High acceleration of the direct aldol reaction cocatalyzed by BINAM-prolinamides and benzoic acid in aqueous media

Article abstract of DOI:10.1016/j.tetasy.2006.05.026

The enantioselective direct aldol reaction, organocatalyzed by recoverable BINAM-prolinamide derivatives can be highly accelerated by a catalytic amount of a carboxylic acid without a detrimental of the obtained enantioselectivities. From the study of sui

Full text of DOI:10.1016/j.tetasy.2006.05.026

Tetrahedron: Asymmetry 17 (2006) 1493–1497
High acceleration of the direct aldol reaction cocatalyzed by
BINAM-prolinamides and benzoic acid in aqueous media
Gabriela Guillena, Mar ´ı a del Carmen Hita and Carmen N a´ jera*
Dpto. Qu ı´ mica Org a´ nica and Instituto de S ı´ ntesis Org a´ nica, Universidad de Alicante, Apdo. 99, E-03080 Alicante, Spain
Received 24 May 2006; accepted 29 May 2006
Abstract—The enantioselective direct aldol reaction, organocatalyzed by recoverable BINAM-prolinamide derivatives can be highly
accelerated by a catalytic amount of a carboxylic acid without a detrimental of the obtained enantioselectivities. From the study of suit-
able acids and reaction conditions, benzoic acid in aqueous DMF or in water was shown to give the best results with high yields and
a
enantioselectivities. Thus, the reaction between p-nitrobenzaldehyde and acetone catalyzed by (S )-BINAM-L-Pro and benzoic acid
can be carried out at À20 °C in only 8.5 h to give the expected product with 86% ee. In the case of butan-2-one, the iso- and the
anti-isomers are obtained in a 1:1 isomer ratio up to 99% ee. Cyclohexanone gives the anti-aldol in up to 99% dr and 97% ee in only
2
h. The opposite diastereoselectivity is obtained in the case of cyclopentanone with lower ee up to 65% for the syn and 85% for the
anti-isomer.
Ó 2006 Elsevier Ltd. All rights reserved.
7
1
. Introduction
Especially attractive is the use of prolinamides or small
8
peptides, derived from proline as catalysts for the aldol
1
Enantioselective organocatalysis is an old tool in organic
synthesis, which has been used to carry out the decarboxyl-
reaction, since the robust and easily formed amide bond
permitted structural modifications, therefore allowing the
fine tuning of catalyst properties. Recently, it has been re-
ported that the synthesis of BINAM-prolinamides (Fig. 1)
and their use for the direct aldol condensation of aldehydes
2
ation of malonic acid. Despite this fact, it has only been in
the last decade when the use of this type of processes has
become a fruitful research area. Organocatalysts have
shown their synthetic usefulness in enantioselective C–C
as well as heteroatom–C bond formation. In both types
of transformations, proline has shown its versatility as
catalyst, its use being especially successful in the direct
enantioselective aldol reaction. Thus, proline is able to
catalyze efficiently the coupling of two carbonyl compo-
nents, ketone–aldehyde or aldehyde–aldehyde,
mild reaction conditions to afford the expected products
with high regio-, diastereo- and enantioselectivities.
Although the use of proline as a catalyst has obvious
advantages, it suffers from some problems, such as a lack
of solubility in most organic solvents and difficult tuning
of its reactivity through structural modifications. There-
fore, several proline derivatives have been synthesized
and used as catalysts.
9
,10
11
and alkyl ketones
and alkoxy ketones gives the
expected products with high regio-, diastereo- and enantio-
selectivities. We have shown that working in DMF as
solvent, the BINAM-prolinamide 1a gave the best perfor-
mances and can be recovered after acidic work up
In general, these direct aldol reactions need rel-
atively long reaction times in order to obtain good yields of
the corresponding aldol.
3
4
9
,11
(Fig. 1).
5
,6
under
NH
NH
HN
HN
O
O
NH
O
NH2
NH
1
a, (Sa)-BINAM-
L
-Pro
1b
*
0
Corresponding author. Tel.: +34 965 90 3728; fax: +34 965 90 3549;
e-mail: cnajera@ua.es
Figure 1. BINAM-derived prolinamides.
957-4166/$ - see front matter Ó 2006 Elsevier Ltd. All rights reserved.
doi:10.1016/j.tetasy.2006.05.026

1494
G. Guillena et al. / Tetrahedron: Asymmetry 17 (2006) 1493–1497
On the other hand, it has been established that Br o¨ nsted
acids enhance the catalytic activity of pyrrolidine-based
organocatalysts, favouring the first steps of the catalytic
time increased (Table 1, entries 6–10), probably due to
the protonation of the catalyst, which lowers its
nucleophilicity.
1
2
cycle. Therefore, it can be expected that the use of such
BINAM-prolinamide catalysts 1 in combination with acids
would provide an acceleration of the reaction rate, facilitat-
ing the formation of the enamine, allowing a further opti-
mization of the reaction conditions. We can also expect
that this combination would protonate the catalyst to form
the corresponding ammonium salt, thus enhancing the sol-
ubility of such catalysts in water and therefore allowing us
to carry out the reaction in that solvent.
Thus, chloroacetic acid gave the best yield and enantio-
selectivity but required 7.5 h to complete the reaction
(Table 1, entry 8). The use of a stronger acid, such as
methanesulfonic acid led to a total inactivation of the
catalyst, with the starting material remaining unchanged
after 4 d (Table 1, entry 11).
Thus, it seems that the acceleration of the reaction was
most effective by the use of 20 mol % of benzoic acid,
allowing us to further optimize the reaction conditions
(Table 2). The influence of the presence of water as a co-
solvent was then evaluated. The reaction in dry DMF, gave
product 2a in similar ee than in aqueous DMF but required
longer reaction time and lower yield (Table 2, compare en-
2
. Results and discussion
At first, we investigated the influence of the amount and
pK of the acid on the reaction between acetone and p-
a
1
0
nitrobenzaldehyde in 1:1 DMF/H O at 0 °C catalyzed by
tries 1 and 2). The reaction in dioxane was accelerated by
the presence of acid but the ee achieved was lower than in
DMF (Table 2, entry 3). Finally, the reaction was carried
out in pure water, achieving the expected product 2a with
almost the same ee but in 20 h (Table 2, entry 4). The use
2
9
(
S )-BINAM-L-Pro 1a (Table 1). A large increase in the
a
reaction rate was observed when 10 mol % of AcOH was
added to the reaction mixture, to give product 2a in a
nearly quantitative yield in only 12 h in comparison with
the 3 d required in the absence of acid with almost the same
ee (Table 1, entries 1 and 2). When the amount of acetic
acid was increased to 20 mol %, that is 1 equiv of acid
per amine group on the catalyst, the reaction rate was
increased further to 3 h without a detrimental effect on
the enantioselectivity achieved (Table 1, entry 3). When
the slightly more acidic benzoic acid was used, the reaction
was over within 1 h, independent of the amount of acid
used (Table 1, entries 4 and 5). In all these cases the yield
was between 90% and 98% and the ee between 75% and
of a mixture of DMF/H O allowed us to decrease the
2
reaction temperature below 0 °C. Thus, the reaction was
carried out at À20 and À40 °C and, as expected, the ee
increased from 76% to 86% and 88%, respectively,
although longer reaction times were required (Table 2,
compare entry 1 with 5 and 6).
Table 2. Optimization of reaction conditions of 4-nitrobenzaldehyde with
7
9%. As the pK of the used acid decreased, the reaction
a
a
acetone in the presence of benzoic acid
OH
O
O
O
cat., PhCO H
Table 1. Direct aldol reaction of 4-nitrobenzaldehyde with acetone
H
2
a
catalyzed by 1a and an acid
solvent , T
O N
2
O N
2
2
a
OH
O
O
O
b
c
Entry Cat.
Me
2
CO Solvent
T (°C) t (h) Yield ee
1
a, acid
H
(
mmol)
(%)
(%)
DMF:H O, 0 oC
2
O N
2
O N
d
2
1
2
3
4
5
6
7
8
1a
1a
1a
1a
1a
1a
1a
1a
1a
1b
1b
27.6
27.6
27.6
27.6
27.6
27.6
13.8
6
DMF:H O
DMF
Dioxane
0
0
1
98
76
77
65
75
86
88
81
83
78
75
61
38
2
a
2
8.5 80
b
c
Entry Acid (mol %)
pK
a
t (h) Yield (%) ee (%)
0
0
3
20
95
99
d
1
2
3
4
5
6
7
8
9
1
1
—
—
4.76 12
4.76
4.20
4.20
72
99
99
90
79
77
75
78
76
77
83
83
84
80
—
H
2
O
d
d
d
d
d
d
AcOH (10)
AcOH (20)
PhCO
PhCO
DMF:H
DMF:H
DMF:H
DMF:H
DMF:H
DMF:H
2
2
2
2
2
2
O
O
O
O
O
O
À20
À40
À20
À20
À20
À20
0
8.5 94
3
36
92
98
90
77
95
96
60
2
H (10)
H (20)
1.5 94
7
18
2
1
2
5
98
90
93
e
f
4-MeOC
4-O NC
ClCH CO
Cl CHCO
6
H
4
CO
CO
2
H (20) 4.47
H (20) 3.44
9
6
27.6
27.6
100
48
2
6
H
4
2
10
11
12
f
2
2
H (20)
H (20)
H (20)
H (20)
2.87
1.29 21
0.23 30
7.5 95
H
2
O
20
g
d
2
2
86
99
—
L-Pro 27.6
DMF:H
2
O
25
54
0
1
F
3
CCO
CH SO
2
a
The reaction was carried out using the indicated equivalent of acetone
per equivalent of aldehyde (0.25 mmol) in the presence of the catalyst
(10 mol %) and benzoic acid (20 mol %) in 0.5 mL of solvent, otherwise
stated.
Isolated products after column chromatography.
Determined by HPLC (Chiracel AS, hexane/isopropanol: 85/15).
1:1.
e
3
3
À2.00
—
a
The reaction was carried out using 27.6 equiv of acetone per equivalent
of aldehyde (0.25 mmol) in the presence of 1a (10 mol %) and the indi-
cated amount of acid in 0.5 mL of solvent.
b
c
d
e
f
b
c
Isolated products after column chromatography.
Determined by HPLC (Chiracel AS, hexane/isopropanol: 85/15), the
absolute configuration for the major enantiomer being R.
5 mol % of catalyst and 10 mol % of benzoic acid was used.
10 mol % of catalyst and 10 mol % of benzoic acid was used.
20 mol % of catalyst and 20 mol % of benzoic acid was used.
d
e
9
In the absence of acid.
g
The starting compounds were recovered after 4 d of reaction time.

G. Guillena et al. / Tetrahedron: Asymmetry 17 (2006) 1493–1497
1495
Thus, it seems that the best balance between ee and reac-
tion time (86 ee, 8.5 h) could be obtained when carrying
out the reaction at À20 °C (Table 2, entry 5). Under these
reaction conditions, the amount of ketone and catalyst was
evaluated. A decrease in the amount of acetone led to an
increase of the reaction time with a slight decrease on the
enantioselectivities achieved (Table 2, entries 5, 7 and 8).
When the catalyst loading and the acid were decreased, the
reaction time was even longer (4 d) thus achieving a 78%
butanone and p-nitrobenzaldehyde, catalyzed by 1a was
performed in DMF in the absence of benzoic acid at rt
product, iso-2b was almost exclusively obtained (Table 3,
9
entry 3). However, in the presence of PhCO H, a decrease
2
in the reaction time from 264 to 31 h was observed and a 5/
1 mixture of regioisomers iso-2b and anti-3b was obtained
with lower regio- and enantioselectivities (Table 3, compare
entries 3 and 4). The addition of water to DMF increased
the enantioselectivity from 75% to 88% for the iso-2b and
from 4% to 78% for the anti-3b but decreased the regio-
selectivity from 5/1 to 7/3 (Table 3, entry 4). Under the
reaction conditions set up for acetone (Table 3, entries 1
9
ee (Table 2, entry 9). The use of catalyst 1b with only one
proline moiety in the presence of acid gave product 2a in
longer reaction times and with lower enantioselectivities
than 1a either in DMF/H O or in pure H O (Table 2,
and 2) either in DMF/H O at À20 °C or in pure water at
2
2
2
entries 10 and 11). Finally, when the reaction was carried
using L-Pro as catalyst (20 mol %) in the presence of benz-
0 °C, a 1:1 mixture of iso-2b and anti-3b was obtained with
high ee for both regioisomers, the reaction rate being
oic acid (20 mol %) in DMF/H O, the reaction did not take
higher in H O (Table 3, entries 6 and 7). Consequently,
2
2
place either at À20 °C or at 0 °C. At 25 °C, product 2a was
the aldol reaction with butan-2-one can be directed either
to the iso-regioisomer in the absence of water and acid or
to the 1/1 mixture of iso and anti-regioisomers in the pres-
achieved after 54 h in 60% yield and 38% ee (Table 2, entry
1
2).
ence of water. For cyclohexanone, the presence of PhCO H
2
The influence of the addition of benzoic acid on the aldol
reaction of several ketones with p-nitrobenzaldehyde cata-
increased the reaction rate from 72 to 2 h, even when work-
ing at À20 °C while the diastereoselectivity increased from
10/1 to 99/1 in favour of the anti-3c diastereomer (Table 3,
compare entries 8 and 9). For the reaction in neat water,
the dr was 19/1 (Table 3, entry 10). In all cases, the ee
lyzed by 1a was studied in aqueous DMF and in H O and
2
then compared with our previous studies in the absence of
9
,10
benzoic acid
(Table 3). When the reaction between 2-
a
Table 3. Reaction of 4-nitrobenzaldehyde with different ketones catalyzed by 1a and benzoic acid
O
O
OH
O
OH
O
OH
R1
+
+
a, R1 = H
b, R1 = Me
R1
R1
anti-
R1
syn-
CHO
NO2
NO2
NO2
NO2
1
a (10 mol %)
2
iso-2
3
3
PhCO H (20 mol%)
solvent , T
(
)
n
O
O
OH
O
OH
+
c, n = 1
d, n = 0
(
)n
( )n
anti-
3
syn-3
1
b
c
d
Entry
R
Solvent
T (°C)
t (h)
Yield (%)
Isomer ratio
ee (%)
Regioselectivity (2/3)
dr (anti/syn)
iso-2
anti-3
syn-3
f
1
2
3
H
H
DMF:H
2
O
À20
0
8.5
20
264
31
20
24
12
72
2
1.5
36
94
99
96
70
99
94
98
98
99
98
99
98
99
—
—
—
—
86
75
96
75
88
93
98
—
—
—
—
—
—
—
—
31
4
78
96
99
93
97
94
26
85
78
—
—
—
—
—
—
—
44
6
34
17
61
65
H
2
O
e
Me
Me
Me
Me
Me
(CH
(CH
(CH
(CH
(CH
(CH
DMF
DMF
25
25
25
À20
0
>50:1
5:1
7:3
1:1
1:1
—
—
—
—
—
100:0
100:0
100:0
100:0
100:0
10:1
99:1
19:1
1:4
4
5
6
7
f
f
DMF:H
DMF:H
2
O
O
2
H
2
O
e
f
f
8
2
2
2
2
2
2
)
)
)
)
)
)
5
5
5
4
4
4
DMF:H
DMF:H
2
O
O
0
9
2
À20
10
11
12
13
H
2
O
0
g
f
f
DMF:H
DMF:H
2
O
O
0
2
À20
1.5
1
1:2
1:2
H
2
O
0
—
a
The reaction was carried out using 26.4 equiv of ketone per equivalent of aldehyde (0.25 mmol) in the presence of 1a (10 mol %) and benzoic acid
20 mol %) in 0.5 mL of solvent, otherwise stated.
(
b
c
d
e
f
Of the isolated products after column chromatography.
Determined by H NMR.
1
Determined by HPLC.
In the absence of benzoic acid.
9
1:1.
g
In the absence of benzoic acid.

1496
G. Guillena et al. / Tetrahedron: Asymmetry 17 (2006) 1493–1497
for the anti-3c diastereomer was kept between 93% and
7%. For the reaction of cyclopentanone with p-nitrobenz-
2. Marckwald, W. Ber. Dtsch. Chem. Ges. 1904, 37, 349–
54.
3
9
3
. (a) List, B. Synlett 2001, 1675–1686; (b) Gr o¨ ger, H.; Wilken,
J. Angew. Chem., Int. Ed. 2001, 40, 529–532; (c) Movassaghi,
M.; Jacobsen, E. N. Science 2002, 298, 1904–1905; (d)
Paraskar, A. S. Synlett 2003, 582–583; (e) List, B. Chem.
Commun. 2006, 819–824.
aldehyde the presence of PhCO H again increased the reac-
2
tion rate from 36 h at 0 °C to 1.5 h at À20 °C (Table 3,
entries 11 and 12). Remarkably, the diastereoselectivity
was reversed, with the product syn-3d being the major
one in a 1/4 and 1/2 diastereomeric ratio. The presence
of acid increased the ee from 17 to 61% for the syn-3d
and from 26% to 85% for the anti-3d diastereomers. When
this condensation was performed in neat water at 0 °C, the
process took place in 1 h with similar regio- and enantio-
selectivities (Table 3, entry 13).
4
. For general reviews on the catalytic enantioselective aldol
reaction, see: (a) Gr o¨ ger, H.; Vogl, E. M.; Shibasaki, M.
Chem. Eur. J. 1998, 4, 1137–1141; (b) Nelson, S. G.
Tetrahedron: Asymmetry 1998, 9, 357–389; (c) Carreira, E.
M. In Comprehensive Asymmetric Catalysis; Jacobsen, E. N.,
Platz, A., Yamamoto, H., Eds.; Springer: Heidelberg, 1999;
Vol. 3, Chapter 29–188; (d) Mahrwald, R. Chem. Rev. 1999,
9
6, 1095–1120; (e) Machajewski, T. D.; Wong, C.-H. Angew.
Chem., Int. Ed. 2000, 39, 1352–1374; (f) Alcaide, B.;
Almendros, P. Eur. J. Org. Chem. 2002, 1595–1601; (g)
Modern Aldol Reactions; Marhrwald, R., Ed.; Wiley-VCH:
Weinheim, 2004; Vols. 1–2, (h) Palomo, C.; Oiarbide, M.;
Garc ´ı a, J. M. Chem. Rev. Soc. 2004, 33, 65–75; (i) Mestres, R.
Green Chem. 2004, 6, 583–603.
3
. Conclusion
In conclusion, benzoic acid has shown to effectively in-
crease the reaction rate as well the yield between p-nitro-
benzaldehyde and several alkyl ketones catalyzed by (S )-
a
BINAM-L-prolinamide 1a in DMF/H O and in neat
5. For the first proline catalyzed intramolecular reaction see:
2
water. This is probably due to the acceleration of the for-
mation of the enamine intermediate. In addition, the pres-
ence of benzoic acid incremented the enantio- and the
diastereoselectivities. For 2-butanone, the presence of ben-
zoic acid favoured the formation of the anti-isomer over
the iso-regioisomer. For cycloalkanones, a noticeable
increase of the reaction rate was also observed. In the case
of cyclohexanone, the corresponding anti-isomer can be
formed exclusively with higher ee. However, for cyclo-
pentanone, the syn-isomer was the major diastereomer
with or without benzoic acid, the ee being much higher in
the presence of the acid. Additional studies about the use
of BINAM-prolinamides and benzoic acid as organo-
catalysts are currently under investigation.
List, B.; Lerner, R. A.; Barbas, C. F., III. J. Am. Chem. Soc.
2
000, 122, 2395–2396.
6
. For latest uses of proline in the direct aldol reaction, see: (a)
C o´ rdova, A.; Ibrahem, I.; Casas, J.; Sund e´ n, H.; Engqvist,
M.; Reyes, E. Chem. Eur. J. 2005, 11, 4772–4784; (b) Reyes,
E.; C o´ rdova, A. Tetrahedron Lett. 2005, 60, 6605–6609; (c)
Liao, W.-W.; Ibrahem, I.; C o´ rdova, A. Chem. Commun. 2006,
6
74–676; (d) Ibrahem, I.; Zou, W.; Xu, Y.; C o´ rdova, A. Adv.
Synth. Catal. 2006, 348, 211–222; (e) Kuchurenko, A. S.;
Struchkova, M. I.; Zlotin, S. G. Eur. J. Org. Chem. 2006,
2000–2004; (f) Suri, J. T.; Mitsumori, S.; Albertshofer, K.;
Tanaka, F.; Barbas, C. F., III. J. Org. Chem. 2006, 71, 3822–
3
2
828; (g) Kumar, I.; Rode, C. V. Tetrahedron: Asymmetry
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Lett. 2006, 47, 3383–3386.
7
. (a) Tang, Z.; Jiang, F.; Yu, L.-T.; Mi, A.-Q.; Jiang, Y.-Z.;
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Acknowledgements
5
755–5760; (c) Tanimori, S.; Naka, T.; Kirihata, M. Synth.
This work was financially supported by the Direcci o´ n
General de Investigaci o´ n of the Ministerio de Educaci o´ n
y Ciencia of Spain (Grant BQU2001-0724-CO2-01 and
CTQ2004-00808/BQU), the Generalitat Valenciana (Grant
CTIOIB/2002/320, GRUPOS03/134 and GV05/157) and
the University of Alicante.
Commun. 2004, 34, 4043–4048; (d) Gryko, D.; Lipi n´ ski, R.
Adv. Synth. Catal. 2005, 347, 1948–1952; (e) Guo, H.-M.;
Cun, L-F.; Gong, L.-Z.; Mi, A. Q.; Jiang, Y.-Z. Chem.
Commun. 2005, 1450–1452; (f) Tang, Z.; Yang, Z.-H.; Chen,
X.-H.; Cun, L-F.; Mi, A.-Q.; Jiang, Y.-Z.; Gong, L.-Z.
J. Am. Chem. Soc. 2005, 127, 9285–9289; (g) Chen, J.-R.;
Lu, H. H.; Li, X.-Y.; Cheng, L.; Wan, J.; Xiao, W.-J.
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