Asymmetric Activation/Deactivation of Racemic Ru Catalysts for Highly Enantioselective Hydrogenation Irrespective of Ketonic Substrates: Molecular Design of Dimethylbinaphthylamine for Enantiomeric Catalysts Discrimination

Article abstract of DOI:10.1002/adsc.200390019

Asymmetric activation and deactivation of racemic catalysts are two extremes in asymmetric catalysis. In a combination of these two protocols, higher enantioselectivity can be achieved by maximizing the difference in catalytic activity between the enantiomers of racemic catalysts through selective activation and deactivation of enantiomeric catalysts. 3,3′-Dimethyl-2,2′-diamino-1,1′-binaphthyl (DM-DABN) is thus designed as a chiral poison (deactivator) for complete enantiomer resolution of racemic BINAP-Ru(II) catalysts. The catalyst system of DM-DABN, 1,2-diphenylethylenediamine (DPEN), and racemic BINAP-Ru(II) led to great success in highly enantioselective hydrogenation irrespective of the ketonic substrates. The lower catalytic activity of the BINAP-Ru(II)/DM-DABN complex stems from the electron delocalization from the Ru center to the diamine moiety in contrast to the BINAP-Ru(II)/DPEN complex where the highest electron densities are localized on the Ru-N region. The present 'asymmetric activation/deactivation protocol' can provide a guiding principle for the rational design of a molecule for enantiomeric discrimination between racemic catalysts.

Full text of DOI:10.1002/adsc.200390019

FULL PAPERS
Asymmetric Activation/Deactivation of Racemic Ru Catalysts for
Highly Enantioselective Hydrogenation Irrespective of Ketonic
Substrates: Molecular Design of Dimethylbinaphthylamine for
Enantiomeric Catalysts Discrimination
Koichi Mikami,* Toshinobu Korenaga, Yukinori Yusa, Masahiro Yamanaka
Department of Applied Chemistry, Tokyo Institute of Technology, Tokyo 152-8552, Japan
Fax: ( ‡ 81)-3-5734-2776, e-mail: kmikami@o.cc.titech.ac.jp
Received: July 15, 2002; Accepted: August 21, 2002
Dedicated to Prof. Noyori in honor of being awarded the Nobel Prize in Chemistry and for his excellent
contribution to catalytic asymmetric hydrogenation.
Abstract: Asymmetric activation and deactivation of irrespective of the ketonic substrates. The lower
racemic catalysts are two extremes in asymmetric catalytic activity of the BINAP-Ru(II)/DM-DABN
catalysis. In a combination of these two protocols, complex stems from the electron delocalization from
higher enantioselectivity can be achieved by max- the Ru center to the diamine moiety in contrast to the
imizing the difference in catalytic activity between the BINAP-Ru(II)/DPEN complex where the highest
enantiomers of racemic catalysts through selective electron densities are localized on the Ru-N region.
activation and deactivation of enantiomeric catalysts. The present −asymmetric activation/deactivation pro-
3
,3'-Dimethyl-2,2'-diamino-1,1'-binaphthyl
(DM- tocol× can provide a guiding principle for the rational
DABN) is thus designed as a chiral poison (deacti- design of a molecule for enantiomeric discrimination
vator) for complete enantiomer resolution of racemic between racemic catalysts.
BINAP-Ru(II)
tem of DM-DABN, 1,2-diphenylethylenediamine Keywords: asymmetric activation; asymmetric deacti-
DPEN), and racemic BINAP-Ru(II) led to great vation; 3,3'-dimethyl-2,2'-diamino-1,1'-binaphthyl; hy-
success in highly enantioselective hydrogenation drogenation; ruthenium(II) complex
catalysts.
The
catalyst
sys-
(
Introduction
tion process is particularly useful in racemic catalysis, as
it can allow the selective activation of one enantiomer of
[
3]
Asymmetric catalysis of organic reactions is an impor- racemic catalysts.
tant subject in modern science and technology. Asym-
[
1]
[1]
In asymmetric catalytic reactions, non-racemic cat-
metric catalysis enjoys this stature because it affords alysts thus prepared via chiral ligand exchange can
large amounts of enantioenriched products, while generate non-racemic products with or without a non-
producing only a small amount of waste material, linear relationship in enantiomeric excesses between
[
4]
through the action of a chiral catalyst. Highly promising catalysts and products. However, racemic catalysts
candidates for such asymmetric catalysts are metal inherently give only a racemic mixture of chiral prod-
complexes bearing chiral but usually non-racemic ucts. Recently, a strategy was reported whereby racemic
ligands. In homogeneous asymmetric catalysis, Sharp- catalysts could be used in catalytic asymmetric synthesis
[
5,6]
less et al. have emphasized the importance of ™chiral in different manners (Figure 2a and b).
An enan-
[
2]
ligand acceleration∫. Here, an asymmetric catalyst is
formed from a −pre-catalyst× via ligand exchange with an
added chiral ligand. The term −asymmetric activation×
may be proposed for this process in analogy to the
activation process of an achiral reagent or catalyst,
which provides an activated but achiral one (shown as
"
Asymmetric Activation"
Chiral
Active
Chiral
Act*
Act*
Highly activated
"
Chiral ligand-Acceleration"
™
Achiral-Act∫ in Figure 1). The asymmetric catalysts
"
Activation"
Act
Achiral
Achiral
Act
thus prepared can be further transformed into highly
activated catalysts by association with chiral activators
Less active
Active
(
shown as ™Act*∫ in Figure 1). This asymmetric activa- Figure 1. The concept of asymmetric activation.
246
¹ 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 1615-4150/03/3451+2-246 ± 254 $ 17.50-.50/0
Adv. Synth. Catal. 2003, 345, No. 1+2

Asymmetric Activation/Deactivation of Racemic Ru Catalysts
FULL PAPERS
a) Asymmetric Deactivation
Favored
Disfavored
Substrate
S-Cat*
S-Cat*
R-Cat*
Product
R-Deact*
k
C lM e
Cl
Me
Ar
Ar
Ru
P
N
Ru
P
P
N
R-Cat*
R-Deact*
P
N
N
H2
H
2
Ar
Cl
Racemic
Ar
Cl
b) Asymmetric Activation
Substrate
(
S)-BINAP-Ru(II)Cl2/(S)-DM-DABN (R)-BINAP-Ru(II)Cl2/(S)-DM-DABN
S-Cat*
S-Act*
S-Cat*
R-Cat*
S-Act*
Product
Product
kact
Figure 3. Rational design of a chiral deactivator for enan-
tiomer discrimination of racemic BINAPs-Ru(II) complexes.
Substrate
R-Cat*
k
Racemic
c) Asymmetric Activation/Deactivation
enantiomer discrimination[9] of racemic BINAP-Ru(II)
Substrate
catalysts in highly enantioselective hydrogenation. The
combination of DM-DABN deactivating one enantiom-
er of a racemic BINAP-Ru(II) complex and DPEN
activating the other enantiomer achieved highly enan-
tioselective hydrogenation regardless of ketonic sub-
S-Cat*
S-Cat*
R-Cat*
S-Act*
Product
S-Act*
kact
R-Deact*
R-Deact*
R-Cat*
Racemic
Figure 2. Strategies of asymmetric catalysis using a racemic
catalyst.
[10]
strates. Furthermore, we theoretically illustrated the
reasons why DM-DABN and DPEN can act as a chiral
deactivator and activator, respectively.
tiomer-selective deactivation strategy for racemic cat-
alysis has been reported, wherein a chiral poison (shown Results and Discussions
as ™Deact*∫ in Figure 2a) selectively complexes with
one enantiomer of a racemic catalyst and the non- Complexation of RuCl (binaps)(dmf)
[
11]
[1a: BINAP-
2
n
complexed enantiomer performs the asymmetric catal- Ru(II)Cl2, 1b: XylBINAP-Ru(II)Cl2] with chiral
ysis (Figure 2a). However, exclusive complexation of diamines has been reported to give stable trans-
[
12b]
one enantiomer of racemic catalysts with chiral poisons RuCl (binaps)(diamine) complexes
which catalyze
2
[
5]
found to be a difficult task. In fact, the chiral poisons hydrogenation of simple ketones in the presence of
have to be used in excess amounts to the catalyst KOH. However, DPEN (3) as a chiral activator forms
[
7]
[12]
enantiomers because of an inherently ineffective dis- both diastereomeric complexes with racemic BINAPs-
[
6d]
crimination. In contrast, we have recently reported that Ru(II)Cl complex (1) in an equal amount.
It is
2
a chiral activator (shown as ™Act*∫ in Figure 2b) both inevitable that the degree of asymmetric induction
selectively complexes and activates rather than deacti- highly depends on substrates by the non-selective
vates one enantiomer of a racemic catalyst (Figure 2b). complexation of DPEN with racemic BINAPs-
The enantioselectivity can be higher than that achieved Ru(II)Cl . Therefore, it is essential to design a poisonous
2
with enantiopure catalyst due to a higher efficiency of diamine to discriminate and maximize catalytic activ-
the catalyst (kact ꢀ k). However, when a chiral activator ities between the two enantiomers of racemic BINAPs-
non-selectively complexes with a racemic catalyst, the Ru(II)Cl (1). (S)-DM-DABN (2) was thus designed as a
2
difference in catalytic activities between the activated deactivator by formation of the RuCl (binaps)(dm-
2
diastereomers (e.g., ™S-Cat*/S-Act*∫ and ™R-Cat*/S- x. As shown in Figure 3, DM-DABN has been rationally
[
6d]
Act*∫) critically depends on the substrates. Thus, the designed as a deactivator according to the enantiomer-
asymmetric activation protocol in combination with the selective steric repulsion of (S)-DM-DABN with the
asymmetric deactivation protocol, namely −asymmetric (R)-BINAPs-Ru(II)Cl and the selective complexation
2
activation/deactivation×, can achieve higher enantiose- of (S)-DM-DABN with (S)-BINAPs-RuCl2.
lectivity regardless of the substrates by maximizing the
difference in catalytic activity between enantiomeric pling reaction of 3-methyl-2-aminonaphthalene, which
catalysts in the racemic mixture (Figure 2c). was derived from commercially available 3-amino-2-
Racemic DM-DABN (2) was synthesized by a cou-
In the enantioselective hydrogenation of ketones naphthoic acid (Figure 4), since methylation of com-
using the racemic TolBINAP-Ru(II) complex and 1,2- mercially available DABN through ortho-functionali-
[
13]
diphenylethylenediamine (DPEN), the suitable mo- zation of aromatic amines
was unsuccessful. The
lecular association of catalysts as well as the degree of resolution of DM-DABN was first attempted by using
asymmetric induction highly depends on the ketonic chiral resolving acids but failed and was finally estab-
substrates.[ We report herein a molecular design lished by employing (S)-BINAP-RuCl (1a) (0.7 molar
6d]
2
of 3,3'-dimethyl-2,2'-diamino-1,1'-binaphthyl (DM- amount) under an argon atmosphere (Figure 4). (R)-2
[8]
[14]
DABN) as a chiral poison (deactivator) for complete with 98% ee ( > 99% ee after one recrystallization)
Adv. Synth. Catal. 2003, 345, 246 ± 254
247

FULL PAPERS
Koichi Mikami et al.
Me
Table 1. Hydrogenation of methyl 3-oxobutanoate (4) by
racemic BINAP-Ru(II) catalysts through chiral poisoning.
CO2H
NH2
Red-Al®
Me
NH2
K3Fe(CN)6
xylene
reflux
DME
r.t.
NH2
NH2
Me
75%
78%
(±)-DM-DABN (2)
1
) RuCl2[(S)-binap](dmf)n (1a)
(
S)-1a/ (S)-2
(47%, based on (±)-2)
00% de
R)-2
49% based on (±)-2)
8% ee
99% ee [α]25: +101.5
(0.7 eq.)
CH2Cl2
(±)-2
1
2
) column chromatgraphy
neutral silica gel
(
(
9
one recryst.
>
D
(c 0.50, CHCl3)
Figure 4. Synthesis of racemic DM-DABN 2 and enantiomer
resolution with enantiopure BINAP-Ru(II) 1.
Complete enantiomer recognition of the racemic
BINAPs-RuCl2 (1) was found to be effective for
[16]
enantioselective hydrogenation of b-keto esters using
enantiopure DM-DABN (2) as the chiral poison (Ta-
ble 1). Hydrogenation of methyl 3-oxobutanoate (4) by
(
3
Æ )-XylBINAP-RuCl (1b) with (S)-2 gave (R)-methyl
2
-hydroxybutanoate (5) with 92.4% ee in quantitative
[
17]
yield. XylBINAP
was thus employed as a more
efficient chiral ligand to give the more stable (S)-
XylBINAP-RuCl /(S)-DM-DABN complex [(S)-1b/
2
[
18]
(
(
S)-2]. A 0.5molar amount of ( S)-2 was added to
Æ )-1b in CH Cl and then stirred for 1 h. Methyl 3-
2
2
Figure 5. X-ray analysis of RuCl [(S)-binap][(S)-dmdabn].
2
oxobutanoate (4), MeOH and H were subsequently
introduced after removal of CH Cl . The reaction
mixture was stirred at room temperature for 16 h to
2
Selective bond lengths [ä] and bond angles [8]: Ru À Cl1
2
2
.418(4), Ru À Cl2 2.401(3), Ru À P1 2.273(3), Ru À P2
.270(4), Ru À N1 2.228(9), Ru À N2 2.263(10); Cl1 ± Ru À
2
2
Cl2 165.34(11), P1 ± Ru À P2 89.80(12), N1 ± Ru À N2 80.1(4). give (R)-methyl 3-hydroxybutanoate (5) with 99.3% ee
in quantitative yield (Run 1). The enantioselectivity
(
99.3% ee) thus obtained was equally high to that
and pure (S)-BINAP-Ru(II)/(S)-DM-DABN [(S)-1a/ (99.9% ee) obtained in our hands by the enantiopure
S)-2] were separated almost quantitatively after puri- (R)-1b catalyst (Run 1 vs. 2). Thus, asymmetric deacti-
(
fication by neutral silica gel column chromatography. vation of racemic catalysts (chiral poisoning strategy) is
The slight decrease in the enantiomer excess of (R)-2 is successful in the case of highly selective complexation
due to decomposition of (S)-1a/(S)-2 through silica gel and deactivation with a well-designed chiral poison.
column, since (R)-2 with 53% ee was obtained in the
By using two different types of chiral diamines as an
earliest fraction after outflow of (S)-1a/(S)-2. This result activator and deactivator, the racemic XylBINAP-
clearly shows that DM-DABN (2) can completely RuCl (1b) catalyst achieves higher enantioselectivities
2
discriminate the enantiomers of the racemic BINAP- than those attained by simple activation. Addition of
Ru(II)Cl complex (1a). The (S)/(S) configuration of a first DM-DABN (2) and then DPEN (3) should lead to
2
BINAP-Ru(II)/DM-DABN diastereomer was con- the two different ruthenium dichloro complexes and
firmed by X-ray analysis of the single-crystal obtained further to the mono- or dihydrido Ru species under
from dichloromethane/ether/hexane (Figure 5). In turn, hydrogenation conditions. The DM-DABN complex is
addition of racemic BINAP-RuCl complex (1a) to the far less catalytically active than the DPEN complex for
2
0
.5molar amount of ( S)-DM-DABN (2), obtained by hydrogenation.
the resolution described above, resulted in selective
formation of the single diastereomeric (S)-1a/(S)-2 2 was stirred for 30 min at room temperature in
complex. While no complex was formed between (R)- dichloromethane. A 0.5molar amount of ( S,S)-3
The mixture of ( Æ )-1b and a 0.5molar amount of ( R)-
1a and (S)-2, as was confirmed in a separate run, the in 2-propanol was added to give selectively the
uncomplexed (R)-1a enantiomer gave an activated two complexes RuCl [(R)-xylbinap][(R)-dmdabn] and
2
catalyst upon addition of enantiopure (S,S)- or (R,R)- RuCl [(S)-xylbinap][(S,S)-dpen] complexes. Enantiose-
2
DPEN.[
15]
lective hydrogenation was performed by the mixture
248
Adv. Synth. Catal. 2003, 345, 246 ± 254

Asymmetric Activation/Deactivation of Racemic Ru Catalysts
FULL PAPERS
Table 2. Hydrogenation of ketones by the racemic [RuCl2
O
H
(
xylbinap)] complex through asymmetric activation/deacti-
O
vation.
H
H
H
H
2
P
P
N
P
P
N
RuCl2[(±)-xylbinap](dmf)n (1b) (0.4 mol %)
Ru
H
Ru
H
(
R)-DM-DABN (2) (0.22 mol %)
N
N
O
OH
*
H
2
DPEN (3) (0.2 mol %)
H2
11
12
Ar
Ar
H2 (8 atm), KOH (0.8 mol %)
r.t., 4 - 6 h, i-PrOH
ketones
O
*
H
O
H
O
O
O
8: R = H
H
H
9a: R = o-Me
9b: R = m-Me
9c: R = p-Me
H
N
H
N
P
P
P
P
R
Ru
H
Ru
H
6
7
10
N
H2
N
H2
H2
(R)-2[a]
Yield[a] [%] ee[b] [%]
14
13
Ketones
3
Scheme 1. The catalytic mechanism of asymmetric hydro-
genation of ketones.
6
6
7
7
8
8
+
-
(S,S)
(S,S)
(S,S)
(S,S)
(S,S)
(S,S)
(S,S)
(S,S)
(S,S)
(S,S)
(S,S)
(S,S)
(R,R)
(R,R)
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
>99
96 (R)
80 (R)
+
-
91 (R)
45 (R)
95 (R)
70 (R)
95 (R)
82 (R)
95 (R)
60 (R)
93 (R)
60 (R)
92 (R)
84 (R)
+
-
9a
9a
9b
9b
9c
9c
+
-
+
-
+
-
The catalytic mechanism of hydrogenation of ketones
has been proposed in the closely related Ru(II)-
[12a,20]
catalyzed transfer hydrogenation (Scheme 1).
In
terms of the reduction process of ketones, which has
been studied in detail, the reactivity of the coordina-
tively saturated complex 11 originates from the dipolar
[
[
b]
b]
1
1
0
0
+
-
dÀ
d‡
dÀ
d‡
H -Ru -N -H arrangement fitting well with the
carbonyl dipole and stabilizing the pericyclic transition
state 12. On the other hand, less attention has been
devoted to the hydrogen-splitting step although the
reaction rate is highly sensitive to the pressure of
hydrogen and becomes rapid under high pressure. The
dependency of the rate on the hydrogen pressure
[
[
a]
b]
"
+" denotes the presence of (R)-2.
Racemic [RuCl2(tolbinap)] was used;
TolBINAP = 2,2'-bis(di-4-tolylphosphanyl)-1,1'-binaphthyl.
after the addition of KOH and different types of ketones indicates the cleavage of hydrogen (13 ! 14) would be
(
6 ±10) to the mixture. The efficiency of this asymmetric the turnover-limiting step. From views of the difference
activation/deactivation protocol was reflected, regard- of the catalytic activity between the ruthenium com-
less of the ketonic substrates, in the high enantioselec- plexes bearing the two different types of diamines, much
tivity in the asymmetric hydrogenation (Table 2). All attention should be devoted to the cleavage of hydrogen
ketones employed can be hydrogenated at room tem- forming the active ruthenium hydride 11.
perature with enantiomeric excesses higher than 90% ee
Thus, the present theoretical study focused on the
in the same quantitative yield. The asymmetric activa- hydrogen-splitting step of amide complex 13. The
tion/deactivation protocol achieved a higher level of hydrogen molecule coordinates on the 16-electron Ru
enantioselectivity than those obtained using the ( Æ )- center of the amide complex 13 to form the initial
XylBINAP-RuCl /(S,S)-DPEN complexes at the same coordination complex 15 followed by the cleavage of
2
temperature and pressure. A superiority of the asym- hydrogen via transition state 14 (Table 3 and Figure 6).
metric activation/deactivation protocol is significantly The amide complexes 13a and 13b as the models of (S)-
shown in the case of 2-naphthyl methyl ketone (7), BINAP-Ru(II)/(S,S)-DPEN and (R)-BINAP-Ru(II)/
where the enantioselectivity of (R)-1-(2-naphthyl)- (R)-DM-DABN were stabilized by 3.2 and 1.7 kcal/
ethanol using (R)-2 is increased up to 91% ee in contrast mol, respectively, through coordination of hydrogen
to only 45% ee without (R)-2. 2,2,4,4-Trimethyl-2- molecule on the Ru center. The activation energy of the
cyclohexenone (10) was also hydrogenated in high hydrogen-cleavage in the series a ( ‡ 11.2 kcal/mol;
enantioselectivity (92% ee) by changing the chirality DPEN model) is 4.0 kcal/mol smaller than that in the
of DPEN from S to R.
series b ( ‡ 15.2 kcal/mol; DM-DABN model). This
To clarify the reasons why DM-DABN and DPEN can activation energy difference clearly indicates that the
act as a deactivator and activator, respectively, we amide complex bearing DPEN preferentially forms the
carried out a density functional calculation for the reactive ruthenium hydride species in comparison with
hydrogen-splitting step of amide complex 13 in a that bearing DM-DABN. What is the origin of the
heterolytic manner via transition state 14 (Scheme 1). 4.0 kcal/mol difference between the series a and b? As
[
19]
Adv. Synth. Catal. 2003, 345, 246 ± 254
249

FULL PAPERS
Koichi Mikami et al.
1
Table 3. The stabilization energies (DE ) through coordina- These structural features illustrate that 14a is an earlier
2
tion of hydrogen molecule and the activation energies (DE )
transition state in comparison with 14b and give a good
of hydrogen-cleavage.
account for the larger activation energy of the hydrogen-
cleavage of 13b.
Furthermore, examination of the natural population
for the Ru, phosphorus, and nitrogen atoms revealed an
interesting tendency of charge distribution in the series a
and b (Table 4). The negative charge on the Ru center in
the amide complex 13a ( À 0.28) is larger than that in 13b
(
À 0.25), while the negative charges become the same
value in the transition states of 14a and 14b ( À 0.37).
1
2
Two nitrogen atoms N and N in the series a are more
negative than those in the series b, while the diamine
moieties in the series a are less negatively charged than
in the series b. The tendency of the nitrogen charge is not
changed through the cleavage of the hydrogen molecule.
The charge distributions of the phosphorus atoms are
comparable in both series a and b. It is noted that the
electron population is highly localized on the ruthenium
shown in Figure 6, the series a and b show almost the and nitrogen atoms in the series a and is delocalized
same structural feature in the amide complex 13, where from the ruthenium to the diamine region in the series b
2
the Ru-P coordination (a: 2.31 ä, b: 2.30 ä) is weaker due to electron-withdrawing aryl groups attached to the
1
than Ru-P coordination (a: 2.26 ä, b: 2.27 ä) due to the nitrogen atoms. Thus, the lower activity of the amide
1
trans influence of the dipolar Ru-N bond (a: 2.01 ä, b: complex bearing DM-DABN for hydrogen-splitting
2
.09 ä). In the transition structures of 14a and 14b, the originates from electron delocalization from the ruthe-
1
cleaving Ru-N and H-H bonds in 14b are 0.1 ä and nium to the diamine region. In the amide complex
1
0.04 ä longer than in 14b and the forming Ru-H and N - bearing DPEN, the highest energy electrons localized
1
H bonds in 14a are 0.04 ä and 0.06 ä shorter than in 14b. on the Ru-N region readily donate to the H-H s*-
Figure 6. 3D structures of the amide complexes 13a and 13b (i and ii) and the hydrogen-splitting transition states 14a and 14b
(
iii and iv) at the B3LYP/631SDD level. Bond lengths are shown in ä.
250
Adv. Synth. Catal. 2003, 345, 246 ± 254

Asymmetric Activation/Deactivation of Racemic Ru Catalysts
FULL PAPERS
Figure 7. The highest occupied Kohn±Sham orbitals of 13a and 13b.
Experimental Section
Table 4. The natural population analysis of the amide com-
plex 13 and the transition state 15 at the B3LYP/631SDD
level.
General
1
13
H NMR and C NMR were measured on aVarian Gemini 300
3
1
(
5
300 MHz) spectrometer and P NMR was measured on aGX-
00 (500 MHz) spectrometer. Chemical shifts of H NMR are
1
expressed in parts per million downfield from tetramethylsi-
1
lane as internal standard (d ˆ 0) in CDCl . Significant H NMR
3
data are tabulated in following order: multiplicity (s: singlet; d:
doublet; t: triplet; q: quartet; sept: septet; bs: broad singlet; bd:
broad doublet; m: multiplet) and coupling constants (J) are
orbital to cleave hydrogen molecule and form the active
ruthenium hydride species. The natural charges of
cleaving hydrogen atoms, H and H in 14, which are
highly polarized, show that hydrogen-splitting step
proceeds in a heterolytic manner.
1
3
reported (Hz). Chemical shifts of C NMR are expressed in
1
2
parts per million downfield from CDCl as internal standard
3
3
1
(
d ˆ 77.0) in CDCl . Chemical shifts of P NMR are expressed
3
in parts per million downfield from 85% H PO as an external
3
4
standard (d ˆ 0) in CDCl . Optical rotations were measured on
3
The highest occupied KohnÀSham orbitals (HO) of
a JASCO DIP-370. Liquid chromatographic analysis (LC) was
conducted on JASCO PU-980, LG-980-02, DG-980-50 and
1
3a and 13b at the B3LYP/631SDD level support the
tendency of charge distribution. As shown in Figure 7, CO-966 instruments equipped with model UV-975spectrom-
the HOs in the series a and b are recognized as an out-of eters as an ultra-violet light detector. Capillary gas chromato-
1
phase interaction between the Ru d-orbital and the N
lone pair orbital. Although the energy levels of the HOs
are almost same (a: À 4.2 eV, b: À 4.1 eV), the highest
energy electrons are delocalized in 13b because the HO
extended from the Ru center to the diamine region
graphic analysis (GC) was conducted on a Shimadzu GC-14B
instrument equipped with FID detector and capillary column
coated with PEG-20M, using He as a carrier gas. Peak areas
were calculated by Shimadzu C-R6A.
1
1
(
1
Figure 7). Indeed, the N -C bond lengths (13b: 1.37 ä,
2
2
4b: 1.39 ä) are shorter than the N -C bond lengths Computational Methods
1
(
1.44 ä) in 13b and 14b due to conjugation between N
All calculations were performed with a Gaussian 98 package.
The B3LYP functional was employed with the basis set
denoted 631SDD consisting of the SDD basis set including
Stuttgart effective core potential for Ru atom and 6-31G(d)
lone pair orbital and p-orbital (Figure 6, ii and iv).
In conclusion, the racemic BINAP-RuCl catalysts (1)
can be completely discriminated with the designed DM-
2
DABN (2) and used as an equally effective catalyst to basis set for the rest. The chemical models consisted of simple
the enantiopure 1 for asymmetric hydrogenation. The diphosphine and diamine ligands, where hydrogen atoms were
success in DM-DABN thus provides a guiding principle used instead of phenyl rings.
for rational design of a molecule for enantiomeric
catalysts discrimination. Furthermore, this −asymmetric
activation/deactivation× strategy leads to highly enan-
tioselective hydrogenation regardless of the ketonic
substrates by maximizing the difference in catalytic
activity between the enantiomeric catalysts in the
racemic mixture. The present −asymmetric activation/
deactivation protocol× can be regarded as a paradigm
shift in racemic catalysis.
Preparation of ( Æ )-DM-DABN
3
-Methyl-2-naphthylamine: To a mixture of 3-amino-2-naph-
thoic acid (80% purity, 12.1 g, 52 mmol) and xylene (350 mL)
¾
was added Red-Al (117 mL, 390 mmol) at room temperature
under argon atmosphere. After stirring for 6 h at 150 8C, the
reaction mixture was cooled and a second portion of Red-Al¾
(
2
39 mL, 130 mmol) was added. After stirring for 18 h at 150 8C,
0% KOH was carefully added at 0 8C. The crude mixture was
¾
filtered over Celite , and the filtrate was washed with 1 N
KOH. After concentration of the organic layer under reduced
Adv. Synth. Catal. 2003, 345, 246 ± 254
251

FULL PAPERS
Koichi Mikami et al.
pressure, the residue was purified by silica-gel chromatography
gen atoms were included but not refined. R ˆ 0.095, wR2 ˆ
0.2116. Crystallographic data (excluding structure factors) for
the structure reported in this paper have been deposited at the
Cambridge Crystallographic Data Center as supplementary
publication no. CCDC-144099. Copies of the data can be
obtained free of charge on application to CCDC, 12 Union
Road, Cambridge CB@ 1EZ, UK (fax:( ‡ 44)1223-336-033; e-
mail: deposit@ccdc.cam.ac.uk).
(
hexane/ethyl acetate ˆ 4:1 to 3:1) followed by recrystalliza-
tion from hexane and ethyl acetate to give 3-methyl-2-
naphthylamine; yield: 6.1 g (75%). H NMR (CDCl3,
1
3
00 MHz): d ˆ 2.36 (s, 3H), 3.79 (br, 2H), 7.01 (s, 1H), 7.22 (t,
J ˆ 7.5Hz, 1H), 7.33 (t, J ˆ 7.5 Hz, 1H), 7.55 (s, 1H), 7.59 (d,
1
3
J ˆ 8.1 Hz, 1H), 7.65(d, J ˆ 8.1 Hz, 1H); C NMR (CDCl ,
3
7
1
5MHz): d ˆ 17.9, 108.6, 122.4, 125.3, 125.4, 127.0, 128.2, 128.7,
33.7, 143.4, 161.0.
3
,3'-Dimethyl-2,2'-diamino-1,1'-binaphthyl
DABN] (on a 1 mmol scale): To a solution of 157 mg
1 mmol) of 3-methyl-2-naphthylamine in 30 mL of dime-
thoxyethane was added 1.32 g (4 mmol) of K Fe(CN) dis-
[( Æ )-DM-
Catalytic Hydrogenation of Methyl 3-Oxobutanoate
by Racemic XylBINAP-Ru(II) Catalyst (1b) with
DM-DABN (2)
(
3
6
solved in 60 mL of 0.5N NaOH solution. The mixture was
stirred at 0 8C for 1 h. Dichloromethane (10 mL) was added
and the layers were separated. The aqueous layer was
extracted with dichloromethane (3 Â 10 mL) and dried over
To the mixture of RuCl [( Æ )-xylbinap](dmf) complex
2
n
(
14.0 mg, 0.0133 mmol) and (S)-DM-DABN (2.2 mg,
0
.0069 mmol) in a 100-mL autoclave was added dichloro-
methane (2.0 mL) under argon atmosphere. After stirring for
h at room temperature, dichloromethane was removed under
MgSO . After removal of the solvent under reduced pressure,
4
the residue was purified by Florisil chromatography (dichloro-
methane), and then by silica-gel chromatography (hexane/
ethyl acetate ˆ 4:1 to 3:1) to give racemic 3,3'-dimethyl-2,2'-
1
reduced pressure. Methanol (1.2 mL) and methyl 3-oxobuta-
noate (1.1 mL, 10 mmol) were then added to the autoclave
under a stream of argon. Hydrogen was introduced at a
pressure of 100 atm. The solution was stirred for 16 h at room
temperature. After concentration under reduced pressure, the
residue was filtered through a short column of silica gel to give
methyl 3-hydroxybutanoate. Chemical yield of methyl 3-
hydroxybutanoate was calculated by GC analysis. The crude
1
diamino-1,1'-binaphthyl; yield: 79.6 mg (51%). H NMR
(
CDCl , 300 MHz): d ˆ 2.45(s, 6H), 3.45(br, 4H), 7.01 (d,
3
J ˆ 8.1 Hz, 2H), 7.15(t, J ˆ 7.5Hz, 2H), 7.22 (t, J ˆ 7.5Hz, 2H),
1
3
7
7
1
.70 (s, 2H), 7.75(d, J ˆ 8.1 Hz, 2H); C NMR (CDCl ,
3
5MHz): d ˆ 18.4, 112.8, 122.4, 123.8, 125.2, 125.8, 127.3,
28.4, 128.9, 132.5, 142.1.
3
-hydroxybutanoate was distilled to give 1.0 g of methyl 3-
1
hydroxybutanoate. H NMR (CDCl , 300 MHz): d ˆ 1.23 (d,
3
J ˆ 6.3 Hz, 3H), 2.43 (dd, J ˆ 8.4, 16.5Hz, 1H), 2. 53 (dd, J ˆ 4.2,
RC oe ms o pl ul et ixo n of ( Æ )-DM-DABN with BINAP-Ru(II)
1
6.5Hz, 1H), 3.00 (br, s, 1H), 3.72 (s, 3H), 4.22 (m, 1H).
13
C NMR (CDCl , 75MHz): d ˆ 22.4, 42.6, 51.6, 64.2, 173.2.
3
GC analysis: column, CP-Chirasil-Dex-CB, i.d. 0.25mm Â
To a mixture of racemic 3,3'-dimethyl-2,2'-diamino-1,1'-bi-
naphthyl (0.73 g, 2.3 mmol) and RuCl [(S)-binap](dmf) (1.6 g,
2
5m, CHROMPACK; carrier gas, nitrogen (75kPa); column
2
2
temp, 75 8C; injection temp, 105 8C; split ratio, 100:1, tR of
1.7 mmol) was added dichloromethane (70 mL) at room
methyl 3-oxobutanoate 6.3 min (0%), t of methyl 3-hydrox-
R
temperature under argon atmosphere. After stirring for 2 h
at room temperature, dichloromethane was removed under
reduced pressure. The residue was purified by neutral silica-gel
chromatography (dichloromethane) to give 0.36 g [49%, based
on ( Æ )-DM-DABN] of (R)-DM-DABN and 1.2 g [47%, based
on ( Æ )-DM-DABN] of (S)-BINAP-Ru(II)/(S)-DM-DABN
complex. (R)-DM-DABN was recrystallized from dichloro-
methane, diethyl ether and hexane to give 99.5% ee of (R)-
DM-DABN. HPLC analysis of (R)-DM-DABN: HPLC
ybutanoate 8.3 min (100%) (R- and S-isomers were not
separated under these conditions). The enantioselectivity was
determined by HPLC analysis after converting to the methyl 3-
benzoyloxybutanoate by benzoyl chloride and pyridine. HPLC
analysis: HPLC (CHIRALCEL OB-H column, hexane/2-
propanol ˆ 90:10, flow rate 0.5mL/min, detection UV ˆ
2
40 nm): t of R-isomer 18.2 min (99.65%), t of S-isomer
R R
2
4.2 min (0.35%). The absolute configuration was determined
by comparison of (S)- and (R)-methyl 3-benzoyloxybutanoates
derived from enantiopure methyl 3-hydroxybutanoate which
was synthesized according to the reported procedure.[
(
CHIRALCEL OD-H column, hexane/2-propanol ˆ 80:20,
flow rate 0.7 mL/min, detection UVˆ 254 nm): t of R-isomer
R
16a]
2
5
1
2.7 min, t of S-isomer 20.3 min. [a] : ‡ 101.5( c 0.50,
R
D
CHCl ).
3
The single crystal growth was carried out from a dichloro-
methane/ether/hexane mixture at room temperature. X-ray
crystallographic analysis was performed on a Bruker SMART
Hydrogenation of 1'-Acetonaphthone through
Asymmetric Activation/Deactivation of Racemic Xyl-
BINAP-Ru(II) Catalyst (1b)
1000 diffractometer (graphite monochromator, Moka radia-
tion, l ˆ 0.71073 ä). The structure was solved by direct
methods and expanded using Fourier techniques. Crystal
data for RuCl [(S)-binap][(S)-dmdabn]: C H Cl N P Ru,
RuCl [( Æ )-xylbinap](dmf) (10.5mg, 0.010 mmol) and ( R)-
2
n
DM-DABN (1.9 mg, 0.0050 mmol) were added to an auto-
clave. Dichloromethane (3.3 mL) was added to the autoclave
2
66 52
2
2 2
reddish-orange, crystal dimensions 0.07 Â 0.17 Â 0.22 mm,
orthorhombic, space group P21212 (No.18), a ˆ 20.822(3), b ˆ under a stream of argon. After stirring at room temperature for
3
2
1
2.130(3), cˆ13.1320(18) ä, Vˆ 6051.2(14) ä , Z ˆ 4, 1calc ˆ
30 min, dichloromethane was removed under reduced pres-
sure. Air present in the autoclave was replaced by argon after
addition of (S,S)-DPEN (1.0 mg, 0.0045mmol). 2-Propanol
(2.8 mL) was added to the autoclave under a stream of argon,
followed by the addition of KOH/2-propanol (0.5M, 40 mL,
À3
À1
.215g cm , (mMoka) ˆ 1.43 cm , Tˆ 103 K, 10304 reflec-
tions were independent and unique, and 6256 with I > 2s(I)
2qmax ˆ 24.78) were used for the resolution of the structure.
The non-hydrogen atoms were refined anisotropically. Hydro-
(
252
Adv. Synth. Catal. 2003, 345, 246 ± 254

Asymmetric Activation/Deactivation of Racemic Ru Catalysts
FULL PAPERS
0
.02 mmol) with stirring for 30 min. 1'-Acetonaphthone
[4] Reviews: a) C. Girard, H. B. Kagan, Angew. Chem. Int.
Ed. 1998, 37, 2923; b) M. Avalos, R. Babiano, P. Cintas,
J. L. Jimenez, J. C. Palacios, Tetrahedron: Asymmetry
(
0.38 mL, 2.5mmol) was added to the autoclave under a
stream of argon, and then hydrogen was introduced at a
pressure of 8 atm. After vigorous stirring for 4 h at room
temperature, the solvent was removed under reduced pressure.
The residue was filtered through a short column of silica gel.
Chemical yield and enantiomeric ratio of 1-(1-naphthyl)etha-
nol were calculated by chiral GC [ > 99%, 96% ee (R)]. The
product can also be isolated by silica gel column chromatog-
raphy (eluent, hexane/EtOAc, 5:1) to give 426 mg (99%) of
1
1
997, 8, 2997; c) K. Mikami, M. Terada, Tetrahedron.
992, 48, 5671; d) H. B. Kagan, C. Girard, D. Guillaneux,
D. Rainford, O. Samuel, S. Y. Zhang, S. H. Zhao, Acta
Chem. Scand. 1996, 50, 345; e) C. Bolm, in Advanced
Asymmetric Synthesis, (Ed.: G. R. Stephenson), Blackie
Academic and Professional, New York, 1996, p. 9; f) R.
Noyori, M. Kitamura, Angew. Chem. Int. Ed. Engl. 1991,
30, 49.
2
8
[12]
25
alcohol. [a] : ‡ 75.5 (c 1.0, CHCl ) {lit. [a] : ‡ 78.9 (c 1,
D
3
D
1
CHCl ), (R)-isomer}. H NMR (300 MHz, CDCl ): d ˆ 1.59 (d,
3
3
[
5] a) N. M. Alcock, J. M. Brown, P. J. Maddox, J. Chem.
Soc. Chem. Commun. 1986, 1532; b) J. M. Brown, P. J.
Maddox, Chirality 1991, 3, 345; c) K. Maruoka, H.
Yamamoto, J. Am. Chem. Soc. 1989, 111, 789; d) J. W.
Faller, J. Parr, J. Am. Chem. Soc. 1993, 115, 804; e) J. W.
Faller, M. Tokunaga, Tetrahedron Lett. 1993, 34, 7359;
f) J. W. Faller, X. Liu Sams, J. Am. Chem. Soc. 1996, 118,
1217; g) R. Sablong, J. A. Osborn, J. W. Faller, J. Organo-
met. Chem. 1997, 527, 65; h) J. W. Faller, A. R. Lavoie,
B. J. Grimmond, Organometallics 2002, 21, 1662.
Jˆ6.6 Hz, 3H, CH ), 1.90 (d, Jˆ3.6 Hz, 1H, HO), 5.59 (dq, Jˆ
3
3
6
7
.6, 6.6 Hz, 1H, CH), 7.37 ± 7.51 (m, 3H, aromatic), 7.60 (d, Jˆ
.6 Hz, 1H, aromatic), 7.70 (d, Jˆ8.1 Hz, 1H, aromatic), 7.78 ±
.81 (m, 1H, aromatic), 8.02-8.05(m, 1H, aromatic).GC
(
column CP-Cyclodextrin-˚-2,3,6-M-19, i.d. 0.25mm
Â
25m, CHROMPACK; carrier gas, nitrogen (75KPa); column
temp, 160 8C; injection temp, 190 8C; split ratio, 100:1),
retention time (t ); (R)-(‡)-isomer: 32.7 min (98.1%), (S)-
R
(
(
À)- isomer: 31.6 min (1.9%), 1'-acetonaphthone: 21.3 min
0%).
[
6] a) K. Mikami, S. Matsukawa, Nature 1997, 385, 613; b) S.
Matsukawa, K. Mikami, Enantiomer 1996, 1, 69; c) S.
Matsukawa, K. Mikami, Tetrahedron: Asymmetry 1997,
Acknowledgements
8
, 815; d) T. Ohkuma, H. Doucet, T. Pham, K. Mikami, T.
Korenaga, M. Terada, R. Noyori, J. Am. Chem. Soc.
998, 120, 1086.
7] Amounts of chiral poisons: 1.0 equiv. for Al (ref. );
We are grateful to Profs. R. Noyori and T. Okumura for their
continuous encouragement and useful discussion of our results
and Drs. H. Kumobayashi and N. Sayo of Takasago Interna-
tional Corp. for providing BINAP ligands. This work was
financially supported by the Ministry of Education, Science,
Sports and Culture of Japan (Nos. 07CE2004, 09238209 and
1
[5c]
[5b]
[
[
[
[5a]
0
1
.7 equiv. for Rh (ref. ); 10 equiv. for Ru (ref. );
.5equiv. for Ti (ref. [ ); 1.0 equiv. for Ir (ref. ).
5c]
[5d]
8] DM-DABN was named after 2,2'-diamino-1,1'-binaphth-
yl (DABN): K. J. Brown, M. S. Berry, J. R. Murdoch, J.
Org. Chem. 1985, 50, 4345.
9] Excellent reviews on kinetic resolutions: a) H. B. Kagan,
J. C. Fiaud, Top. Stereochem. 1988, 18, 249; b) J. M.
Keith, J. F. Larrow, E. N. Jacobsen, Adv. Synth. Catal.
10208204) and the Research Fellowships of the Japan Society for
the Promotion of Science for Dr. T. Korenaga.
2
001, 323, 5 .
References and Notes
[
10] Our preliminary communication: K. Mikami, T. Kore-
naga, T. Ohkuma, R. Noyori, Angew. Chem. Int. Ed.
[
1] a) R. E. Gawley, J. Aube, Principles of Asymmetric
Synthesis, Pergamon, London, 1996; b) E. N. Jacobsen,
A. Pfaltz, H. Yamamoto (Eds.), Comprehensive Asym-
metric Catalysis, Vol. 1 ± 3, Springer, Berlin, 1999; c) R.
Noyori, Asymmetric Catalysis in Organic Synthesis,
Wiley, New York, 1994; d) H. Brunner, W. Zettlmeier,
Handbook of Enantioselective Catalysis, VCH, Wein-
heim, 1993; e) Catalytic Asymmetric Synthesis, Vol. I and
II (Ed.: I. Ojima), VCH, New York, 1993, 2000; f) H. B.
Kagan, Comprehensive Organic Chemistry, Vol. 8, Per-
gamon, Oxford, 1992; g) Asymmetric Catalysis (Ed.:
Bosnich, B.), Martinus Nijhoff Publishers, Dordrecht,
2
000, 39, 3707.
[
[
11] M. Kitamura, M. Tokunaga, T. Ohkuma, R. Noyori, Org.
Synth. 1993, 71, 1.
12] a) Review: R. Noyori, T. Ohkuma, Angew. Chem. Int.
Ed. Engl. 2001, 40, 40; b) T. Ohkuma, H. Ooka, S.
Hashiguchi, T. Ikariya, R. Noyori, J. Am. Chem. Soc.,
1
995, 117, 2675; c) X-ray analysis of trans-RuCl (tolbi-
2
omplex: H. Doucet, T. Ohkuma, K. Murata, T. Yokoza-
wa, M. Kozawa, E. Katayama, A. F. England, T. Ikariya,
R. Noyori, Angew. Chem. Int. Ed. Engl. 1998, 37, 1703;
d) T. Ohkuma, D. Ishii, H. Takeno, R. Noyori, J. Am.
Chem. Soc. 2000, 122, 6510, and references cited therein.
1
986.
[
2] a) Review: D. J. Berrisford, C. Bolm, K. B. Sharpless,
Angew. Chem. Int. Ed. Engl. 1995, 34, 1059; b) E. N.
Jacobsen, I. Marko, M. B. France, J. S. Svendsen, K. B.
Sharpless, J. Am. Chem. Soc. 1989, 111, 737; 1988, 110,
[13] a) W. Fuhrer, H. W. Gschwend, J. Org. Chem. 1979, 44,
1133; b) J. M. Muchowski, M. C. Venuti, J. Org. Chem.
1980, 45, 4798.
[14] The % ee was determined by chiral HPLC analysis
2
5
1
968.
(CHIRALCEL OD-H); [a]
D
: ‡ 101.5( c 0.50, CHCl
).
3
[
3] Review: K. Mikami, M. Terada, T. Korenaga, Y. Matsu-
moto, M. Ueki, R. Angelaud, Angew. Chem. Int. Ed.
Engl. 2000, 39, 3532.
Absolute configuration was determined by comparison
of the CD spectrum with that of commercially available
(R)-DABN {[a] : ‡ 130.6 (c 0.50, CHCl )}.
3
0
D
3
Adv. Synth. Catal. 2003, 345, 246 ± 254
253

FULL PAPERS
Koichi Mikami et al.
[
[
15] a) P. Mangeney, T. Tejero, A. Alexakis, F. Grosjean, J.
Normant, Synthesis 1988, 255; b) S. Pikul, E. J. Corey,
Org. Synth. 1993, 71, 22.
S. Dapprich, J. M. Millam, A. D. Daniels, K. N. Kudin,
M. C. Strain, O. Farkas, J. Tomasi, V. Barone, M. Cossi,
R. Cammi, B. Mennucci, C. Pomelli, C. Adamo, S.
Clifford, J. Ochterski, G. A. Petersson, P. Y. Ayala, Q.
Cui, K. Morokuma, P. Salvador, J. J. Dannenberg, D. K.
Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman,
J. Cioslowski, J. V. Ortiz, A. G. Baboul, B. B. Stefanov, G.
Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. Gom-
perts, R. L. Martin, D. J. Fox, T. Keith, M. A. Al-Laham,
C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W.
Gill, B. Johnson, W. Chen, M. W. Wong, J. L. Andres, C.
Gonzalez, M. Head-Gordon, E. S. Replogle, J. A. Pople,
Gaussian 98, revision A.11, Gaussian, Inc, Pittsburgh,
PA, 2001.
16] Hydrogenation of b-keto esters by enantiopure RuCl2-
(
binap)(dmf) : a) R. Noyori, T. Ohkuma, M. Kitamura,
n
H. Takaya, N. Sayo, H. Kumobayashi, S. Akutagawa, J.
Am. Chem. Soc. 1987, 109, 5856; b) review: D. J. Ager,
S. A. Laneman, Tetrahedron: Asymmetry 1997, 8, 3327,
and references cited therein.
[
17] XylBINAP ˆ 2,2'-bis(di-3,5-xylylphosphanyl)-1,1'-bi-
naphthyl: a) K. Mashima, Y. Matsumura, K. Kusano, H.
Kumobayashi, N. Sayo, Y. Hori, T. Ishizaki, S. Akutaga-
wa, H. Takaya, J. Chem. Soc. Chem. Commun. 1991, 609;
b) K. Mashima, K. Kusano, N. Sato, Y. Matsumura, K.
Nozaki, H. Kumobayashi, N. Sayo, Y. Hori, T. Ishizaki, S.
Akutagawa, H. Takaya, J. Org. Chem. 1994, 59, 3064.
18] Indeed, (S)-2 in (S)-1b/(S)-2 did not exchange at all with
another diamine such as DPEN even after 24 h at
ambient temperature. However, (S)-2 in (S)-1a/(S)-2 did
slowly exchange.
[20] a) R. Noyori, S. Hashiguchi, Acc. Chem. Res. 1997, 30,
97; b) M. Yamakawa, H. Ito, R. Noyori, J. Am. Chem.
Soc. 2000, 122, 1466; c) D. A. Alonso, P. Brandt, S. J. M.
Nordin, P. G. Andersson, J. Am. Chem. Soc. 1999, 121,
9580; d) A. Aranyos, G. Csjernyik, K. J. Szab o¬ , J. E.
B‰ckvall, Chem. Chem. 1999, 351; e) O. P a¡ mies, J. E.
B‰ckvall, Chem. Eur. J. 2001, 7, 5052; f) K. Abdur-
Rashid, A. J. Lough, R. H. Morris, Organometallics 2001,
20, 1047; g) K. Abdur-Rashid, M. Faatz, A. J. Lough,
R. H. Morris, J. Am. Chem. Soc. 2001, 123, 7473.
[
[
19] All calculations were performed with a Gaussian 98
package: M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E.
Scuseria, M. A. Robb, J. R. Cheeseman, V. G. Zakrzew-
ski, J. A. Montgomery, Jr., R. E. Stratmann, J. C. Burant,
254
Adv. Synth. Catal. 2003, 345, 246 ± 254