N-Tosyl-(Sa)-binam-L-prolinamide as highly efficient bifunctional organocatalyst for the general enantioselective solvent-free aldol reaction

Article abstract of DOI:10.1055/s-0028-1087349

N-Tosyl-(S_a)-binam-L-prolinamide (5 mol%) and benzoic acid (1 mol%) were used as catalysts in the enantioselective direct aldol reaction between different ketones and aldehydes under solvent-free conditions in the presence or absence of water. Under these reaction conditions it was possible to reduce the amount of required ketone to two equivalents to give the corresponding aldol products with high yields, regio-, diastereo- and enantioselectivities. The aldol reaction between aldehydes or the intramolecular aldol reaction can be also performed with excellent results. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0028-1087349

LETTER
3031
N-Tosyl-(S )-binam-L-prolinamide as Highly Efficient Bifunctional
a
Organocatalyst for the General Enantioselective Solvent-Free Aldol Reaction
N
G
-Tosyl-(
S
)-binam
a
-
L
-
prolinam
b
ide as
H
igh
r
ly Effici
i
e
nt Bif
e
unctional
O
l
rganoc
a
a
talyst Guillena,* Carmen Nájera,* Santiago F. Viózquez
a
Departamento Química Orgánica and Instituto de Síntesis Orgánica, Universidad de Alicante, Apdo. 99, 03080 Alicante, Spain
Fax +34(965)903549; E-mail: gabriela.guillena@ua.es; E-mail: cnajera@ua.es
Received 30 July 2008
Abstract: N-Tosyl-(S )-binam-L-prolinamide (5 mol%) and benzo-
a
NH
NH
O
ic acid (1 mol%) were used as catalysts in the enantioselective direct
aldol reaction between different ketones and aldehydes under sol-
vent-free conditions in the presence or absence of water. Under
these reaction conditions it was possible to reduce the amount of re-
quired ketone to two equivalents to give the corresponding aldol
products with high yields, regio-, diastereo- and enantioselectivi-
ties. The aldol reaction between aldehydes or the intramolecular al-
dol reaction can be also performed with excellent results.
NH
NH
O
O
NH
NHTs
NH
1
2
Key words: organocatalyzed, aldol reactions, solvent-free, enantio-
selective, ketone
Figure 1 Binam-L-prolinamide-derived catalysts
6
derivative 2 (Figure 1) in the aldol reaction under sol-
vent-free reaction conditions.7
Bisprolinamides derived from 1,1¢-binaphthyl-2,2¢-di-
1
amine (binam) have been studied as organocatalysts in
Catalyst 2 was prepared by a modified procedure of the
2
6
the aldol reaction under different reaction conditions us-
standard method using p-toluenesulfonyl chloride and
3
4
ing alkyl and a-functionalized ketones as source of nu-
commercially available (S)-1,1¢-binaphthyl-2,2¢-diamine
cleophile, with recoverable catalyst (S )-binam-L-Pro 1
Figure 1), providing the best results. For this prolina-
a
[(S )-binam], and subsequently coupling with the in situ
a
3
a,c
(
generated Fmoc-L-Pro chloride followed by deprotection
with piperidine, giving compound 2 in 80% overall yield
after purification.
mide, the addition of an equimolecular amount of benzoic
acid as co-catalyst in the reaction has been crucial to get a
great acceleration in the reaction. Under these reaction
Then, catalyst 2 was tested in the reaction between cyclo-
hexanone and 4-nitrobenzaldehyde under solvent-free
conditions, to optimize the reaction conditions. The addi-
tion of a small amount of benzoic acid (1 mol%) was re-
quired to obtained the aldol product 4aa in shorter
reaction times (Table 1, compare entries 1 and 2). Under
the same reaction conditions, catalysts 1 showed to be less
efficient, giving after three hours, 90% of conversion,
with the aldol products being obtained with a slightly low-
er diastereo- and enantioselectivity (Table 1, entry 3). In
some organocatalyzed aldol reactions, the addition of wa-
ter to an organic solvent has shown to have a positive ef-
3c
conditions, less reactive ketones, such as butanone and
a-functionalized ketones can be used as donors, even in
4
water. Recently, we have described the aldol reaction un-
der solvent-free conditions, reducing the loading of cata-
lyst 1 and the amount of required ketone to two
equivalents to give the corresponding products with a high
5
level of selectivity. Mechanistic studies showed that the
derived enamines from one or both proline moieties are
formed but only one prolinamide works as catalyst. In ad-
dition, the presence of benzoic acid as co-catalyst in-
creased the reaction rate but decreased the enantio-
selectivity.
8
fect on the reaction rates. Thus, the effect of the use of
With these previous results in our hands, we thought that
it would be of interest to achieve the replacement of one
of the proline residue in catalyst 1, by an acidic moiety
such as a sulfonamide group, which might allow the re-
duction of the amount of acid co-catalyst. In addition, the
sulfonamide moiety can activate the carbonyl group of the
acceptor aldehydes through a hydrogen bond acting as a
bifunctional catalyst. Here, we report the application of
some amount of water in the reaction catalyzed by 2 was
studied (Table 1). First, the addition of 1 mL of water was
evaluated. Under these conditions, the reaction was much
slower, giving the aldol product 4aa with lower conver-
sion, diastereo- and enantioselectivity (Table 1, compare
entries 2 and 4). Decreasing gradually the amount of add-
ed water, caused an acceleration in the reaction rate as
well as an increase in the conversion, diastereo- and enan-
tioselectivity (Table 1, entries 5–7), with the best results
being obtained by adding only 33 mL (7 equiv) of water to
the reaction mixture (Table 1, entry 7). This effect was in
contrast to that observed when proline was used as cata-
lyst for the aldol reaction under solvent-free-conditions, in
which the addition of small amounts of water caused a de-
the unsymmetrical (S )-binam-L-prolinamide sulfonamide
a
SYNLETT 2008, No. 19, pp 3031–3035
x
x
.x
x
.2
0
0
8
Advanced online publication: 12.11.2008
DOI: 10.1055/s-0028-1087349; Art ID: G24208ST
©
Georg Thieme Verlag Stuttgart · New York

3
032
G. Guillena et al.
LETTER
Table 1 Effect of the Addition of Water in the Reaction between 4-Nitrobenzaldehyde and Cyclohexanone under Solvent-Free Conditionsa
Entry
H O (mL)
T (°C)
Time (h)
Conversion (%)c anti/synd
ee (%)e
99
2
1b
2
–
0
0
0
0
0
0
0
0
12
6
97
99
90
44
39
70
99
97
97:3
99:1
96:4
86:16
93:7
86:11
99:1
97:3
–
97
3f
4
–
3
94
1000
500
125
33
33
26
23
18
4
94
5
96
6
98
7
98
8f
3
92
a
Reaction conditions: 4-nitrobenzaldehyde (0.25 mmol), cyclohexanone (2 equiv), 2 (5 mol%), BzOH (1 mol%), 0 °C, unless stated otherwise.
The reaction was carried out in the absence of BzOH.
b
c
d
e
f
Conversion based on the unreacted aldehyde.
1
Determined by H NMR of the crude product.
Determined by chiral-phase HPLC analysis for the anti isomer.
Reaction conditions: catalyst 1 (5 mol%), BzOH (1 mol%).
7
c
crease in the reaction rate. Applying these reaction con- gave the expected anti isomer (major isomer) in only 50%
ditions using catalyst 1 (5 mol%), benzoic acid (1 mol%), yield under wet conditions compared to 93% yield ob-
the achieved result in terms of conversion, diastereo- and tained under anhydrous conditions, with comparable dia-
enantioselectivity were poorer than those obtained using stereo- and enantioselectivities achieved for both cases
catalyst 2 (Table 1, compare entries 7 and 8) where longer (Table 2, entries 11 and 12). When acetone was allowed
reaction time was required.
to react with 4-nitrobenzaldehyde the aldol 4da was ob-
tained in good yield and enantioselectivity in only 16
hours under wet conditions, whereas 48 hours were re-
quired under anhydrous conditions (Table 2, entries 13
and 14). Longer reaction times were required with less re-
active 2-butanone giving mainly the iso isomer 5ea with
excellent enantioselectivity under wet conditions
The scope of catalyst 2 under anhydrous and wet solvent-
free reaction conditions was studied using several aromat-
ic aldehydes and cyclic, acyclic and a-functionalized ke-
tones (Scheme 1 and Table 2). For cyclic ketones the
reaction proceeded smoothly in all cases giving mainly
the anti isomers 4 (Table 2, entries 1–8), except for cyclo-
pentanone, with isomer syn-4ba being in this case the ma-
(
Table 2, entries 15 and 16).
jor isomer (Table 2, entries 9 and 10). The reaction times In all cases, with the exception of 2-butanone, the
were shorter under wet than under anhydrous conditions. achieved diastereo- and enantioselectivities were better
Whereas 4-nitrobenzaldehyde gave the corresponding al- than those previously obtained using catalyst 1 (5 mol%)
dol product in a short reaction time, nonactivated alde- and benzoic acid (10 mol%) under solvent-free condi-
hydes such as benzaldehyde or tolualdehyde needed tions, with longer reaction times being required with cat-
5
longer reaction time to give the aldol product in moderat- alyst 2. Finally, several a-functionalized ketones were
ed yields (36–76%) but in excellent diastereo- and enantio- reacted with 4-nitrobenzaldehyde under wet and anhy-
selectivities (96% de and 96% ee; Table 2, entries 3–6). drous solvent-free conditions. Whereas a-methoxyace-
Also heteroaromatic aldehydes such as furfural were suit- tone gave mainly the anti isomer 4fa in shorter reaction
able donor for this process yielding mainly anti-4ad in time under anhydrous conditions (Table 2, entries 17 and
better yield (52%) under wet than under anhydrous condi- 18), a-methylsulfanylacetone gave as main product iso-
tions and in high diastereo- and enantioselectivity 5ga with good enantioselectivity, with catalyst 2 (10
(
Table 2, entries 7 and 8). Cyclopentanone gave mainly mol%) and benzoic acid (5 mol%) being needed to obtain
aldol syn-4ba with slightly better diastereo- and enantio- 87% yield under anhydrous conditions (Table 2, entries
selectivity under anhydrous than under wet conditions 19 and 20).
(
Table 2, entries 9 and 10). Surprisingly, 4-pyranone,9
O
O
OH
O
OH
O
OH
2
(5 mol%)
CHO
BzOH (1 mol%)
+
+
+
solvent-free, 0 °C
R1
R2
R1
R2
anti-4
R1
R2
syn-4
R2
R1
iso-5
O N
2
NO2
NO2
NO2
3
Scheme 1 Reaction between 4-nitrobenzaldehyde and different ketones under solvent-free conditions
Synlett 2008, No. 19, 3031–3035 © Thieme Stuttgart · New York

LETTER
N-Tosyl-(S )-binam-L-prolinamide as Highly Efficient Bifunctional Organocatalyst
3033
a
Table 2 Reaction of Aldehydes with Cyclohexanone under Anhydrous and Wet Solvent-Free Conditionsa
Entry
Major product
Time (h)
Yield (%)b
4/5
anti/sync
ee (%)d
O
OH
1
2
6
4
94
98
–
–
99:1
99:1
98
98
e
NO2
anti-4aa
O
OH
OH
OH
3
4
120
72
76
74
–
99:1
98
e
anti-4ab
O
5
6
120
91
36
52
–
–
98:2
>99:1
96
98
e
anti-4ac
O
7
8
O
26
91
27
52
–
–
96:4
97:3
92
94
e
anti-4ad
O
OH
OH
9
0
15
7.5
90
88
–
–
32:68
37:63
84
78
e
1
NO2
syn-4ba
O
1
1
1
2
24
19
93
50
–
–
97:3
98:2
89
94
e
O
NO2
anti-4ca
O
OH
OH
OH
1
1
3
4
48
16
86
88
–
–
–
–
84
86
e
e
e
NO2
NO2
NO2
4
da
O
1
1
5
6
96
68
62
81
23:77
36:64
>99:1
>99:1
90
98
iso-5ea
O
1
1
7
8
48
65
73
76
78:22
72:28
82:18
84:16
93
94
OMe
anti-4fa
O
OH
1
2
9f
0
120
67
87
40
17:83
19:81
>99:1
>99:1
88
86
e,f
SMe
NO2
iso-5ga
a
b
c
d
e
f
Reaction conditions: 4-nitrobenzaldehyde (0.25 mmol), ketone (2 equiv), 2 (5 mol%), BzOH (1 mol%), 0 °C, unless stated otherwise.
Yield after purification by column chromatography.
1
Determined by H NMR of the crude product.
Determined by chiral-phase HPLC analysis for the major isomer.
H O (33 mL) was added to the reaction mixture.
2
Catalyst 2 (10 mol%) and BzOH (5 mol%) were used.
Synlett 2008, No. 19, 3031–3035 © Thieme Stuttgart · New York

3
034
G. Guillena et al.
LETTER
Scheme 2 Reaction between 4-nitrobenzaldehyde and propanal under solvent-free conditions
Aldehydes can also act as donors in this transformation. In Table 3 Intramolecular Aldol Reaction under Anhydrous and Wet
Solvent-Free Conditionsa
this case catalyst 2 (10 mol%) and benzoic acid (5 mol%)
have to be used at 25 °C to perform the reaction. When
Entry
Product
n
Time
h)
Yield
(%)
9/10c
ee
(%)
propanal (5 equiv) and 4-nitrobenzaldehyde were allowed
to react under these conditions 1,3-diol 7 was obtained un-
der anhydrous conditions in seven days with 80% yield,
b
d
(
1
9a
9a
9b
9b
2
2
1
1
27
16
48
48
95
90
84
85
100:0
100:0
67:36
70:30
90
88
83
90
6
6% de and 94% ee, after in situ reduction of the corre-
2e
3
sponding aldol with NaBH in MeOH (Scheme 2). When
4
the same reaction was carried out by adding 33 mL of wa-
ter (wet conditions), compound 7 was obtained after 2.5
days with 86% yield, 78 de and 92% ee (Scheme 2). These
results are superior in terms of reaction time, yield, dia-
stereo- and enantioselectivity to those reported previously
using catalyst 1 (5 mol%) and benzoic acid (10 mol%).5b
In this case six days of reaction time was required to yield
4e
a
Reaction conditions: triketone 8 (0.25 mmol), 2 (5 mol%), BzOH (1
mol%), 25 °C.
b
Yield after purification by column chromatography for compound 9.
Determined by H NMR of the crude product.
Determined by chiral-phase HPLC analysis of the isolated product.
c
d
e
1
4
5% of compound 7 with 56% de and 85% ee after final
H O (14 mL, 3 equiv) was added to the reaction mixture.
2
reduction.
Finally, these anhydrous and wet solvent free-conditions
were applied to the intramolecular aldol reaction (Robin-
and 71% ee. The synthesis of bicyclic compound 9b,
which is an important intermediate in the total synthesis of
steroids was optimized using L-proline (3 mol%) in DMF
as solvent at room temperature.1 Under these conditions
product 10a (n = 1) was obtained with 100% yield and
1
0
son-type annulation), which has been extensively used
in the synthesis of natural products (Scheme 3 and
0d
1
o,2
Table 3). Using catalyst 2 (5 mol%) and benzoic acid
(
1 mol%) at room temperature under anhydrous solvent-
9
3% ee, with a subsequent dehydration step being neces-
free conditions the 6-(enolendo)-exo,trig cyclization of
triketone 8a (n = 2), led directly to the corresponding
sary to achieve the desired product. Under solvent-free
conditions and using catalyst 2, the main product ob-
served was the indanone derivative 9b. After column
chromatography purification, the only product isolated
with similar yield, under both anhydrous and wet condi-
tions, was compound 9b, with better enantioselectivity
obtained under wet conditions (Table 3, entries 3 and 4).
1
1
Wieland–Miescher ketone 9a in only 27 hours with ex-
cellent yield and enantioselectivity (Table 3, entry 1).
1
(5 mol%)
O
O
O
BzOH (1 mol%)
solvent-free, 25 °C
O
In conclusion, sulfonamide 2 in the presence of a small
amount of benzoic acid has shown to be a useful catalyst
to perform the intramolecular and the ketone–aldehyde or
aldehyde–aldehyde intermolecular aldol reaction under
solvent-free conditions.12 The aldol products were ob-
tained with good yields and excellent diastereo- and enan-
tioselectivities, generally, with the addition of a small
amount of water (3–7 equiv) to the reaction mixture lead-
ing to an increase in the reaction rates. In general, these re-
sults are superior in terms of diastereo- and enantio-
selectivities to those previously reported using L-proline
or prolinamide 1 as catalyst under solvent-free conditions.
Further scope and application of sulfonamide catalyst 2 in
this and other related transformations are currently being
studied.
O
n
O
n
O
n
OH
10
8
9
Scheme 3 Intramolecular aldol reaction under solvent-free conditi-
ons
A shorter reaction time was required when a small amount
of water (14 mL, 3 equiv) was added to the reaction mix-
ture (wet conditions), giving ketone 9a with similar yield
and ee (Table 3, entry 2). These results are better when
compared to those reported in the literature using MeCN
as solvent in the presence of HClO and 50 mol% of L-
proline at 80 °C, affording compound 9a in 73% yield
4
1
0c
Synlett 2008, No. 19, 3031–3035 © Thieme Stuttgart · New York

LETTER
N-Tosyl-(S )-binam-L-prolinamide as Highly Efficient Bifunctional Organocatalyst
3035
a
Acknowledgment
(5) (a) Guillena, G.; Hita, M. C.; Nájera, C.; Viózquez, S. F.
Tetrahedron: Asymmetry 2007, 18, 2300. (b) Guillena, G.;
Hita, M. C.; Nájera, C.; Viózquez, S. F. J. Org. Chem. 2008,
This work was financially supported by the Dirección General de
Investigación of the Ministerio de Educación y Ciencia of Spain
73, 5933.
(
Proyects CTQ2007-62771/BQU and Consolider Ingenio 2010
(
(
6) Chen, T.; Gao, J.; Shi, M. Tetrahedron 2006, 62, 6289.
7) For some other organacatalyzed enantioselective aldol
reactions under solvent-free conditions, see: (a) Rodríguez,
B.; Rantanen, T.; Bolm, C. Angew. Chem. Int. Ed. 2006, 45,
CSD2007-00006), and the University of Alicante (GRJ06-05).
References and Notes
6
924. (b) Hayashi, Y.; Aratake, S.; Itoh, T.; Okano, T.;
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(
1) For comprehensive books and selected reviews, see:
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138. (b) Berkessel, A.; Gröger, H. Asymmetric Organo-
(
2
(
5
(
(
8) Nyberg, A. I.; Usano, A.; Pinko, P. M. Synlett 2004, 1891.
9) Chen, J.-R.; Li, X.-Y.; Xing, X.-N.; Xiao, W.-J. J. Org.
Chem. 2006, 71, 8198.
catalysis: From Biomimetic Concepts to Applications in
Asymmetric Synthesis; Wiley-VCH: Weinheim, 2005.
(
(
c) Seayad, J.; List, B. Org. Biomol. Chem. 2005, 3, 719.
d) Kočovský, P.; Malkov, A. V. Tetrahedron 2006, 62,
(
10) (a) Eder, U.; Wiechert, R.; Sauer, G. Ger. Patent, DE
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255. (e) Lelais, G.; McMillan, D. W. C. Aldrichimica Acta
Z. G.; Parrish, D. R. Ger. Patent, DE 2102623, 1971; Chem.
Abstr. 1972, 76, 59072. (c) Eder, U.; Sauer, G.; Wiechert, R.
Angew. Chem. Int. Ed. Engl. 1971, 10, 496. (d) Hajos,
Z. G.; Parrish, D. R. J. Org. Chem. 1974, 39, 1615.
2
006, 39, 79. (f) Marigo, M.; Jørgensen, K. A. Chem.
Commun. 2006, 2001. (g) Guillena, G.; Ramón, D. J.
Tetrahedron: Asymmetry 2006, 17, 1465. (h) List, B. Chem.
Commun. 2006, 819. (i) Gaunt, M. J.; Johansonn, C. C. C.;
McNally, A.; Vo, N. T. Drug Discovery Today 2007, 12, 8.
(
e) Hajos, Z. G.; Parrish, D. R. In Org. Synth., Coll. Vol. VII;
Freeman, J. P., Ed.; Wiley: New York, 1990, 363.
f) Buchschacher, P.; Fürst, A.; Gutzwiller, J. In Org. Synth.,
Coll. Vol. VII; Freeman, J. P., Ed.; Wiley: New York, 1990,
68. (g) Kwiatkowski, S.; Syed, A.; Brock, C. P.; Watt, D.
S. Synthesis 1989, 818. (h) Tietze, L. F.; Utecht, J. Synthesis
993, 927.
(
j) Pellissier, H. Tetrahedron 2007, 63, 9267. (k)
(
Enantioselective Organocatalysis; Dalko, P. I., Ed.; Wiley-
VCH: Weinheim, 2007. (l) List, B. Chem. Rev. 2007, 107,
3
5413. (m) Enders, D.; Grondal, C.; Hüttl, M. R. M. Angew.
Chem. Int. Ed. 2007, 46, 1570. (n) Guillena, G.; Ramón, D.
J.; Yus, M. Tetrahedron: Asymmetry 2007, 18, 693. (o) de
Figueiredo, R. M.; Christmann, M. Eur. J. Org. Chem. 2007,
1
(
(
11) Wieland, P.; Miescher, K. Helv. Chim. Acta 1950, 33, 2215.
12) Representative Experimental Procedure:
2575. (p) Kotsuki, H.; Ikishima, H.; Okuyama, A.
Anhydrous Solvent-Free Conditions: To a mixture of the
corresponding aldehyde (0.25 mmol), catalyst 2 (0.0125
mmol, 6.5 mg) and benzoic acid (0.0025 mmol, 0.3 mg) at
Heterocycles 2008, 75, 493. (q) Kotsuki, H.; Ikishima, H.;
Okuyama, A. Heterocycles 2008, 75, 757.
2) Guillena, G.; Nájera, C.; Ramón, D. J. Tetrahedron:
(
(
0
°C was added the corresponding ketone (0.5 mmol). The
reaction was stirred until the aldehyde was consumed
monitored by TLC). Then, the crude product was diluted in
CH Cl (10 mL), silica gel was added and the solvent was
Asymmetry 2007, 18, 2249.
3) (a) Guillena, G.; Hita, M. C.; Nájera, C. Tetrahedron:
Asymmetry 2006, 17, 729. (b) Gryko, D.; Kowalczyk, B.;
Zawadzki, L. Synlett 2006, 1059. (c) Guillena, G.; Hita,
M. C.; Nájera, C. Tetrahedron: Asymmetry 2006, 17, 1493.
(
2
2
evaporated in vacuo. The resulting residue was purified by
flash chromatography (hexanes–EtOAc) to yield the pure
aldol product.
Wet Solvent-Free Conditions: To a mixture of the
corresponding aldehyde (0.25 mmol), catalyst 2 (0.0125
(
d) Corrigendum: Guillena, G.; Hita, M. C.; Nájera, C.
Tetrahedron: Asymmetry 2007, 18, 1031. (e) Guizzetti, S.;
Benaglia, M.; Pignataro, L.; Puglisi, A. Tetrahedron:
Asymmetry 2006, 17, 2754. (f) Ma, G.-N.; Zhang, Y.-P.;
Shi, M. Synthesis 2007, 197. (g) Guizzetti, S.; Benaglia, M.;
Raimondi, L.; Celentano, G. Org. Lett. 2007, 9, 1247.
4) (a) Guillena, G.; Hita, M. C.; Nájera, C. Tetrahedron:
Asymmetry 2006, 17, 1027. (b) Corrigendum: Guillena, G.;
Hita, M. C.; Nájera, C. Tetrahedron: Asymmetry 2007, 18,
mmol, 6.5 mg), benzoic acid (0.0025 mmol, 0.3 mg), H O
2
(
33 mL) was added at 0 °C followed by the corresponding
ketone (0.5 mmol). The reaction was stirred until the
(
aldehyde was consumed. The crude product was diluted in
CH Cl (10 mL), MgSO was added and filtered. To the
2
2
4
filtrate, silica gel was added and the above purification
procedure was followed to obtain the aldol product.
1030. (c) Guillena, G.; Hita, M. C.; Nájera, C. Arkivoc 2007,
(
iv), 260. (d) Corrigendum: Guillena, G.; Hita, M. C.;
Nájera, C. Arkivoc 2007, (i), 146. (e) Guillena, G.; Hita,
M. C.; Nájera, C. Tetrahedron: Asymmetry 2007, 18, 1272.
Synlett 2008, No. 19, 3031–3035 © Thieme Stuttgart · New York