Direct asymmetric syn-aldol reactions of linear aliphatic ketones with primary amino acid-derived diamines

Article abstract of DOI:10.1002/adsc.201000419

We have designed a novel class of chiral diamine organocatalysts based on natural primary amino acids that efficiently catalyze syn-selective aldol reactions of challenging linear ketones, such as 2-butanone, and aromatic aldehydes. In the presence of trifluoroacetic acid (TFA) as Bronsted acid and 2,4-dinitrophenol (DNP) as co-catalyst, syn-aldol products have been obtained with excellent enantioselectivities of up to> 99% ee.

Full text of DOI:10.1002/adsc.201000419

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DOI: 10.1002/adsc.201000419
Direct Asymmetric syn-Aldol Reactions of Linear Aliphatic
Ketones with Primary Amino Acid-Derived Diamines
Anneleen L. W. Demuynck,a Jozef Vanderleyden,b and Bert F. Selsa,*
a
Centre for Surface Chemistry and Catalysis, Katholieke Universiteit Leuven, Kasteelpark Arenberg 23, B-3001 Leuven,
Belgium
Fax : (+32)-016-321-998; e-mail: bert.sels@biw.kuleuven.be
Centre of Microbial and Plant Genetics, Katholieke Universiteit Leuven, Kasteelpark Arenberg 20, B-3001 Leuven,
b
Belgium
Received: May 28, 2010; Revised: August 19, 2010; Published online: October 12, 2010
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.201000419.
Abstract: We have designed a novel class of chiral
diamine organocatalysts based on natural primary
amino acids that efficiently catalyze syn-selective
aldol reactions of challenging linear ketones, such
as 2-butanone, and aromatic aldehydes. In the pres-
ence of trifluoroacetic acid (TFA) as Brønsted acid
and 2,4-dinitrophenol (DNP) as co-catalyst, syn-
aldol products have been obtained with excellent
enantioselectivities of up to> 99% ee.
in asymmetric catalysis since the presence of a hydro-
gen on the nitrogen can play a role in both promoting
the formation of an active intermediate and control-
ling the stereoselectivity.[4b,c] As a consequence of
these findings, primary amines have been further es-
tablished during the past few years as valuable enam-
ine catalysts, complementing the traditional secondary
amino catalysis.[4,5] Although important progress has
been accomplished recently, the development of pri-
mary amine catalysts is still far behind in comparison
with secondary amine catalysis. Hence, it is of great
interest to further explore the potential of primary
Keywords: aldol reaction; aliphatic ketones; chiral
diamines; syn-diastereoselectivity; primary amino
acids
In the past decade intensive research has focused on
the design of various chiral pyrrolidine derivatives for
asymmetric enamine-catalyzed reactions.[1] Within this
ꢀaminocatalytic gold rushꢁ, an overwhelming number
of novel, highly efficient organocatalysts appeared in
the literature and at present, asymmetric aminocataly-
sis is widely regarded as a well-established and power-
ful synthetic tool for the enantioselective functionali-
zation of carbonyl compounds.[2] However, in spite of
the tremendous success of secondary amines as enam-
ine-based catalysts, primary amino acids have only
rarely been considered for this kind of catalysis. In
2004/2005 the first reports on primary amines as en-
amine-based catalysts appeared in the literature.[3,4]
Simple acyclic amino acids and peptides, as well as
thiourea-primary amines have been successfully ap-
plied in asymmetric aldol reactions, Mannich reac-
tions and/or Michael additions. These initial investiga-
tions demonstrate that enamine intermediates derived
from primary amines can be generated effectively.
Figure 1. Overview of catalysts screened in the model reac-
Moreover, primary amine catalysts show advantages tion.
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Anneleen L. W. Demuynck et al.
COMMUNICATIONS
amines as organocatalysts in order to extend the utili- been less investigated. Only Cheng and co-workers
ty of enamine-based catalysis. have reported a highly selective primary amine cata-
Besides, there was growing attention for the appli- lyst for syn-aldol reactions with a broad range of ke-
cation of primary amine-based catalysts in direct tones.[5i,6a–c,7a] For methyl alkyl ketones, such as 2-buta-
asymmetric aldol reactions, in particular because they none, good results with high enantioselectivities for
exhibit syn-diastereoselectivity. Recently, the research syn-aldol products are quite rare to date. Hence, de-
groups of Cheng, Barbas, Lu and Gong have inde- spite some remarkable advances, the design of simple
pendently reported successful applications of primary and efficient organocatalysts that enable syn-aldol re-
amines based on different chiral structural scaffolds actions with high enantioselectivity remains an impor-
as highly enantioselective syn-aldol catalysts.[6] Subse- tant challenge.
quently, stimulated by this pioneering work, those and
In this paper, we present a novel class of chiral dia-
other groups designed similar catalysts for syn-aldol mine catalysts based on l-(iso)leucine, l-valine and l-
reactions with comparable results.[7] Some theoretical phenylalanine for highly enantioselective syn-aldol re-
studies on the origins of the observed syn-selectivity actions of linear aliphatic ketones. Inspired by the
for selected catalyst–ketone combinations have been previous reports on syn-selective aldol reactions, we
published as well.[8] Nevertheless, in general, these prepared a series of different types of primary amino
primary amines have proved to be effective syn-aldol acid derivatives, as illustrated in Figure 1 (6–9). Con-
catalysts only for a limited group of ketone donors, sidering the target substrate group as mentioned
this is mainly for cyclic ketones and protected or un- above, we selected the aldol reaction of 2-butanone
protected (di)hydroxyacetones. Direct asymmetric and 4-(trifluoromethyl)benzaldehyde as a model reac-
syn-aldol reactions with linear aliphatic ketones have tion to evaluate the synthesized organocatalysts. To-
gether with some commercially available primary
amino acids and their derivatives (Figure 1, 1–5), cata-
Table 1. Results of catalyst screening.[a]
Table 2. Effect of additives and optimization of reaction
conditions.[a]
Entry Additive
Yield[b]
[%]
rr[b]
b/l
dr[b] syn/
anti
ee[c]
[%]
1
2
3
4
5
6
7
8
9
TFA
34
28
7/1
4/1
96
7
n.d.
Entry Catalyst Yield[b] [%] rr[b] b/l dr[b] syn/anti ee[c] [%]
TfOH[d]
n.d.[e] 1/1
1
2
3
4
5
6
7
8
1
2
3
4
5
6
<5
<1
30
10
74
65
29
28
<10
10
n.d.[d] n.d.
n.d.
n.d.
74
41
74
28
18
40
n.d.
38
n.d.
n.d.
96
94
97
HCOOH[d]
CCl3COOH
mNO2bzac
H3PW12O40
p-TSA
DNP
TFA/
PhCOOH
TFA/AcOH 44
<10
no reaction
<10
15
23
n.d.
n.d.
n.d.
15/1
2/1
n.d.
3/2
5/2
2/1
4/1
3/2
5/2
n.d.
1/1
n.d.
n.d.
5/2
7/2
3/1
3/1
n.d.
13/1 3/2
5/2
4/1
9/1
n.d.
n.d.
52
96
94
96
[f]
3/1
3/2
5/2
4/1
7/2
24
42
7a[e]
7b
8a
8b
6/1
7/2
9
n.d.
11/1
n.d.
n.d.
5/2
10
11
11/1 7/2
10/1 4/1
95
96
10
11
12
13
14
15
16
TFA/
31
8c/TFA <5
8d <1
9a/TFA 71
9b/TFA 39
9c/TFA 59
9d/TFA 82
mNO2bzac
TFA/DNP
12
13
14
49
9/1
9/1
9/1
4/1
4/1
4/1
95
97
97
TFA/DNP[g] 58
TFA/DNP[f,g] 92
7/1
7/1
8/1
[a]
All reactions were performed under neat conditions in
10a (2 mL) with 11a (0.25 mmol), 15 mol% of catalyst 9b
and 15 mol% additive at room temperature, and ana-
lyzed after 20 h, unless indicated otherwise.
Determined with chiral GC and verified with H NMR
(rr=regioisomer ratio, b/l=ratio of branched and linear
96
[a]
All reactions were performed under neat conditions in
10a (2 mL) with 11a (0.25 mmol) and 20 mol% catalyst
at room temperature and analyzed after 20 h, unless indi-
cated otherwise.
Determined with GC and verified with H NMR (rr=re-
gioisomer ratio, b/l=ratio of branched and linear prod-
[b]
[c]
1
[b]
[c]
1
products).
The ee of the major syn-isomer, determined by chiral
GC.
20 mol% of catalyst.
Not determined.
After 68 h of reaction.
ucts).
[d]
[e]
[f]
The ee of the major syn-isomer, determined by chiral
GC.
[d]
[e]
Not determined.
After 68 h of reaction.
[g]
Reaction in 0.5 mL of 2-butanone.
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Direct Asymmetric syn-Aldol Reactions of Linear Aliphatic Ketones
lysts 6–9 were tested in the model reaction under neat ried out in aqueous and non-aqueous solvents with
conditions and ambient temperature. The screening different
polarities
(Supporting
Information,
results are summarized in Table 1. Table S1). The catalytic performance of diamines 9a–
From the examined amino acids and their deriva- d in the model reaction was found to be highly sol-
tives, l-isoleucine and the t-BuO-l-threonine give the vent-dependent. In general, neither polar nor apolar
best results (Table 1, entries 3 and 5). Amide catalysts organic solvents improve the efficiency of 9b in the
6 and 7a, b derived from the l-Val-l-Phe dipeptide, model reaction. The use of water as a solvent or co-
show rather moderate activity and poor enantioselec- solvent increases the activity of 9b compared to the
tivity for the syn-product (entries 6–8). Furthermore, reaction in purely organic solvents, with only a mar-
the catalytic performance of amides 8a–d in the ginal decline in ee with respect to the solvent-free re-
model reaction is inferior to all previously tested action (Supporting Information, Table S1, entries 3–7
amines (entries 9–12). To our delight, primary-tertiary and 10). For catalyst 9d, a similar trend is observed
diamine catalysts 9a–d display good diastereoselectivi- (Supporting Information, Table S1, entries 14–16).
ties and very high enantioselectivities of up to 97% ee Nevertheless, optimal activity and selectivity were ob-
for the syn-product in the presence of TFA as Brøn- tained under neat conditions. From Table 2, it be-
ACHTUNGTRENNUNGsted acid (entries 13–16). Very recently, Li et al. re- comes clear that the choice of the acidic additive also
ported the design of a primary-tertiary diamine de- has an important influence on the activity and selec-
rived from l-phenylalanine.[9] Despite excellent enan- tivity of catalyst 9b. Interestingly, in spite of successful
tioselectivities in the aldol reaction of hydroxyace- applications with previously reported primary-tertiary
tone, poor results were obtained with 2-butanone as diamines, TfOH almost completely suppresses the
the ketone donor under neat conditions (36% ee for model reaction catalyzed by 9b (Table 2, entry 2). In
the syn-isomer). Hence, to the best of our knowledge, the optimized reactions with acetone and 2-pentanone
these are the highest ee values achieved for the syn- catalyzed by 9d (vide infra, Table 4, entries 9 and 13),
selective aldol reaction of 2-butanone and an aromat- replacing TFA by TfOH could not improve the over-
ic aldehyde catalyzed by a primary amino acid-de- all performance of catalyst 9d. Considering these ob-
rived organocatalyst.
servations, it is important to notice that, in absence of
Encouraged by these results, we chose catalyst 9b a diamine catalyst, the aldol reactions of 2-butanone,
for further optimization. The model reaction was car- acetone and 2-pentanone are catalyzed by TfOH
Table 3. Reaction of 10a with different aromatic aldehydes under optimized conditions.[a]
Entry
R
Cat.
Yield[b] [%]
rr[b] b/l
dr[b] syn/anti
ee[c] [%]
1
2
4-CF3C6H4
9a
9b
9c
9d
9b
9c
9b
9c
9b
9c
9b
9c
9c
94
92
97
92
99
97
98
99
90
98
39
36
18
4/1
9/1
9/1
11/1
13/1
10/1
6/1
3/1
4/1
3/1
4/1
4/1
3/1
7/2
3/1
6/1
8/1
7/2
3/1
5/2
96
97
97
98
94
96
96
98
96
>99
86
87
91
3
4[d]
5
4-NO2C6H4
2-NO2C6H4
2-ClC6H4
1-Napth
6
7
8
9
10
11
12
13[e]
4/1
>20/1
>20/1
7/1
9/1
9/1
[a]
All reactions were performed under neat conditions with 10a (0.5 mL) and aldehyde (0.25 mmol) in the presence of
15 mol% 9/TFA/DNP at room temperature and analyzed after 68 h, unless indicated otherwise.
Determined with GC or HPLC and verified with H NMR (rr=regioisomer ratio, b/l=ratio of branched and linear prod-
[b]
1
ucts).
[c]
[d]
[e]
The ee of the major syn-isomer, determined by chiral GC or HPLC.
After 20 h of reaction.
Reaction without DNP.
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Anneleen L. W. Demuynck et al.
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Table 4. Reaction of different ketones and aromatic aldehydes under optimized conditions.[a]
Entry
Cat.
R1, R2, R
Yield[b] [%]
Dr[b] syn/anti
ee[c] [%]
1
2
3
4
5
6
7
8
9b
9a
9b
9c
9d
9a
9b
9c
9d
9a
9b
9c
9d
9c
9d
9b
9b
9b
9c
9c
9c
9b
9c
9c
H, Me, 4-CF3
Me, Me, 4-CF3
92
50
29
93
69
92
95
97
93
60
31
87
93
98
99
93
98
99
> 99
96
87
92
93
96
4/1
8/1
7/1
5/1
6/1
-
-
-
-
5/2
5/2
2/1
2/1
23/1
25/1
11/1
5/1
9/2
5/1
9/2
9/2
1/7
1/2
1/2
97
98
> 99
96
99
64
62
77
83
45
52
50
42
97
98
90
86
84
91
89
88
60[h]
70
68
H, H, 4-CF3
9
[d]
10
11
12
13
14
15
16
17[e]
18
19[f]
20
21[f]
22[g]
23
24[f]
H, Et, 4-CF3
H, OH, 2-NO2
H, OH, 4-NO2
H, OH, 4-CF3
H, OH, 4-Cl
-(CH2)3-, 4-Br
[a]
All reactions were performed under neat conditions with ketone (0.5 mL) and aldehyde (0.25 mmol) in the presence of
15 mol% 9/TFA/DNP at room temperature and analyzed after 68 h, unless indicated otherwise.
Determined with GC or HPLC and verified with H NMR.
The ee of the major syn-isomer, determined by chiral GC or HPLC.
Regioisomer ratio (rr) for 9a, 9b, 9c and 9d: b/l (branched/linear)=1/1, 1/1, 2/1, 2/1 respectively.
NMP:hexane 1:1 (200 mL) was used as the solvent.
Reaction without DNP.
0.5 mmol cyclohexanone and water (0.5 mL) as the solvent.
The ee of the major anti-isomer, determined by chiral HPLC.
[b]
[c]
[d]
[e]
[f]
1
[g]
[h]
through general acid catalysis (Supporting Informa- examined. Therefore, different aromatic aldehydes
tion, Table S2, entries 2, 6 and 10), yielding the aldol were tested as aldol acceptors under optimized condi-
product as a racemic mixture. With TFA under neat tions. As shown in Table 3, excellent yields and enan-
conditions, no reaction occurs without a catalyst, sug- tioselectivities of up to >99% ee, and good diastereo-
gesting that a weaker Brønsted acid is required for ef- selectivities are achieved (entries 1–10), except for the
ficient catalysis with diamines 9a–d. Very recently, reaction with 1-naphthylaldehyde, the catalytic perfor-
2,4-dinitrophenol (DNP) has been employed as a co- mance of 9b and 9c is rather moderate (entries 11–
catalyst to improve the activity and enantioselectivity 13).
of inefficient primary amine-based organocatalysts in
In order to further explore the scope and limita-
asymmetric aldol reactions.[10] Here, the addition of tions of this novel catalytic system, diamines 9a–d
DNP significantly improved the activity of the 9b/ were applied in the aldol reaction of various ketones
TFA catalytic system in the model reaction (Table 2, and aromatic aldehydes under optimized conditions
entry 12).
(Table 4). From all ketones, the symmetrical 3-penta-
Next, the application of the 9/TFA/DNP catalytic none displays the highest stereoselectivity with an
system in the aldol reaction of 2-butanone was further enantiomeric excess of up to >99% for the syn-prod-
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Direct Asymmetric syn-Aldol Reactions of Linear Aliphatic Ketones
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uct (entries 2–5). Furthermore, good stereoselectivity
and good to excellent enantioselectivities (98% ee,
entry 15) are achieved with hydroxyacetone as the
ketone donor (entries 14–21). With acetone, despite
good activity, the enantioselectivity of 9a–d is rather
moderate (entries 6–9) and with 2-pentanone the se-
lectivity for the branched product is limited (en-
tries 10–13), as also was observed for the cyclohexane-
diamine catalyst by Cheng and co-workers.[6a] In ac-
cordance with previous reports on primary amine-
based aldol catalysts, 9b and 9c exhibit anti-selectivity
for aldol reactions of cyclic ketone donors (en-
tries 22–24).
In summary, we have designed simple chiral di-
ACHTUNGTRENNUNGamine organocatalysts based on natural primary
amino acids with hydrophobic side chains that effi-
ciently catalyze syn-aldol reactions of challenging
linear aliphatic ketones, such as 2-butanone, and un-
protected hydroxyacetone. Excellent yields and enan-
tioselectivities of up to >99%, and good diastereose-
lectivities were obtained in the presence of TFA as
Brønsted acid and DNP as co-catalyst. This work il-
lustrates the synthetic utility of primary amino acid-
based structures for the development of effective or-
ganocatalysts that promote direct syn-selective aldol
reactions.
[3] For selected examples, see: aldol reaction: a) A. Heine,
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Experimental Section
Representative Procedure for the Aldol Reaction
Organocatalyst 9b·TFA (15 mol%) and co-catalyst DNP
(15 mol%) were mixed together with the ketone (0.5 mL) at
room temperature, followed by the addition of the aldehyde
(0.25 mmol). The reaction mixture was stirred for a given
time and extracted with ethyl acetate and water. The organ-
ic phase was analyzed with NMR and chiral GC to calculate
yields and regio- and diastereomeric ratios. The enantiomer-
ic excess (ee) was determined with chiral GC or chiral
HPLC.
[4] For reviews on primary amines, see: a) F. Peng, Z.
Shao, J. Mol. Catal. 2008, 285, 1–13; b) L.-W. Xu, Y.
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[5] For selected examples, see: aldol reaction: a) B.-L.
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Biomol. Chem. 2007, 5, 2913–2915; b) J. Zhou, V. Wak-
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Acknowledgements
We are grateful to the IWT (ALWD) (instituut voor Innovatie
door Wetenschap en Technologie), IAP (Interuniversity At-
traction Poles) and Methusalem long-term structural funding
of the Flemish government (CASAS) for the financial sup-
port.
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Adv. Synth. Catal. 2010, 352, 2421 – 2426
ꢂ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Adv. Synth. Catal. 2010, 352, 2421 – 2426