Silver oxide as a novel catalyst for carbon-carbon bond-forming reactions in aqueous media

Article abstract of DOI:10.1246/cl.2010.652

Silver oxide was found to be an excellent catalyst for allylation reactions of allyltributyltins with aldehydes in aqueous media. Despite the very low solubility of silver oxide in the media, the reactions proceeded smoothly, and the catalyst was recovere

Full text of DOI:10.1246/cl.2010.652

doi:10.1246/cl.2010.652
652
Published on the web May 19, 2010
Silver Oxide as a Novel Catalyst for Carbon-Carbon
Bond-forming Reactions in Aqueous Media
Masaharu Ueno, Arata Tanoue, and Shū Kobayashi*
Department of Chemistry, School of Science and Graduate School of Pharmaceutical Sciences,
The University of Tokyo, and The HFRE Division, ERATO, Japan Science Technology Agency (JST),
Hongo, Bunkyo-ku, Tokyo 113-0033
(Received April 9, 2010; CL-100350; E-mail: shu_kobayashi@chem.s.u-tokyo.ac.jp)
Silver oxide was found to be an excellent catalyst for allylation
Table 1. Allylation reactions of benzaldehyde with allyltins in the
presence of a catalytic amount of silver source in aqueous media
reactions of allyltributyltins with aldehydes in aqueous media.
Despite the very low solubility of silver oxide in the media, the
reactions proceeded smoothly, and the catalyst was recovered and
reused. When ¡-methyl-substituted allyltributyltin was used as the
nucleophile, the corresponding ¡-adducts were obtained exclusively
in high yields with good anti-selectivity. A reaction mechanism
including transmetalation from tin to silver is proposed.
R1
SnBu3 10 mol% Silver source
γ
α
+
PhCHO
R2 R3
(1.5 equiv)
THF:H2O = 1:1 (0.2 M)
rt, 24 h
OH
α
OH
γ
R1
R3
α
γ
+
Ph
Ph
R1 R2
γ-adduct (B)
R3 R2
α-adduct (A)
Silver
source
Yield
/%
Entry Allyltin
A/B
syn/anti
Organic reactions in water are now of great interest. Water
is an inexpensive, safe, and environmentally benign solvent.1
However, the use of water in organic synthesis is limited
because most organic materials are hydrophobic, and thus are
not soluble in water. In addition, many active catalysts are not
stable in water; rather, they decompose even in the presence of a
small amount of water. To address these issues, we have
searched for efficient catalysts, that can work in aqueous media.2
In the course of our investigations, we found that silver
oxide (Ag2O)3 was an excellent catalyst for the reaction of
allyltins with aldehydes in aqueous media. For example,
allyltributyltin (1) reacted with benzaldehyde in the presence
of 10 mol % of Ag2O in THF/H2O (1/1) at room temperature
to afford the corresponding homoallylic alcohol in 87% yield
(Table 1, Entry 1). While silver carbonate (Ag2CO3)4 also
showed a high yield (86%, Entry 2), AgOAc and AgF were
less effective (38 and 48% yields, respectively using 20 mol %
of the catalyst), and only trace amounts of the product were
obtained using AgCl and AgOTf. It should be noted that more
acidic silver salts gave lower yields.5 Moreover, the use of water
was found to be essential for the silver catalysis, because the
allylation reaction proceeded sluggishly in absolute THF.
We next examined the reactions with ¡-methyl-substituted
allyltributyltin 2.6 Very interestingly, the reaction of 2 with
benzaldehyde proceeded faster than that of 1, and the corre-
sponding ¡-adduct7-10 was obtained exclusively in high yields
using Ag2O and Ag2CO3 (Table 1, Entries 3 and 4). As for the
relative stereochemistry, the anti-adduct was obtained preferen-
tially.11 We further tested the reactions of (Z)- and (E)-
crotyltributyltin (3Z and 3E) with benzaldehyde. In these cases,
although Ag2O gave the £-adducts (B) selectively, the reactions
proceeded sluggishly in the presence of Ag2CO3.
1
2
3
4
5
6
7
8
1
1
2
2
3Z
3Z
3E
3E
Ag2O
Ag2CO3
Ag2O
Ag2CO3
Ag2O
Ag2CO3
Ag2O
87
86
84
73
86
<5
20
6
®
®
®
®
>99/1
>99/1
14/86a
48/52
16/84b
76/24
18/82 (A)
18/82 (A)
18/82 (B)
®
17/83 (B)
16/84 (B)
Ag2CO3
SnBu3
SnBu3
SnBu3
SnBu3
1
2
3Z (E/Z = 1/99) 3E (E/Z = 95/5)
aE/Z ratio of ¡-adduct(A) = 10/1. bE/Z ratio of ¡-adduct
(A) = 6/1.
3 mol% Ag2O
+
RSnBu3
PhCHO
Product
THF-d8:D2O = 9:1 (0.2 M)
(1.0-1.1 equiv)
rt, Time (h)
90
80
70
60
50
40
30
20
10
α-methyl allyltin (2)
isomerization of 2 to 3
allyltin (1)
(Z)-crotyltin (3Z)
0
0
0.5
1
1.5
2
Time/h
Figure 1. Reaction profiles of 1, 2, and 3Z with benzaldehyde and
isomerization of 2 to 3.
We were very interested in the unique catalytic activities of
the silver salts and the selectivities of the allylations in aqueous
media, and further examined the reaction course carefully. The
profiles of the reactions of 1, 2, and 3Z with benzaldehyde and
isomerization of 2 to 3 in the presence of Ag2O are shown in
Figure 1. In the allylations of 1 and 2, the reactions completed
within 1 h in the presence of 3 mol % of Ag2O. On the other
hand, when we used (Z)-crotyltributyltin (3Z) as the nucleophile,
less than 10% yield of the product was obtained in 1 h. It is
known that ¡-methyl-substituted allyltributyltin 2 is gradually
isomerized to the corresponding crotyltributyltin 3.6 We care-
fully examined the isomerization rate from 2 to 3, which was
found to be much slower than the rate of the allylation reaction 2
with benzaldehyde. It was concluded that the allylation reaction
Chem. Lett. 2010, 39, 652-653
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/

653
OH
Table 2. Substrate scope of aldehydes with allyltins in aqueous
media
RCHO
SnBu3
Transmetalation
Me
[Ag]
R
Fast
Fast
Me
OH
Me
OH
SnBu3
3 mol% Ag2O
2
R4CHO
+
R3
R4
+
Isomerization
Isomerization
(Slow)
R4
R3
1 or 2 (1.5 equiv)
THF:H2O = 9:1 (0.2 M)
rt, Time (h)
R3
OH
α-adduct
γ-adduct
[Ag]
Me
SnBu3
RCHO
Transmetalation
Me
R
Time Yield
syn/anti
(¡)
Entry R4
R3
¡/£
Me
[Ag]
Slow : from 3Z
Very Slow : from 3E
Slow
3Z or 3E
/h
/%
1
2
3
4
5
6
7a
8b
1-Naphthyl
2-Furyl
H
H
H
H
H
H
H
Me
Me
Me
Me
5
24
24
5
24
5
12
4
4
4
3
4
4
79
87
®
®
®
®
®
®
®
®
®
®
®
®
®
®
O
OH
R
H
quantg
74
R
4-Pyridyl
Ph
(4-Br)C6H4
PhCH=CH
PhCH2CH2
1-Naphthyl
Lewis acid catalyzed mechanism
(Traditional pathway)
Slow
Me
76
76
67
99
78
89
84
62
Scheme 1. Assumed reaction pathway.
of 2 with benzaldehyde did not proceed via the corresponding
isomerized crotyltin 3.
>99/1 13/87
>99/1 14/86
>99/1 14/86
>99/1 10/90
>99/1 11/89
>99/1 13/87
>99/1 14/86
9b,c 2-Furyl
Based on these experiments, we assume the overall reaction
pathway as shown in Scheme 1. It is likely that the key step is
transmetalation from 2 to a crotylsilver species, which rapidly
reacts with an aldehyde to afford the formal ¡-addition product.
Thus, two £-additions could result in providing the ¡-addition
product. It is noted that the transmetalation of 2 seems to be
much faster than the isomerization from 2 to 3Z or 3E. In
contrast, transmetalation of crotyltin 3Z or 3E is supposed to be
slow, probably because of steric hindrance at the £-position of
3Z or 3E. In addition, conventional addition of 3Z or 3E to an
aldehyde in the presence of a silver salt also seems to be slow
because of the low Lewis acidity of the silver salt.
We then surveyed the substrate scope of this allylation
(Table 2). Not only aromatic aldehydes (Entries 1-5), ¡,¢-
unsaturated aldehyde (Entry 6), but also aliphatic aldehyde
(Entry 7) reacted smoothly in the presence of a catalytic amount
of silver oxide to afford the desired compounds in good to
excellent yields. It is noted that in all cases only ¡-adducts were
obtained in high yields when ¡-methyl-substituted allyltributyl-
tin 2 was used (Entries 8-14). In addition, it was found that the
reactions of 2 with aldehydes proceeded faster than those of 1,
and the corresponding ¡-adducts were obtained exclusively with
good anti-selectivity.
In conclusion, we have found that allylation reactions of
allyltins with aldehydes proceed smoothly in the presence of a
catalytic amount of silver oxide in aqueous media. The use of
silver oxide as a catalyst in organic synthesis is rare, and parti-
cularly interesting is the use of aqueous media for the reactions. In
addition, it is unique that the ¡-addition adducts were obtained
exclusively in high yields with good anti-selectivity when ¡-
methyl-substituted allyltributyltin was used as the nucleophile.
Further investigations to clarify the detailed mechanism of this
reaction, including the role of water, are now in progress.
10b
11b
4-Pyridyl
Ph
12d,e (4-MeO)C6H4 Me
13f
14c
(4-Br)C6H4
PhCH2CH2
Me
Me
96
66
4
a3.0 equiv allyltin 1 was used. bMeOH:H2O = 9:1 was used as
solvent. c5 mol % Ag2CO3 was used instead of Ag2O. di-
e
PrOH:H2O = 9:1 was used as solvent. 7 mol % 1,10-phenanthro-
f
line was added. MeCN:H2O = 9:1 was used as solvent. g84%
yield in 5 h.
2
3
Recent work: a) M. Kokubo, C. Ogawa, S. Kobayashi, Angew. Chem.,
Int. Ed. 2008, 47, 6909. b) M. Kokubo, T. Naito, S. Kobayashi,
Tetrahedron 2010, 66, 1111. c) S. Kobayashi, M. Kokubo, K.
Kawasumi, T. Nagano, Chem. Asian J. 2010, 5, 490.
Ag2O-catalyzed oxidation reactions in aqueous media, see: a) F.
Derikvand, F. Bigi, R. Maggi, C. G. Piscopo, G. Sartori, J. Catal.
2010, 271, 99; Previously, we found that an allylsilane reacted with an
aldehyde in the presence of Ag2O, but substrates were limited. b) S.
Kobayashi, K. Hirano, M. Sugiura, unpublished.
4
5
Au(PPh3)Cl/Ag2CO3-catalyzed reaction in aqueous media, see: D. Ye,
J. Wang, X. Zhang, Y. Zhou, X. Ding, E. Feng, H. Sun, G. Liu, H.
Jiang, H. Liu, Green Chem. 2009, 11, 1201.
Asymmetric allylation of allyltins with aldehydes catalyzed by
AgOTf/BINAP complex, see: a) A. Yanagisawa, H. Nakashima, A.
Ishiba, H. Yamamoto, J. Am. Chem. Soc. 1996, 118, 4723. b) A.
Yanagisawa, H. Nakashima, Y. Nakatsuka, A. Ishiba, H. Yamamoto,
Bull. Chem. Soc. Jpn. 2001, 74, 1129.
6
7
V. J. Jephcote, E. J. Thomas, J. Chem. Soc., Perkin Trans. 1 1991, 429.
Selected examples of ¡-addition via the corresponding isomerized
crotyltin, see: a) J. A. Marshall, W. Y. Gung, Tetrahedron Lett. 1989,
30, 309. b) J.-P. Quintard, G. Dumartin, B. Elissondo, A. Rahm, M.
Pereyre, Tetrahedron 1989, 45, 1017.
8
9
Selected examples of formal ¡-addition via two £-additions using
stoichiometric amounts of Lewis acid, see: a) D. Furlani, D. Marton,
G. Tagliavini, M. Zordan, J. Organomet. Chem. 1988, 341, 345. b) J.
Becker, R. Fröhlich, K. Salorinne, D. Hoppe, Eur. J. Org. Chem. 2007,
3337. c) M. Yasuda, T. Azuma, K. Tsuruwa, S. A. Babu, A. Baba,
Tetrahedron Lett. 2009, 50, 3209.
Marshall et al. reported InCl3-promoted formal ¡-addition of ¡-alkoxy
allyltins to aldehyde and application to natural product synthesis, see:
a) J. A. Marshall, A. W. Garofalo, J. Org. Chem. 1996, 61, 8732. b)
J. A. Marshall, A. Piettre, M. A. Paige, F. Valeriote, J. Org. Chem.
2003, 68, 1771.
This work was partially supported by a Grant-in-Aid for
Science Research from the Japan Society for the Promotion of
Science (JSPS) and Global COE Program (Chemistry Innovation
through Cooperation of Science and Engineering), The Univer-
sity of Tokyo, MEXT, Japan.
10 Formal ¡-addition of 1,4-cyclohexadienyltributyltin to aldehydes
using AgOTf/BINAP catalyst has been reported, see: R. Umeda, A.
Studer, Org. Lett. 2008, 10, 993.
References and Notes
11 Recently, we reported Zn(OH)2-catalyzed ¡-addition of ¡-substituted
allylboronates to aldehydes in aqueous media: S. Kobayashi, T. Endo,
U. Schneider, M. Ueno, Chem. Commun. 2010, 46, 1260.
12 Supporting Information is available electronically on the CSJ-Journal
Web site, http://www.csj.jp/journals/chem-lett/.
1
a) Aqueous-Phase Organometallic Catalysis, 2nd ed., ed. by B.
Cornils, W. A. Herrmann, Wiley-VCH, Weinheim, 2004. b) Organic
Reactions in Water, ed. by U. M. Lindström, Blackwell, Oxford, 2007.
c) S. Kobayashi, Pure Appl. Chem. 2007, 79, 235.
Chem. Lett. 2010, 39, 652-653
© 2010 The Chemical Society of Japan
www.csj.jp/journals/chem-lett/