Catalytic organic reactions on the surface of silver(I) oxide in water

Article abstract of DOI:10.1246/cl.140818

We have developed Ag₂O-catalyzed allylation reactions of aldehydes with allylsilanes in water, providing homoallylic alcohols in high yields with high anti-selectivities. It was found that the reactions proceeded not in the water solution but o

Full text of DOI:10.1246/cl.140818

Received: August 31, 2014 | Accepted: September 14, 2014 | Web Released: September 20, 2014
CL-140818
Catalytic Organic Reactions on the Surface
of Silver(I) Oxide in Water
Masaharu Ueno, Arata Tanoue, and Shū Kobayashi*
Department of Chemistry, School of Science, The University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-0033
(E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp)
Table 1. Effect of solvents
We have developed Ag2O-catalyzed allylation reactions of
aldehydes with allylsilanes in water, providing homoallylic
alcohols in high yields with high anti-selectivities. It was found
that the reactions proceeded not in the water solution but on the
surface of Ag2O. We could reduce the amount of the catalyst
to less than 0.01 mol %. Moreover, an asymmetric allylation
reaction was performed in water with high yield and moderate
enantioselectivity by using a chiral phosphine ligand.
O
OH
Ag2O (10 mol%)
PhCHO
+
Si
Solvent, rt, 5 h
O
Ph
(1.5 equiv)
Entry
Solvent
Yield/% Entry
Solvent
Yield/%
1
2
3
4
5
6
7
8
DME/H2O = 6/1
THF/H2O = 6/1
MeCN/H2O = 6/1
acetone/H2O = 6/1
hexane/H2O = 6/1
MeOH/H2O = 6/1
EtOH/H2O = 6/1
iPrOH/H2O = 6/1
70
52
42
69
61
65
81
80
9
EtOH
H2O
H2O
H2O
80
80
NR
86
31
22
23
62
10
11a
12b
13 neat (No solvent)
14 THF
15c EtOH/H2O = 1/1
16c
H2O
Recently, from the viewpoint of the environment and
sustainable development, alternative media to commonly used
organic solvents for organic reactions have been extensively
studied. In particular, attention has been paid to water as a
solvent because it is safe, benign, environmentally friendly, and
inexpensive as compared with organic solvents.1 Furthermore,
unique reactivities and selectivities, which are observed only in
water, have been reported.2-4
Catalytic reactions are also important from the standpoint
of green sustainable chemistry.5 In catalytic reactions in water,
solubility of organic substrates and catalysts is an important
issue. Water-soluble catalysts are not necessarily efficient
because organic substrates are not soluble in water. To address
this issue, surfactant-like amphiphilic catalysts have been
developed;6 however, separation of such catalysts after the
reaction poses another problem. We report here a different
catalysis in water: catalytic reactions proceeding efficiently
on the surface of an inorganic solid (Ag2O). Allylation reac-
tions of water-insoluble substrates proceeded smoothly in 100%
water.
We have recently developed enantioselective 1,4-additions
of diboron to α,β-unsaturated carbonyl compounds and nitrile
catalyzed by Cu(OH)2 with a chiral bipyridine ligand in water.2f
Interestingly, all the components (catalyst, chiral ligand, and
substrates) are insoluble in water, but the reactions proceeded
smoothly, effective asymmetric environments were created on
the surface of the metal catalyst, and very efficient catalyst
turnover was achieved with high enantioselectivity. Inspired by
this work, we investigated silver(I) oxide (Ag2O)-catalyzed
allylation reactions of aldehydes in water.
The allylation reaction of aldehydes to provide homoallylic
alcohols is one of the most important carbon-carbon bond-
forming reactions.7 Recently, we developed Zn(OH)2-dimethyl-
phenanthroline complex-catalyzed syn-selective allylation reac-
tions of aldehydes with allylboronates in aqueous media.3 This
reaction system was applied to asymmetric reactions using a
chiral bipyridine ligand instead of dimethylphenanthroline.3b
We also reported8 Ag2O-catalyzed anti-selective allylation
reactions of aldehydes with allyltin in aqueous media.4 In
searching for more environmentally friendly reagents, we found
that allysilanes9 also worked as allylation reagents.
c
aWithout catalyst. b1.0 mol % Ag2O was used. PhCH2CH2CHO was
used instead of PhCHO.
First, we optimized the reaction conditions for the allylation
reaction of benzaldehyde with allylmethylpinacolsilane in the
presence of Ag2O (Table 1). By the screening of cosolvents,
it was found that the reaction proceeded smoothly not only in
polar solvents (Entries 1-4) and a nonpolar solvent (Entry 5),
but also in protic solvents (Entries 6-8). When ethanol (Entry 9)
or water (Entry 10) was used as a solvent, the reaction proceeded
smoothly with high yield. In this reaction, Ag2O was necessary
as a catalyst (Entry 11); however, 1.0 mol % of the catalyst was
sufficient to promote the reaction (Entry 12). It is noted that
water is essential for the reaction because the yield decreased
when the reaction was conducted under neat conditions or using
THF as a solvent (Entries 13 and 14). When we conducted the
reaction of 3-phenylpropanal, a better yield was obtained by
using only water rather than water-organic solvent mixtures as
solvents. Water is best not only for the environment but also as a
medium for this reaction (Entries 15 and 16).
Previously, we found that the structure of the boronate parts
of allylboronates strongly affected the reactivity in the Zn(OH)2-
catalyzed allylation reactions of aldehydes with allylboronates
in aqueous media.3c Therefore, we synthesized several types of
allysilanes and examined the reactivities in water (Table 2).
Interestingly, the allylation reaction proceeded smoothly when
allylmethylpinacolsilane was used; however, the reaction did not
proceed at all when allyltrimethylsilane or allyltrimethoxysilane
was used as an allylation reagent (Entries 1-3). Yoshida et al.
reported that 10 mol % of silver acetate-catalyzed allylation
reactions of aldehydes with allyldimethyl(2-pyridyl)silane in an
organic solvent.10a In that study, they also used a stoichiometric
amount of Ag2O with moderate yield (74%). They also reported
the allylation of aldehydes, ketones, and imines by using
10 mol % of CuI and 2.0 equiv of CsF.10b When allyldimethyl(2-
pyridyl)silane was employed in our system, the reaction
proceeded sluggishly (Entry 4). We also conducted the reactions
Chem. Lett. 2014, 43, 1867–1869 | doi:10.1246/cl.140818
© 2014 The Chemical Society of Japan | 1867

Table 2. Reactivity of allylsilanes
Table 4. Substrate scope
Ag2O (1.0 mol%)
OH
OH
Ag2O (1.0 mol%)
H2O, rt
O
O
Si
PhCHO
+
+
Si
R1
H2O, rt, 3 h
(1.5 equiv)
Ph
R1
H
O
R2
= >99/<1 (R2 = Me)
R2
Entry
Allylsilane
Yield/% Entry
Allylsilane
Yield/%
(1.5 equiv)
/
O
O
Si
Entry
Aldehyde (R1)
R2
Time/h Yield/%
syn/anti
Si
1
2
3
86
5
6
7
11
2
O
O
O
1
2
3
4
5
6
7
8
Ph
H
H
H
H
H
H
H
H
3
3
4
2
3
2
3
24
86
96
48
85
90
75
62
49
®
®
®
®
®
®
®
®
SiMe3
Si
1-Naphthyl
4-MeOC6H4
4-MeC6H4
4-BrC6H4
PhCH=CH
PhCH2CH2
nC9H19
NR
NR
O
OiPr
OiPr
Si(OMe)3
Si
NR
O
O
Ph
Si
N
Si
4
10
8
95
9a
10a
11a
12a
13a
14a
15a
16a
Ph
Me
Me
Me
Me
Me
Me
Me
Me
3
3
3
3
3
3
4
3
94
87
76
92
91
92
92
83
12/88
16/84
10/90
11/89
16/84
14/86
11/89
11/89
1-Naphthyl
4-MeOC6H4
4-MeC6H4
4-BrC6H4
PhCH=CH
PhCH2CH2
nC9H19
Table 3. Allylation reactions with α-methylallylsilane
Ag2O
(Additive)
OH
O
+
Si
PhCHO
Ph
Solvent, rt, 3 h
O
/
= >99/<1
(1.5 equiv)
Ag2O
/mol %
Additive
/mol %
Yield
/%
a5.0 mol % of DMAP was added.
Entry
Solvent
syn/anti
1
MeCN/H2O = 6/1
10
10
10
1.0
®
®
84
78
72
94
12/88
22/78
10/90
12/88
2
3
4
H2O
H2O
H2O
0.2 µm filter
(twice)
DMAP/48
DMAP/5
sat. Ag2O aq. soln. (A)
Ag2O
H2O, 28 °C, 12 h
Ag: 25.9 ppm
Concentration
(x50)
conc. Ag2O aq. soln. (B)
Ag: 30.4 ppm
(Supernatant)
using allysilanes derived from 2,2-dimethylpropanediol, cate-
chol, or iPrOH instead of pinacol, and in all cases, the reactions
did not proceed well (Entries 5-7). Almost the same reactivity
was observed when we used allylphenylpinacolsilane as an
allylation reagent (Entry 8).
O
OH
PhCHO
Si
+
rt, 3 h
O
Ph
(1.5 equiv)
THF:(A) = 4:1 (1.0 mL): N.R.
THF:(B) = 4:1 (1.0 mL): 82%
We next examined the reactions with α-methyl-substituted
allylmethylpinacolsilane (Table 3). Similar to the reactions with
allyltin,4 the corresponding α-adduct was obtained exclusively in
high yields with good anti-selectivities. Accordingly, a reaction
mechanism including transmetalation from silicon to silver was
proposed.11 The anti-selectivity was decreased in water (Entries
1 and 2). To improve the selectivity, we screened additives and
found that the addition of 4-dimethylaminopyridine (DMAP)
was effective for obtaining high diastereoselectivities (Entry 3).
Even in the presence of 1.0 mol % catalyst, the yield and the
selectivity remained at high levels (Entry 4). We then surveyed
the substrate scope under the optimal conditions (Table 4). In all
cases, the reactions proceeded smoothly to afford the desired
homoallylic alcohols in moderate to high yields.
When we observed the reaction medium, Ag2O seemed to be
insoluble in the system. It is known that Ag2O is slightly soluble
in water, and in fact, we observed 25.9 ppm of silver in a solution
after filtration (0.2 ¯m filter, twice), using inductively coupled
plasma analysis. However, the reaction did not proceed when
this solution was used as the catalyst. In contrast, solid Ag2O was
formed by concentrating the solution, and the reaction proceeded
in the presence of this solid (Scheme 1). Judging from these
experiments, the reaction proceeded only on the surface of
Ag2O, not in a partially dissolved solution. It was assumed from
these results that not all of the Ag2O was used as the catalyst.
Scheme 1. Confirmation experiments whether the reaction
proceeded “in the water solution” or “on the surface of Ag2O.”
Table 5. Improvement of TON
O
Ag2O (mol%)
OH
PhCHO
Si
+
H2O, rt, Time
O
Ph
(1.5 equiv)
¹1
Entry Ag2O/mol % (S/C) Time/h Yield/% TON TOF/h
1
2
3
4
0.1 (1000)
6
10
20
32
75.8
86.9
84.3
38.4
758
4340
8430
7680
127
434
422
240
0.02 (5000)
0.01 (10000)
0.005 (20000)
We then performed the reaction using low catalyst loading of
Ag2O (Table 5), and even in the presence of 0.01 mol % Ag2O,
the reaction proceeded with high productivity (Entry 3).12
Finally, we performed an asymmetric variant of this reaction
(Scheme 2). The addition of a chiral phosphine ligand13 was
found to be effective, and a high yield with moderate
enantioselectivity was observed.
1868 | Chem. Lett. 2014, 43, 1867–1869 | doi:10.1246/cl.140818
© 2014 The Chemical Society of Japan

Ag2O (1 mol%)
Ligand (3 mol%)
2013, 8, 2033.
OH
O
O
Si
PhCHO
+
4
5
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H2O (1.0 mL), rt, 3 h Ph
(1.5 equiv)
Ar
Ph
Ph
O
O
O
O
O
O
P
N
P
N
P
N
Ph
Ph
Ar
6
75%, 36% ee (S)
Ar = Ph; 89%, 57% ee (R)
81%, 0% ee
Ar = 3,5-Me2C6H3; 88%, 63% ee (R)
0.2 mol% Ag2O, 0.6 mol% Ligand (Ar = Ph)
Slow addition 6 h then 12 h; 89%, 53% ee
Scheme 2. Asymmetric allylation in water.
In conclusion, we have developed Ag2O-catalyzed allyla-
tion reactions of aldehydes with allylsilanes in water. The
reactions proceeded smoothly to afford homoallylic alcohols in
high yields with high anti-selectivities. It is suggested that only
the surface of Ag2O was used as a catalyst, and we could reduce
the amount of the catalyst to less than 0.01 mol %. Moreover, an
asymmetric allylation reaction was performed to afford the
desired product in high yield with moderate enantioselectivity
by using a chiral phosphine ligand. Further investigations to
elucidate the reaction mechanism as well as to develop general
asymmetric reactions are now in progress.
7
8
This work was partially supported by a Grant-in-Aid for
Science Research from the Japan Society for the Promotion of
Science (JSPS).
9
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Chem. Lett. 2014, 43, 1867–1869 | doi:10.1246/cl.140818
© 2014 The Chemical Society of Japan | 1869