1a catalyzed reaction in the same
solvent to confirm the new character of our catalyst
As high concentration of O is thought to generate a
2
(
R,S,S)-1a. A large difference was observed in our cat-
alyst (R,S,S)-1a. Our catalyst hardly dissolved in CCl
and BINOL was obtained only in 16% yield with 8% ee
entry 4).
vanadium(V) complex, we examined the coupling reac-
tion under air. Although the reaction rate was slightly
decreased under air, (S)-BINOL was obtained in 99%
yield with higher enantioselectivity (88% ee) than that
4
(
2
produced under O (Table 2, entry 4). The results of
If the amount of VOSO
4
for complexation with the
coupling reaction of various 2-naphthol derivatives were
summarized in Table 3.
corresponding ligand was insufficient, a mononuclear
vanadium(IV) complex 2 (Fig. 3) would be formed. An
attempt to deliberately prepare a mononuclear vana-
dium(IV) catalyst 2 for comparison with (R,S,S)-1a
resulted in a switch of major product enantiomer to
(R)-BINOL with 13% ee (15% yield) (Table 2, entry 1).
In order to avoid the formation of a mononuclear
vanadium catalyst, the use of 4 equiv of VOSO was
4
For elucidating the mechanism of the coupling reaction
promoted by (R,S,S)-1a, a mononuclear vanadium(IV)
complex (S)-3 was synthesized in a manner similar to
that of complex (R,S,S)-1a (Fig. 4). Oxidative coupling
reactions of 2-naphthol in the presence of either complex
(R,S,S)-1a or (S)-3 were carried out in CH Cl at 20 °C
2
2
effective during catalyst preparation (entry 3). Since the
(R,S,S)-1a complex consists of paramagnetic vana-
dium(IV), (R,S,S)-1a was characterized by HRMS, IR,
under air. In these experiments, the amount of the
mononuclear catalyst (S)-3 used was twice that of
(R,S,S)-1a. The complex (R,S,S)-1a and (S)-3 catalyzed
reactions were second order in 2-naphthol for up to
about 15 h. The calculated rate constants for (R,S,S)-1a
13
and ESR spectroscopy. An ESR study revealed no
V–O–V linkage in the structure of (R,S,S)-1a.
R
N
R
N
N
V
O
O
O
O
1a: R = t-Bu
1b: R = i-Pr
O
O
V
V
O
V
N
O
O
O
O
O
O
O
O
V
N
O
O
V
N
1
1
c: R = Bn
d: R = Ph
O
O
O
O
O
O
O
O
1e: R = H
R
R
(
R,S,S)-1
(
S,S,S)-1
Gong's Catalyst
Figure 2. Proposed structure of our catalyst and GongÕs catalyst.

H. Somei et al. / Tetrahedron Letters 45 (2004) 1841–1844
1843
0
0
0
Table 2. Activity of the catalyst derived from various amounts of VOSO
4
, (R)-3,3 -diformyl-2,2 -dihydroxy-1,1 -binaphthyl, and (S)-tert-leucine
a
b
c
Entry
VOSO
4
(mol equiv)
Atmosphere
Catalyst (mol %)
Time (h)
Yield (%)
Ee (%)
1
2
3
4
5
1
2
O
O
O
2
2
2
5
5
5
5
1
24
24
15
64
83
99
99
13 (R)
78 (S)
83 (S)
88 (S)
90 (S)
d
4
24
48
4
4
air
air
168
a
b
c
Number shows molar equivalent to diformyl compound.
Isolated yield.
Determined by HPLC (DAICEL Chiralpak AS).
d
4
Residual VOSO did not mediate the coupling reaction.
Table 3. Coupling reaction of 2-naphthol derivatives catalyzed by (R,S,S)-1a
R2
R1
R2
R1
3
(
R,S,S)-1a (5 mol %)
R
OH
OH
3
R3
CH Cl , air
2
2
R
OH
R2
R1
a
b;c
Entry
Substrate
Temperature (°C)
Time (h)
Yield (%)
Ee (%)
1
2
3
1
2
3
4
5
6
7
R ¼ H, R ¼ OMe, R ¼ H
30
0
24
72
99
83
91
94
43
51
35
86
89
93
89
68
92
45
1
2
3
R ¼ H, R ¼ OMOM, R ¼ H
R ¼ H, R ¼ Bn, R ¼ H
R ¼ H, R ¼ OBn, R ¼ H
R ¼ H, R ¼ Br, R ¼ H
R ¼ R ¼ H, R ¼ OMOM
R ¼ OMe, R ¼ R ¼ H
1
2
3
0
72
24
1
2
3
30
30
0
1
2
3
48
72
1
2
3
1
2
3
30
240
a
b
c
Isolated yield.
Determined by HPLC (DAICEL Chiralpak AS).
Absolute configuration was assumed to be (S) by optical rotation of purified products.
under a higher catalyst loading an increased reaction
opportunity for the substrate molecules, which are
activated with respective catalyst molecules would give
the product with a lower reaction rate and enantio-
selectivity. This is truly the case. Using either 50 mol %
or 100 mol % of (R,S,S)-1a in the coupling reaction of
2-naphthol in CH Cl at rt for 24 h under air, (R)-BI-
2 2
NOL was obtained in 58% yield (4% ee) and 35% yield
9% ee), respectively.
(
In conclusion, a new type of dual activation catalysis for
the homolytic coupling of 2-naphthols promoted by
16
Figure 4. Kinetic analysis of either (R,S,S)-1a or (S)-3 catalyzed oxi-
dative coupling reactions of 2-naphthol; a ¼ initial concentration of
dinuclear vanadium(IV) complex was realized. The
dual activation mechanism was supported by the kinetic
analysis and catalyst loading effects.
2
-naphthol (0.2 M), x ¼ concentration of BINOL.
and (S)-3 catalyzed coupling reactions were kðR;S;SÞ-1a
¼
ꢀ
1
ꢀ1
ꢀ1 ꢀ1
0
:1738 M
h
and kðSÞ-3 ¼ 0:0036 M h , respectively,
as shown in Figure 4. The coupling reaction in the
presence of 5 mol % of complex (R,S,S)-1a was thus
shown to be about 48 times faster than that using
Acknowledgements
This work was supported by Grant-in-Aid for Scientific
Research on Priority Areas (no 15036243, Reaction
Control of Dynamic Complexes) from Ministry of
Education, Culture, Sports, Science, and Technology,
Japan and the Asahi Glass Foundation. We thank Prof.
Shunichi Fukuzumi and Dr. Tomoyoshi Suenobu at
Osaka University for ESR analyses, and the technical
staff of the Materials Analysis Center, ISIR, Osaka
University.
1
0 mol % of complex (S)-3. The higher entropy depen-
dence, reaction rate, and enantioselectivity using com-
plex (R,S,S)-1a than those of (S)-3 catalyzed reaction
are attributable to the simultaneous activation of two
14;15
molecules of 2-naphthol to produce BINOL.
In the (R,S,S)-1a catalyzed reaction, two 2-naphthol
molecules should be activated simultaneously. However

1844
H. Somei et al. / Tetrahedron Letters 45 (2004) 1841–1844
Santoni, G.; Licini, G. M.; Schulzke, C.; Meier, B. Coord.
Chem. Rev. 2003, 237, 53.
References and notes
7
. (a) Luo, Z.; Liu, Q.; Gong, L.; Cui, X.; Mi, A.; Jiang, Y.
Angew. Chem., Int. Ed. 2002, 41, 4532; (b) Luo, Z.; Liu,
Q.; Gong, L.; Cui, X.; Mi, A.; Jiang, Y. Chem. Commun.
1
. (a) Sasai, H.; Arai, T.; Satow, Y.; Houk, K. N.; Shibasaki,
M. J. Am. Chem. Soc. 1995, 117, 6194; (b) Konsler, R. G.;
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2
002, 914.
1
0780; (c) Amouri, H. E.; Gruselle, M. Chem. Rev. 1996,
0
8
. R refers to the axial chirality of 1,1 -binaphthyl and S to
the carbon central chirality on the amino acid derived
moieties.
. Zhang, H.-C.; Huang, W.-S.; Pu, L. J. Org. Chem. 2001,
6
0. (R,S,S)-1a was prepared as follows: Condensation of (R)-
96, 1077; (d) Molenveld, P.; Engbersen, J. F. J.; Rein-
houdt, D. N. Chem. Soc. Rev. 2000, 29, 75.
. (a) Sasai, H.; Arai, T.; Watanabe, S.; Shibasaki, M. Catal.
Today 2000, 62, 17; (b) Shibasaki, M.; Sasai, H.; Arai, T.;
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2
9
6, 481.
1
0
0
0
3
,3 -diformyl-2,2 -dihydroxy-1,1 -binaphthyl with (S)-tert-
leucine in the presence of sodium acetate afforded diimine
compound. Addition of an aqueous solution of VOSO to
(
d) Steinhagen, H.; Helmchen, G. Angew. Chem., Int. Ed.
Engl. 1996, 35, 2339; (e) Zhang, F.-Y.; Yip, C.-W.; Cao, R.;
Chan, A. S. C. Tetrahedron: Asymmetry 1997, 8, 585; (f)
Shibasaki, M.; Yoshikawa, N. Chem. Rev. 2002, 102, 2187;
4
the diimine solution gave a black solution that was
subsequently evaporated to yield a dark green powder.
This powder was used as catalyst (R,S,S)-1a after washing
(
(
g) Braunstein, P.; Morise, X. Chem. Rev. 2000, 100, 3541;
h) Wheatley, N.; Kalck, P. Chem. Rev. 1999, 99, 3379; (i)
with water and CCl
diastereomeric complex (S,S,S)-1a was also prepared from
the (S)-configuration of corresponding binaphthyl deriv-
4
(93% yield in two steps). The
Rowlands, G. J. Tetrahedron 2001, 57, 1865; (j) Inanaga, J.;
Furuno, H.; Hayano, T. Chem. Rev. 2002, 102, 2211.
. (a) Smrcina, M.; Pol ꢀa kov ꢀa , J.; Vyskocil, S.; Kocovsky, P.
J. Org. Chem. 1993, 58, 4534; (b) Nakajima, M.; Miyoshi,
I.; Kanayama, K.; Hashimoto, S.; Noji, M.; Koga, K. J.
Org. Chem. 1999, 64, 2264; (c) Li, X.; Yang, J.; Kozlowski,
M. C. Org. Lett. 2001, 3, 1137; (d) Irie, R.; Matsutani, T.;
Katsuki, T. Synlett 2000, 1433; (e) Sridhar, M.; Vadivel, S.
K.; Bhalerao, T. U. Tetrahedron Lett. 1997, 38, 5695; (f)
Takemoto, M.; Suzuki, Y.; Tanaka, K. Tetrahedron Lett.
3
4
ative, (S)-tert-leucine, and VOSO .
1
1
1
1. In the case of catalyst of the type developed by Gong, the
catalytic activity of both (R,S,S)-catalyst and (S,S,S)-
catalyst were reported to be similar.
2. Results of coupling reaction using other catalyst: (R,S,S)-
1
b (52%, 66% ee (S)), (R,S,S)-1c (30%, 8% ee (R)), (R,S,S)-
d (21%, rac), (R)-1e (6%, rac).
1
ꢀ
1
3. Data for (R,S,S)-1a: IR (cm ): 1654 (C@N), 1604 (C@O),
2002, 43, 8499.
9
C
74 (V@O); FAB-HRMS: found: m=z 699.1085. Calcd for
4
5
6
. (a) Hon, S.-W.; Li, C.-H.; Kuo, J.-H.; Barhate, N. B.; Liu,
Y.-H.; Wang, Y.; Chen, C.-T. Org. Lett. 2001, 3, 869; (b)
Barhate, N. B.; Chen, C.-T. Org. Lett. 2002, 4, 2529.
. (a) Chu, C.-Y.; Hwang, D.-R.; Wang, S.-K.; Uang, B. J.
Chem. Commun. 2001, 980; (b) Chu, C.-Y.; Uang, B.-J.
Tetrahedron: Asymmetry 2003, 14, 53.
. For a review on vanadium in organic synthesis, see: (a)
Hirao, T. Chem. Rev. 1997, 97, 2707; (b) The Schiff base
ligands derived from the reaction of aromatic aldehydes
with amino acids are well-established in organic chemistry,
see: Casella, L.; Gullotti, M.; Pintar, A.; Colonna, S.;
Manfredi, A. Inorg. Chim. Acta 1988, 144, 89; (c)
Sureshan, C. A.; Bhattacharya, P. K. J. Mol. Catal. A:
Chem. 1998, 136, 285; (d) Dejmek, M. M.; Selke, R.
Angew. Chem., Int. Ed. 1998, 37, 1540; (e) Rehder, D.;
þ
34
H
33
N
2
O
8
V
2
: [M+H] , 699.1116; mp over 300 °C. The
eight peaks, which can be assigned as vanadium(IV)
R,S,S)-1a complex were observed by using ESR.
4. Thermodynamic data for (R,S,S)-1a catalyzed coupling
(
1
z
ꢀ2
reaction: DE ¼ 0:74 kcal/mol, DS ¼ ꢀ7:8 ꢁ 10 kcal/mol
z
K, and DH ¼ 0:16 kcal/mol. For (S)-3 catalyzed coupling
z
ꢀ2
reaction: DE ¼ 23 kcal/mol, DS ¼ ꢀ1:5 ꢁ 10 kcal/mol
z
K, and DH ¼ 21 kcal/mol.
1
1
5. Catalyst (S)-3 gave an almost racemic BINOL.
6. Quite recently Gao and co-workers reported excellent
results on oxidative coupling reaction of 2-naphthol
promoted by dinuclear copper catalysts. Gao, J.; Reibens-
pies, J. H.; Martell, A. E. Angew. Chem., Int. Ed. 2003, 42,
6
008.