Homogeneous silicone modified primary amine-Br?nsted acid salt catalyzed aldol reaction: unexpected synergistic effect of polysiloxane with remarkable improvement of efficiency and stereoselectivity

Article abstract of DOI:10.1016/j.tetlet.2008.09.145

We have developed a new strategy to improve the stereoselectivity in enamine catalysis by the introduction of super-hydrophobic long-chain silicone/polysiloxane as support/functional group for a model aldol reaction. The homogeneous direct aldol reaction

Full text of DOI:10.1016/j.tetlet.2008.09.145

Tetrahedron Letters 49 (2008) 7037–7041
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Tetrahedron Letters
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Homogeneous silicone modified primary amine-Brønsted acid salt catalyzed
aldol reaction: unexpected synergistic effect of polysiloxane with
remarkable improvement of efficiency and stereoselectivity
a,b,*
a
a
a
a
a,*
Li-Wen Xu
, Ya-Dong Ju , Li Li , Hua-Yu Qiu , Jian-Xiong Jiang , Guo-Qiao Lai
a
Key Laboratory of Organosilicon Chemistry and Material Technology of Ministry of Education, Hangzhou Normal University, Hangzhou 310012, PR China
State Key Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, Lanzhou 730000, PR China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
We have developed a new strategy to improve the stereoselectivity in enamine catalysis by the introduc-
tion of super-hydrophobic long-chain silicone/polysiloxane as support/functional group for a model aldol
reaction. The homogeneous direct aldol reaction of cyclic ketones with different aromatic aldehydes cata-
lyzed by polysiloxane-derived primary amines has been reported with high yields, good diastereoselec-
tivity, and up to 99% ee.
Received 5 August 2008
Revised 18 September 2008
Accepted 25 September 2008
Available online 30 September 2008
Ó 2008 Elsevier Ltd. All rights reserved.
Keywords:
Enamine catalysis
Primary amine
Aldol reaction
Polysiloxane
Organocatalysis
9
The asymmetric aldol condensation, an ancient and fundamen-
tal organic reaction, is one of the most powerful methods for con-
structing carbon–carbon bonds in organic synthesis, creating the
b-hydroxy carbonyl structural unit which is found in many natural
2005. Interestingly, other amino acids with primary amine groups
1
0
received considerable attention as organocatalyst, and similarly,
some progress has been made in the primary amine-based organ-
ocatalysis. In this vein, several groups have reported recently the
successful application of chiral diamine-derived primary amine
1
1
1
products and drugs. Since the pioneering work of List and Barbas
2
12
in 2000, the development of organocatalytic stereoselective meth-
thiourea catalysts in Michael addition, and the primary–tertiary
1
3
ods for asymmetric direct aldol reactions has been an interesting
subject of intense research. Among the numerous reports on
diamine catalyzed aldol and other reactions.
Recent findings by Lu and co-worker showed that silicone group
in O-TBS-threonine (TBS-protection of hydroxyl group in -threo-
nine) exhibited important functionality in aldol and Mannich reac-
tion compared to unsubstituted threonine. It is reasoned that
TBS-protection of hydroxyl group resulted in hydrophobic catalyst
and associated strongly with hydrophobic reactants. In this con-
text, we demonstrated a new strategy that involves hydrophobic
interactions by the introduction of super-hydrophobic long-chain
polysiloxane as support/functional group to primary amine
catalyst.
Polysiloxanes are hydrophobic, chemically resistant inorganic
polymer, in which the chain is entirely inorganic, with alternating
Si and O atoms, and organic side groups are attached to the silicon
atoms. Most applications of polysiloxanes were derived from the
extraordinary flexibility and super-hydrophobicity of the siloxane
backbone. Although many applications of polysiloxanes include
high-performance elastomers, membranes, electrical insulators,
water repellent sealants, adhesives, protective coatings, and
hydraulic, heat transfer, dielectric fluids, biomaterials, and catalyst
organocatalysts,
lyst in a wide range of organic reactions. The success of
L
-proline is arguably the most famous organocata-
L
3
L
-proline
1
4
and its derivatives promoted the studies of other abundant chiral
4
organic molecules as catalysts in organic transformation. In fact,
primary amino acid catalysis is effectively exploited by enzymes
such as type I aldolases, decarboxylases, and dehydratases, each
5
of which contain catalytically active lysine or threonine residues.
6
7
Although Hajos and Prrich , as well as Eder et al. used phenylala-
nine in the first organocatalytic intramolecular aldol condition of
achiral triones, and Buchschacher et al. reported b-amino acids,
8
3
3
such as b -homoproline, b -homophenylalanine, and also c-amino
acids are effective catalysts in intramolecular asymmetric aldol
condensations (Hajo–Parrich–Eder–Sauer–Wiechert reactions or
Robinson annulation), these findings have not been recognized
either, until primary amino acids were reinvestigated from 2004/
*
Corresponding authors. Tel./fax: +86 0571 28865135.
E-mail addresses: liwenxu@hznu.edu.cn (L.-W. Xu), licpxulw@yahoo.com (G.-Q.
Lai).
1
5
supports have been reported in the past decades, to the best of
0
040-4039/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetlet.2008.09.145

7
038
L.-W. Xu et al. / Tetrahedron Letters 49 (2008) 7037–7041
our knowledge, very few examples are known where the polysilox-
anes were used as a linker or as a group or even as a supporter in
organocatalyst. Only Siegel16 and Bergbreiter reported recently
used polysiloxanes as soluble inorganic polymer supports for a Le-
wis base organocatalyst, Cinchona alkaloid, in the Michael addition
of thiol and a,b-unsaturated ketones and esters, which gave low ee
value (ꢀ20% ee).
17
To explore the potential of using hydrophobic polysiloxane in
organocatalysis, we prepared different nitrogen amounts of poly-
siloxane with chiral primary amine backbone (Scheme 1).18 The
O
HO HN
O
NH
2
O
H N
NH2
2
Me
Si
Me
i
-PrOH/reflux
TMS
O Si
TMS
n
m
24hr
Me
Me
Si
Me
1
: (R,R)-DPEN
TMS
O Si
TMS
2
: MW = ~5500
m
n
Me
3
a: 1.0 w% N amount
O
b: 0.35 w% N amount
HO HN
NH
2
O
O
Me
Si
Me
i
-PrOH/reflux
H N
2
NH2 TMS
O Si
TMS
Me
Si
Me
n
m
4
:
Me
24hr
TMS
O Si
TMS
n
(1
S,2S
)-cyclohexane-1,2-
m
Me
diamine
2: MW = ~5500
5: 1.1 w% N amount
Scheme 1. The preparation of polysiloxane-modified primary amine catalysts.
Table 1
Screening results of polysiloxane containing primary amine-brønsted acid catalyzed aldol reaction
a
Ο
CHO
OH
polysiloxane-modified
primary amine catalysts
solvent or solvent-free
O N
2
Ο
NO2
7a
6a
8a
Entry
Cat.
Sol.
Acid
T (h)
Y (%)/drb
Ee% (anti/syn)c
1
2
3
4
5
6
7
8
3a
3a
3a
3a
3a
3a
3a
3b
5
3a
3a
3a
3a
3a
3a
3a
3a
DCM
Hex.
THF
DMF
2
H O
Brine
—
—
—
—
—
—
—
—
—
—
—
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TfOH
TFA
96
24
96
24
24
24
16
24
12
24
24
24
24
24
24
12
38
34 (5.7/1)
58 (3.8/1)
46 (5.7/1)
66 (6.1/1)
73 (5.7/1)
64 (5.7/1)
84 (6.1/1)
46 (4.6/1)
95 (4.0/1)
53 (5.7/1)
84 (6.1/1)
36 (5.7/1)
53 (1/1.2)
92 (6.7/1)
83 (4.6/1)
97 (6.7/1)
78 (6.7/1)
97/91
82/40
99/89
99/80
98/72
98/79
98/80
87/30
d
9
96/82
e
10
11
12
13
14
15
16
17
97/70
97/50
95/76
8/64
99/86
99/91
99/86
99/87
TsOH
4-Nitrobenzoic acid
TfOH
TfOH
4-Nitrobenzoic acid:TfOH(1:1)
4-Nitrobenzoic acid:TfOH(1:1)
f
g
f
h
a
The reaction were performed with 1.0 mmol of p-nitrobenzaldehyde, 10 equiv of cyclohexanone, primary amine-based catalyst (20 mol %), Brønsted acid (40 mol %),
solvent or neat, at room temperature.
b
Isolated yield; dr number is anti/syn in parenthesis, and determined by 1H NMR analysis.
The ee values were determined by HPLC.
The absolute configuration of adduct product is opposite in comparison with that of catalyst 3.
In entry 10, 10 mol % of primary amine catalyst, 20 mol % of acid.
In entries 14 and 16 and 20 mol % of primary amine catalyst, 20 mol % of acid.
In entry 15, 20 mol % of catalyst and 10 mol % of acid.
In entry 17, 10 mol % of catalyst, 10 mol % of acid.
c
d
e
f
g
h

L.-W. Xu et al. / Tetrahedron Letters 49 (2008) 7037–7041
7039
primary amine (1,2-diphenylethylenediamine and cyclohexane-1,2-
diamine)-substituted polysiloxane was prepared from the one-
pot ring-opening of polysiloxane containing side group of epoxide
with chiral diamine in i-PrOH under reflux for 24 h. The functional
polysiloxane was completely soluble in acetone, toluene, chloro-
form, etc. we focused on undertaking a systematic investigation
of this polysiloxane-modified primary amine salt as potential cat-
alysts for the model reaction of cyclohexanone (6) with 4-nitro-
benzaldehyde (7a). As shown in Table 1, among the standard
reaction parameters, solvent choice and Brønsted acid proved par-
ticularly important. While reactions carried out in non-polar or
low-polar solvents, such as THF, DCM, and hexane, were slow or
only with moderate enantioselectivity (Table 1, entries 1–3), those
run in highly polar and/or protic solvents, such as DMF, water,
and brine, proceeded with useful rates along with excellent
enantioselectivity (entries 4–6). And we were then pleased to find
that polysiloxane-modified primary amine (silicone-DPEN) 3a
exhibited high catalytic activity under solvent-free conditions. It is
on problem that neat cyclohexanone was suitable as both reactant
and solvent in terms of enantioselectivity and catalyst reactivity.
In this case, the corresponding adduct was cleanly isolated in 84%
yield with a promising 98% ee after 16 h, and the anti-isomer was
favorably formed with 6.1:1 dr (entry 7). Under the same condi-
tions, polysiloxane derivative (silicone-DPEN) 3b with low loading
of chiral active primary amine salt catalyst showed decreased
enantioselectivity with lower yield (entry 8, 46% yield, 87% ee of
anti-isomer). Similarly, polysiloxane-modified primary amine 5 al-
most exhibited the same enantioselectivity and catalytic activity
to 3a (entry 9). Even with 10 mol % of 3a, the corresponding anti-ad-
duct was obtained with 97% ee in acceptable yield (entry 10). To our
surprise, Brønsted acid had a significant impact on both reactivity
and diastereoselectivity. The enantioselectivity could be obtained
above 95% ee when the aldol reaction was conducted in the pres-
ence of TFA and TsOH (entries 11, 12), however, the employment
of 4-nitrobenzoic acid decreased the enantioselectivity of anti-iso-
mer to only 8 with poor dr (entry 13). Interestingly, the amount
of triflic acid (TfOH) used considerably affected the reactivity of
the catalyst. 20 mol % or 10 mol % of acid in combination with
20 mol % of catalyst was suitable for the reaction in terms of enanti-
oselectivity, diastereoselectivity, and reactivity (Table 1, entries 14
and 15: 99% ee of anti-isomer and 91% ee of syn-iomer). The
employment of two different Brønsted acid, triflic acid with 4-nitro-
benzoic acid, further enhanced the enantioselectivity and reactivity
(entries 16, 17).
For comparison, other similar silica gel-supported primary
amine catalyst and the (R,R)-1,2-diphenylethylenediamine (DPEN)
were also investigated. We found that the primary diamine 1
(DPEN) reacted very slowly in the presence of TfOH under sol-
vent-free conditions to give a poor yield and enantioselectivity of
8a (Scheme 2, 15% yield, 37% ee for major anti-isomer).
Silica gel-supported primary amines (silica-DPEN) were also
1
9
prepared according to the reported method and were used in
the present aldol reaction. As shown in Scheme 3, all the silica
gel-supported DPEN (silica-DPEN) with different amounts of nitro-
gen gave good yield and excellent enantioselectivities (up to 75%
yield, 95% ee). However, these results were not better than those
of polysiloxane-modified primary amine catalysts 3 and 5. All the
above mentioned experimental results showed that polysiloxane-
modified primary amine gave privileged reactivity and enantio-
selectivity in this aldol reaction.
With optimized conditions established for the model reaction of
4-nitrobenzaldehyde with cyclohexanone, we explored the scope
OH
CHO
Ο
1
: (R,R)-DPEN (10mol%)
TfOH (20 mol%)
solvent-free/48 hr/rt
O N
2
Ο
NO2
H N
2
NH2
8a: 15%yield, 37%ee (anti)
6a
7a
DPEN:
Scheme 2. DPEN-catalyzed aldol reaction under solvent-free condition.
NH NH
2
Si
9
a: 0.187mmol/g for N amount
b: 0.546mmol/g for N amount
9
CHO
9:Silica gel supported Cat.
OH
Ο
9
or 10 (20 mol%)
TfOH (40 mol%)
solvent-free/48 hr/rt
O N
Ο
2
NO2
8
a
6
a
7a
Cat. 9a: solvent-free, 24 h,yield = 75%, dr (anti/syn) = 2.4/1, ee% = 95 (anti)/70(syn
)
reused data, for the second time: yield = 71% , ee% = 87/64
reused data, for the third time:
yield = 56% , ee% = 89/63
Cat. 9b: solvent-free, 48 h, 45% yield, dr (anti/syn) = 3/1, ee% = 80 (anti)/61(syn
)
Scheme 3. Silica gel-supported primary amine-catalyzed aldol reaction under solvent-free condition.

7
040
L.-W. Xu et al. / Tetrahedron Letters 49 (2008) 7037–7041
Table 2
Enantioselective aldol reactions of cyclic ketones to aromatic aldehydes
O
O
OH
polysiloxane-modified
R
primary amine catalyst 3a
CHO
+
R
solvent-free/rt
n
n
7a-d
6a-d
8a-k
Entrya
1
R-
Ketone
Product
Time (h)
12
Yieldb (%)
dr (anti/syn)c
Ee% (anti/syn)d
p-NO
2
O
8a
97
6.7/1
99/86
2
3
p-NO
2
2
O
O
8b
8c
24
24
70
53
4.0/1
2.7/1
65/45
82/61
o-NO
4
5
6
7
8
9
p-NO
p-NO
o-NO
o-NO
2
2
2
2
O
O
O
8d
8e
8f
96
16
40
60
22
30
24
22
94
71
65
96
53
95
>19/1
>19/1
>99/1
>19/1
13.3/1
3.2/1
98/—
98/—
97/—
94/—
99/81
95/68
99/80
O
O
8g
8h
8i
m-NO
m-NO
p-CN
2
O
O
O
2
1
0
1
8j
5.7/1
1
p-CN
8k
60
65
2.1/1
96/64
O
a
The reactions were performed with 1.0 mmol of aldehydes, 10 equiv of ketones, 20 mol % of primary amine-based catalyst 3a, 10 mol % of triflic acid, and 10 mol % of 4-
nitrobenzoic acid, neat (solvent-free), stirred at room temperature.
b
Isolated yield.
c
Determined by 1H NMR analysis.
d
The ee values were determined by HPLC.
of useful cyclic ketones and aldehydes substrates using homoge-
neous polysiloxane-modified primary amine as a more practical
reacting partner. As expected, under these conditions, a wide range
of aromatic benzaldehydes underwent reaction with cyclic ketones
in high yields and enantioselectivities in most cases (Table 2). The
results of these trials indicated that the reaction is dramatically
dependent on the electronic effect of the substituent. Unsubstitut-
ed aromatic aldehydes or with electron donating groups at
aromatic ring were less reactive, because benzaldehyde, p-
fluorobenzaldehyde, p-methyl benzaldehyde, and 1-naphthylalde-
hyde afforded only low to moderate yields but with excellent
enantioselectivity (below 20% yield, up to 99% ee). However,
o-, m-, and p-nitrobenzaldehyde as well as p-cyanobenzaldehyde
underwent smoothly with cyclic ketones in good yields and excel-
lent enantiselectivities except few examples.
organocatalyst, and also is a useful material for mediating aldol
reactions.
In conclusion, an enantioselective direct aldol reaction of cyclic
ketones with different aromatic aldehydes catalyzed by polysilox-
ane-derived primary amines has been reported with high yields,
good diastereoselectivity, and up to 99% ee. The aromatic alde-
hydes with electron-withdrawing groups are suitable substrates
for this reaction. Although the reaction scope of substrates seems
to be limited, this work provides a new strategy to improve the ste-
reoselectivity in enamine catalysis by the introduction of super-
hydrophobic long-chain silicone/polysiloxane as support/func-
tional group for a model aldol reaction, also provide a practical
and convenient method for the synthesis of optically enriched
hydroxyl ketones.
Although the functional polysiloxane was not soluble in hexane,
the simple separation of catalyst except column chromatography
and ultrafiltration was not easy.16 However, polysiloxane is a via-
ble functional group for the modification of primary amine-based
Acknowledgments
This work was supported in part by Hangzhou Normal Univer-
sity and the National Nature Science Foundation of China (Project

L.-W. Xu et al. / Tetrahedron Letters 49 (2008) 7037–7041
7041
No. 20572114). Xu L.W. is greatly indebted to Prof. Shibasaki and
Dr. Kanai, Graduate School of Pharmaceutical Sciences, The Univer-
sity of Tokyo, for their help.
10. Xu, L. W.; Lu, Y. Org. Bimol. Chem. 2008, 6, 2047 and references cited therein.
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02.
9
M
w
ꢀ 5500), and 10 mL of i-PrOH. And then the solution was refluxed for
4 h. After completion of the reaction, the solvent was removed under reduced
pressure, the resulting crude product was washed several times with H O,
6
7
.
.
Hajos, Z. G.; Parrish, D. R. J. Org. Chem. 1974, 39, 1615.
(a) Eder, U.; Sauer, G.; Wiechert, R. Angew. Chem. 1971, 83, 492; Angew. Chem.,
Int. Ed. Engl. 1971, 10, 496; (b) Shimizu, I.; Naito, Y.; Tsuji, J. Tetrahedron Lett.
2
2
ethanol, and hexane. The pure product was confirmed by NMR (see
Supplementary data) and elementary analysis (the N% amount was shown in
Scheme 1).
1
980, 21, 487.
Buchschacher, P.; Cassal, J. M.; Fürst, A.; Meier, W. Helv. Chim. Acta 1977, 60,
68.
8
.
.
2
19. (a) Adima, A.; Moreau, J.; Wong Chi Man, M. Chirality 2000, 12, 411; (b) Miao,
9
(a) Pizzarello, S.; Weber, A. L. Science 2004, 303, 1151; (b) Clemente, F. R.; Houk,
K. N. J. Am. Chem. Soc. 2005, 127, 11294; (c) Davies, S. G.; Sheppard, R. L.; Smith,
A. D.; Thomson, J. E. Chem. Commun. 2005, 3802.
T.; Wang, L. Tetrahedron Lett. 2007, 48, 95.