Scandium triflate catalyzed allylation of acetals and gem-diacetates: A facile synthesis of homoallyl ethers and acetates

Article abstract of DOI:10.1055/s-2001-13379

Scandium triflate catalyzes efficiently the allylation reactions of acetals and gem-diacetates with allyltrimethylsilane at ambient temperature to afford the corresponding homoallyl ethers and acetates in high yields. Also it is found to be effective for

Full text of DOI:10.1055/s-2001-13379

LETTER
673
Scandium Triflate Catalyzed Allylation of Acetals and gem-Diacetates:
A Facile Synthesis of Homoallyl Ethers and Acetates
Jhillu S. Yadav,* Basi V. Subba Reddy, P. Srihari
Organic Chemistry Division 1, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Fax +91(40)7170512; E-mail: yadav@iict.ap.nic.in
Received 11 December 2000
OMe
Sc(OTf)3, TMOF
Abstract: Scandium triflate catalyzes efficiently the allylation reac-
Si
tions of acetals and gem-diacetates with allyltrimethylsilane at am-
bient temperature to afford the corresponding homoallyl ethers and
acetates in high yields. Also it is found to be effective for the direct
formation of homoallyl ethers from aldehydes and allylsilane in
presence of trimethylorthoformate.
R-CHO
+
R
CH2Cl2, rt
Scheme 1
Key words: scandium triflate, allyltrimethylsilane, homoallyl
ethers, homoallyl acetates
homoallyl ethers from aldehydes and allylsilane in the
presence of trimethylorthoformate using scandium triflate
as catalyst. The reaction of benzaldehyde with allyltrime-
thylsilane and trimethylorthoformate in presence of cata-
lytic amount of scandium triflate (10 mol%) in dichloro-
methane afforded the corresponding homoallyl ether in
Lewis acid promoted carbon-carbon bond forming reac-
tions are of great interest because of their unique reactivi-
ty and selectivity. Among various Lewis acid catalyzed
C C bond forming reactions, the allylation of carbonyl
compounds or their equivalents with allylsilanes is one of
9
0% yield. Similarly, several aromatic aldehydes were re-
acted with allyltrimethylsilane in the presence of trimeth-
ylorthroformate to give the desired homoallylethers in
high yields. The reactions proceeded smoothly at room
temperature and completed within 5-8 hours (Table 1).
The reactions underwent quite cleanly without any side
reactions and the desired products were obtained in 70
1
the most useful synthetic reactions in organic synthesis.
2
-4
Generally, strong Lewis acids are required to promote
the addition of allylsilanes to carbonyl compounds and ac-
etals due to the inherent low nucleophilicity of allylsi-
lanes. These reactions are usually carried out under
strictly anhydrous conditions. The presence of even a
small amount of water causes lower yields probably due
to the rapid decomposition or deactivation of the promot-
ers and hydrolysis of acetals. Even though, conventional
methods require stoichiometric amount of Lewis acids,
9
0% yield. Further, the reactions of acetals and gem-diac-
etates with allyltrimethylsilane in the presence of scandi-
um triflate(5 mol%) in dichloromethane resulted in the
formation of homoallylic ethers and acetates in good
yields(Scheme 2).
5
-7
several catalytic processes have been developed to re-
alize high efficiency by using novel catalysts. However,
they cannot be recovered and reused because they decom-
pose under quenching conditions. Lanthanide triflates are
found to be highly oxophilic and form strong but labile
bonds with oxygen donor ligands. This feature has often
allowed sub-stoichiometric amount of the Lewis acid to
be used to promote a variety of reactions. Indeed, such
Lewis acids are effective catalysts in many fundamental
OR'
OR'
Sc(OTf)3,
+
Si
CH2Cl2, rt
R
R
OR'
R'=Me, Ac
Scheme 2
Both activated and unactivated aromatic acetals and gem-
diacetates were converted into the corresponding homoal-
lylic ethers and acetates in high yields(Table 2). In the
case methoxy substituted aldehyde diacetates, bis-allylat-
ed products were observed in 5-10% yield along with the
desired homoallylic acetate. This is due to the low nucleo-
philicity of allylsilane and also the low electrophilicity of
electron rich aldehydes which requires strong Lewis acids
for activation. Strong Lewis acids promoted carbocation
formation becomes increasingly facile as aromatic alde-
hyde increases in electron density that results in diallyla-
tion. The ketone dimethylacetals and diacetates did not
work under the present reaction conditions. However, ali-
phatic aldehyde acetals gave moderate yields of products
8
reactions, including Diels - Alder reactions, Michael re-
actions, Friedel-Crafts acylations and Mukaiyama aldol
9
reactions. Also they have been used to catalyze the addi-
tion of allyl metals and less reactive allylsilanes to carbo-
nyl compounds. Even though, scandium triflate has been
1
0
employed as catalyst in carbonyl addition reactions, it
has not been used in the substitution reactions of acetals
and gem-diacetates. And also this is the first report on the
use of scandium triflate for the direct formation of ho-
moallyl ethers from aldehydes and allylsilane in the pres-
ence of trimethylorthoformate(Scheme 1).
1
1
In continuation of our work, we report here a mild and
highly efficient procedure for the one pot synthesis of
Synlett 2001 No. 5, 673–675 ISSN 0936-5214 © Thieme Stuttgart · New York

6
74
J. S. Yadav et al.
LETTER
Table 1 Sc(OTf) -Catalyzed Conversion of Acetals into Homoallyl
Table 2 Sc(OTf) -Catalyzed Conversion of Acetals into Homoallyl
3
3
Ethers
Ethers
a
All products were characterized by ’HNMR, IR and Mass spectra
Isolated and unoptimized yields after purification
b
a
All products were characterized by ’HNMR, IR and Mass spectra
Isolated and unoptimized yields after purification
c
5
-10% of bis-allylated products were isolated
b
after long reaction times due to their intrinsic lower reac- diacetates with allyltrimethylsilane. And also scandium
tivity. Several examples illustrating this novel and effi- triflate catalyzes efficiently the direct formation of ho-
cient procedure for the preparation of the homoallylic moallyl ethers from aldehydes and allylsilane in presence
ethers and acetates are summarized in the Table 2. We of trimethylorthoformate. The procedure offers several
have examined the catalytic activity of various lanthanide advantages including mild reaction conditions, greater se-
triflates such as Sc(OTf) , Yb(OTf) and Y(OTf) in the lectivity, regeneration of the catalyst, high yields of prod-
3
3
3
allylation reaction. Among these catalysts, Sc(OTf) was ucts, cleaner reactions and simple experimental/isolation
3
found to be more effective in terms of conversion and re- procedures which makes it a useful and attractive strategy
action time. The catalyst was easily recovered from the for the preparation of homoallylic ethers and acetates.
aqueous layer while work-up and reused without signifi-
cant loss of activity.
Acknowledgement
In conclusion, Sc(OTf) is found to be an efficient and re-
usable catalyst for the allylation of both acetals and gem-
3
BVS and PSH thank CSIR, New Delhi for award of fellowship.
Synlett 2001, No. 5, 673–675 ISSN 0936-5214 © Thieme Stuttgart · New York

LETTER
Scandium Triflate Catalyzed Allylation of Acetals and gem-Diacetates
675
References and Notes
(12) General Procedure: (a)Preparation of homoallylethers from
aldehydes: A mixture of aldehyde (5 mmol),
IICT Communication No. :4620
trimethylorthoformate(6 mmol), allyltrimethylsilane
(
(
6 mmol) and scandium triflate (10 mol%) in dichloromethane
15 mL) was stirred at room temperature for an appropriate
(
(
1) (a) Sakurai, H. Pure Appl. Chem. 1982, 54, 1; (b) Hosomi, A.;
Sakurai, H. Tetrahedron Lett. 1976, 16, 1295.
2) (a) Flemming, I.; Dunogues, J.; Smithers, R. Org. Reactions
time. After completion of the reaction as indicated by TLC,
the reaction mixture was diluted with water (10 mL) and
extracted twice with dichloromethane (2 10 mL). The
combined organic layers were dried, concentrated in vacuo
and the resulting product was purified by column
chromatography on silica gel(Merck, 100-200 mesh, ethyl
acetate: hexane, 0.5:95) to afford pure homoallyl ether.
(
or gem-diacetate(5 mmol), allyltrimethyl silane(6 mmol) and
scandium triflate (5 mol%) in dichloromethane (10 mL) was
stirred at room temperature for an appropriate time. After
complete conversion as indicated by TLC, the reaction
mixture was diluted with water (10 mL) and extracted twice
with dichloromethane (2 15 mL). The combined organic
layers were dried over anhydrous Na SO concentrated in
1989, 37, 57. (b) Hunter, R.; Tomlinson, G. D. Tetrahedron
Lett. 1989, 30, 2013.
(
(
3) (a) Hosomi, A.; Endo, M.; Sakurai, H. Chem Lett. 1976, 941.
(b) McCluskey A.; Mayer, D. M.; Young, D. J. Tetrahedron
Lett. 1997, 38, 5217.
4) (a) Tanaka, H.; Yamashita, S.; Ikemoto, Y.; Torii, S.
Tetrahedron Lett. 1988, 29, 1721. (b) Komatsu, N.; Uda, M.;
Suzuki, H.; Takahashi, T; Domae, T; Wada, M. Tetrahedron
Lett. 1997, 38, 7215.
b) Allylation of acetals or gem-diacetates:A mixture of acetal
(5) (a) Mukaiyama, T.; Nagaoka, H.; Murakami, M; and
Ohshima, M. Chem. Lett. 1985, 977. (b) Tsunoda, T.; Suzuki,
M.; Noyori, R. Tetrahedron Lett. 1980, 21, 71.
(6) (a) Sakurai, H.; Sasaki, K.; Hosomi, A. Tetrahedron Lett.
(
2
4,
1981, 22, 745. (b) Trehan, A.; Vij, A.; Walia, M. Kaur, G.;
vacuo and purified by column chromatography on silica gel
Merck 100 200 mesh, ethyl acetate: hexane, 0.5:95) to
Verma, R. D.; Trehan. S. Tetrahedron Lett. 1993, 34, 7335.
7) (a) Noyori, R.; Murata, S.; Suzuki, M. Tetrahedron 1981, 37,
(
afford pure homoallyl ether or acetate. Representative data for
homoallyl ether: compound 2b (Table 1): H NMR (CDCl ):
3899. (b) Wang, M. W.; Chen, Y. J.; Wang, D. Synlett 2000,
1
3
385.
2
.35 (m, 1H). 2.50(m, 1H), 3.20 (s, 3H), 4.05 (t, 1H,
J = 6.8Hz), 5.05 (m, 2H), 5.75 (m, 1H), 6.0 (s. 2H), 6.75-6.85
m, 3H). EIMS: m/z (%) :206 M+(15), 165 (100), 150 (90),
(
(
8) (a) Kobayashi, S.; Eur. J. Org. Chem. 1999. 15 (b) Kobayashi,
S. Synlett 1994, 689. (C) Long bottom, D. Synlett. 1999, 2023.
9) (a)Yang, Y.; Wang, M. W.; Wang, D. J. Chem. Soc. Chem.
Commun. 1997, 1651. (b) Fang, X, Watkin, J. G.; Warner, B.
P. Tetrahedron Lett. 2000, 41, 447. (c) Bonini, B. F.;
(
1
1
15 (30), 77 (45). IR (neat) : 3076.6, 2980.5, 2823.6, 1641.3,
486.9, 1244.8, 1040.7, 998, 978. Compound 4d (R = Ac,
Table 2) NMR (CDCl ): 2.10 (s, 3H), 2.60 (m, 2H), 5.15 (m,
2
(
1
3
Franchini, M. C.; Fochi, M.; Mazzanti, G.; Nanni, C.; Ricci,
A. Tetrahedron Lett. 1998, 39, 6737 (d)Kobayashi, S.;
Basujima, T.; Nagayama, S. J. Chem. Soc.Chem. Commun.
H), 5.65 (m, 1H),5.75 (t, 1H, J = 6.8Hz), 7.15 (m, 1H), 7.45
+
m, 2H). EIMS: m/z :258(M ), 219, 217, 177, 173, 148, 142,
11, 75. IR (KBr):3055, 2980, 1740, 1620, 1580, 970.
1998, 19.
(
(
10) (a) Aggarwal, V. K.; Vennall, G. P. Tetrahedron Lett. 1996,
37, 3745. (b) Aggarwal, V. K.; Vennall, G. P.Synthesis 1998,
Article Identifier:
437-2096,E;2001,0,05,0673,0675,ftx,en;S11100ST.pdf
1822.
1
11) Yadav, J. S.; Subba Reddy, B. V.; Kiran Kumar Reddy, G. S.
Tetrahedron Lett. 2000, 41, 2695.
Synlett 2001, No. 5, 673–675 ISSN 0936-5214 © Thieme Stuttgart · New York