Prolinamide-Derived Ionic-Liquid-Supported Organocatalyst for Asymmetric Mono- and Bis-Aldol Reactions in the Presence of Water

Article abstract of DOI:10.1002/ejoc.201500775

A novel recyclable prolinamide-derived ionic-liquid-supported organocatalyst of asymmetric cross-aldol reactions in aqueous medium has been developed. In its presence, aromatic aldehydes react with cyclic or linear ketones to afford chiral aldol adducts i

Full text of DOI:10.1002/ejoc.201500775

FULL PAPER
DOI: 10.1002/ejoc.201500775
Prolinamide-Derived Ionic-Liquid-Supported Organocatalyst for Asymmetric
Mono- and Bis-Aldol Reactions in the Presence of Water
Aleksander S. Kucherenko,[a] Vasiliy V. Gerasimchuk,[a] Vladislav G. Lisnyak,[a,b]
Yulia V. Nelyubina,[c] and Sergei G. Zlotin*[a]
Keywords: Aldol reactions / Organocatalysis / Water / Supported catalysts / Bis-aldols
A novel recyclable prolinamide-derived ionic-liquid-sup-
ported organocatalyst of asymmetric cross-aldol reactions in
aqueous medium has been developed. In its presence, aro-
matic aldehydes react with cyclic or linear ketones to afford
chiral aldol adducts in moderate to high yields and with ex-
cellent dr (anti/syn up to 96:4) and ee (81–99%) values that
do not tend to decline over ten recycles of the catalyst.
Furthermore, it allows a highly enantioselective catalytic
synthesis of linear bis-aldols in aqueous medium.
Introduction
tions,[11] because these hybrid systems[12] are potentially re-
coverable and recyclable. Polystyrene-supported organocat-
alysts are nearly insoluble in two-phase reagents, where the
addition of an organic cosolvent to an aqueous system is
commonly needed to ensure high-molecular catalyst swell-
ing in the reaction medium.[13] Ionic-liquid-supported pro-
linamide 1, which has a much lower molecular weight than
polymer-supported organocatalysts, efficiently catalyzes
asymmetric aldol reactions in “pure” water.[14] However, its
activity significantly drops after three regenerations because
of gradual leaching of catalyst 1 into organic solution dur-
ing isolation of products. Later on, more robust C2-sym-
metric catalysts 2 bearing two prolinamide units modified
with ionic groups were designed. They retained high per-
formance of asymmetric catalysis in water over 15 cycles.[15]
Yet, one had to pay for higher recyclability by a noticeable
reduction of catalytic activity (efficient loading is 2–5 mol-
% for 1 vs. 10 mol-% for 2) in this case.
Herein we propose to address this issue by an approach
based on the modification of the cation unit present in cata-
lyst 1 with a Brønsted-acidic group (Scheme 1). Many
asymmetric organocatalytic aldol reactions are known to
proceed at a higher rate and with better enantioselectivity
in the presence of acidic additives.[16] We expected that a
remote carboxy group in catalyst 3 would act as an acidic
cocatalyst and simultaneously assist in retaining the catalyst
in the aqueous phase during the product extraction. Re-
cently, we reported a favorable impact of the incorporated
carboxy group on catalytic performance and recyclability of
ionic-liquid-supported primary-amine-derived chiral organ-
ocatalysts in asymmetric Michael reactions.[17] To the best
of our knowledge, no attempt has been made to improve
catalytic performance of prolinamide-derived supported or-
ganocatalysts in asymmetric aldol reactions by the incorpo-
ration of the Brønsted-acidic group.
The catalytic asymmetric aldol reaction is one of the
most important methods for the enantioselective formation
of carbon–carbon bonds in organic compounds.[1] During
the past decade, a number of metal-free organocatalytic ver-
sions of this reaction have been developed.[2] Efficient or-
ganocatalysts, such as proline derivatives, significantly wid-
ened applications and amazingly improved stereo- and
enantioselectivity of the aldol reaction.[3] Moreover, par-
ticular organocatalysts bearing hydrophobic structural
units were designed to allow asymmetric aldol reactions to
proceed in the presence of water,[4] where similar enzymatic
reactions (synthesis of carbohydrates from glycerin alde-
hyde phosphates and dihydroxyacetone[5]) occur in nature
in the presence of aldolases, which can be considered or-
ganocatalyst prototypes.[6] α-Amino acid amides[7] and
some other chiral compounds[8] are regarded as very prom-
ising catalysts of asymmetric aldol reactions in the aqueous
environment. Among them, water-tolerant supported pro-
linamide derivatives tagged to polymers[9] or ionic groups
–
–
(imidazolium cations and PF6 or NTf2 anions)[10] attract
attention as promising candidates for industrial applica-
[a] Zelinsky Institute of Organic Chemistry, Russian Academy of
Sciences
47, Leninsky Prosp., 119991 Moscow, Russia
E-mail: zlotin@ioc.ac.ru
http://zioc.ru
[b] Higher Chemical College of the Russian Academy of Sciences,
D. I. Mendeleev University of Chemical Technology
9, Miusskaya square, 125047 Moscow, Russia
[c] A. N. Nesmeyanov Institute of Organoelement Compounds of
Russian Academy of Sciences
28, Vavilova str, 119991 Moscow, Russia
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201500775.
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S. G. Zlotin, et al.
FULL PAPER
Scheme 1. Research strategy.
on the stereoinduction process. Catalyst loading reduction
down to 2 mol-% did not change enantioselectivity (Table 1,
Results and Discussion
To verify this hypothesis, we prepared hybrid catalyst 3 Entry 9), whereas reaction temperature reduction to 2 °C
by a synthetic scheme that included a reaction of prolinam- improved it up to 95% ee, though at the expense of the
ide 4,[14] which bears an α,α-diphenylvalinol structural unit, product yield (Table 1, Entry 10).
with benzyl 5-(1H-imidazol-1-yl)pentanoate 5[18] followed
Importantly, the reaction between compounds 7g and 8h
by a one-pot replacement of the bromide anion for the PF6– (1:3 ratio) catalyzed by 3 afforded bis-aldol product 10a
anion and removal of protecting groups in hexafluorophos- along with expected mono-aldol product 9m in a 19:81 ratio
phate 6 by catalytic (Pd/C) hydrogenation (Scheme 2). The (1H NMR spectroscopic data) (Table 1, Entry 15). A conve-
relatively low melting point of imidazolium salt 3 (105– nient enantioselective synthesis of linear bis-aldols in the
107 °C) allows its categorization as a task-specific chiral presence of organometallic catalysts has been developed re-
ionic liquid.
cently.[19] However, in asymmetric organocatalytic aldol re-
The catalytic performance of amide 3 was tested in asym- actions, these valuable compounds were generated in a very
metric aldol reactions of aromatic (heteroaromatic) alde- low yield[20] and, according to the best of our knowledge,
hydes 7 with ketones 8a–h or aldehyde donors 8i,j in the no data on their synthesis in aqueous medium have been
presence of water under comparable conditions (catalyst reported so far. To improve a poor yield of bis-aldol adduct
loading 5 mol-%, 7/8 ratio 1:3, 25 equiv. of water, room 10a, we studied aldolization of compounds 7g and 8h cata-
temp.). The reactions were carried out over the period of lyzed by compound 3 in the presence of water (25 equiv.) at
time required for a full conversion of 7, though no longer various 7g/8h molar ratios. It was found that the amount of
than 24 h (48 h in the case of 9g) (Table 1).
8h directly correlated with the 10a yield that reached 58%
Under the studied conditions, carbonyl compounds 7 in the experiment where a 7g/8h molar ratio of 1:0.33 was
and 8 generated corresponding aldol products 9 in moder- used (Table 2, Entries 1–4). A further decrease in the
ate to high yields and with superior dr and ee values as amount of ketone 8h did not noticeably enhance the yield
compared with corresponding reactions catalyzed by analog of bis-aldol product 10a. Under the optimal conditions,
1 (Table 1, Entries 1, 5 and 8). An advantage of catalyst 3 compounds 7a and 8h generated the corresponding bis-
over 1 was especially noticeable in the reaction of 7a with aldol product 10b in 49% yield (Table 2, Entry 5). Alterna-
acetone (8h) (Table 1, Entry 8) where the ee values of prod- tively, bis-aldol product 10a was prepared in 51% yield by
uct 9h improved from 40% to 91%, which testified to a reaction of aldehyde 7g with mono-aldol adduct 9m cata-
favorable impact of the carboxy group present in catalyst 3 lyzed by 3 under the proposed conditions (Table 2, Entry 6).
Scheme 2. Synthesis of task-specific chiral ionic liquid 3.
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Ionic-Liquid-Derived Organocatalyst for Asymmetric Aldol Reactions in H2O
Table 1. Mono-aldol reactions of aldehydes 7 with ketones 8a–h or aldehyde donors 8i,j in the presence of 3.[a]
Entry[a]
Ar
R1, R2
Time [h]
9, 10a
Yield [%][b]
dr (anti/syn)[c]
ee (9) [%[d]
]
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
4-O2NC6H4 (7a)
3-PhOC6H4 (7b)
–(CH2)4– (8a)
–(CH2)4– (8b)
4
24
24
24
24 (38)
24
48
24
24
24
24
24
24
24
24
24
24
99 (99[e]) (9a)
47 (9b)
96:4 (94:6)
92 (89[e])
95
96:4
96:4
92:8
–
–
–
–
–
–
–
–
–
–
4-O2NC6H4 (7a) –CH2–CH(Me)–(CH2)2– (8c)
99 (9c)
90
94
4-O2NC6H4 (7a)
4-O2NC6H4 (7a)
4-O2NC6H4 (7a)
4-O2NC6H4 (7a)
4-O2NC6H4 (7a)
4-O2NC6H4 (7a)
4-O2NC6H4 (7a)
2-O2NC6H4 (7c)
2-FC6H4 (7d)
2-pyridyl (7e)
4-BrC6H4 (7f)
2-BrC6H4 (7g)
4-O2NC6H4 (7a)
4-O2NC6H4 (7a)
–CH2–O–CH2– (8d)
H, nPr (8e)
86 (9d)
73 (95[e]) (9e)
92 (9f)
94 (82[e])
92
H, n-hexyl (8f)
H, –CH2–c-Pr (8g)
H, CH3 (8h)
H, CH3 (8h)
H, CH3 (8h)
H, CH3 (8h)
H, CH3 (8h)
H, CH3 (8h)
H, CH3 (8h)
H, CH3 (8h)
CH3, H (8i)
16 (9g)
Ͼ 99
93 (85[f], 89[g], 96[h]) (9h)
60 (9h)
91 (40[f], 50[g], 48[h]
)
91[i]
95[j]
98
40 (9h)
96 (9i)
93 (9j)
87
92
80 (9k)
85 (9l)
81
97[k] (95[k,l]) (9m + 10a)
95 (9n)
–
91[m] (91[l,m]
)
80:20
82:18
97
91
iPr, H (8j)
68 (9o)
[a] Unless otherwise specified, all reactions were carried out with 3 (3.82 mg, 0.005 mmol), 7 (0.10 mmol), 8 (0.30 mmol), and water
(0.045 mL). [b] Isolated yield of product 9 after column chromatography on silica gel. [c] 1H NMR spectroscopic data. [d] HPLC analysis
data for anti isomers (Chiralpak AD-H and OJ-H). [e] Reported data[14] on corresponding reactions in the presence of catalyst 1 (5 mol-
%) are given in parentheses. [f] Data of this research for catalyst 1 (5 mol-%) are given in parentheses. [g] Data for 1 (5 mol-%)/AcOH
(5 mol-%) catalytic system are given in parentheses. [h] Data for 1 (5 mol-%)/stearic acid (5 mol-%) catalytic system are given in parenthe-
ses. [i] The reaction was carried out in the presence of 3 (2 mol-%) at room temp. [j] The reaction was carried out in the presence of 3
(2 mol-%) at 2 °C. [k] Total yield of 9m and 10a (81:19 ratio) (1H NMR spectroscopic data). [l] The reaction was carried out with 3
(120 mg, 0.159 mmol), 7g (12.0 g, 64.9 mmol), 8h (16 mL, 194 mmol), and water (15 mL). [m] ee of mono-aldol 9m.
Table 2. Bis-aldol reactions of aldehydes 7 with ketones 8 or 9 in the presence of 3.[a]
Entry Aldehyde Ketone
Aldehyde/ketone molar ratio
Time [h]
Ar1, Ar2
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
7g
7g
7g
7g
7a
7g
7g
8h
8h
8h
8h
8h
9m
9h
1:2
1:1
1:0.5
1:0.33
1:0.33
1:0.33
1:0.33
24
48
48
60
60
60
60
2-BrC6H4, 2-BrC6H4 (10a)
2-BrC6H4, 2-BrC6H4 (10a)
2-BrC6H4, 2-BrC6H4 (10a)
2-BrC6H4, 2-BrC6H4 (10a)
4-O2NC6H4, 4-O2NC6H4 (10b)
2-BrC6H4, 2-BrC6H4 (10a)
2-BrC6H4, 4-O2NC6H4 (10c)
29
34
41
Ͼ 99
Ͼ 99
Ͼ 99
Ͼ 99
Ͼ 99
Ͼ 99[d]
84
58
49
51[d]
55
[a] Reaction conditions: organocatalyst 3 (3.82 mg, 0.005 mmol), aldehyde 7 (0.10 mmol), ketone 8h or 9h (0.33–2.00 mmol) and water
(0.045 mL). [b] Isolated yield of product 10 after column chromatography on silica gel and recrystallization from the hexane/EtOAc (4:1)
solvent system. [c] HPLC analysis data. [d] The reaction was carried out with 3 (16.0 mg, 0.02 mmol), 7g (1.14 g, 6.2 mmol), 9m (0.50 g,
2.05 mmol), and water (1 mL, 25 equiv.).
This method appeared to be applicable to the synthesis of
Pure enantiomers of products 10a and 10b (Ͼ 99% ee)
bis-aldol 10c, which bears different aldehyde units from were attained after the single crystallization of correspond-
compounds 7g and 9h (Table 2, Entry 7). Both mono- and ing chromatographically isolated samples from the hexane/
bis-alolization catalytic reactions were scalable (Table 1, En- EtOAc (4:1) solvent system. Minor amounts (ca. 20%) of
try 15 and Table 2, Entry 6). Furthermore, the 7 g scale meso-10a or meso-10b were separated in this way from cor-
preparation of the bromides 9m + 10a (81:19) efficiently responding enantiomers and were not analyzed. Bis-aldol
proceeded in the presence of just 0.2 mol-% of catalyst 3 10a was obtained as transparent needles with a (1R,5R) ab-
with no decrease of product yield or reaction selectivity solute configuration (X-ray data) (Figure 1 and Supporting
(Table 1, Entry 15, see Supporting Information).
Information). The absolute (R) configuration was assigned
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S. G. Zlotin, et al.
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Figure 1. (1R,5R)-1,5-bis(2-bromophenyl)-1,5-dihydroxypentan-3-one (10a).
to mono-aldol compounds 9 based on the comparison of hydrophilic AcOH, nor ambiphilic stearic acid[22] appeared
specific optical rotations of compounds 9h and 9m with re- to be an efficient cocatalyst in asymmetric aldol reactions
ported data.[21]
between 7a and 8h in the presence of water. These results
To clarify the role of the auxiliary COOH function in strongly support the observation that the covalent linkage
the catalytic aldol reactions in the aqueous environment, we between the carboxy group and the catalyst unit is essential
carried out the reaction between aldehyde 7a and acetone for attaining a high level of enantioselectivity in the studied
8h in the presence of carboxy-group-free catalyst 1 (5 mol- reactions. Apparently, a combination of H-bonding ability
%) with or without acidic additive (AcOH or stearic acid, and adjacent location of the carboxy group incorporated
5 mol-%) under similar conditions. In all cases, aldol 9h was into catalyst 3 allowed efficient transfer of hydrophilic 8h
generated in high yields but with significantly lower molecules from the aqueous phase to a convenient position
enantioselectivity (40–50% ee) than in the corresponding 3- within the organic/water interfacial region, where the active
catalysed reaction (Table 1, Entry 8). The different level of sites (pyrrolidine rings) of 3 and hydrophobic aldehyde 7
stereoinduction displayed by catalyst 3 on the one hand and molecules are located, and the enamine-type catalytic trans-
the 1/AcOH or 1/stearic acid catalytic systems on the other formation occurs in a highly enantioselective manner
hand was unexpected, because a COOH functionality was (Scheme 3).
present in all these systems, and each system had similar
Furthermore, the presence of the auxiliary COOH moi-
acidic properties and hydrogen-bonding abilities. Neither ety significantly improved recyclability of catalyst 3 as com-
pared with catalyst 1 in the asymmetric mono-aldol reac-
tion between compounds 7a and 8h (Table 3). After comple-
tion of the reaction, aldol 9h was extracted with Et2O, and
fresh portions of the starting compounds were added to the
Table 3. Recycling of catalyst 3 in the reaction of 7a with 8h in the
presence of water.[a,b]
Entry
Time [h]
Conversion [%]
ee (9h) [%]
1
2
3
4
5
6
7
8
9
24
24
24
24
24
24
24
24
24
30
93 (85)
96 (82)
99 (64)
96
96
98
96
97
90
93
91 (40)
91 (40)
92 (39)
92
94
93
94
93
93
94
10
[a] Reaction conditions: organocatalyst 3 (3.82 mg, 0.005 mmol),
7a (15.1 mg, 0.10 mmol), 8h (17.4 mg, 0.30 mmol), and water
(0.045 mL). [b] Corresponding data for catalyst 1 are given in pa-
rentheses.
Scheme 3. Plausible catalytic cycle of asymmetric aldol reactions
catalyzed by compound 3 between compounds 7 and 8h in the pres-
ence of water.
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Ionic-Liquid-Derived Organocatalyst for Asymmetric Aldol Reactions in H2O
Scheme 4. Derivatization of 10.
8 (0.30 mmol), catalyst 3 (3.82 mg, 5 mol-%), and H2O (0.045 mL,
remaining suspension of 3 in water. In this manner, catalyst
3 was successfully recycled ten times without any reduction
of the reaction rate or process enantioselectivity, whereas
catalyst 1 was significantly deactivated already in the third
cycle.
The possibility for further derivatization of bis-aldols ad-
ducts 10 was demonstrated through reduction of (1R,5R)-
10a to axially symmetric 1,3,5-triol 11 with NaBH4 fol-
lowed by conventional treatment of 11 with benzaldehyde
to afford a mixture of diastereomeric (HPLC data) ketals
12 (Scheme 4).
25 equiv.) was stirred at ambient temperature for the time given in
Table 1. The reaction mixture was extracted with Et2O (3ϫ
0.5 mL). The combined extracts were dried with anhydrous Na2SO4
and concentrated under reduced pressure (700 Pa). The conversion
was determined by 1H NMR spectroscopy of crude products. Prod-
ucts 9 were then purified by column chromatography on silica gel
(n-hexane/ethyl acetate, 1:1 to 1:3).
General Procedure for Bis-Aldol Reactions in the Presence of Water:
A suspension of aldehyde 7 (0.10 mmol), ketone 8h or 9h (0.03–
0.20 mmol), catalyst 3 (3.82 mg, 5 mol-%), and H2O (0.045 mL,
25 equiv.) was stirred at ambient temperature for 24 h. The reaction
mixture was extracted with Et2O (3ϫ 0.5 mL). The combined ex-
tracts were dried with anhydrous Na2SO4 and concentrated under
reduced pressure (700 Pa). The resulting raw bis-aldol products
were purified by column chromatography on silica gel (n-hexane/
ethyl acetate, 1:5 to 1:10) and crystallized from hexane/EtOAc (4:1)
to afford bis-aldol products 10. 10a: m.p. 47–49 °C (ref.[23] n/a
Conclusions
We have developed a prolinamide-derived ionic-liquid-
supported recyclable organocatalyst for asymmetric aldol
reactions of aromatic aldehydes with linear or cyclic ketones
data), 10b: m.p. 53–55 °C (ref.[20e] n/a data) and 10c: m.p. 74–75 °C.
1
in the presence of water. Under the proposed conditions, For the H and 13C NMR spectra or HPLC data for compounds
10a–c, and X-ray data for compound 10a, see Supporting Infor-
mation.
corresponding aldol adducts were generated in moderate to
high yields and with excellent dr (anti/syn up to 96:4) and
ee (81–99%) values that did not change over ten recycles Gram-Scale Procedure for the Synthesis of 10a from 7g and 9m in
the Presence of Water: A suspension of 2-bromobenzaldehyde
(1.14 g, 6.2 mmol), 4-(2-bromophenyl)-4-hydroxybutan-2-one (9m)
(0.50 g, 2.05 mmol), catalyst 3 (16 mg, 0.02 mmol), and H2O
(1 mL, 25 equiv.) was stirred at ambient temperature for 60 h. The
reaction mixture was extracted with Et2O (3ϫ 25 mL). The com-
bined extracts were dried with anhydrous Na2SO4 and concentrated
under reduced pressure (700 Pa). The resulting orange viscous
product was then recrystallized (hexane/EtOAc, 4:1) to afford
0.44 g (51%) of white needles, m.p. 47–49 °C.
of the catalyst. Furthermore, the catalyst allowed a highly
enantioselective synthesis of linear bis-aldol adducts that
have not been so far prepared in aqueous medium.
Experimental Section
General: 1H and 13C NMR spectra were recorded with a Bruker
AM 300 spectrometer in CDCl3 and [D6]DMSO. The chemical
shifts of 1H and 13C signals were measured relative to Me4Si or
CDCl3, respectively. The high-resolution mass spectra (HRMS)
were measured with a Bruker microTOF II spectrometer using elec-
trospray ionization (ESI). The measurements were taken either in
the positive ion mode (interface capillary voltage 4500 V) or in the
negative ion mode (3200 V) in a mass range m/z = 50–3000 Da;
external or internal calibration was done with electrospray calibr-
ant solution (Fluka). Syringe injection was used for solution in
MeCN/H2O (1:1, v/v) (flow rate 3 μL/min). Nitrogen was applied
as a dry gas, and the interface temperature was set at 180 °C. Spe-
cific optical rotations [α]2D0 were measured with a Jasco DIP-360
polarimeter at 589 nm. Silica gel 0.060–0.200 µm (Acros) was used
for column chromatography. Prolinamide 4[14] and benzyl 5-(1H-
imidazol-1-yl)pentanoate (5)[18] were synthesized according to
known methods. Compounds 7 and 8 were purchased from Aldrich
and used without purification. The solvents were purified by stan-
dard procedures. For experimental details and spectral or HPLC
data see Supporting Information.
Recycling of Catalyst 3: A suspension of aldehyde 7a (15.1 mg,
0.10 mmol), acetone 8h (0.30 mmol), catalyst 3 (3.82 mg, 5 mol-%),
and H2O (0.045 mL, 25 equiv.) was stirred at ambient temperature
for 24 h. Aldol product 9h was extracted with Et2O (3ϫ 0.5 mL).
Fresh portions of reagents were added to the remaining aqueous
suspension of catalyst 3, and the reaction was performed again as
described above.
Acknowledgments
We gratefully acknowledge financial support of the Russian Foun-
dation of Basic Research (Project 14-03-92701).
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Eur. J. Org. Chem. 2015, 5649–5654
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Received: June 11, 2015
Published Online: July 30, 2015
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