The effects of aromatic and aliphatic anionic surfactants on Sc(OTf)3-catalyzed Mukaiyama aldol reaction in water

Article abstract of DOI:10.1016/S0040-4039(01)00037-5

Aromatic (2c and 2d) and aliphatic (2a and 2b) anionic surfactants were employed in Sc(OTf)₃-catalyzed aldol reactions of some labile silyl enol ethers (3a and 3b) with aromatic aldehydes in water. The results indicated that the aromatic surfac

Full text of DOI:10.1016/S0040-4039(01)00037-5

TETRAHEDRON
LETTERS
Pergamon
Tetrahedron Letters 42 (2001) 1803–1805
The effects of aromatic and aliphatic anionic surfactants on
Sc(OTf) -catalyzed Mukaiyama aldol reaction in water
3
a
a
a,
a
b,
Hong-Yu Tian, Yong-Jun Chen, Dong Wang, * Yan-Ping Bu and Chao-Jun Li *
a
Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, PR China
b
Department of Chemistry, Tulane University, New Orleans, LA 70118, USA
Received 3 January 2001; accepted 8 January 2001
Abstract—Aromatic (2c and 2d) and aliphatic (2a and 2b) anionic surfactants were employed in Sc(OTf) -catalyzed aldol reactions
3
of some labile silyl enol ethers (3a and 3b) with aromatic aldehydes in water. The results indicated that the aromatic surfactants
have a better ability to inhibit the hydrolysis of labile silyl enol ethers and promote the aldol reaction, leading to higher yields of
aldol adducts. © 2001 Elsevier Science Ltd. All rights reserved.
Organic reactions in water, which avoid the use of
harmful organic solvents, are now of great interest
especially in relation to today’s environmental con-
SO Na
3
NaO S
SO Na SO3Na
3
3
1
n=3
R=n-C H , or n-C H
6 13
1
4
9
cerns. Recently, great progress has been made on the
lanthanide triflate catalysed Mukaiyama aldol reaction
n
R
2
R
R
R
in aqueous media, and even in pure water. However,
3
some surfactants must be added to promote the reac-
In our investigations, aliphatic anionic surfactants
sodium dodecyl sulfate (2a) and sodium dodecylsul-
fonate (2b), and aromatic anionic surfactants sodium
p-dodecylphenylsulfate (2c) and6 sodium p-dode-
cylphenylsulfonate (2d), were used. Among them, 2c is
not commercially available and can be prepared by
Fridel–Crafts reaction, followed by Zn–Hg reduction
tions in water alone. These surfactants can be anionic,
such as sodium dodecyl sulfate (SDS), or non-ionic. In
addition, although some very labile silyl enol ethers can
react smoothly with aldehydes in water in the presence
of a Lewis acid-surfactant-combined catalyst, such as
4
scandium trisdodecylsulfate (STDS), they decompose
rapidly in a Sc(OTf) –SDS–H O system, leading to a
7
3
2
and reaction with sulphur trioxide/pyridine. Since
low yield of the desired products. In order to improve
the reaction yield, it is necessary to use a large excess o5f
the labile silyl enol ethers. More recently, we reported
that the calix[6]arene derivatives 1 bearing sulfonate
groups on the upper rim and the alkyl groups on the
lower rim have a special ability as surfactants in stabi-
lizing labile silyl enol ethers and in promoting the
aqueous Mukaiyama reaction. In view of the molecular
structure, it should be noted that the calix[6]arene
derivative can be considered as a bundle of aromatic
anionic surfactants, while SDS is an aliphatic anionic
surfactant. Therefore, considering the different effects
of aromatic and aliphatic surfactants, it would be inter-
esting to investigate the roles of aromatic and aliphatic
aqueous solutions of 2c are likely to decompose at
ambient temperature within several days, freshly pre-
pared 2c must be used in the aqueous reaction. Initial
reactions of 1-trimethylsiloxy-cyclohex-1-ene (3a) with
aldehydes 4a–d were carried out in water at room
temperature in the presence of the surfactants 2a–d (20
mol%) and the catalyst Sc(OTf) (10 mol%) to afford
3
the aldol products 5a–d (Eq. (1)). The experimental
results are summarized in Table 1. In the case of alkyl
sulfonate surfactant 2a, the silyl enol ether 3a in the
reaction mixtures disappeared (as shown by TLC) after
1
–1.5 h; however, a large amount of the aldehydes
remained, leading to lower yields of aldol products
40–54%, entry 1, 5, 9). On the other hand, in the
presence of the corresponding aryl sulfonate surfactant
c, the yields of the aldol products were very high
88–99%, entry 3, 7, 11, 14) with the same reaction
(
anionic surfactants in the Sc(OTf) -catalyzed Mukaiy-
3
ama aldol reaction in water.
2
(
time. For sodium alkyl and aryl sulfonates (2b and 2d),
the same trend was observed: sodium alkyl sulfate 2b
*
0
Corresponding authors.
040-4039/01/$ - see front matter © 2001 Elsevier Science Ltd. All rights reserved.
PII: S0040-4039(01)00037-5

1
804
H.-Y. Tian et al. / Tetrahedron Letters 42 (2001) 1803–1805
furnished lower yields (55–70%, entry 2, 6, 10) and
sodium aryl sulfate 2d higher yields (78–93%, entry 4, 8,
It was assumed that micelles were formed in these
reaction systems and that the Lewis acid-catalyzed reac-
tions would proceed smoothly in these micelles. On the
other hand, there is a competition between the hydroly-
1
2) of the aldol products, respectively. For p-chloro-
benzaldehyde 4d, both sodium alkyl and aryl sulfates
2a and 2c) gave higher yields (entry 13 and 14) due to
(
sis of the silicon substrate and the Sc(OTf) -catalyzed
3
a higher reactivity of 4d. Meanwhile, a significant dif-
ference between 2a and 2c (74 and 98% yields, respec-
tively) was observed. Both aromatic and aliphatic
surfactants offer a similar stereoselectivity (syn/anti) of
the aldol reaction products.
aldol reaction of silyl enol ether with aldehydes in
aqueous media. The hydrolysis of the silicon substrates
has a strong effect on the yield of the corresponding
aqueous aldol reaction. In the case of 2a and 2b, the
silyl enol ethers (3a and 3b) in the reaction mixture
O
OH
O
OH
OTMS
H O, r.t.
2
R
R
RCHO
+
+
cat. Sc(OTf) /2a-d
(1)
(2)
3
syn
anti
3a
4a-d
5a-d
O
OH
OTMS
H2O, r.t.
RCHO
+
Ph
R
cat. Sc(OTf) /2a-d
Ph
3
3b
4a-b
5e-f
The reactions of 1-trimethylsiloxy-1-phenyl-ethylene 3b
with aldehydes 4a–b were also carried out in water in
the presence of the surfactants 2b and 2d (Eq. (2)).
Again a trend was noticed in which the yields of the
aldol products 5e and 5f were lower (43–49%; entry 15,
disappeared (as shown by TLC) after 1 h of the reac-
tion, leading to lower yields of the aldol products.
Therefore, although hydrolysis of some labile silyl enol
ether can be prevented by using alkyl sulfates and
sulfonates to some extent, the hydrolysis reaction still
surpasses the desired aldol reaction in aqueous media.
From these results we can conclude that the surfactants
with aryl groups can inhibit the hydrolysis of labile silyl
enol ethers (3a and 3b) and promote the aldol reactions
more effectively than the alkyl surfactants. These
results suggest that the micellar phases with 2a–d
1
7) for the aliphatic anionic surfactant than those for
the aromatic anionic surfactant under the same reaction
conditions. It should be noted that no reaction was
observed between 3b and aldehydes either in the
absence of surfactants or with sulfonated calix[6]arene
5
itself as the surfactant.
Table 1. Aqueous aldol reactions of silyl enol ethers with aldehydes in the presence of various surfactants
Entry
Aldehyde (R)
Silyl enol ether
Surfactant
Reaction time (h)
Product (R)
Yielda (%) (syn/anti)
1
2
3
4
5
6
7
8
9
Ph, 4a
4a
4a
3a
3a
3a
3a
2a
2b
2c
2d
2a
2b
2c
2d
2a
2b
2c
2d
2a
2c
2b
2d
2b
2d
1
1
1
1
1.5
1.5
1.5
1.5
1
1
1
1
1
1
1
1
1
Ph, 5a
5a
5a
47 (72/28)
55 (68/32)
99 (80/20)
85 (75/25)
40 (61/39)
61 (63/37)
98 (69/31)
93 (73/27)
54 (52/48)
70 (49/51)
88 (50/50)
78 (51/49)
74 (77/23)
98 (77/23)
49
4a
5a
p-CH OPh, 4b 3a
p-CH OPh, 5b
3
3
4b
4b
4b
3a
3a
3a
5b
5b
5b
PhCHꢀCH, 4c 3a
4c
4c
4c
PhCHꢀCH, 5c
5c
5c
5c
p-ClPh, 5d
5d
Ph, 5e
5e
10
11
12
13
14
15
16
17
18
3a
3a
3a
3a
3a
3b
3b
3b
3b
p-ClPh, 4d
4d
4a
4a
4b
4b
61
43
58
p-CH O, 5f
3
1
5f
a
Yields based on 1H NMR.

H.-Y. Tian et al. / Tetrahedron Letters 42 (2001) 1803–1805
1805
provide an excellent reaction medium for organic reac-
tions by passivating the lability of 3a and 3b by seques-
tration within their hydrophobic cavities. It should also
be noted that the yield of the reaction of 3a with a
simple aliphatic aldehyde, such as nonyl aldehyde, in
the presence of aromatic surfactant 2d was very low
Organic Synthesis; Kobayashi, S., Ed.; Lanthanide triflate-
catalyzed carbonꢁcarbon bond-forming reactions in
organic synthesis. Springer: Berlin, 1999 and references
cited therein; Kobayashi, S.; Nagayama, S.; Busujima, T.
J. Am. Chem. Soc. 1998, 120, 8287; Kobayashi, S.;
Nagayama, S.; Busujima, T. Tetrahedron 1999, 55, 8739;
Loh, T. P.; Li, X. R. Tetrahedron 1999, 55, 10789;
Lubineau, A.; Auge, J.; Queneau, Y. Synthesis 1994, 741.
. (a) Kobayashi, S.; Wakabayashi, T.; Nagayama, S.; Oya-
mada, H. Tetrahedron Lett. 1997, 38, 4559; (b) Kobayashi,
S.; Wakabayashi, T.; Oyamada, H. Chem. Lett. 1997, 831;
(
10%). The silyl enol ether 3a disappeared within 0.5 h,
generating a complicated mixture of products. The p–p
interaction between the molecules of aryl surfactant
and aryl aldehydes would be more favourable for acti-
vation of the substrates in the hydrophobic cavities. An
alternative interpretation of the low yield between 3a
and nonyl aldehyde is that two micelle populations
exist, one of which is stabilized by nonyl aldehyde and
the other which is stabilized by 2d, thereby effectively
keeping the two reactants in separate microenviron-
ments. In summary, aromatic surfactants provide better
protection of labile silyl enol ethers and promote the
aldol reaction more effectively than aliphatic surfac-
3
4
(
c) Kobayashi, S.; Busujima, T.; Nagayama, S. Synlett
999, 545.
. (a) Kobayashi, S.; Wakabayashi, T. Tetrahedron Lett.
998, 39, 5389; (b) Manabe, K.; Mori, Y.; Wakabayashi,
Y.; Nagayama, S.; Kobayashi, S. J. Am. Chem. Soc. 2000,
22, 7202.
1
1
1
5
. Tian, H. Y.; Chen, Y. J.; Wang, D.; Zeng, C. C.; Li, C. J.
Tetrahedron Lett. 2000, 41, 2529.
. The critical micelle concentrations of 2a, 2b, and 2d are
8
tants.
6
0.0081, 0.0098, and 0.0016 mol/L, respectively, see: The
Application of Surfactants in Pharmaceutics; Zhong, J. F.,
Ed.; The People’s Press of Hygiene: Beijing, 1996, p. 114.
The critical micelle concentration of 2c is not known and
should be comparable to the others. Based on the concen-
trations of surfactants being used, all reactions should
proceed above the critical micelle concentrations.
. Borgulya, J.; Bernauer, K. Synthesis 1983, 29.
. A typical experimental procedure is as follows: A mixture
of benzaldehyde (0.2 mmol), silyl enol ether 3 (0.3 mmol),
Acknowledgements
We gratefully acknowledge financial support by the
National Natural Science Foundation of China (No.
20028203).
7
8
References
surfactant 2 (0.2 equiv.), and Sc(OTf) (10 mg, 0.1 equiv.)
3
in water (1 mL) was stirred at room temperature until the
silyl enol ether disappeared on TLC. The mixture was then
passed through a short silica gel pad and extracted with
ethyl ether. The combined organic layer was dried over
1
. (a) Li, C. J.; Chan, T. H. Organic Reactions in Aqueous
Media; Wiley: New York, 1997; (b) Organic Synthesis in
Water; Grieco, P. A., Ed.; Blackie Academic and Profes-
sional: London, 1998; (c) Cornils, B.; Hermann, W. A.
Aqueous-Phase Organometallic Chemistry: Concepts and
Applications; Wiley-VCH: Weinheim, 1998.
MgSO and concentrated under reduced pressure. The
4
residue was purified by flash chromatography on silica gel
to afford the desired aldol adduct.
2
. Kobayashi, S. In Lanthanides: Chemistry and Use in
.
.