Mechanism of silver(I)-catalyzed Mukaiyama aldol reaction: Active species in solution in AgPF6-(S)-BINAP versus AgOAc-(S)-BINAP systems

Article abstract of DOI:10.1016/S1381-1169(01)00049-8

Silver(I)-diphosphine complex is an effective catalyst for Mukaiyama Aldol reaction in polar solvents. AgPF₆-(S)-BINAP cationic chiral complex indicated a good activity and could afford fairly high enantioselectivity in the reaction of aromatic aldehydes and silyl enol ethers. On the other hand, AgOAc-(S)-BINAP system afforded the aldol product of the absolute configuration opposite to that by AgPF₆-(S)-BINAP and very high catalytic activity was shown. The structure and equilibrium state of Ag(I)-BINAP complexes in solution were examined to understand the reaction mechanism. In AgPF₆ system [Ag((S)-BINAP)₂]PF₆ (1a), [Ag((S)-BINAP)]PF₆ (1b), [Ag₂((S)-BINAP)](PF₆)₂ (1c) and AgPF₆ are present in solution. The active species of the aldol reaction in DMF is [Ag((S)-BINAP)]PF₆ (1b), which exists as a minor species in solution. For this cationic Ag(I) catalyst, cyclic transition state containing substrate and silyl enol ether is assumed. In AgOAc-(S)-BINAP system, active species is also monomeric AgOAc((S)-BINAP) (2b) species which exists as a major component in solution and strong interaction was observed with a silyl enol ether. The reaction by AgOAc-(S)-BINAP catalyst is concluded to proceed as follows: nucleophile forms a complex with AgOAc-(S)-BINAP species and is activated. This complex attacks aldehydes to afford aldol adduct via acyclic transition state.

Full text of DOI:10.1016/S1381-1169(01)00049-8

Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
Mechanism of silver(I)-catalyzed Mukaiyama aldol reaction:
active species in solution in AgPF6-(S)-BINAP versus
AgOAc-(S)-BINAP systems
Munetaka Ohkouchi, Dai Masui, Motowo Yamaguchi, Takamichi Yamagishi∗
Department of Applied Chemistry, Graduate School of Engineering, Tokyo Metropolitan University,
Minami-Ohsawa Hachioji, Tokyo 192-0397, Japan
Received 17 November 2000; accepted 5 January 2001
Abstract
Silver(I)-diphosphine complex is an effective catalyst for Mukaiyama Aldol reaction in polar solvents. AgPF6-(S)-BINAP
cationic chiral complex indicated a good activity and could afford fairly high enantioselectivity in the reaction of aromatic
aldehydes and silyl enol ethers. On the other hand, AgOAc-(S)-BINAP system afforded the aldol product of the absolute
configuration opposite to that by AgPF6-(S)-BINAP and very high catalytic activity was shown. The structure and equilib-
rium state of Ag(I)-BINAP complexes in solution were examined to understand the reaction mechanism. In AgPF6 system
[Ag((S)-BINAP)2]PF6 (1a), [Ag((S)-BINAP)]PF6 (1b), [Ag2((S)-BINAP)](PF6)2 (1c) and AgPF6 are present in solution. The
active species of the aldol reaction in DMF is [Ag((S)-BINAP)]PF6 (1b), which exists as a minor species in solution. For
this cationic Ag(I) catalyst, cyclic transition state containing substrate and silyl enol ether is assumed. In AgOAc-(S)-BINAP
system, active species is also monomeric AgOAc((S)-BINAP) (2b) species which exists as a major component in solution and
strong interaction was observed with a silyl enol ether. The reaction by AgOAc-(S)-BINAP catalyst is concluded to proceed
as follows: nucleophile forms a complex with AgOAc-(S)-BINAP species and is activated. This complex attacks aldehydes
to afford aldol adduct via acyclic transition state. © 2001 Elsevier Science B.V. All rights reserved.
Keywords: Mukaiyama aldol reaction; AgPF6-(S)-BINAP systems; AgOAc-(S)-BINAP systems
1. Introduction
low substrate/catalyst ratio for obtaining aldol prod-
ucts in high yield and high stereoselectivities, since
these Lewis acid catalysts are easily hydrolyzed by
water and reaction activity is lost. Although, transi-
tion metals have weak Lewis acidity, some of them
are air-insensitive, can be handled in air and are
stable in the presence of water. Therefore, develop-
ment of transition metal complex catalyst will be
important to cultivate the new possibility of this re-
action. These transition metal catalysts could work in
water-containing solvent or in protic solvent where
strong Lewis acids easily lose their activity. Recently,
Mukaiyama aldol reaction is one of the very use-
ful methods for organic synthesis to construct a C–C
linkage [1–3]. Many Lewis acid catalysts were de-
veloped for this reaction, such as titanium [4–9], tin
[10–14], boron [15–19]. However, these catalysts re-
quire low temperature, dry conditions and relatively
∗ Corresponding author. Tel.: +81-426-77-2847;
fax: +81-426-77-2821.
E-mail address: yamagisi@ecomp.metro-u.ac.jp (T. Yamagishi).
1381-1169/01/$ – see front matter © 2001 Elsevier Science B.V. All rights reserved.
PII: S1381-1169(01)00049-8

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M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
several transition metal catalysts were reported for
the Mukaiyama aldol reaction: Sodeoka et al. re-
ported Pd-catalyzed reaction in wet DMF [20,21],
and Kobayashi et al. reported Sc-catalyzed reaction in
wet THF [22–27]. Although, most of previous Lewis
acid catalysts require halogen-containing solvent such
as CHCl3 or CH2Cl2 for the reaction, these transition
metal catalysts enabled the reaction to proceed in
non-halogen solvent such as DMF and THF.
system, the cationic character of the catalyst is im-
portant for obtaining high activity and enantioselec-
tivity and AgPF6-(S)-BINAP afforded the best results.
In AgOAc-(S)-BINAP system, which is not likely to
afford cationic species, much higher catalytic activity
was observed and the preferential absolute configura-
tion of the aldol product was reversed although optical
yield was low.
We became interested in the difference between
AgPF6 and AgOAc systems. We examined the struc-
ture of the active Ag(I)-BINAP species in solution and
the solid state structure of Ag(I)-BINAP complexes
in order to understand the reaction mechanism of the
Ag(I)-catalyzed Mukaiyama aldol reaction.
Silver(I)-catalyzed reactions were also reported.
Ito et al. reported silver(I)-BPPFA catalyzed asym-
metric aldol reaction of isocyanides with aldehyde
[28–31]. Yanagisawa and Yamamoto et al. reported
Ag(I)-BINAP catalyzed asymmetric allylation of alde-
hyde using allylbutyltin [32,33] and asymmetric aldol
reaction between aldehyde and tin enolate [34,35].
They also reported AgF-BINAP catalyzed asymmetric
aldol reaction between aldehyde and trimethoxysilyl
enol ether to afford high enantioselectivity [36].
We already reported silver(I) catalyzed asymmet-
ric Mukaiyama aldol reaction in polar solvents [37].
Cationic Ag(I)-(S)-BINAP catalyst easily caused the
reaction even in water-containing DMF or THF and
afforded the aldol product in a quantitative yield and
in high enantioselectivity of up to 80% e.e. In this
2. Results and discussion
2.1. Reaction with AgPF6 and AgOAc catalysts
Tables 1 and 2 summarized the results obtained
for asymmetric Mukaiyama aldol reactions bet-
ween aldehydes and silyl enol ethers catalyzed by
AgPF6-(S)-BINAP and AgOAc-(S)-BINAP. Two
catalytic systems are in a striking contrast. The
absolute configurations of aldol products from aro-
matic aldehydes in AgPF6-(S)-BINAP catalyzed re-
action were opposite in all cases to those obtained
by AgOAc-(S)-BINAP catalyzed reaction and the
Table 1
Mukaiyama aldol reaction with AgPF6 catalysts
a
Entry
1
Substrates
PhCHO
Nucleophiles
Time (h)
2
Yield (%)
100
e.e. (%)
69(S)
96
36
80(S)
2b
3
4
1-NpCHO
2-NpCHO
CyCHO
2
2
24
82
100
83
54(−)
58(−)
47(−)
5
24
0
6
PhCHO
a Catalyst, AgPF6-(S)-BINAP; solvent, DMF; temperature, 25◦C; S/C/Nu = 50/1/100.
b The reaction performed at −30◦C.

M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
3
Table 2
Mukaiyama aldol reaction with AgOAc catalystsa
Entry
Substrates
Nucleophiles
Yield (%)
5 min
e.e. (%)
1 h
4 h
1
PhCHO
100
11(R)
11(R)
100 (72 h)
2b
3
4
1-NpCHO
2-NpCHO
CyCHO
100
83
17(+)
15(+)
6(+)
100
5
100
100
0
6
PhCHO
a Catalyst, AgOAc((S)-BINAP); solvent, DMF; temperature, 25◦C; S/C/Nu = 50/1/100.
b The reaction performed at −30◦C.
reaction of aliphatic aldehydes also afforded the same
results. With AgPF6-(S)-BINAP catalyst, the enan-
tioselectivity was relatively high (ca. 70% e.e.) even
at 25◦C, while AgOAc-(S)-BINAP catalyst indicated
only low enantioselectivities (ca. 10–15% e.e.). The
reaction of benzaldehyde with acetophenone silyl enol
ether by AgPF6-(S)-BINAP proceeded fairly fast at
25◦C and terminated in 2 h, but with AgOAc-BINAP
the reaction proceeded extremely fast and finished
in 5 min to afford 100% yield at 25◦C (the sub-
strate/catalyst ratio is 50). Using ketene silyl acetal 3
as nucleophile the AgPF6 system could not cause the
reaction at 25◦C, but with the AgOAc system the aldol
reaction also finished in 5 min to give 100% yield.
2.2. Structure of AgPF6-BINAP complex in solution
and solid state
We investigated the structure of AgPF6-(S)-BINAP
complex in solution with 31P NMR method. Fig. 1a
shows 31P NMR spectra of AgPF6-BINAP sys-
tem in DMF at −50◦C, where BINAP and AgPF6
were mixed in 1:1 ratio. Three double doublet
peaks were observed. The characterization of
silver(I)-monophospine complexes in solution were
performed by Muetterties and Alegranti [38], and Goel
and Pilon [39]. Silver atom has two isotopes 107Ag
and 109Ag in nature. Both of them have a nuclear spin
and shows spin–spin coupling with 31P atom in 31P
NMR spectra. They reported that spin–spin coupling
Fig. 1. 31P NMR spectra of AgPF6-BINAP system: (a) 31P NMR
spectra of AgPF6-(S)-BINAP solution in DMF at −50◦C; (b)
benzaldehyde (50 eq.) was added to (a); (c) silyl enol ether was
added to (b).
constant (J-value) between phosphorous and silver
changes depending on the number of coordinated
phosphorous atoms in the silver complex [38,39],
and we could determine the number of coordinated

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M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
Table 3
J-values and, the number of coordinated P atoms and ligands
d (ppm)
J109
J107
Pa
Lb
Complex
Ag–31
P
Ag–31
P
−1.59
13.69
17.46
802
336
260
696
294
224
1
2
4
0.5
1
2
[Ag2(BINAP)](PF6)2 (1c)
[Ag(BINAP)]PF6 (1b)
[Ag(BINAP)2]PF6 (1a)
a The number of coordinated phosphorous atoms.
b The number of the coordinated ligands. Measurement at −50◦C in DMF
.
phosphorous atoms by measuring of J-value in 31P
NMR spectra.
still revealed a sharp 1a peak at the same position, but
1b and 1c peaks disappeared and new broad peak was
observed at 11.7 ppm. This result shows that equilib-
rium exists between 1b, 1c and presumably AgPF6.
Addition of benzaldehyde to AgPF6-BINAP solu-
tion in DMF caused a change of peak style. The peak
of 1b became broad (Table 1b), while addition of silyl
enol ether to AgPF6-(S)-BINAP solution did not cause
an apparent change in the spectra. Addition of silyl
enol ether to the above solution containing benzalde-
hyde, new doublet appeared at 12.7 ppm (Fig. 1c).
Thus, [Ag((S)-BINAP)]PF6 (1b) is considered to in-
teract with benzaldehyde and silyl enol ether in solu-
tion.
In the reaction of benzaldehyde with silyl enol ether
5 catalyzed by AgPF6-(S)-BINAP, the reaction pro-
ceeded slowly in a syn-selective manner. Syn-selective
reaction suggests an acyclic transition state [52–54],
but slowness of the reaction for 5 suggests the cyclic
transition state. For cyclic silyl enol ether 6, the reac-
tion did not proceed (Table 4) to afford no information
Table 3 summarizes the obtained J-values of these
three peaks in 31P NMR spectra at −50◦C and the
number of coordinated phosphorous atoms on the
complex species. These results show that the Ag(I)
complex of 17.46 ppm has four coordinated phospho-
rous atoms, that is, two BINAP ligands. Therefore,
this complex is [Ag((S)-BINAP)2]PF6 (1a). We pre-
pared [Ag((S)-BINAP)2]PF6 (1a) in THF and isolated
it as clear crystals (see experimental section). 31P
NMR of the isolated complex revealed the same
peak with the in situ species of 17.46 ppm. The
complex of 13.69 ppm peak and −1.59 ppm peak
are considered to be [Ag((S)-BINAP)]PF6 (1b) and
[Ag2((S)-BINAP)](PF6)2 (1c), respectively.
Though, [Ag((S)-BINAP)2]PF6 complex is a major
species among Ag complexes in solution, it can not
catalyze the aldol reaction and is stable in solution,
not in equilibrium with other Ag(I) species. In fact,
on raising the temperature to 25◦C, 31P NMR spectra
Table 4
The reaction using (E)- and (Z)-silyl enol ethera
Entry
1
Substrates
Nucleophiles
Catalyst
Time (h)
24
Yield
27
e.e. (%)
–
Syn/anti
AgPF6-(S)-BINAP
84/16
AgOAc-(S)-BINAP
2
3
PhCHO
PhCHO
1
24
98
0
23(−)
84/16
74/26
AgPF6-(S)-BINAP
AgOAc-(S)-BINAP
3
100
1
4
a The reaction between benzaldehyde (50 eq.) and silyl enol ether (100 eq.) performed in DMF at 25◦C.

M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
5
2.3. Synthesis of AgOAc-diphosphine complex
We synthesized the AgOAc-diphosphine complex
to understand solid state structure and equilibrium
state in solution. The diphosphine ligands used were
dppp, dppb, dppf, (S)-BINAP, and (S)-Tol-BINAP. We
could synthesize all AgOAc-diphosphine complexes
in a quantitative yield. These complexes were stable
in water and can be readily handled in air.
2.4. Mukaiyama aldol reaction using
AgOAc-diphosphine complex
Mukaiyama aldol reaction catalyzed by AgOAc-
diphosphine complexes was examined and results
were summarized in Table 5. The (S)-BINAP and
(S)-Tol-BINAP complexes were effective and the
reaction proceeded with 100% yield in 5 min and af-
forded the same results as that of in situ conditions.
Triarylphosphine type ligands including dppf were
more effective than alkyldiarylphosphine type ligands.
Fig. 2. Equilibrium state of AgPF6-BINAP system.
2.5. Solid state structure of AgOAc-diphosphine
complex
of the transition state. However, the 31P NMR results
indicating the interaction of Ag(I) with aldehyde and
silyl enol ether and the high enantiomeric excess by
AgPF6-(S)-BINAP will support the cyclic transition
state to afford the aldol adduct. Fig. 2 summarizes
the equilibrium state in solution by AgPF6-(S)-BINAP
system and proposed reaction mechanism.
We could obtain single crystals of AgOAc-dppb co-
mplex and AgOAc-(S)-BINAP complex by recrystal-
lization from CH2Cl2–Et2O and CHCl3–cyclohexane,
respectively. We examined the X-ray crystal structure
analysis, and obtained following crystal structure and
crystallographic data (Figs. 3 and 4, Table 6).
Table 5
The Mukaiyama Aldol reaction catalyzed by AgOAc-diphosphine complexa
Entry
Complex
Yield
5 min
e.e. (%)
1 h
1
2
3
4
5
6
7
AgOAc-(S)-BINAP
AgOAc-(S)-Tol-BINAP
AgOAc-dppf
AgOAc-dppp
AgOAc-diop
100
100
100
88
58
24
11(R)
11(R)
–
–
0
–
0
100
100
44
AgOAc-dppb
AgOAc-(S)-(R)-BPPFA
0
19
a The reaction between benzaldehyde (50 eq.) and acetophenone silyl enol ether (100 eq.) performed in DMF at 25◦C.

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M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
Fig. 3. The ORTEP drawing of [AgOAc(dppb)]2.
Fig. 4. The ORTEP drawing of AgOAc((S)-BINAP).

M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
7
Table 6
Crystallographic data for [Ag(OAc)(dppb)]2 and AgOAc(BINAP)·CHCl3
[Ag(OAc)(dppb)]2
AgOAc(BINAP)
Chemical formula
Fw
C60 H62 Ag2 O4 P4
1186.72
C47 H36 Ag Cl3 O2 P2
908.92
Diffractometer
Crystal system
Space group
a (Å)
Bruker SMART CCD
Triclinic
Bruker SMART CCD
Orthorhombic
P21212 (number 18)
16.1652 (9)
24.0891 (13)
10.8762 (6)
¯
P1 (number 2)
10.2536 (10)
11.7167 (12)
12.9938 (13)
65.590 (2)
71.166 (2)
76.483 (2)
1336.3 (2)
1
b (Å)
c (Å)
α (◦)
β (◦)
γ (◦)
V (Å3)
4235.2 (4)
4
1.425
0.778
298
Z
ρcalculated (g/cm3)
1.475
0.899
100
µ (mm−1
T (K)
)
λ (Mo K␣) (Å)
0.71073
56.36
5712
5427
317
0.0438
0.1174
0.876
0.71073
58.28
9335
5918
537
0.0477
0.1082
0.916
2θmax (◦)
Number of independent datas
Number of observed reflections (no) [|Fo|2 > 2.0σ(|F|2)]
Number of parameters refined (nv)
R1 (observed reflections)a [F2 > 2σ(F2)]
wR2 (observed reflections)b (F2)
GOFc
ꢀ
ꢀ
a R1 = (|Fo − (1/k)Fc|)/ |Fo|.
ꢁ
ꢂ
ꢀ
ꢀ
b wR2 =
w(Fo2 − (1/k)Fc2)2/ w|Fo2|2 , w = 1/[σ(Fo2) + (0.0828P)2 + 4.0051P] ([Ag(OAc)(dppb)]2), w = 1/[σ2(Fo2) +
(0.00.0483P )2 + 0.0000P ] (AgOAc(BINAP)), P = (Fo2 + 2Fc2)/3.
ꢁ
ꢂ1/2
ꢀ
c GOF =
w(Fo2 − (1/k)Fc2)2/(no − nv)
.
The solid AgOAc-dppb complex was a binuclear
complex bridged by acetate ion and phosphorous
atoms of dppb ligand coordinated to silver atoms
separately (Fig. 3). Concerning the crystal structure
of AgOAc-diphosphine complexes only a few exam-
ples were known so far. The [AgOAc(dppm)]2 [40]
was reported to have the similar binuclear structure.
As for AgNO3 system, many diphosphine com-
plexes were known with dppf [41], dppb [42], hexane
(dpph) [43], and dppm [44] ligands. In these com-
plexes diphosphine ligand coordinated to two silver
atoms, thus, forming diphosphine bridged binuclear
complex [45–50]. 1 Therefore, we supposed that
silver-diphosphine complexes generally form similar
diphosphine bridged complexes. To our knowledge,
mononuclear complex (silver:diphosphine
=
1:1
form) was scarce [51]. 2 However, AgOAc-(S)-BINAP
complex was a mononuclear complex (Fig. 4) differ-
ent from those described above.
2.6. Structure of AgOAc-diphosphine complex in
solution
In DMF or THF solution, AgPF6-diphosphine
complex formed an equilibrium state between several
species described above.
Table 7 summarizes 31P NMR data of AgOAc-
diphosphine complexes and the number of the
1 Several phosphine-bridged silver-diphosphine complexes were
reported: AgCl complex; dppf [45]; dppp [46]; AgI complex [47];
AgClO4-dppm complex [48]; AgOCOCH2Ph-dppm complex [49];
AgNO3-dppe complex [50].
2 One example of mononuclear complex in AgCl system was
reported. The ligand was 2,11-bis(diphenylphosphinomethyl)benzo-
[c]phenanthrene.

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M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
Table 7
31P NMR parameter for AgOAc-diphosphine complex in CDCl3
Complex
BINAP
Temperature (◦C)
δ
J107
J109
Pa
Lb
Ag–31
P
Ag–31
P
25
−50
11.63(dd)
10.81(dd)
15.90(dd)
4.75(dd)
342
342
225
676
397
403
255
796
2
4
1
1
2
0.5
Major
Minor
Minor
Tol-BINAP
25
−50
9.77(dd)
10.81(dd)
15.88(dd)
4.93(dd)
344
349
226
663
403
407
262
770
2
4
1
1
2
0.5
Major
Minor
Minor
dppf
dppp
25
−50
25
−50
−5.16(d, br)
−5.45(dd)
0.17(s, br)
−0.98(dd)
−6.26(dd)
409
498
433
2
1
433
218
498
253
2
4
1
2
Major
Minor
dppb
25
−50
−80
1.01(s, br)
0.34(dd)
−0.58(dd)
420
433
491
492
2
2
1
1
In CH2Cl2
a The number of coordinated phosphorous atoms.
b The number of the coordinated ligands.
coordinated phosphorous atoms and ligand. AgOAc-
(S)-BINAP and AgOAc-(S)-Tol-BINAP showed one
sharp double doublet at 25◦C, and lowering the
temperature to −50◦C three kinds of peaks were
observed. Other complexes showed broad singlet or
doublet peak at 25◦C and one or two kinds of sharp
double doublet at −50◦C. These results show that
these complexes are labile.
solid state structure, binuclear (2b-2) or polymeric
complex (2b-3) (Fig. 6). We examined the possibil-
ity of scrambling of (S)-BINAP and (S)-Tol-BINAP
in this 1:1 complex. AgOAc (1 eq.), (S)-BINAP
(0.5 eq.) and (S)-Tol-BINAP (0.5 eq.) were mixed
in CDCl3 and this mixture was analyzed by 31P
NMR. The result was shown in Fig. 5b. In the chem-
ical shift region of 1:1 complex, only two species
were observed indicating the exact superposition
of AgOAc-BINAP (2b) and AgOAc-Tol-BINAP
(2b ) species. As 1:2 complex, AgOAc(BINAP)2
(2a) and AgOAc(Tol-BINAP)2 (2a ) were ob-
served. All peaks could be assigned and no peaks
were observed more intricate than double dou-
blet except AgOAc((S)-BINAP)((S)-Tol-BINAP). 3
If 2b-2 and 2b-3 are present in solution, it is
fully expected that more intricate peak appears
in the region of 1:1 species, similarly to the
Equilibrium state of AgOAc-(S)-BINAP in CDCl3
at −50◦C is shown in Fig. 5a. The complex of
15.90 ppm peak has a coupling constant of 225 Hz
(
107Ag–31P) and is concluded to have four coordi-
nated phosphorous atoms, that is, two BINAP ligands.
Therefore, this complex is Ag((S)-BINAP)2OAc. In
AgPF6-BINAP system, bis-BINAP species was a ma-
jor component and was very stable and gave a sharp
double doublet even at 25◦C in 31P NMR in solu-
tion. However, Ag((S)-BINAP)2OAc species exists
as a minor species and is in equilibrium with other
Ag species and major species was 1:1 silver-BINAP
complex.
This major complex has coordinative acetate an-
ion (since this complex has a chemical shift differ-
ent from [Ag((S)-BINAP)]PF6), three possibilities
are considered: the mononuclear (2b-1) similarly to
3 We examined the synthesis of the Ag(I) complex from
BINAP (1 eq.), Tol-BINAP (1 eq.), and AgOAc (1 eq.). 31P
NMR analysis of the mixture indicated the formation of three
Ag(I) complexes. All species had coupling constants corre-
sponding to four phosphorous atoms coordinated complex and
were assigned as AgOAc(BINAP)2, AgOAc(Tol-BINAP)2 and
AgOAc(BINAP)(Tol-BINAP).

M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
9
Fig. 5. 31P NMR spectra of AgOAc-(S)-BINAP system: (a) 31P NMR spectra of AgOAc-(S)-BINAP in CDCl3 at −50◦C; (b) spectra of
the complexes prepared from AgOAc (1 eq.), (S)-BINAP (0.5 eq.), and (S)-Tol-BINAP (0.5 eq.) (2a –2c have (S)-Tol-BINAP ligand).
Ag((S)-BINAP)((S)-Tol-BINAP) species which was
really observed. On addition of 1 eq. (S)-Tol-BINAP
to the solution of AgOAc-(S)-BINAP 1:1 system,
double doublet at 13.5 ppm largely diminished and 2a,
2a and AgOAc((S)-BINAP)((S)-Tol-BINAP) were
observed. Therefore, the 1:1 complex of 13.5 ppm
(Fig. 5a) is concluded to be mononuclear (2b-1) one.
We also examined the scrambling experiment for
AgOAc-dppb system. AgOAc-dppb and AgOAc-dppp
complexes afforded double doublet signal with a
large coupling constant (J ∼ 500 Hz), respectively
at −50◦C in CDCl3. AgOAc-dppb and AgOAc-dppp
were mixed in CDCl3 and the mixture was analyzed at
−50◦C. New eight doublets were observed in addition
Fig. 6. Structure of AgOAc-(S)-BINAP complex.

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M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
Fig. 7. Scrambling experiment between AgOAc-dppb and
AgOAc-dppp: (᭹), AgOAc-dppp; (ꢀ), AgOAc-dppb.
to AgOAc-dppb and AgOAc-dppp peaks. This result
shows that AgOAc-dppb holds dimeric structure in
solution similar to solid state structure (Fig. 7).
2.7. Reaction mechanism of Mukaiyama aldol
reaction catalyzed by AgOAc-(S)-BINAP catalyst
By addition of DMF to the complex solution in
CDCl3 (0.4 eq. volume for CDCl3), the peak of 2b-1
in 31P NMR spectra became broad. This result shows
that new equilibrium was caused by DMF. When ben-
zaldehyde and nucleophile such as silyl enol ether or
ketene silyl acetal were added, the peak form was
changed to indicate that the substrate and nucleophile
interact with the monomeric silver complex. Because
of broadness of the peak, it was difficult to analyze
the structure and equilibrium state.
Therefore, we examined 31P NMR spectra of the
complex in THF. 4 Although, 2a indicated a broad
peak at 25 and −50◦C, sharp peaks were observed
at −90◦C. Addition of benzaldehyde to the solution
at −90◦C caused only a small change in the spectra
(Fig. 8b). When ketene silyl acetal 3 was added to the
above AgOAc-(S)-BINAP complex in THF, the spec-
tra changed and new peaks appeared (Fig. 8c). The
same spectra was observed by addition of the ketene
silyl acetal to AgOAc-(S)-BINAP solution without
Fig. 8. 31P NMR spectra of AgOAc-(S)-BINAP in THF at −90◦C:
(a) 31P NMR spectra of AgOAc-(S)-BINAP in THF; (b) 50 eq. of
benzaldehyde was added to (a); (c) 100 eq. of ketene silyl acetal
3 was added to (b).
shows that these Ag(I) species have one BINAP lig-
and. The spectra change caused only by the addition
of ketene silyl acetal indicates that 2b-1 strongly inter-
acts with ketene silyl acetal in THF. It suggests that the
activation of ketene silyl acetal by 2b-1 occurs in so-
lution of AgOAc-(S)-BINAP. These strong interaction
of the nucleophile with AgOAc-(S)-BINAP species
was not observed in the case of AgPF6-(S)-BINAP
system. With this AgOAc catalyst, silyl enol ether
or ketene silyl acetal is very rapidly hydrolyzed in
methanol to afford no aldol product at all, while with
AgPF6-(S)-BINAP catalyst the aldol product was
benzaldehyde. The J
were 342 Hz (10.72 ppm), 338 Hz (10.23 ppm), 314 Hz
(3.34 ppm) and 301 Hz (3.34 ppm), respectively. This
values of these species
107Ag–31P
4 We examined the Mukaiyama aldol reaction in THF. Although,
the reaction activity was lower than in DMF, the aldol product
was obtained in 78% yield and 11% e.e. (R) after 5 h at 25◦C.
The reaction in THF afforded the same optical yield and absolute
configuration as those by the reaction in DMF.

M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
11
Table 8
The reactions in MeOHa
Entry
Ag complex
Yield (%)
1
2
3
AgOAc-(S)-BINAP
AgPF6-(S)-BINAP
AgOTf-(S)-BINAP
0b
4b
32b
a The reaction between benzaldehyde (50 eq.) and acetophe-
none silyl enol ether (100 eq.) performed at 25◦C
b Methanolysis of silyl enol ether also performed.
obtained in 4% yield and with AgOTf-(S)-BINAP
catalyst the aldol reaction occurred in 32% yield in
methanol (Table 8). This will indicate the different
degree of the activation of the nucleophile by Ag(I)
species. The presence of four peaks will suggest that
coordination mode of the ketene silyl acetal is not
simple and coordination of the nucleophile by silyl
oxygen or carbon–carbon double bond is possible.
We observed the formation of silver enolate
species in a wet THF-d8 solution using cationic
AgPF6-BINAP catalyst by 1H NMR [37]. By the
coordination to the AgOAc-(S)-BINAP species, the
nucleophile is expected to be activated to afford the
silver enolate species as an intermediate in solution.
However, 1H NMR analysis in DMF-d7, THF-d8
and CDCl3 at low temperature, did not indicate the
formation of the silver enolate species in solution.
This interaction of the nucleophile with AgOAc-
BINAP species was also observed in UV–VIS spectra
(Fig. 9). In the DMF solution of AgOAc-(S)-BINAP
system in the presence of the 50 equivalent substrate,
no absorption maxima was observed in 350–600 nm
region. However, addition of ketene silyl acetal 3 to
the above solution afforded the new absorption max-
ima at 413 nm with an isobestic point at 366 nm. This
absorption was also observed without the substrate 4.
The peak at 413 nm may be ascribed to MLCT band
of the AgOAc-BINAP - 3 complex.
Fig. 9. UV–VIS spectra of AgOAc-(S)-BINAP in DMF.
indicated that this AgOAc-BINAP species interact
strongly with nucleophiles (silyl enol ether or ketene
silyl acetal) to activate the nucleophile. In the reaction
of benzaldehyde using silyl enol ethers 5 and 6 by
AgOAc-(S)-BINAP catalyst, the reaction proceeded in
a syn-selective manner for both nucleophiles (Table 4).
These results strongly suggest the reaction proceeded
through an acyclic transition state [52–54]. We con-
sider that nucleophile was activated by silver atom and
acetate anion [55–57], 5 and the reaction proceeded
by the direct attack of Ag-nucleophile complex to the
substrate via acyclic transition state (Fig. 10).
3. Experimental
3.1. General Method
1H NMR and 31P NMR spectra were obtained with
a JEOL-EX270 and JEOL-LA400 NMR spectrometer
using tetramethylsilane (Me4Si) as an internal stan-
dard and 85% H3PO4 as an external standard, respec-
tively. J-values are given in Hz. Mass spectra were
recorded on a JEOL LX-1000 in m-nitrobenzyl al-
While AgPF6-(S)-BINAP system afforded fairly
high enantioselectivity in the reaction of benzalde-
hyde and acetophenone silyl enol ether, the catalyst
AgOAc-(S)-BINAP afforded low enantioselectivity
with an inversion of chiral induction. The reaction
by AgCl-(S)-BINAP catalyst also indicated the same
chiral induction as that by AgOAc-(S)-BINAP though
the reactivity of AgCl catalyst is much lower than
that of AgOAc species. NMR and UV–VIS spectra
5 Several examples were known of the activation of silicon atom
by carboxylates or alkoxides; Sn promoted Aldol reaction [55];
Pd-catalyzed ␲-allyl alkylation with BSA [56]; trifluoromethyla-
tion with TMS-CF3 [57].

12
M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
follows. To a mixture of AgPF6 (4.0 mg, 0.013 mmol)
and (S)-BINAP (8.4 mg 0.013 mmol) was added
DMF (206 ␮l, solvent) under nitrogen atmosphere
and the solution was stirred at room temperature for
10 min. Benzaldehyde (69 ␮l, 0.67 mmol, substrate)
was added to the solution and the mixture was stirred
for 10 min. Acetophenone silyl enol ether (276 ␮l,
1.35 mmol, nucleophile) was added and the mixture
was stirred at 25◦C for 2 h. It was filtrated through
a short silica gel column (EtOAc/hexane (1:1)) and
concentrated under reduced pressure to afford clear
oil. The oil was hydrolyzed with 1 M HCl THF/water
(1:1) solution, extracted with diethylether, dried over
anhydrous sodium sulfate, concentrated in vacuo to
give pale clear oil. This oil was purified by silica gel
column chromatography (EtOAc/hexane (1:1)).
3.3. Aldol reaction product
The yield was determined by 1H NMR analy-
sis [58,59]. 6 Enantiomeric excess was determined
by HPLC analysis using Daicel Chiralcel OB-H, or
OD-H. The absolute configuration of the products
was determined by their optical rotation.
3.4. 1-Hydroxy-1,3-diphenyl-3-propanone
1H NMR (CDCl3, 270 MHz) δ 3.38 (d, 2H, J =
5.9 Hz, CH2), 3.56 (d, 1H, J = 3.0 Hz, OH), 5.35
(dt, 1H, J = 3.0 Hz, 5.9 Hz, CH), 7.26–7.62 (m,
8H, Ph), 7.95 (d, 2H, J = 7.3 Hz, Ph). FAB–MS
calcd. for C15H14O2, 226 (M+); found 227 ((M +
1)+), 209 ((M-OH)+), HPLC (OB-H column, eluent;
Fig. 10. Equilibrium state and proposed reaction mechanism of
AgOAc-(S)-BINAP system.
cohol matrix with a fast atom bombardment ioniza-
tion method. Column chromatography was performed
with Merck silica gel (230–400 mesh). HPLC for the
separation of enantiomers was carried out on a Shi-
madzu LC-6A and LC-10A chromatograph using Chi-
ralcel OB-H and OD-H columns (Daicel Chemical In-
dustry) at room temperature. The mobile phase was
n-hexane/propan-2-ol (9:1) and detection was done by
a UV–VIS detector and polarimeter detector.
hexane/2-propanol (9:1), flow rate 0.9 ml/min) tR
=
22.5 min ((R)-isomer), tR = 33.5 min ((S)-isomer);
(OD-H column, eluent; hexane/2-propanol (9:1), flow
rate 0.5 ml/min) tR = 22.5 min ((R)-isomer), tR
=
24.5 min ((S)-isomer).
3.5. 1-Hydroxy-1-(1-naphthyl)-3-phenyl-3-propanone
1H NMR (270 MHz, CDCl3) δ 3.42–3.61 (m,
2H, CH2), 3.70 (d, 1H, J = 3.0 Hz, OH), 6.15 (m,
1H, CH), 7.25–8.07 (m, 12H, Ar), HPLC (OB-H
column, eluent; hexane/2-propanol (9:1), flow rate
3.2. Experimental procedure of Mukaiyama aldol
reaction
A typical experimental procedure for silver(I)-
catalyzed asymmetric Mukaiyama aldol reaction of
benzaldehyde and acetophenone silyl enol ether is as
6 As for aldol reaction product see also [37].

M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
13
0.9 ml/min), tR = 32.8 min ((+)-isomer), tR
60.3 min ((−)-isomer).
=
formed and were filtered, [Ag((S)-BINAP)2]PF6 (1a)
was gave as a clear crystal in 97% yield.
31P NMR at 25◦C (162 MHz, CDCl3) δ 17.4 (dd,
J
= 260 Hz, J
= 226Hz), −143
109Ag–31P
107Ag–31P
3.6. 1-Hydroxy-1-(2-naphthyl)-3-phenyl-3-propanone
(m, J
= 166 Hz, PF6), FAB–MS calculated
19F–31P
for C88H64AgF6P5, 1353 ((M-PF6)+); found, 1353
((M-PF6)+), 731 ((M-BINAP-PF6)+), elemental anal-
ysis; calculated C, 70.55; H, 4.31, found, C, 70.27; H,
4.54.
1H NMR (270 MHz, CDCl3) δ 3.46 (m, 2H, CH2),
3.70 (s, 1H, OH), 5.52 (m, 1H, CH), 7.25–7.98
(m, 12H, Ph), HPLC (OB-H column, eluent;
hexane/2-propanol (9:1), flow rate 0.9 ml/min), tR
42.2 min ((+)-isomer), tR = 52.5 min ((−)-isomer).
=
3.10. Preparation of AgOAc-(S)-BINAP complex
3.7. 1-Hydroxy-2-methyl-1,3-diphenyl-3-propanone
To a Schlenck tube (S)-BINAP (200 mg, 0.32 mmol)
and AgOAc (53.6 mg, 0.32 mmol) were placed under a
nitrogen atmosphere. The Schlenck tube was charged
with THF (5 ml) and the mixture was stirred for 30 min
to give a white suspension. The solvent was removed
under reduced pressure and white solid was obtained.
This solid was dissolved in CH2Cl2 (5 ml) and filtered.
The filtrate was concentrated to 1 ml and Et2O (10 ml)
was added, and cooled at −30◦C. Formed crystals
were filtered to give AgOAc-(S)-BINAP as clear crys-
tal in 93% yield. The single crystal was obtained by
slow recrystallization from CHCl3–cyclohexane (1:6)
at room temperature.
1H NMR (CDCl3, 270 MHz) anti-isomer: δ 1.07
(d, 3H, J = 7.2 Hz, CH3), 3.6–3.8 (m, 1H, CHCH3),
4.99 (d, 1H, J = 8.3 Hz, CHOH), 7.2–8.2 (m, 10H,
Ph), syn-isomer: δ 1.19 (d, 3H, J = 7.2 Hz, CH3),
3.6–3.8 (m, 1H, CHCH3), 5.23 (d, 1H, J = 3.3 Hz,
CHOH), 7.2–8.2 (m, 10H, Ph), HPLC (OD-H column,
eluent; hexane/2-propanol (9:1), flow rate 0.5 ml/min),
tR = 13.3 min (syn (−)-isomer), tR = 14.8 min (syn
(+)-isomer), tR = 15.9 min (anti (−)-isomer), tR
17.5 min (anti (+)-isomer).
=
31P NMR at 25◦C (162 MHz, CDCl3) δ 11.63 (dd,
3.8. 2-(α-Hydroxybenzyl)cyclohexanone
J
= 397 Hz, J
= 342 Hz), at −50◦C
109Ag–31P
107Ag–31P
1H NMR (CDCl3, 270 MHz) anti-isomer:
δ
(162 MHz, CDCl3) δ 15.90 (dd, J
J
= 255 Hz,
109Ag–31P
1.29–2.62 (m, 9H, c-C6H9), 3.96 (s, 1H, OH), 4.78
(d, 1H, J = 8.8 Hz, CH), 7.23–7.36 (m, 5H, Ph),
syn-isomer: δ 1.29–2.62 (m, 9H, c-C6H9), 3.01 (s, 1H,
OH), 5.39 (s, 1H, CH), 7.23–7.36 (m, 5H, Ph), HPLC
(OD-H column, eluent; hexane/2-propanol (9:1), flow
rate 0.5 ml/min), tR = 13.3 min (syn (−)-isomer),
tR = 14.8 min (syn (+)-isomer), tR = 15.9 min
(anti-isomer), tR = 17.5 min (anti-isomer).
= 225 Hz, minor species Ag(BINAP)2OAc),
107Ag–31P
10.81 (dd, J
342 Hz, major species, Ag(BINAP)OAc), 4.75 (dd,
=
403 Hz, J
=
109Ag–31P
107Ag–31P
J
= 796 Hz, J
= 676 Hz, minor
109Ag–31P
107Ag–31P
species, Ag2(BINAP)(OAc)2), FAB–MS calculated
for C46H35AgO2P2 (AgOAc(BINAP)), 791 ((M)+);
found, 731 ((M-OAc)+), elemental analysis; calcu-
lated for AgOAc(BINAP) C, 69.97; H, 4.47, found,
C, 69.89; H, 4.57.
3.9. Preparation of [Ag(BINAP)2]PF6 (1a)
3.11. Preparation of [AgOAc(dppb)]2 complex
To a Schlenck tube (S)-BINAP (258 mg, 0.414 mmol)
and AgPF6 (49.6 mg, 0.196 mmol) were placed under
a nitrogen atmosphere. The schlenck tube was charged
with THF (2 ml) and the mixture was stirred for 30 min
to give a white suspension. The solvent was removed
in reduced pressure and white solid was obtained.
This solid was dissolved in CHCl3 (5 ml) and filtered.
The filtrate was concentrated for 1 ml, Et2O (10 ml)
was added, and cooled at −30◦C. The clear crystals
[AgOAc(dppb)]2 was obtained as a clear crystal by
same procedure for AgOAc-(S)-BINAP complex. The
single crystal was obtained by recrystallization from
CH2Cl2–Et2O (1:10) solution at −30◦C.
31P NMR at 25◦C (162 MHz, CDCl3) δ 1.01
(s, br), at −50◦C (162 MHz, CDCl3) δ 0.34 (dd,
J
= 491 Hz, J
= 420 Hz), at −80◦C
109Ag–31P
107Ag–31P
(162 MHz, CH2Cl2) δ −0.58 (dd, J
=
109Ag–31P

14
M. Ohkouchi et al. / Journal of Molecular Catalysis A: Chemical 170 (2001) 1–15
492 Hz, J
= 433 Hz) FAB–MS calculated for
and polarization factors by SAINT program [60].
An empirical absorption correction was also applied
using the SADABS program [61].
107Ag–31P
C62H66Ag2O4P4 ([AgOAc(dppb)]2), 1214 ((M)+);
found, 1154 ((M-OAc)+), 985 ((M-AgOAc)+), 545
((M-AgOAc-dppb)+), elemental analysis; calculated
for [AgOAc(dppb)]2·H2O C, 60.27; H, 5.31, found,
C, 60.20; H, 5.39. TG–DTA analysis indicated the
presence of one H2O.
The structure was solved by direct and Fourier
methods. The data were refined by full-matrix least
ꢁ
ꢀ
squares [62], minimizing the function
w Fo2−
ꢂ2
(1/k)Fc2 . The handedness of structure was tested
by refining the Flack parameter [63,64].
The calculations of initial structure were carried out
by using PC version of SIR92 [65] 7 and the refine-
ment was carried out by using SHELX-97 programs
[62]. The crystallographic data were listed in Table 6.
3.12. Preparation of AgOAc-dppf complex
AgOAc-dppf was obtained as an orange crystal by
the same procedure for AgOAc-(S)-BINAP complex.
31P NMR at 25◦C (162 MHz, CDCl3) δ −5.16 (d,
br, JAg–P = 409 Hz), at −50◦C (162 MHz, CDCl3)
4. Conclusion
δ −5.45 (dd, J
= 498 Hz, J
=
109Ag–31P
107Ag–31P
433 Hz), FAB–MS calculated for C72H66Ag2Fe2O4P4
([AgOAc(dppf)]2), 1446 ((M)+); found, 1386
((M-OAc)+), 1217 ((M-Oac-AgOAc)+), 661 ((M-
OAc-AgOAc-dppf)+), elemental analysis; calculated
for [AgOAc(dppf)]2·H2O C, 59.21; H, 4.42, found,
C, 58.90; H, 4.34.
We examined structure and equilibrium state in
solution to understand the reaction mechanism. We
found a difference in equilibrium state in solution be-
tween AgPF6 system and AgOAc system. In AgPF6
system, [Ag(BINAP)2]PF6 (1a) was a major species,
but 1a was stable and inactive. [Ag(BINAP)]PF6 (1b)
which exists as a minor species was active species.
From the change in NMR spectra by addition of sub-
strate and nucleophile and high enantioselectivity in
the case of AgPF6-BINAP, the reaction is consid-
ered to proceed through the six-membered transition
state. In AgOAc-diphosphine systems, mode of co-
ordination of diphosphine differs depending on the
structure of diphosphines. AgOAc-dppb formed a
diphosphine bridged dimer in the solid state and in
solution. AgOAc-(S)-BINAP complex formed phos-
phine chelated monomeric complex in solid state
and in solution, 1:1 monomeric AgOAc((S)-BINAP)
complex exists as a major species and interacts
strongly with the nucleophile to cause the activation
and the reaction was considered to proceed through
the acyclic transition state by the direct attack of the
Ag-nucleophile species to the substrate.
3.13. Preparation of AgOAc-dppp complex
AgOAc-dppp was obtained as clear powder by the
same procedure for AgOAc-(S)-BINAP complex.
31P NMR at 25◦C (162 MHz, CDCl3) δ 0.17
(s, br), at −50◦C (162 MHz, CDCl3) δ −0.98 (dd,
J
= 498 Hz, J
= 433 Hz, major
109Ag–31P
107Ag–31P
species, AgOAc(dppp)), −6.26 (dd, J
=
109Ag–31P
253 Hz,
J
=
218 Hz, minor species,
107Ag–31P
AgOAc(dppp)2) FAB–MS calculated for C60H62Ag2-
O4P4 ([AgOAc(dppp)]2), 1186 ((M)+); found,
1126 ((M-OAc)+), 957 ((M-OAc-AgOAc)+), 531
((M-OAc-AgOAc-dppf)+), elemental analysis; cal-
culated for [AgOAc(dppp)]2·H2O C, 59.20; H, 5.14,
found, C, 59.00; H, 5.14.
3.14. X-ray crystallography
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fined after data collection with SAINT program [60].
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