Highly diastereo- and enantioselective direct aldol reactions promoted by water-compatible organocatalysts bearing central and axial chiral elements

Article abstract of DOI:10.1002/adsc.200800335

Two novel bifunctional primary amine catalysts 1 (R_A,S,S) and 2 (R_A,R,R), which bear both central and axial chiral elements, have been developed to promote highly diastereoselective and enantioselective aldol reactions of arylaldehyd

Full text of DOI:10.1002/adsc.200800335

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DOI: 10.1002/adsc.200800335
Highly Diastereo- and Enantioselective Direct Aldol Reactions
Promoted by Water-Compatible Organocatalysts Bearing Central
and Axial Chiral Elements
Fang-Zhi Peng,a Zhi-Hui Shao,a,* Xue-Wei Pu,a and Hong-Bin Zhanga
a
Key Laboratory of Medicinal Chemistry for Natural Resource (Yunnan University), Ministry of Education, School of
Chemical Science and Technology, Yunnan University, Kunming 650091, Peopleꢀs Republic of China
Fax : (+86) 871-5033538; e-mail: zhihui_shao@hotmail.com
Received: June 2, 2008; Revised: July 17, 2008; Published online: August 29, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800335.
Abstract: Two novel bifunctional primary amine
catalysts 1 (RA,S,S) and 2 (RA,R,R), which bear
both central and axial chiral elements, have been
developed to promote highly diastereoselective and
enantioselective aldol reactions of arylaldehydes
with cyclic and acyclic ketones in the presence of
water at roomtemperature. The catalyst 2 (RA,R,R)
afforded the desired products with high levels of
anti diastereoselectivity (up to 99:1) and enantiose-
lectivity (up to 98%), showing that the two chiral
elements of catalyst 2 (RA,R,R) are matched, and
enhance the stereochemical control. In addition, the
catalyst 2 (RA,R,R) was found to catalyze the direct
aldol reaction of 4-nitrobenzaldehyde with 2-cyclo-
hexanone under neat reaction conditions at room
temperature with the high anti diastereoselectivity
(98:2) and enantioselectivity (98%).
reactions in aqueous medium remains an important
but challenging goal[7] as water often inhibits the cata-
lystꢀs activity or alters enantioselectivity by interrupt-
ing ionic interactions and hydrogen bonds critical for
stabilizing the transition states of the reactions. Early
studies with small organic molecules in an aqueous
medium had limited success.[8] Recently Hayashi,[9]
Barbas III and Takabe[10] independently reported pro-
line-derived chiral catalysts for highly enantioselective
aldol reactions in the presence of water. Following
these seminal reports, a number of organocatalysts
have been designed for the direct asymmetric aldol
reactions in aqueous medium.[11–13] However, only a
few organocatalysts among themreally seemto work
in the presence of a large excess of water,[9,10,11a–e]
while other catalysts have different drawbacks. Some
[12]
of themwork in aqueous organic mixed solvents
or
require the use of a large excess of ketones[11f–i] and
others are supported or dendritic systems.[13] On the
other hand, the catalysts which really seemto work in
the presence of a large excess of water are often suf-
fering fromlimitations such as the requirement of low
temperature[9b,11a,c] and the high catalyst loading.[11e]
Therefore, fromthe practical viewpoint, the develop-
Keywords: aldol reaction; asymmetric catalysis; cat-
alyst design; central and axial chirality; water
The asymmetric aldol reaction is one of the most im- ment of new chiral organocatalysts with appropriate
portant carbon-carbon bond forming reactions in or- catalyst loading for the direct asymmetric aldol reac-
ganic synthesis.[1] Asymmetric organocatalysis has tions in the presence of a large excess of water at
grown explosively to become the main focus of re- roomtemperature is still highly desirable.
search in asymmetric synthesis in recent years.[2] Since
As part of our interest in asymmetric organocataly-
the pioneering findings by List,[3] Barbas,[4] and their sis based on chiral primary amines,[14] we report a
co-workers that l-proline could work as an active or- novel type of organocatalyst for promoting the direct
ganocatalyst in intermolecular direct aldol reactions, aldol reaction in water at roomtemperature with high
numerous organocatalysts have been developed for diastereoselectivity and enantioselectivity.
the direct asymmetric aldol reactions.[5]
For the design of novel chiral catalysts, we are in-
Water is an ideal solvent for chemical reactions due terested in organocatalysts bearing two different types
to its low cost, safety and environmentally benign of stereogenic structure in the same molecule as this
nature.[6] The development of highly stereoselective would facilitate catalyst tunability. On the other hand,
organocatalysts that promote direct asymmetric aldol to achieve a catalytic asymmetric aldol reaction in
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water, the incorporation of an appropriate hydropho- ing tertiary amines 5a and 5b in 74% and 76% yield,
bic group into the organic catalyst may be feasible as respectively.[16] Subsequent deprotection of 5a and 5b
it would generate a hydrophobic organocatalyst. with KOH in 96% EtOH solution at 508C afforded
Given these considerations, we designed bifunctional primary amines 1 and 2 in 90% and 89% yield, re-
organocatalysts 1 and 2 (Figure 1) possessing the ax- spectively.
The efficacy of bifunctional primary amines 1 and 2
as chiral organocatalysts was initially evaluated using
the reaction of cyclohexanone 7 with 4-nitrobenzalde-
hyde 8 in the presence of 7 mol% of the bifunctional
primary amines and TfOH in water at room tempera-
ture (Table 1). Under the same conditions, the cata-
lyst 2 (RA,R,R) gave the desired product 9a in higher
diastereoselectivity (98:2) and enantioselectivity
Figure 1. Designed bifunctional catalysts 1 and 2.
(98%) (entry 2), whereas the catalyst 1 (RA,S,S) af-
forded the product 9a in lower diastereoselectivity
(79:21) and enantioselectivity (85%) (entry 1). These
ially chiral binaphthyl unit. It was hypothesized that results seemto indicate that the two chiral elements
the axially chiral binaphthyl group of the catalysts in the primary amine catalyst 2 (RA,R,R) are matched,
would play a double role in the catalytic process. One enhancing the stereochemical control, whereas two
role is to act as the second chiral element to facilitate chiral elements in the chiral primary amine catalyst 1
catalyst tunability, and another role is to act as a hy- (RA,S,S) are mismatched. Interestingly, very recently
drophobic group to enhance the hydrophobic hydra- we[17] developed two bifunctional amine-thiourea or-
tion of the catalysts and assembly with organic sub- ganocatalysts 6a and 6b possessing both central and
strates in water. As a result, water can be sequestered axial chiral elements for the enantioselective Michael
fromthe transition state and high stereocontrol may
reaction between 1,3-dicarbonyl compounds and
be expected. To the best of our knowledge, this is the nitro-olefins in which two chiral elements of the
first highly diastereoselective and enantioselective amine-thiourea catalyst 6b (RA,S,S) was matched,
direct aldol reaction in the presence of a large excess whereas two chiral elements of the amine-thiourea
of water promoted by a primary amine catalyst with catalysts 6a (RA,R,R) was mismatched (Figure 2).
both central and axial chiral elements.
Additionally, the reaction became very sluggish in
The two bifunctional chiral organocatalysts 1 and 2, the absence of TfOH (entry 3). This experiment indi-
carrying the same absolute configuration of the bi- cated that strong acidic additive such as TfOH was es-
naphthyl unit, but differing in the two stereogenic sential for catalytic activity. As shown in the Table 1,
centers of the cyclohexyl scaffold, were easily synthe- although the central chiral element of the primary
sized by following the procedure described in amine catalysts predominates the absolute configura-
Scheme 1. The reaction of (R)-2,2’-di(bromomethyl)- tion of the product 9a, the axial chirality significantly
1,1’-binaphthyl 3[15] with DAB (1,3-dimethyl-5-acetyl- influences the diastereoselectivity and enantioselectiv-
barbituric
acid)-mono-protected
(1S,2S)-
and ity of this reaction. Next, the primary amine 2 was
(1R,2R)-cyclohexyldiamine 4a and 4b in CH2Cl2 in taken as the catalyst of choice and evaluated for the
the presence of Et3N as the base gave the correspond- same reaction in neat reaction conditions. To our de-
Scheme 1. Synthesis of bifunctional organocatalysts 1 and 2.
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Highly Diastereo- and Enantioselective Direct Aldol Reactions
Table 1. Direct aldol reaction of 2-cyclohexanone 7 with 4-nitrobenzaldehyde 8 catalyzed by primary amines 1 and 2.[a]
COMMUNICATIONS
Entry
Catalyst (mol%)
TfOH (mol%)
Solvent
Yield[b] [%]
anti/syn[c]
ee[d] [%]
1
2
3
4
5
1 (7)
2 (7)
2 (7)
2 (7)
2 (3.5)
7
7
0
7
H2O
H2O
H2O
Neat
H2O
81
96
trace
97
96
79:21
>98:2
–
98:2
98:2
85
98
–
98
98
3.5
[a]
The reaction was performed with cyclohexanone 7 (0.6 mmol) and 4-nitrobenzaldehyde 8 (0.2 mmol) in the presence of
catalysts 1 and 2 with specified catalyst loading at roomtemperature.
[b]
[c]
[d]
Combined yields of diastereomers.
Diastereoselectivities were determined by H NMR analysis of the products.
1
The ees of the anti product were determined by HPLC using a Chiralpak AD-H column.
conditions.[11i] Since we got high diastereoselectivity
(98:2) and enantioselectivity (98%) on performing
the aldol reaction in water by using 7 mol% of 2/
TfOH, the possibility of decreasing the catalyst load-
ing was evaluated with the hope to improve the cata-
lyst efficiency. Fortunately, by using water as the sol-
vent, 3.5 mol% of 2/TfOH was efficient enough to
mediate the aldol reaction in a good yield (96%) with
an excellent diastereomeric ratio of 98:2 as well as
high enantioselectivity of 98% ee (entry 5).
Figure 2. Bifunctional amine-thiourea organocatalysts 6a
and 6b.
To examine the generality of the reaction, the aldol
reactions between aryl aldehydes with ketones includ-
ing cyclic and acyclic ketones were tested by using
light, the same level of diastereoselectivity (98:2) and catalyst 2/TfOH. The results are summarized in
enantioselectivity (98%) was achieved (entry 4). It Table 2. The aldol reactions of aryl aldehydes with ke-
should be noted that few universal catalysts give high tones proceeded smoothly to give the desired adducts
diastereoselectivity and enantioselectivity for this re- in water. With 2-cyclohexanone, high diastereoselec-
action, both in the presence of water and under neat tivities (94:6~99:1) and enantioselectivities (90-98%)
Table 2. 2/TfOH-catalyzed direct aldol reactions in water.[a]
Entry
1
Product
Time [h]
10
Yield [%][b]
96
anti/syn[c]
ee [%][d]
>98:2
98
2
16
93
98:2
97
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Table 2. (Continued)
Entry
Product
Time [h]
18
Yield [%][b]
94
anti/syn[c]
ee [%][d]
3
99:1
98:2
97:3
98:2
94:6
95:5
96
4
30
21
17
60
72
83
95
95
96
94
90
5
80
6
93
7[e]
61[f]
8[e]
23[g]
9
12
18
48
94
80:20
90:10
–
86
92
87
10[h]
91
11[i]
71[j]
[a]
The reaction was performed with arylaldehydes (0.2 mmol), ketones (0.6 mmol), 2/TfOH (3.5 mol%) and water (0.5 mL)
at roomtemperature.
Combined yields of diastereomers.
Diastereoselectivities were determined by H NMR analysis of the products.
The ees were determined by chiral-phase HPLC analysis of the anti product.
The reaction was performed with 7 mol% catalyst 2.
35% benzaldehyde was recovered.
72% 4-methoxybenzaldehyde was recovered.
The reaction was run at 128C.
[b]
[c]
[d]
[e]
[f]
1
[g]
[h]
[i]
20 equiv. of acetone was used in this reaction.
23% 4-nitrobenzaldehyde was recovered.
[j]
were obtained (entries 1–8). In the case of 2-cyclopen- hexanone in our catalytic system. However, the dia-
tenone with 4-nitrobenzaldehyde, lower diastereose- stereoselectivity and enantioselectivity were increased
lectivity (80:20) and enantioselectivity (86%) were to 90:10 dr and 92% ee, respectively, when the reac-
achieved at roomtemperature compared to 2-cyclo-
tion was conducted at 128C. It should be noted that
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Highly Diastereo- and Enantioselective Direct Aldol Reactions
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Experimental Section
Typical Procedure for the Direct Asymmetric Aldol
Reaction Catalyzed by Bifunctional Catalyst 2
To a stirred suspension of 2/TfOH (3.5 mol%) and ketone
(0.6 mmol) in water (0.5 mL) at room temperature after
20 min, the aryl aldehyde (0.2 mmol) was added. The mix-
ture was allowed to stir for the stated time, then the reaction
mixture was quenched with saturated NH4Cl solution, ex-
tracted with EtOAc and dried over Na2SO4. The crude prod-
uct was purified by flash silica gel chromatography.
Figure 3. Proposed transition state model.
the diastereoselectivity in this reaction represents the
best result so far. In addition to cyclic ketones, an acy-
clic ketone was also evaluated as the substrate. For
example, acetone gave the desired product with 87%
ee.[18] Absolute configurations of 9a–i were deter-
mined by comparing the retention time on HPLC of
the products with literature data.[11g,13a,19]
Acknowledgements
We thank National Natural Science Foundation of China
(20702044) for financial support.
To account for the stereochemical outcome of the
current reaction, we propose that the primary amine-
catalyzed aldol reaction occurs via the transition state
as depicted in Figure 3, where the enamine attacks
the aldehyde fromthe Si face, leading to the forma-
tion of the major stereoisomer. In this model, the pro-
tonated tertiary amine serves as a directing hydrogen
bonding donor.[19] We belive that chiral binaphthyl
group of the organocatalyst facilitates the formation
of a hydrophobic core with substrates in water, thus
promoting the aldol reaction in water. This may be
supported by the observation that the catalyst
(1R,2R)-N1,N1-dimethylcyclohexane-1,2-diamine (3.5
mol%) with decreased hydrophobicity affords only a
very small amount of product in the reaction of cyclo-
hexanone (0.6 mmol) and 4-nitrobenzaldehyde
(0.2 mmol) in water (0.5 mL) at room temperature
after 10 h.
In summary, we have developed two novel bifunc-
tional enamine catalysts 1 (RA,S,S) and 2 (RA,R,R),
which bear both central and axial chiral elements, for
promoting the diastereoselective and enantioselective
aldol reaction in water between aryl aldehydes with
cyclic and acyclic ketones. The catalyst 2 (RA,R,R) af-
forded the desired products with high levels of diaste-
reoselectivity (up to 99:1) and enantioselectivity (up
to 98%), showing that two chiral elements of 2 are
matched, and enhance the stereochemical control. In
addition, the catalyst 2 was found to catalyze the
direct aldol reaction of 4-nitrobenzaldehyde with 2-
cyclohexanone under neat reaction conditions at
roomtemperature with the high anti diastereoselec-
tivity (98:2) and enantioselectivity (98%). Mechanis-
tic studies and applications of these bifunctional en-
amine organocatalysts in other asymmetric reactions
are under examination in our laboratory.
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