1,4-Conjugate addition of allyltrimethylsilane to α,β-unsaturated ketones

Article abstract of DOI:10.1016/S0040-4039(02)02373-0

α,β-Unsaturated ketones smoothly undergo conjugate addition with allyltrimethylsilane in the presence of a catalytic amount of elemental iodine under very mild and convenient conditions to afford the corresponding Michael adducts in high yields with high

Full text of DOI:10.1016/S0040-4039(02)02373-0

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 43 (2002) 9695–9697
1,4-Conjugate addition of allyltrimethylsilane to
a,b-unsaturated ketones
J. S. Yadav,* B. V. S. Reddy, K. Sadasiv and G. Satheesh
Division of Organic Chemistry, Indian Institute of Chemical Technology, Hyderabad 500 007, India
Received 20 June 2002; revised 8 October 2002; accepted 18 October 2002
Abstract—a,b-Unsaturated ketones smoothly undergo conjugate addition with allyltrimethylsilane in the presence of a catalytic
amount of elemental iodine under very mild and convenient conditions to afford the corresponding Michael adducts in high yields
with high selectivity. © 2002 Elsevier Science Ltd. All rights reserved.
The conjugate addition of allylsilanes to electrophilic
alkenes, referred to as the Sakurai–Hosomi reaction has
been recognized as a particularly efficient method of
carbonꢀcarbon bond formation and has been exten-
sively applied in organic synthesis, especially in natural
product synthesis.1 Lewis acids such as TiCl4, AlCl3,
and BF3·OEt2 have been employed as the most effective
promoters for these conjugate allylations.2 Subse-
quently, modified reagents such as allylbarium3 and
allylcopper4 and allylindium5 reagents have been devel-
oped to avoid strongly acidic conditions. With these
modified reagents the allylation of acyclic enones is far
more difficult than that of cyclic ketones. Recently,
organotantalum reagents have also been developed for
the allylation of enones by transmetalation of organotin
compounds.6 However, many of these methods involve
the use of highly toxic tin compounds, expensive
reagents and require a stoichiometric amount or even
an excess amount of the Lewis acid to obtain reason-
able reaction rates and acceptable yields of products.
Thus, there is still scope to develop a simple and
efficient method for the conjugate allylation of a,b-
unsaturated ketones. Furthermore, the development of
new reagents, which are more efficient and lead to
convenient procedures with improved yields, is well
appreciated.
In continuation of our interest on catalytic applications
of elemental iodine for various organic transforma-
tions,7 we report herein a mild and convenient method
for the conjugate allylation of both cyclic and acyclic
enones with allyltrimethylsilane using elemental iodine
as an efficient catalyst. Thus, treatment of 2-benzyli-
denecyclopentanone with allyltrimethylsilane in the
presence of
5
mol% of elemental iodine in
dichloromethane afforded the corresponding 2-(1-
phenyl-3-butenyl)cyclopentanone in 90% yield with
80:20 diastereoselectivity (Scheme 1).
Similarly, 2-benzylidenecyclohexanone, 2-benzylidene-
1-tetralone and benzylideneacetone reacted well to give
the corresponding 1,4-adducts in excellent yields. Cyclic
enones such as 2-cyclohexenone, 2-cyclopentenone and
acyclic enones such as chalcones afforded the corre-
sponding Michael adducts in excellent yields. In all
cases the reactions proceeded smoothly at ambient tem-
perature with high selectivity. No 1,2-adduct was
Scheme 1.
Keywords: Michael addition; a,b-unsaturated ketones; iodine catalysis; 1,4-adducts.
* Corresponding author. Fax: 91-40-7160512; e-mail: yadav@iict.ap.nic.in
0040-4039/02/$ - see front matter © 2002 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(02)02373-0

9696
J. S. Yada6 et al. / Tetrahedron Letters 43 (2002) 9695–9697
obtained under these reaction conditions. The products
were characterized by H NMR, IR, and mass spec-
giving the best results. This method is equally effective
for the allylation of both cyclic and acyclic enones. The
results of the conjugate allylation of enones, both cyclic
and acyclic substrates, are presented in Table 1. Inter-
1
troscopy and also by comparison with authentic com-
pounds.3,5 Dichloromethane is the solvent of choice
Table 1. Iodine-catalyzed conjugate allylation of a,b-unsaturated ketones8

J. S. Yada6 et al. / Tetrahedron Letters 43 (2002) 9695–9697
9697
estingly, a catalytic amount of TMSI was also found to
be equally effective for this conversion. However, the
use of allyltri-n-butyltin in place of allyltrimethylsilane
did not yield any product under these reaction condi-
tions, perhaps because iodine does not interact with
allyltri-n-butyltin. Furthermore, the reactions of
enones, with allylsilane in the presence of metal triflates
such as scandium or ytterbium triflate afforded 1,2-
adducts predominantly. In addition, attempted allyla-
tion with allylsilane and solid acids such as K10 clay
and Amberlyst-15 gave only 1,2-adducts. Thus, the
combination of allyltrimethylsilane and iodine could be
the method of choice for conjugate allylation of enones.
There are many advantages in the use of elemental
iodine as catalyst for this transformation, which include
high yields of products, mild reaction conditions,
greater selectivity, cleaner reaction profiles and opera-
tional simplicity. No additives or acidic promoters are
required for the reaction to proceed. The catalyst is
readily available at low cost and is highly efficient in
promoting conjugate allylations.
3. Yanagisawa, A.; Habaue, S.; Yasue, K.; Yamamoto, H. J.
Am. Chem. Soc. 1994, 116, 6130.
4. (a) Lipshutz, B. H.; Ellsworth, E. L.; Dimock, S. H.;
Smith, R. A. J. J. Am. Chem. Soc. 1990, 112, 4404; (b)
Lipshutz, B. H.; Hackmann, C. J. Org. Chem. 1994, 59,
7437.
5. (a) Lee, P. H.; Ahn, H.; Lee, K.; Sung, S.-Y.; Kim, S.
Tetrahedron Lett. 2001, 42, 37; (b) Lee, P. H.; Lee, K.;
Sung, S.-Y.; Chang, S. J. Org. Chem. 2001, 66, 8646.
6. Shibata, I.; Kano, T.; Kanazawa, N.; Fukuoka, S.; Baba,
A. Angew. Chem., Int. Ed. 2002, 41, 1389.
7. (a) Yadav, J. S.; Reddy, B. V. S.; Hashim, S. R. J. Chem.
Soc., Perkin Trans. 1 2000, 3025; (b) Yadav, J. S.; Reddy,
B. V. S.; Sabitha, G.; Reddy, G. S. K. K. Synthesis 2000,
1532; (c) Kumar, H. M. S.; Reddy, B. V. S.; Reddy, E. J.;
Yadav, J. S. Chem. Lett. 1999, 857; (d) Yadav, J. S.;
Reddy, B. V. S.; Rao, C. V.; Chand, P. K.; Prasad, A. R.
Synlett 2001, 1638.
8. General procedure: To a stirred solution of the a,b-unsatu-
rated ketone (1 mmol), and iodine (0.2 mmol) in
dichloromethane (10 mL), allyltrimethylsilane (1.5 mmol)
was added slowly at 0°C and the mixture stirred at rt for
the appropriate time (Table 1). After complete conversion
as indicated by TLC, the reaction mixture was quenched
with water (15 mL) and extracted with dichloromethane
(2×15 mL). The combined extracts were washed with a
15% solution of sodium thiosulphate, dried over anhy-
drous Na2SO4 and concentrated in vacuo. The resulting
product was purified by column chromatography on silica
gel (Merck, 100–200 mesh, ethyl acetate–hexane, 1:9) to
afford the pure product.
In summary, this paper describes a simple and efficient
method for the conjugate allylation of a,b-unsaturated
ketones using the cheap and readily available elemental
iodine as catalyst. In addition to its efficiency, simplic-
ity and mild reaction conditions, this method provides
high yields of products with high selectivity, which
makes it a useful and attractive process for the synthe-
sis of d,o-unsaturated ketones of synthetic importance.
1
Spectral data for selected products: 2e: H NMR (CDCl3)
Acknowledgements
l: 2.0 (s, 3H), 2.30–2.35 (dd, 2H, J=5.5, 7.0, 12.5 Hz),
2.70–2.75 (dd, 2H, J=6.0, 12.5 Hz), 3.25–3.30 (m, 1H),
4.97–5.0 (2d, 2H, J=17.0, 10.2 Hz), 5.58–5.65 (ddt, 1H,
J=17.0, 10.2, 7.0 Hz), 7.20–7.35 (m, 5H). 13C NMR
(CDCl3, proton decoupled, 50 MHz) l: 30.7, 40.6, 40.8,
B.V.S. thanks CSIR, New Delhi for the award of a
fellowship.
49.5, 116.7, 126.4, 127.5, 128.4, 136.2, 144.1, 207.8. EIMS:
+
’
m/z: 188 M IR (KBr) w: 3015, 2951, 1705, 1440, 1398,
References
1108, 963, 770.
2i: H NMR (CDCl3) l: 1.35–1.40 (m, 1H), 1.60–1.64 (m,
1H), 1.82–2.75 (m, 6H), 2.25–2.41 (m, 3H), 5.05 (2d, 2H,
J=17.8, 10.7 Hz), 5.75 (ddt, 1H, J=17.8, 10.7, 7.0 Hz).
13C NMR (CDCl3, proton decoupled, 50 MHz) l: 25.0,
1
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React. 1989, 37, 57–575.
30.7, 38.6, 40.7, 41.3, 47.5, 116.7, 135.6, 211.4. EIMS: m/z:
138 M IR (KBr) w: 3025, 2950, 1690, 1443, 1390, 1108,
+
’
970, 763.