Heterogeneously catalyzed aerobic oxidative biaryl coupling of 2-naphthols and substituted phenols in water

Article abstract of DOI:10.1021/ja050436k

The oxidative coupling reaction can efficiently be promoted by supported ruthenium catalyst Ru(OH)_x/Al₂O₃. A variety of 2-naphthols and substituted phenols can be converted to the corresponding biaryl compounds in moderate to excellent yields using molecular oxygen as a sole oxidant in water without any additives. The catalysis is truly heterogeneous in nature, and Ru(OH)_x/Al₂O₃ can easily be recovered after the reaction. The catalyst can be recycled seven times with the maintenance of the catalytic performance, and the total turnover number reaches up to 160. The results of competitive coupling reactions suggest that the present oxidative biaryl coupling reaction proceeds via the homolytic coupling of two radical species and the Ru(OH)_x/Al₂O₃ catalyst acts as an one-electron oxidant. Two radical species are coupled to give the corresponding biaryl product, and the one-electron reduced catalyst is reoxidized by molecular oxygen. The amounts of O₂ uptake and H ₂O formation were almost one-quarter and one-half the amount of substrate consumed, respectively, supporting the reaction mechanism. The kinetic data and kinetic isotope effect show that the reoxidation of the reduced catalyst is the rate-limiting step for the coupling reaction.

Full text of DOI:10.1021/ja050436k

Published on Web 04/12/2005
Heterogeneously Catalyzed Aerobic Oxidative Biaryl Coupling
of 2-Naphthols and Substituted Phenols in Water
Mitsunori Matsushita,† Keigo Kamata,‡ Kazuya Yamaguchi,†,‡ and
Noritaka Mizuno*,†,‡
Contribution from the Department of Applied Chemistry, School of Engineering, The UniVersity
of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Core Research for EVolutional
Science and Technology (CREST), Japan Science and Technology Agency (JST), 4-1-8 Honcho,
Kawaguchi, Saitama, 332-0012, Japan
Received January 22, 2005; E-mail: tmizuno@mail.ecc.u-tokyo.ac.jp.
Abstract: The oxidative coupling reaction can efficiently be promoted by supported ruthenium catalyst
Ru(OH)x/Al2O3. A variety of 2-naphthols and substituted phenols can be converted to the corresponding
biaryl compounds in moderate to excellent yields using molecular oxygen as a sole oxidant in water without
any additives. The catalysis is truly heterogeneous in nature, and Ru(OH)x/Al2O3 can easily be recovered
after the reaction. The catalyst can be recycled seven times with the maintenance of the catalytic
performance, and the total turnover number reaches up to 160. The results of competitive coupling reactions
suggest that the present oxidative biaryl coupling reaction proceeds via the homolytic coupling of two radical
species and the Ru(OH)x/Al2O3 catalyst acts as an one-electron oxidant. Two radical species are coupled
to give the corresponding biaryl product, and the one-electron reduced catalyst is reoxidized by molecular
oxygen. The amounts of O2 uptake and H2O formation were almost one-quarter and one-half the amount
of substrate consumed, respectively, supporting the reaction mechanism. The kinetic data and kinetic isotope
effect show that the reoxidation of the reduced catalyst is the rate-limiting step for the coupling reaction.
reaction,3 they are often hazardous, expensive, stoichiometric,
and difficult to remove from the reaction solution.
Introduction
The syntheses of the biaryl compounds are of importance
because they are used as building blocks of many alkaloids and
natural products, auxiliaries, and ligands for a wide range of
organic functional transformations.1 Especially, homochiral 1,1′-
binaphthalene derivatives have successfully been utilized as
chiral inducers for stereo- and enantioselective reactions because
of their axial dissymmetry and molecular flexibility.2 Therefore,
much attention has been paid to the development of efficient
syntheses of 1,1′-binaphthalene-2,2′-diol (BINOL) derivatives,
which are versatile sources of the various 1,1′-binaphthalene
skeletons. The oxidative coupling of 2-naphthols is one of the
most useful methods for the syntheses of BINOL derivatives.
Although various kinds of stoichiometric oxidation reagents such
as Fe, Cu, Mn, and Ti salts have been employed for the
In the laboratory scale synthesis as well as the manufacture
of large-volume petrochemicals, the environmentally unaccept-
able processes should be replaced by “greener” catalytic ones
with clean, safe, and inexpensive oxidant of molecular oxygen.
While some efficient catalytic procedures using molecular
oxygen as a sole oxidant for the above reaction have been
developed,4-6 most of them are homogeneous systems, and there
are few heterogeneous catalysts despite their significant advan-
(3) (a) Toda, F.; Tanaka, K.; Iwata, S. J. Org. Chem. 1989, 54, 3007. (b) Dewar,
M. J. S.; Nakaya, T. J. Am. Chem. Soc. 1968, 90, 7134. (c) Ding, K.; Wang,
Y.; Zhang, L.; Wu, Y. Tetrahedron 1996, 52, 1005. (d) Hovorka, M.;
Gu¨nterova´, J.; Za´vada, J. Tetrahedron Lett. 1990, 31, 413. (e) Doussot, J.;
Guy, A.; Ferroud, C. Tetrahedron Lett. 2000, 41, 2545. (f) Nishino, H.;
Itoh, N.; Nagashima, M.; Kurosawa, K. Bull. Chem. Soc. Jpn. 1992, 65,
620.
(4) Examples of catalytic aerobic oxidative biaryl coupling (homogeneous
system): (a) Sharma, V. B.; Jain, S. L.; Sain, B. Tetrahedron Lett. 2003,
44, 2655. (b) Noji, M.; Nakajima, M.; Koga, K. Tetrahedron Lett. 1994,
35, 7983. (c) Hwang, D.-R.; Chen, C.-P.; Uang, B.-J. Chem. Commun.
1999, 1207. (d) Yadav, J. S.; Reddy, B. V. S.; Gayathri, K. U.; Prasad, A.
R. New J. Chem. 2003, 27, 1684. (e) Umare, P. S.; Tembe, G. L. React.
Kinet. Catal. Lett. 2004, 82, 173. (f) Gupta, R.; Mukherjee, R. Tetrahedron
Lett. 2000, 41, 7763. (g) Nishino, H.; Satoh, H.; Yamashita, M.; Kurosawa,
K. J. Chem. Soc., Perkin Trans. 2 1999, 1919. (h) Umare, P. S.; Tembe,
G. L. React. Kinet. Catal. Lett. 2004, 82, 173.
† The University of Tokyo.
‡ CREST.
(1) (a) Taylor, W. I.; Buttersby, A. R. OxidatiVe Coupling of Phenols; Marcel
Dekker: New York, 1967. (b) Bringmann, G.; Walter, R. Angew. Chem.,
Int. Ed. Engl. 1990, 29, 977. (c) Nasir, M. S.; Cohen, B. I.; Karlin, K. D.
J. Am. Chem. Soc. 1992, 114, 2482. (d) Solomon, E. I.; Sundaram, U. M.;
Machonkin, T. E. Chem. ReV. 1996, 96, 2563.
(2) (a) Noyori, R. Asymmetric Catalysis in Organic Synthesis; Wiley: New
York, 1994. (b) Noyori, R.; Takaya, H. Acc. Chem. Res. 1990, 23, 345. (c)
Noyori, R. Tetrahedron 1994, 50, 4259. (d) Trost, B. M. Pure Appl. Chem.
1992, 64, 315. (e) Hayashi, T.; Kudo, A.; Ozawa, F. Pure Appl. Chem.
1992, 64, 421. (f) Mikami, K.; Shimizu, M. Chem. ReV. 1992, 92, 1021.
(g) Kagan, H. B.; Riant, O. Chem. ReV. 1992, 92, 1007. (h) Blaser, H.-U.
Chem. ReV. 1992, 92, 935. (i) Rosini, G.; Franzini, L.; Raffaelli, A.;
Salvadori, P. Synthesis 1992, 503. (j) Pu, L. Chem. ReV. 1998, 98, 2405.
(k) Kocˇovsky´, P.; Vyskocˇil, Sˇ.; Sm´rcina, M. Chem. ReV. 2003, 103, 3213.
(l) Chen, Y.; Yekta, S.; Yudin, A. K. Chem. ReV. 2003, 103, 3155.
(5) Examples of catalytic aerobic oxidative biaryl coupling (heterogeneous
system): (a) Prasad, M. R.; Kamalakar, G.; Kulkarni, S. J.; Raghavan, K.
V. J. Mol. Catal. A 2002, 180, 109. (b) Armengol, E.; Corma, A.; Garc´ıa,
H.; Primo, J. Eur. J. Org. Chem. 1999, 1915, 5. (c) Kantam, M. L.; Kavita,
B.; Figueras, F. Catal. Lett. 1998, 51, 113. (d) Mastrorilli, P.; Muscio, F.;
Suranna, G. P.; Nobile, C. F.; Latronico, M. J. Mol. Catal. A 2001, 165,
81. (e) Sakamoto, T.; Yonehara, H.; Pac, C. J. Org. Chem. 1997, 62, 3194.
(f) Iwai, K.; Yamauchi, T.; Hashimoto, K.; Mizugaki, T.; Ebitani, K.;
Kaneda, K. Chem. Lett. 2003, 32, 58. (g) Fujiyama, H.; Kohara, I.; Iwai,
K.; Nishiyama, S.; Tsuruya, S.; Masai, M. J. Catal. 1999, 188, 417.
9
6632
J. AM. CHEM. SOC. 2005, 127, 6632-6640
10.1021/ja050436k CCC: $30.25 © 2005 American Chemical Society

Coupling of 2-Naphthols and Substituted Phenols in Water
A R T I C L E S
Table 1. Oxidative Coupling of 1aa
tages from environmental and economical standpoints, easy
separation from the reaction mixture, and recycling of catalysts.7
Cu2+-exchanged MCM-41,5a,b Fe3+-exchanged MCM-41,5b
Fe3+-exchanged montmorillonite,5c Cu2+-exchanged mont-
morillonite,5d CuSO4/Al2O3,5e K/Cu-Mg-Al-CO3 hydrotalcite,5f
6i
and vanadium-Shiff-base/SiO2 are examples of the heteroge-
neous catalysts for the oxidative biaryl coupling reaction. How-
ever, the reported heterogeneous systems have disadvantages:
(i) Turnover numbers are still low (e33), (ii) the catalysts are
deactivated, (iii) forced reaction conditions (g403 K) are re-
quired to attain high yield of products, and/or (iv) these reactions
are typically performed in environmentally undesirable halo-
genated organic solvents such as chlorobenzene and chloroform.
Recently, organic reactions in water are of great interest8 be-
cause water is much safer than the usual organic solvents, which
are often inflammable, explosive, and carcinogenic, and can
diminish the pollution problems. The separation and recovery
of catalysts and products can be achieved by the phase separation
and/or extraction with some loss of catalysts and products (in
some cases). These properties make the use of water favorable
not only in laboratory-scale organic syntheses but also in
industries. Many water-soluble ligands have now been developed
for transition metal-catalyzed reactions in water.8 Phase-transfer
agents and surfactants are also used to retain contact between
hydrophobic substrates and the catalyst in the water phase.8
In this paper, we report that the supported ruthenium hy-
droxide catalyst (Ru(OH)x/Al2O3) is found to act as an effective
heterogeneous catalyst for the aerobic biaryl coupling of
2-naphthols and substituted phenols in water without any
additives (eqs 1 and 2). Ru(OH)x/Al2O3 can easily be separated
by the filtration and reused several times with retention of the
high catalytic activity. The catalysis is truly heterogeneous in
nature because the filtrate after removal of the solid catalyst is
completely inactive. As far as we know, the heterogeneous
catalytic aerobic biaryl coupling reactions of 2-naphthols and
substituted phenols in water have never been reported so far.
Furthermore, we investigate the reaction mechanism of the
present oxidative coupling reaction in detail.
conv. of
yield of
entry
catalyst
solvent
1ab (%)
1bb (%)
1
2
3
4
5
6
7
8
9
Ru(OH)x/Al2O3
Ru(OH)x/Al2O3
Ru(OH)x/Al2O3
Ru(OH)x/Al2O3
Ru(OH)x/Al2O3
Ru(OH)x/Al2O3
Ru(OH)x/TiO2
Ru(OH)x/SiO2
Cu(OH)x/Al2O3
water
toluene
51
99
87
24
18
<1
44
7
50
84
78
19
12
1,2-dichloroethane
1,2-dimethoxyethane
acetonitrile
ethanol
water
26
5
60
8
water
water
water
water
water
water
water
water
76
8
10 Fe(OH)x/Al2O3
11 Pd(OH)x/Al2O3
12 Rh(OH)x/Al2O3
13 RuCl3‚nH2O
<1
<1
39
<1
14
<1
10
11
32
<1
<1
32
7
14 RuCl2(bpy)3
15 [RuCl2(p-cymene)]2
16 RuO2 (anhydrous)
17 Ru(OH)x‚nH2O
18 Ru(OH)x‚nH2O + Al2O3 water
19 CuCl2
water
water
10
9
4
c
water
water
water
c
20 Al2O3
21 none
a Reaction conditions: 1a (0.3 mmol), catalyst (metal: 5 mol %), solvent
(1.5 mL), 353 K, 2 h, under 1 atm of molecular oxygen. b Conversions and
yields were determined by 1H NMR analyses of the crude reaction mixture
and based on 1a. c Al2O3 (64 mg).
1,2-dichloroethane gave the corresponding BINOL (1b) in high
yields (entries 2 and 3), and polar organic 1,2-dimethoxyethane,
acetonitrile, and ethanol were poor solvents (entries 4-6). It
was found that water was also an effective solvent for the present
oxidative coupling with Ru(OH)x/Al2O3, 50% yield of 1b at
51% conversion of 1a under the conditions (entry 1). In this
case, no byproducts such as perylenes, quinones, and polymers
could be detected by H and 13C NMR analyses of the crude
1
reaction mixture. Because the use of organic solvents such as
toluene and 1,2-dichloroethane is undesirable from an environ-
mental point of view and the highest selectivity was observed
in water, we hereafter used water as a solvent.
Next, the catalytic activities of the oxidative coupling of
1a in water were compared with various catalysts (Table 1).
(6) Examples of enantioselective biaryl coupling: (a) Nakajima, M.; Miyoshi,
I.; Kanayama, K.; Hashimoto, S.-I.; Noji, M.; Koga, K. J. Org. Chem. 1999,
64, 2264. (b) Li, X.; Yang, J.; Kozlowski, M. C. Org. Lett. 2001, 3, 1137.
(c) Chu, C.-Y.; Hwang, D.-R.; Wang, S.-K.; Uang, B.-J. Chem. Commun.
2001, 980. (d) Somei, H.; Asano, Y.; Yoshida, T.; Takizawa, S.; Yamataka,
H.; Sasai, H. Tetrahedron Lett. 2004, 45, 1841. (e) Luo, Z.; Liu, Q.; Gong,
L.; Cui, X.; Mi, A.; Jiang, Y. Angew. Chem., Int. Ed. 2002, 41, 4532. (f)
Gao, J.; Reibenspies, J. H.; Martell, A. E. Angew. Chem., Int. Ed. 2003,
42, 6008. (g) Irie, R.; Masutani, K.; Katsuki, T. Synlett 2000, 1433. (h) Li,
X.; Hewglay, J. B.; Mulrooney, C. A.; Yang, J.; Kozlowski, M. C. J. Org.
Chem. 2003, 68, 5500. (i) Tada, M.; Taniike, T.; Kantam, L. M.; Iwasawa,
Y. Chem. Commun. 2004, 2542. (j) Barhatre, N. B.; Chen, C.-T. Org. Lett.
2002, 4, 2529.
Results and Discussion
(7) Sheldon, R. A.; van Bekkum, H. Fine Chemical through Heterogeneous
Catalysis; Wiley: Weinheim, 2001.
Ru(OH)x/Al2O3-Catalyzed Oxidative Biaryl Coupling in
Water. First, the oxidative coupling of 2-naphthol (1a) was
carried out using Ru(OH)x/Al2O3 at 353 K under 1 atm of
molecular oxygen in various solvents. The results are sum-
marized in Table 1. The reaction did not proceed at all without
Ru(OH)x/Al2O3. The use of nonpolar solvents of toluene and
(8) (a) Li, C.; Chan, T.-H. Organic Reactions in Aqueous Media; Wiley: New
York, 1997. (b) Grieco, P. A. Organic Synthesis in Water; Blacky Academic
Professional: London, 1998. (c) Anastas, P. T.; Warner, J. C. Green
Chemistry: Theory and Practice; Oxford University Press: London, 1998.
(d) Sheldon, R. A. Green Chem. 2000, 2, G1. (e) Anastas, P. T.; Bartlett,
L. B.; Kirchhoff, M. M.; Williamson, T. C. Catal. Today 2000, 55, 11. (f)
Thematic issue on “Organic Reaction in Water”. AdV. Synth. Catal. 2002,
3-4, 219-451 and references therein.
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A R T I C L E S
Matsushita et al.
Ru(OH)x/Al2O3 and Cu(OH)x/Al2O3 gave 1b in 50% and 60%
yields, respectively (entries 1 and 9). No reaction proceeded in
the presence of Al2O3 (entry 20), showing that supported
transition metals such as ruthenium and copper are active species
for the coupling reaction. The homogeneous RuCl3‚nH2O and
CuCl2 catalysts showed moderate catalytic activities (entries 13
and 19), but they were lower than those of Ru(OH)x/Al2O3 and
Cu(OH)x/Al2O3. The Ru2+ complexes such as RuCl2(bpy)3 and
[RuCl2(p-cymene)]2 were not effective for the reaction (entries
14 and 15). RuO2 (anhydrous) showed no catalytic activity (entry
16), while 10% yield of 1b was obtained in the case of
Ru(OH)x‚nH2O (entry 17). It is noted that the catalytic activity
of a mixture of Ru(OH)x‚nH2O and Al2O3 powders was
comparable to that of Ru(OH)x‚nH2O and much lower than that
of Ru(OH)x/Al2O3 (entry 18), indicating that the supporting
ruthenium hydroxide species on Al2O3 is essential for the
appearance of the high activity of Ru(OH)x/Al2O3 in water. The
Ru(OH)x/TiO2 and Ru(OH)x/SiO2 were less active than
Ru(OH)x/Al2O3 (entries 7 and 8).
leached into the reaction solution, and the observed catalysis is
truly heterogeneous in nature.11
The scope of the present Ru(OH)x/Al2O3-catalyzed oxidative
coupling in water with regard to various kinds of 2-naphthols
was examined. The results are summarized in Table 2. All of
the products were isolated and fully characterized by comparison
of the physical data with the authentic data. When the oxidation
of 1a was carried out at 373 K, the complete conversion of 1a
was achieved and 98% of 1b was isolated after 4 h (entry 1). A
larger-scale reaction (25-fold scaled up) showed the same results
as for the small-scale experiments in Table 2. A mixture of 1a
(1.08 g, 7.5 mmol) and Ru(OH)x/Al2O3 (Ru: 5 mol %) in 37.5
mL of water was heated at 373 K for 4 h, giving analytically
pure 1b in 97% isolated yield (1.04 g) after the recrystallization.
Further, Ru(OH)x/Al2O3 catalyst could be reused with the
maintenance of the high catalytic performance; even in the
seventh recycling run, 1a was quantitatively converted to 1b at
the same reaction rate as that for the first run (Table 3). The
turnover number (TON) for one run was 20, and the total
TON reached up to 160 after the seventh recycling. The
TONs (for one run) reported for other catalysts were as
follows: CuCl(OH)/TMEDA (homogeneous, TON ) 90),4b
photoactivated [RuII(salen)(NO)] (homogeneous, 48),6g vanadium-
Schiff-base/SiO2 (heterogeneous, 33),6i oxovanadium(IV) com-
plex (homogeneous, 33),6j methyltrioxorhenium (homogeneous,
18),4a divanadium(IV) complex (homogeneous, 17),6d CuSO4/
Al2O3 (heterogeneous, 13),5e Cu2+-exchanged MCM-41 (het-
erogeneous, 12),5a VO(acac)2 (homogeneous, 10),5h Mn(acac)3
(homogeneous, 9),5h dicopper(II) complex (homogeneous, 9),6f
RuCl3/[bimi]PF6 (homogeneous, 9),4d Fe3+-exchanged mont-
morillonite (heterogeneous, 7),5c Fe3+-exchanged MCM-41
(heterogeneous, 3).5b It is noted that air can be used as an
oxidant instead of molecular oxygen in the present system
(entries 2 and 4). Also, the catalytic oxidative coupling of
substituted 2-naphthols 2a-6a efficiently proceeded to afford
the corresponding BINOL derivatives 2b-6b in moderate to
high yields (entries 3-8). Deuterium labeled 2-naphthol 7a
was also oxidized to give the corresponding BINOL 7b in
excellent yield with almost the same reaction rate as that for
1a (entry 9).
The catalytic activity (yield of 1b) of transition metal
hydroxide catalysts supported on Al2O3 decreased in the order
of Cu(OH)x (60%) > Ru(OH)x (50%) > Fe(OH)x (8%) .
Pd(OH)x (<1%), Rh(OH)x (<1%), and Cu(OH)x/Al2O3 gave the
highest yield of 1b. The catalytic activity decreased with an
increase in the redox potential of the transition metal ions,
Cu2+/Cu+ (0.15 V vs SHE), Ru3+/Ru2+ (0.25 V), Fe3+/Fe2+
(0.77 V).9 Unidentified byproducts were formed at higher
conversion of 1a in the case of Cu(OH)x/Al2O3, resulting in
low selectivity to 1b; when the oxidative coupling was carried
out with Cu(OH)x/Al2O3 under the conditions in Table 2, only
66% yield of 1b was obtained at >99% conversion of 1a.
Further, copper species were leached into the reaction solution
to some extent (∼7%) under the conditions, and the recovered
Cu(OH)x/Al2O3 catalyst was less active than the fresh catalyst.10
In contrast, no leaching of ruthenium species was observed, and
the recovered catalyst after the reaction could be reused without
loss of the catalytic activity in the case of Ru(OH)x/Al2O3 (see
below). Therefore, the oxidative coupling reactions hereafter
were carried out with Ru(OH)x/Al2O3 in water.
It is important to verify that the observed catalysis is caused
by solid Ru(OH)x/Al2O3 or leached ruthenium species. There-
fore, the following control experiment was carried out. A
catalytic oxidative coupling of 1a was carried out under the
conditions in Table 2, and Ru(OH)x/Al2O3 and product 1b were
removed from the reaction mixture by the filtration at the
reaction temperature after complete consumption of 1a. The
substrate 1a (0.3 mmol) was again added to the filtrate, and the
mixture was heated at 373 K. No conversion of 1a was observed.
As above-mentioned, no ruthenium species was present in the
reaction solution (ICP-AES analysis, below the detection limit
of 7 ppb). Moreover, the first-order dependence of the reaction
rate on the amount of Ru(OH)x/Al2O3 (Ru: ∼6.5 mol %, Figure
1) was observed. These observations can rule out any contribu-
tion to the observed catalysis from ruthenium species that
The present procedure was further applied to the oxidative
coupling of substituted phenols. The product selectivities
depended on the substitution pattern of phenols. For 2,4-di-
tert-butylphenol (8a), the ortho-ortho coupling product 8b was
mainly obtained (entry 10). 2,6-Di-tert-butylphenol (9a) gave
the para-para coupling product of diphenoquinone 9b and a
small amount of diphenol without formation of the correspond-
ing benzoquinone (entry 11). In the case of 2,6-dimethylphenol
(10a), the major product was diphenoquinone 10b (entry 12).
These high selectivities to the para-para coupling products
for the oxidative coupling of 2,6-disubstituted phenols are very
similar to those of the laccese and tyrosinase enzyme-catalyzed
system, where Cu2+ acts as a one-electron oxidant to generate
the radical species from the starting phenol and Cu+ is
reoxidized by molecular oxygen.12
(9) Cotton, F. A.; Wilkinson, G.; Gauss, P. L. Basic Inorganic Chemistry, 3rd
ed.; John Wiley & Sons: New York, 1995.
(10) The Cu(OH)x/Al2O3-catalyzed oxidative coupling of 1a under the conditions
in Table 2 gave 66% yield of 1b at >99% conversion of 1a (first run). In
the recycling experiment using the recovered Cu(OH)x/Al2O3, 41% yield
of 1b was obtained at 88% conversion of 1a under the same reaction
conditions.
(11) Sheldon, R. A.; Wallau, M.; Arends, I. W. C. E.; Schuchardt, U. Acc. Chem.
Res. 1998, 31, 485.
(12) (a) Schouten, A. J.; Challa, G.; Eeedijk, J. J. Mol. Catal. 1980, 9, 41. (b)
Pandey, G.; Muralikrishna, C.; Bhalerao, U. T. Tetrahedron Lett. 1990,
31, 3771.
9
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Coupling of 2-Naphthols and Substituted Phenols in Water
A R T I C L E S
a
Table 2. Oxidative Coupling of 2-Naphthols and Substituted Phenols in Water Catalyzed by Ru(OH)x/Al2O3
a Reaction conditions: substrate (0.3 mmol), Ru(OH)x/Al2O3 (Ru: 5 mol %), water (1.5 mL), 373 K, under 1 atm of molecular oxygen. b Conversions
1
were determined by H NMR and/or GC analyses of the crude reaction mixture and based on the starting materials. c Isolated yields based on the starting
materials. All isolated products were identified by 1H and 13C NMR spectroscopic analyses (Supporting Information). d Under 1 atm of air instead of molecular
oxygen. e Unidentified byproducts were formed. f Substrate (0.2 mmol), water (1 mL). g 363 K. h Diphenol (6% select.) was formed. i Diphenol, C-O coupling
products, and unidentified byproducts were formed.
Reaction Mechanism. According to the literature,4-6,13 the
oxidative biaryl coupling can proceed through the following
three mechanisms: (i) the homolytic coupling (A• + B•), (ii)
the heterolytic coupling (A+ + B), and (iii) the radical-anion
coupling (A• + B-). To clarify the mechanism of the present
Ru(OH)x/Al2O3-catalyzed oxidative coupling, we attempted the
competitive reaction of two substrates with different oxidation
potentials (Eox), 1a (Eox ) 1.15 V vs SCE14) and 3-carbo-
methoxy-2-naphthol (11a, 1.55 V14). Because the Eox value of
1a is smaller than that of 11a, the homo-coupling should lead
(13) (a) Sm´rcina, M.; Vyskoˇcil, S.; Ma´ca, B.; Pola´×f0ek, M.; Claxton, T. A.;
Abbott, A. P.; Koˇcovsk’y, P. J. Org. Chem. 1994, 59, 2156. (b) Hovorka,
M.; Sˇcigel, R.; Gunterova´, J.; Tich×c6, M.; Za´vada, J. Tetrahedron 1992,
48, 9503. (c) Hovorka, M.; Za´vada, J. Tetrahedron 1992, 48, 9517.
(14) The data were taken from ref 13a.
9
J. AM. CHEM. SOC. VOL. 127, NO. 18, 2005 6635

A R T I C L E S
Matsushita et al.
fact shows that the present oxidative biaryl coupling reaction
proceeds via the homolytic coupling of two radical species.
The competitive oxidations of structurally similar 2-naphthols
1a-4a under 1 atm of argon were carried out, and the respective
initial rates (consumption of substrates) were expressed by R01a
,
Figure 1. Dependence of the initial rate (R01a) on the amount of Ru(OH)x/
Al2O3 (Wcat). Reaction conditions: 1a (0.20 M), Ru(OH)x/Al2O3 (Ru: ∼6.5
mol %), water (1.5 mL), 373 K, under 1 atm of molecular oxygen. R01a
values were determined by the slopes of the conversion versus time curves
at low conversion (<10%) of 1a. Slope from the inset ) 1.00.
R
02a, R03a, and R04a. The relative reactivity decreased in the
following order: 3a (R03a/R01a ) 1.87) > 4a (R04a/R01a ) 1.23)
> 1a (1.00) > 2a (R02a/R01a ) 0.72). As shown in Figure 2,
the reactivity for 1a-4a increased with an increase in the
HOMO orbital energies. Because the Eox values increase with
a decrease in the HOMO orbital energies of neutral molecules
of the parent molecules such as 1a and 11a,13a the good
correlation between log(R0/R01a) and the HOMO orbital energy
shows that the reaction rates increased with a decrease in Eox
values, supporting the radical-radical homolytic coupling
mechanism. In the competitive oxidation of 1a and 7a with the
same oxidation potential, 1b, 7b, and the cross-coupling product
7c were obtained in 25%, 24%, and 51% selectivities (1:1:2
ratio), respectively, at quantitative conversion of 1a and 7a (eq
4). These selectivities were almost unchanged in the conversion
range of 0-99%. These results also support the present reaction
mechanism.
Table 3. Recycling of the Ru(OH)x/Al2O3 Catalyst for the
Oxidative Coupling of 1aa
recycle no.
conv. of 1ab (%)
yield of 1bc (%)
fresh
>99
>99
>99
>99
>99
>99
>99
>99
98
98
93
97
94
91
96
93
1
2
3
4
5
6
7
a Reaction conditions: 1a (0.3 mmol), Ru(OH)x/Al2O3 catalyst (metal:
5 mol %), water (1.5 mL), 373 K, 4 h, under 1 atm of molecular oxygen.
The reaction rates for the recycle runs were the same as that for the first
run with fresh Ru(OH)x/Al2O3 catalyst. b Conversions were determined by
1H NMR analyses of the crude reaction mixture and based on 1a. c Isolated
yields based on 1a.
to a mixture reflecting the relative reactivity and homo-coupled
1b would predominate. In the heterolytic coupling, more easily
oxidizable 1a is oxidized to form the cationic species, which
undergoes electrophilic attack to more nucleophilic 1a. In this
case, the homo-coupling product 1b would also be a main
product. If the present oxidative coupling in water proceeds via
the heterolytic coupling mechanism, water and hydroxyl groups
on the catalyst can act as nucleophiles, resulting in the formation
of hydroxylated products.15 However, in the present system, no
hydroxylation proceeded. Therefore, we exclude the heterolytic
coupling mechanism. Alternatively, the radical-anion coupling
mechanism would lead to the cross-coupling product (11c)
predominantly because the more stable anion from 11a would
attack the radical formed from more oxidizable 1a.16 With
Ru(OH)x/Al2O3 catalyst, the homo-coupling product of 1b was
obtained as a major product under the conditions in eq 3. This
The reaction of 1a was carried out at 373 K under 1 atm of
argon, and the Ru(OH)x/Al2O3 was recovered under argon. The
ESR spectrum of the recovered catalyst was then measured at
290 K under argon. As shown in Figure 3a, a sharp signal at g
) 2.0044 was observed. The signal is possibly assignable to a
radical species derived from 1a, for example, a naphthoxy
radical. The signal was not observed after the treatment of
Ru(OH)x/Al2O3 without 1a. The Ru(OH)x/Al2O3 catalyst re-
(15) (a) Yamaguchi, K.; Matsushita, M.; Mizuno, N. Angew. Chem., Int. Ed.
2004, 43, 1576. (b) Kamata, K.; Kasai, J.; Yamaguchi, K.; Mizuno, N.
Org. Lett. 2004, 6, 3577.
(16) For the anion-radical coupling mechanism, more easily reducible 11a is
reduced to form the anionic species, while the radical from 1a prefers to
act as an electrophilic radical. Therefore, the radical from 1a preferably
interacts with the anion from 11a rather than that from 1a, giving the cross-
coupling products predominantly. See ref 13a.
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6636 J. AM. CHEM. SOC. VOL. 127, NO. 18, 2005

Coupling of 2-Naphthols and Substituted Phenols in Water
A R T I C L E S
Figure 4. A ESR spectrum of the nitroxide spin adduct obtained by the
treatment of phenyl N-tert-butylnitron with the Ru(OH)x/Al2O3 recovered
after the reaction with 1a under argon. Ru(OH)x/Al2O3 (5 mol %) was
reacted with 1a (0.3 mmol) in water under 1 atm of argon at 373 K. After
4 h, the catalyst was recovered and treated with phenyl N-tert-butylnitron
(0.1 M) in toluene at room temperature for 1 h. After the treatment, the
catalyst was recovered and the ESR spectrum of the filtrate was measured
at 178 K under argon. The dotted line was obtained by the simulation (see
text). Asterisks were assigned to the signals of the Mn marker.
Figure 2. Relative initial rates for the competitive oxidative coupling of
1a-4a as a function of the HOMO orbital energies. Reaction conditions:
1a (0.15 mmol) + 2a, 3a, or 4a (0.15 mmol), Ru(OH)x/Al2O3 (Ru: 5 mol
%), water (1.5 mL), 353 K, under 1 atm of argon. The initial rates were
determined by the slopes of the conversion versus time curves below 5%
conversions of the substrates.
Scheme 1
the following parameters: g ) 2.0067, AN ) 1.54, AH ) 0.11
mT. The signal intensity of Figure 4 corresponded to the 0.2
mol % nitroxide spin adducts with respect to the amount of
ruthenium species loaded. According to the literature,17 this
signal is assignable to the nitroxide spin adduct formed by the
reaction of phenyl N-tert-butylnitron with the radical species.
The ESR results indicate that the radical species is formed on
the surface of Ru(OH)x/Al2O3 (intermediate A in Scheme 1).
The XPS measurements showed that the Ru 3p3/2 and 3d5/2
binding energies of the fresh Ru(OH)x/Al2O3 was detected at
463.5 eV (full width at the half-maximum (fwhm) 4.7 eV) and
281.8 eV (fwhm 2.4 eV), respectively, indicating that the
oxidation state of ruthenium species on Ru(OH)x/Al2O3 is +3.18
The Ru 3p3/2 and 3d5/2 binding energies of the catalyst recovered
after the reaction with 1a under argon were detected at 462.9
eV (fwhm 4.5 eV) and 281.4 eV (fwhm 2.6 eV), respectively,
and were by 0.6 and 0.4 eV lower than those of the fresh one,
respectively. These results suggest that the average oxidation
state of the ruthenium species of the recovered catalyst is lower
Figure 3. ESR spectra of Ru(OH)x/Al2O3 measured at 290 K under argon.
(a) Ru(OH)x/Al2O3 (5 mol %) was treated with 1a (0.3 mmol) in water
under 1 atm of argon at 373 K. After 4 h, the catalyst was recovered and
the spectrum was measured. (b) The recovered catalyst (a) was treated with
phenyl N-tert-butylnitron at room temperature for 1 h. Asterisks were
assigned to the signals of the Mn marker.
covered was successively treated with phenyl N-tert-butylnitron
(0.1 M) in toluene at room temperature for 1 h under 1 atm of
argon. After the treatment, the catalyst was recovered by the
filtration under argon and the ESR spectrum of the catalyst was
again measured. The signal almost disappeared with the
treatment using phenyl N-tert-butylnitron (Figure 3b). The ESR
spectrum of the filtrate was also measured at 178 K under argon.
As shown in Figure 4, a very sharp triplet signal was observed
and the spectrum was well reproduced by the simulation with
(17) Church, D. F. Anal. Chem. 1994, 66, 419.
(18) (a) Mitchell, P. C. H.; Scott, C. E.; Bonnelle, J. P.; Grimblot, J. G. J. Catal.
1987, 107, 482. (b) Nagai, M.; Koizumi, K.; Omi, S. Catal. Today 1997,
35, 393. (c) Murata, S.; Aika, K. J. Catal. 1992, 136, 110. (d) Aika, K.;
Ohya, A.; Ozaki, A.; Inoue, Y.; Yasumori, I. J. Catal. 1985, 92, 305. (e)
Williams, G. P. In CRC Handbook of Chemistry and Physics, 82nd ed.;
Lida, D. R., Ed.; CRC Press: Washington, DC, 2001; Section 10, pp 200-
205.
9
J. AM. CHEM. SOC. VOL. 127, NO. 18, 2005 6637

A R T I C L E S
Matsushita et al.
Figure 5. Relationship between the amounts of 1a consumed and O2 uptake
for the oxidative coupling of 1a with molecular oxygen by Ru(OH)x/Al2O3.
Reaction conditions: 1a (0.6-1.0 mmol), Ru(OH)x/Al2O3 (Ru: 5 mol %),
water (3 mL), 373 K, under 1 atm of molecular oxygen. Slope (O2
uptake/1a consumed) ) 0.27.
Figure 7. Dependence of the initial rate (R01a) on the concentration of 1a
([1a]). Reaction conditions: 1a (0.12-0.29 M), Ru(OH)x/Al2O3 (Ru: 5
mol %), water (1.5 mL), 373 K, PO ) 1.0 atm. R01a values were determined
2
by the slopes of the conversion versus time curves at low conversion (<10%)
of 1a. Slope from the inset ) 0.11.
amount of 1b formed was approximately one-half the amount
of ruthenium loaded.
On the basis of the above experimental results, we here
propose the possible reaction mechanism for the present oxi-
dative coupling reaction (Scheme 1). This reaction can be
divided into two steps. The reaction proceeds with a typical
radical mechanism via the one-electron transfer from a substrate
to Ru(OH)x/Al2O3 to give the reduced form and a radical species
(intermediate A) (step 1 in Scheme 1). Two radical species are
then coupled to give the corresponding biaryl coupling product,
and the reduced catalyst reacts with molecular oxygen to
regenerate the oxidized form with the formation of water (step
2 in Scheme 1).
As mentioned, Al2O3 showed no catalytic activity for the
present oxidative coupling reaction. Therefore, the one-electron
oxidation probably occurs on the ruthenium species. Among
the ruthenium species supported on various metal oxides,
Ru(OH)x/Al2O3 was found to be the most active catalyst, and
the order of catalytic activity decreased as follows: Ru(OH)x/
Al2O3 > Ru(OH)x/TiO2 > Ru(OH)x/SiO2 (entries 1, 7, and 8 in
Table 1). This order is in good accord with that of their
basicities,19 indicating that the basic sites of the catalysts also
play a role in the present oxidative coupling reaction. The proton
abstraction from the substrate or cation radical species would
occur during the reaction (step 1 in Scheme 1), which is
promoted by the basic sites of the catalysts.
The reaction rate for the oxidative coupling of 1a was almost
independent of the concentration of 1a (0.12-0.29 M, Figure
7), while the first-order dependence of the reaction rate on the
partial pressure of molecular oxygen (0.2-1.0 atm, Figure 8)
was observed. A kinetic isotope effect (kH/kD) of 1.0 ((0.1)
was observed for the oxidative coupling of 2-naphthol (1a) and
2-naphthol-1,3,4,5,6,7,8-d7 (7a) at 373 K, showing that the C-H
bond cleavage is not included in the rate-limiting step. Kinetic
Figure 6. Relationship between the amounts of 1a consumed and H2O
formed for the oxidative coupling of 1a with molecular oxygen by
Ru(OH)x/Al2O3. Prior to the reaction, Ru(OH)x/Al2O3 was treated with
D2O at 373 K for 1 h. After the treatment, H2O from Ru(OH)x/Al2O3
can be negligible. The reaction was carried out in D2O, and the amount
of H2O (and HDO) formation was determined by 1H NMR. Two moles
of HDO were regarded as one mole of H2O. Reaction conditions: 1a
(0.06-0.45 mmol), Ru(OH)x/Al2O3 (Ru: 15 µmol), D2O (1.5 mL), 363 K,
under 1 atm of molecular oxygen. Slope (water formed/1a consumed) )
0.51.
than that (+3) on the fresh Ru(OH)x/Al2O3. Because the binding
energies of Ru 3p3/2 and 3d5/2 of Ru(0) have been reported to
be 461.5 and 280.0 eV, respectively,18 the oxidation state of
ruthenium species of the recovered catalyst is not zero. The
lower oxidation state of the ruthenium species of the recovered
catalyst is possibly assigned to Ru2+, which is formed by the
one-electron transfer from 1a.
The measurements of O2 uptake and H2O formation during
the oxidative coupling of 1a were carried out. As shown in
Figures 5 and 6, the amounts of O2 uptake and H2O formation
were almost one-quarter and one-half of the amount of 1a
consumed, respectively. Under the anaerobic conditions, the
(19) The basicities of the catalysts (the equilibrium amount of acetic acid
adsorption, see Experimental Section) decreased in the order of Ru(OH)x/
Al2O3 (0.71 mmol g-1) > Ru(OH)x/TiO2 (0.46 mmol g-1) > Ru(OH)x/
SiO2 (0.23 mmol g-1).
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Coupling of 2-Naphthols and Substituted Phenols in Water
A R T I C L E S
radiation (hν ) 1486.6 eV). The X-ray anode was run at 200 W, and
the voltage was kept at 10 kV. The pass energy was fixed at 20.0 eV
to ensure sufficient resolution to accurately determine peak positions.
The binding energies were calibrated by using Al 2p signal at 74.8
eV. 2-Naphthols and substituted phenols were commercially obtained
from Tokyo Kasei, Aldrich, and Fluka (reagent grade) and were used
without further purification. 2-Naphthol-1,3,4,5,6,7,8-d7 (99.5% purity,
97.5 at. % D) was obtained from ISOTEC and used without further
purification. Alumina (KHS-24, BET surface area: 160 m2 g-1) was
supplied from Sumitomo Chemical Co., Ltd. All products (1b-10b)
1
have been identified by comparison of their H and 13C NMR signals
with the literature data (see Supporting Information).4-6
Preparation of Ru(OH)x/Al2O3 Catalyst. The Ru(OH)x/Al2O3
catalyst was prepared according to the procedure reported previously.20
The powder Al2O3 (2.0 g) calcined at 823 K for 3 h was vigorously
stirred with 60 mL of an aqueous solution of RuCl3 (8.3 mM) at room
temperature. After 15 min, the pH of the solution was slowly adjusted
to 13.2 by addition of an aqueous solution of NaOH (1.0 M), and the
resulting slurry was stirred for 24 h. The solid was then filtered off,
washed with a large amount of water, and dried in vacuo to afford
2.1 g of Ru(OH)x/Al2O3 as a dark green powder. The content of
ruthenium was 2.0-2.1 wt %. The content of ruthenium is con-
trollable by changing the concentration of the starting ruthenium
solution. The XRD pattern of Ru(OH)x/Al2O3 was the same as that of
the parent Al2O3 support, and no signal due to Ru metal (clusters) and
RuO2 was observed. Particles of Ru metal (clusters) and RuO2 were
not detected by TEM micrograph. XPS shows that binding energies of
Ru 3d5/2 and Ru 3p3/2 of Ru(OH)x/Al2O3 were detected at 281.8 eV
(full width at the half-maximum, fwhm 2.4 eV) and 463.5 eV (fwhm
4.7 eV), respectively, showing that the oxidation state of ruthenium
species on Ru(OH)x/Al2O3 is +3.18 The IR spectrum showed a very
broad ν(OH) band in the range of 3000-3700 cm-1. These facts suggest
that ruthenium(III) hydroxide is highly dispersed on Al2O3. Other
transition metal hydroxide supported on Al2O3 catalysts, M(OH)x/Al2O3
(M ) Pd, Rh, Pt, V, Fe, and Cu) and ruthenium hydroxide on SiO2
Figure 8. Dependence of the initial rate (R01a) on the partial pressure of
molecular oxygen (PO ). Reaction conditions: 1a (0.20 M), Ru(OH)x/Al2O3
2
(Ru: 5 mol %), water (1.5 mL), 373 K, P ) 0.2-1.0 atm. R01a values
O2
were determined by the slopes of the conversion versus time curves at low
conversion (<10%) of 1a. Slope from the inset ) 1.01.
data and kinetic isotope effect show that the radical formation
smoothly proceeds (step 1 in Scheme 1) and that the reoxidation
is the rate-limiting step (step 2 in Scheme 1). The reaction rate
of stoichiometric oxidation of 1a (under 1 atm of argon at 353
K) was 4.3 mM min-1 and faster than that of catalytic oxidation
under the same reaction conditions, consistent with the idea that
step 2 is the rate-limiting step.
Conclusion
and TiO2 were prepared by
Ru(OH)x/Al2O3.
a procedure similar to that for
We can develop the efficient aerobic oxidative biaryl coupling
system with Ru(OH)x/Al2O3 catalyst. From the standpoint of
“green chemistry and technology”, the present system has the
following significant advantages: (i) the use of molecular
oxygen as a sole oxidant, (ii) the use of water as a solvent, (iii)
the applicability to various 2-naphthols and substituted phenols,
(iv) the high turnover number, (v) simple workup procedures,
for example, catalyst/product separation, (vi) reusability of
Ru(OH)x/Al2O3, and (vii) the use of easily prepared and inex-
pensive Ru(OH)x/Al2O3 catalyst. The mechanistic and kinetic
investigations show that the Ru(OH)x/Al2O3-catalyzed aerobic
oxidative biaryl coupling reaction proceeds through the radical-
radical coupling mechanism. The catalyst acts as a one-electron
oxidant to form the reduced form and a radical species. The
reduced catalyst is reoxidized by molecular oxygen. The latter
step is included in the rate-limiting step.
A Typical Procedure for the Oxidative Biaryl Coupling. Into a
Pyrex-glass vial were successively placed Ru(OH)x/Al2O3 (Ru: 5 mol
%), 2-naphthol (1.08 g, 7.5 mmol), and water (37.5 mL). A Teflon-
coated magnetic stir bar was added, and the reaction mixture was
vigorously stirred (800 rpm) at 373 K under 1 atm of molecular oxygen
(equipped with a balloon). After 4 h, the catalyst and the product were
separated by filtration, and the solid mixture was washed with a large
amount of acetonitrile. Acetonitrile solution of the crude product was
then evaporated in vacuo, and the crude product was purified by
recrystallization from benzene to give analytically pure pale-yellow
crystals of BINOL (1.04 g, 97% isolated yield). The separated
Ru(OH)x/Al2O3 was washed with an aqueous solution of NaOH (pH
) 13.2) and water, and then dried in vacuo before recycling.
Measurements of Acetic Acid Adsorption. The measurements of
the amount of acetic acid adsorption were carried out in water. A typical
procedure was as follows: The finely ground catalyst (Ru(OH)x/Al2O3,
Ru(OH)x/TiO2, or Ru(OH)x/SiO2, 0.10 g) was placed in a Pylex-glass
vial with a Teflon-coated magnetic stir bar, and 1.0 mL of D2O solution
of acetic acid (0.36 M) was added. The operations were carried out
under 1 atm of argon at 300 K. The amounts of acetic acid adsorbed
on the catalyst were evaluated by the decreases in the acetic acid
concentrations in the liquid phase, which were determined by GC and
1H NMR analyses. The equilibrium of acetic acid adsorption on the
catalyst was reached within ca. 10 min in each case. The equilibrium
amount of acetic acid adsorption was regarded as a measure of the
basicity of the catalyst.
Experimental Section
General. NMR spectra were recorded on a JEOL JNM-EX-270
spectrometer. 1H and 13C NMR spectra were measured at 270 and 67.5
MHz, respectively, in CDCl3 or CD3CN with TMS as an internal
standard. GC analyses were performed on a Shimadzu GC-14B with a
flame ionization detector equipped with a TC-1 capillary column
(internal diameter ) 0.25 mm, length ) 30 m). Mass spectra were
recorded on a Shimadzu GCMS-QP2010 at an ionization voltage of
70 eV equipped with a DB-WAX capillary column (internal diameter
) 0.25 mm, length ) 30 m). ESR measurements at 290 K (X-band)
were performed with a JEOL JES-RE-1X spectrometer. XPS measure-
ments were carried out on JEOL JPS-90 using monochromated Al KR
(20) (a) Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2002, 41, 4538.
(b) Yamaguchi, K.; Mizuno, N. Angew. Chem., Int. Ed. 2003, 42, 1480.
(c) Yamaguchi, K.; Mizuno, N. Chem.-Eur. J. 2003, 9, 4353.
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A R T I C L E S
Matsushita et al.
The Calculation of HOMO Orbital Energies. The calculations
were carried out with the program package Gaussian 98.21 The HOMO
orbital energies of 1a-4a were calculated at the B3LYP level with
6-311G(d, p) (C, H, O) and the triple-ú level basis sets with the efficient
core potential proposed by Stevens, Basch, and Krauss (Br).22 The order
of HOMO orbital energies was 3a (-5.47 eV) > 4a (-5.57 eV) > 1a
(-5.78 eV) > 2a (-5.98 eV).
Acknowledgment. We thank Mr. Y. Nakagawa (The Uni-
versity of Tokyo) for the help of the calculation of the HOMO
orbital energies of 1a-4a. This work was supported by the Core
Research for Evolutional Science and Technology (CREST)
program of the Japan Science and Technology Corp. (JST) and
a Grant-in-Aid for Scientific Research from the Ministry of
Education, Culture, Science, Sports, and Technology of Japan.
(21) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M.
A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann,
R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin,
K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi,
R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.;
Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.;
Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz,
J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.;
Komaromi, I.; Gomperts, R.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham,
M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.;
Johnson, B.; Chen, W.; Wong, M. W.; Andres, J. L.; Gonzalez, C.; Head-
Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98, revision A.9;
Gaussian, Inc.: Pittsburgh, PA, 1998.
Supporting Information Available: Detailed presentation of
physical data of 1b-10b, Figures S1-S3. This material is
available free of charge via the Internet at http://pubs.acs.org.
JA050436K
(22) (a) Stevens, W.; Basch, H.; Krauss, J. J. Chem. Phys. 1984, 81, 6026. (b)
Stevens, W. J.; Krauss, M.; Basch, H.; Jasien, P. G. Can. J. Chem. 1992,
70, 612. (c) Cundari, T. R.; Stevens, W. J. J. Chem. Phys. 1993, 98, 5555.
9
6640 J. AM. CHEM. SOC. VOL. 127, NO. 18, 2005