LETTER
Allylation of Aldehydes, Aldimines and Ring Opening of Terminal Aromatic Epoxides
233
H
O
H
H
SnBu3
Sc(OTf)3 (5 mol %)
PEG Solvent,
Ar
+
Ar
H
55–60 °C, 24 h
OH
Ar = Ph, 2Br-C6H4
Yield upto 90%
Scheme 1 Scandium triflate catalyzed regioselective ring opening of terminal aromatic epoxides using PEG as a solvent medium
by restricting their widespread application. Recently, in References
situ generated allylindium reagents have also been uti-
(
1) Green Chemistry, ACS Symposium Series 626; Anastas, P.
lized to achieve the synthesis of bishomoallyl alcohols
from terminal epoxides in which both regioisomers are
T.; Williamson, T. C., Eds.; American Chemical Society:
Washington, and references therein.
1
6
present. Therefore, a totally regiocontrolled procedure
for the ring opening of oxiranes by allyl transfer agents is
highly demanding.
(2) (a) Cave, G. W. V.; Raston, C. L.; Scott, J. L. Chem.
Commun. 2001, 2159. (b) Toda, F.; Tanaka, K. Chem. Rev.
2
000, 100, 1025. (c) Cave, G. W. V.; Raston, C. L. Chem.
Commun. 2000, 2199. (d) Toda, F. Acc. Chem. Res. 1995,
8, 480. (e) Toda, F. Synlett 1993, 303.
We tested the ring opening of styrene epoxides using 5
mol% scandium triflate at 55–60 °C. The reactions pro-
ceed smoothly in PEG with nearly quantitative conversion
resulting in the formation of predominantly one regio-
isomer of the corresponding unsaturated alcohols
2
(3) (a) Pereyre, M.; Quintard, J.-P.; Rahm, A. Tin in Organic
Synthesis; Butterworths: London, 1987, 216.
(
b) Nishigaichi, Y.; Takuwa, A.; Narutan, Y.; Maruyama, K.
Tetrahedron 1993, 49, 7395. (c) Yamamoto, Y.; Asao, N.
Chem. Rev. 1993, 93, 2207.
1
7
(
Scheme 1).
(
4) Keck, G. E.; Enholm, J. J. Org. Chem. 1985, 50, 146.
To check the reusability of the solvent as well as the cata-
lyst, after the initial reaction, the reaction mixture was ex-
(5) Choudary, B. M.; Chidara, S.; Rajasekhar, C. V. Synlett
002, 1694.
(6) (a) Hachiya, I.; Kobayashi, S. J. Org. Chem. 1993, 58, 6958.
b) Fukuzawa, S.; Sato, K.; Fujinami, T.; Sasaki, S. J. Chem.
2
1
8
tracted with dry diethyl ether and the residual diethyl
ether was removed from the reaction mixture by concen-
trating on the rotavapor. The subsequent cycle was con-
ducted adding the reactants to the spent reaction medium.
This protocol avoids water to recover the metal triflate.
Therefore, the activity of the catalyst remains almost con-
stant and recycled for several times without significant re-
duction of the yield. Thus, the scandium triflate, which is
expensive, can be reused for several times using PEG as
an effective recyclable medium.
(
Soc., Chem. Commun. 1993, 853. (c) Yanagisawa, A.;
Inoue, H.; Morodome, M.; Yamamoto, H. J. Am. Chem. Soc.
1993, 115, 10356.
(7) Akiyama, T.; Iwai, J. Tetrahedron Lett. 1997, 38, 853.
(8) (a) Namboodiri, V. V.; Verma, R. S. Green Chem. 2001,
146. (b) Chandrasekhar, S.; Narasihmulu, C.; Sultana, S. S.;
Reddy, N. R. Org. Lett. 2002, 4, 4399.
(
9) Harris, J. M. In Polyethylene Glycol Chemistry.
Biotechnological and Biomedium Applications; Jarris, J. M.,
Ed.; Plenum Press: New York, 1992, 3.
(
10) Typical Procedure for the Catalytic Allylation of
Aldehydes: A mixture of p-nitrobenzaldehyde (1 mmol),
allyltributylstannane (1.2 mmol), benzoic acid (1 mmol),
PEG (2 g) and scandium triflate (5 mol%) was placed in a 25
mL round bottomed flask. The mixture was stirred at r.t. for
In conclusion, polyethylene glycol, which is non-toxic,
thermally stable, inexpensive and recyclable reaction
medium for allylation reactions, with several advantages
including high yields, total regioselectivity, shortened
reaction timings, simple experimental and work-up proce-
dures, makes it a useful and attractive strategy for the syn-
thesis of homoallylic alcohols, amines and bishomoallylic
alcohols. In fact, this forms the first report, PEG as a
recyclable medium for Lewis acid catalyzed allylation
reactions and cleaner and greener recyclable reaction me-
dium thus substituting for volatile organic solvents. The
expensive scandium triflate is recycled for several times
without significant loss of activity. The reusability of the
catalyst makes the reaction economical and commercially
applicable.
3
h (monitored by TLC). The reaction mixture was extracted
with dry Et O and the crude product was purified by column
2
chromatography to obtain homoallylic alcohol in excellent
1
yields. The products were characterized by H NMR data.
1
H NMR (200 MHz, CDCl ): d = 2.01 (d, 1 H, OH), 2.52 (m,
3
2
5
H, CH ), 4.80 (t, 1 H, CH), 5.14–5.20 (m, 2 H, 2 × vinyl),
2
.82 (m, 1 H, vinyl), 7.50 (d, 2 H, aromatic), 8.20 (d, 2 H,
aromatic).
(
11) Kobayashi, S.; Wakabayashi, T.; Nagayama, S.; Oyamada,
H. Chem. Lett. 1997, 831.
(12) Typical Procedure for the One-Pot Catalytic Allylation
of Aldimines: A mixture of p-nitrobenzaldehyde (1 mmol),
p-nitroaniline (1 mmol) allyltributylstannane (1.2 mmol),
benzoic acid (1 mmol), PEG (2 g) and scandium triflate 20
mol% was placed in a 25 mL round bottomed flask. The
mixture was stirred at r.t. for 4 h (monitored by TLC). The
Acknowledgment
K. J. thanks the Indian Institute of Chemical Technology, India for
the award of a research fellowship and S. M. thanks the Council of
Scientific and Industrial Research, India, for the award of a senior
research fellowship.
reaction mixture was extracted with Et O and the crude
2
product was purified by column chromatography to obtain
homoallylic amine in good yields. The products were
1
characterized by H NMR data.
Synlett 2004, No. 2, 231–234 © Thieme Stuttgart · New York