Chiral phosphoproline-catalyzed asymmetric Michael addition of ketones to nitroolefins: An experimental and theoretical study

Article abstract of DOI:10.1039/c1ob06143g

A novel pyrrolidine-based chiral phosphoproline is an effective bifunctional organocatalyst for the asymmetric Michael addition of ketones to nitroolefins giving high levels of diastereo- and enantio-selectivities (up to > 99:1 dr and 96% ee). anti-SR Tra

Full text of DOI:10.1039/c1ob06143g

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PAPER
Chiral phosphoproline-catalyzed asymmetric Michael addition of ketones to
nitroolefins: an experimental and theoretical study†
Zhiping Zeng,a Ping Luo,a Yao Jiang,a Yan Liu,a Guo Tang,a Pengxiang Xu,a Yufen Zhao*a,b and
G. Michael Blackburn*c
Received 11th July 2011, Accepted 22nd July 2011
DOI: 10.1039/c1ob06143g
A novel pyrrolidine-based chiral phosphoproline is an effective bifunctional organocatalyst for the
asymmetric Michael addition of ketones to nitroolefins giving high levels of diastereo- and
enantio-selectivities (up to > 99 : 1 dr and 96% ee). anti-SR Transition state has the lowest barrier which
controls the stereoselectivity, in agreement with experimental results.
proline derivatives have been developed and used as excellent
Introduction
organocatalysts.
During recent years, organocatalysis has developed rapidly and
The pyrrolidine ring of these compounds is now regarded as one
of the “privileged” backbones for asymmetric catalysis.4i Further
investigations and applications have been made with water as
an additive or reaction solvent,10 and this may indicate that a
hydrogen-bond donor function is valuable catalyst structure. To
the best of our knowledge, there has been only a few pyrrolidine-
type organocatalysts having a phosphine oxide function linked
to the “privileged” chiral pyrrolidine to be used successfully4c,4e,11
(Scheme 1). It has been suggested that its P O group (compound
I, II) may mimic a carboxyl group and interact favourably with
the nitro group by dipole interactions mediate by a single water
molecule (compound III) and thereby control both enantio- and
diastereo-selectivity.4c However, these organocatalysts still have
some drawbacks, such as high catalyst loading (10–30 mol %) and
low reaction temperature, which will limit their application in the
pharmaceutical industry. We have therefore sought to improve the
performance of the diphenyl-pyrrolidyl-methyl-phosphine oxide
catalyst through the introduction of a chiral functional group
hydroxyl, as shown (Scheme 1, IV).
much attention has been given extensively to the design and
application of organocatalysts to construct asymmetric carbon-
carbon and carbon-heteroatom bonds for the preparation of
enantiomerically pure compounds.1
Among well-developed methods, the asymmetric Michael addi-
tion of different carbon-centered nucleophiles to electron deficient
nitroolefins is widely recognized as one of the most important
and versatile methods for the formation of carbon-carbon bonds
with high diastereoselectivities as well as high enantioselectivities
for the formation of two contiguous stereocentres in a single
step.2 Because of the versatile reactivity of the nitro group, the
resulting nitroalkanes can readily be transformed into versatile
synthetic building blocks, such as amines, nitrile oxides, ketones,
and carboxylic acids.3 As a result, the design and development of
new and efficient chiral organocatalysts to achieve high levels of
enantio- and/or diastereoselectivity in Michael conjugate addi-
tions remains a strong challenge in synthetic organic chemistry.4
Pioneering work by Hanessian,5 List,6 Ender,7 Alexakis,8 and
Ley9 has served to show that (S)-proline is a very good catalyst
for asymmetric Michael addition reactions, the key step being
the formation of an enamine intermediate. Because of the poor
solubility of proline in many organic solvents, numerous novel
aDepartment of Chemistry and The Key Laboratory for Chemical Biology of
Fujian Province, College of Chemistry and Chemical Engineering, Xiamen
University, Xiamen, 361005, P. R. China. E-mail: yfzhao@xmu.edu.cn;
Fax: +8605922185780; Tel: +8605922185780
bThe Key Laboratory for Bioorganic Phosphorus Chemistry and Chemical
Biology (Ministry of Education), Department of Chemistry, School of
Sciences, Tsinghua University, Beijing, 100084, P. R. China
cKrebs Institute, Department of Molecular Biology & Biotechnology,
Sheffield University, S10 2TN, UK. E-mail: g.m.blackburn@sheffield.ac.uk;
Fax: +44 114 2222800; Tel: +44 114 2229462
† Electronic supplementary information (ESI) available: Synthetic, spec-
tral and crystallographic details. CCDC reference number 826958. For
ESI and crystallographic data in CIF or other electronic format see DOI:
10.1039/c1ob06143g
Scheme 1 Structure comparison of designed catalysts.
This catalyst design has a hydroxyl group introduced as an
intramolecular hydrogen bond donor for activation of the nitro
group to replace the water molecule described in Zhong’s work.4c
We here report the asymmetric Michael addition of cyclic ketones
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to nitroolefins promoted by this simple bifunctionalorganocata-
lystusing low catalyst loading (5 mol %) in solvent-free reactions,
and achieving high levels of diastereo- and enantio-selectivity. In
addition, we have sought to explain these results using a DFT
theoretical study.
eluting with CH2Cl2–MeOH (20 : 1) to give compound 5 (452 mg,
0.92 mmol, 92%) as a white solid.
General experimental procedure for the Michael addition
The trans-nitrostyrene (38 mg, 0.25 mmol, 1 eq) in solution in
cyclohexanone (0.52 mL, 5 mmol, 20 eq) was treated with catalyst
4 (4 mg, 0.0125 mmol (5 mmol %), 0.050eq) in the presence of
p-nitrobenzoic acid (0.025 mmol, 0.010 eq) and KOH (50 mL 1
M). The resulting mixture was stirred at r. t.. After the reaction
was complete (monitored by TLC), the mixture was purified by
flash chromatography (hexane : EtOAc = 9/1) to give the product.
Experimental
Synthesis of the catalyst
Synthesis of the compound 3SR
A solution of 1 (24 mmol, 4.776 g, yellow oil) in dichloromethane
(30 mL) was treated dropwise with 2 (28.8 mmol, 5.820 g, solid)
and triethylamine (2 mL). The reaction was monitored by TLC
until the compound 1 had disappeared. Solvent was then removed
under reduced pressure to give a white solid, purified by multi-
recrystallization in ethyl acetate to give product 3SR (5.144 g) as
a colorless amorphous solid (yield 54%).
Computation details
All the geometries were fully optimized with GAUSSIAN12 at the
B3LYP/6-31G* level followed by frequency calculations to deter-
mine the nature of the stationary points. The reported energies
come from the gas phase on the DFT gas-phase geometries.
Synthesis of the compound 4
Results and discussion
Compound 3SR (2.5 mmol, 1.000 g, white solid) in methanol
(30 mL) was treated dropwise with concentrated hydrochloric
acid (2 mL) with ice cooling. The reaction was monitored by MS
until the reactant 3SR vanished. Solvent was then removed under
reduced pressure to give 4 as a white solid (0.710 g, 91%).
Synthesis of the catalyst
The newly designed S-2-[R-(diphenyl-phosphinoyl)-hydroxy-
methyl]-pyrrolidinium chloride 4 was easily prepared from S-
2-acetyl-pyrrolidine-1-carboxylic acid tert-butyl ester 1 as its
hydrochloride salt (Scheme 2). First, compound 1 was coupled
with diphenylphosphine oxide 2, to giveS-2-(diphenylphosphanyl-
hydroxy-methyl)-pyrrolidine-1-carboxylic acid tert-butyl ester
3 as a mixture of epimers. Subsequently, through multi-
recrystallization of 3, one chiral pure stereoisomer 3SR was
obtained. Finally, the Boc protect group was removed using
concentrated HCl and MeOH to give 4 as the hydrochloric
acid salt of the newly designed pyrrolidine-hydroxyl phosphine
oxide. The absolute configuration of 4 was determined by
X-ray diffraction of the derived R-(diphenyl-phosphinoyl)-[S-
1-(quinoline-8-sulfonyl)-pyrrolidin-2-yl]-methanol 5 which was
readily crystallized from ether/ethyl acetate (see ESI†). Because 4
Synthesis of the compound 5
Compound 4 (1.0 mmol, 0.300 g, white solid) was added in
a 50 mL flask containing 10 mL water, and then potassium
carbonate (10 mmol, 0.560 g, solid) and quinoline-8-sulfonyl
chloride (2.0 mmol, 0.460 g, solid) was added to this solution, and
then 10 mL CH2Cl2 was added to solute the quinoline-8-sulfonyl
chloride. The reaction was tracked by TLC until the reactant
vanished. The resulting mixture was extracted with CH2Cl2 (3 ¥
30 mL). The combined organic phases were dried over anhydrous
Na2SO4, filtered and concentrated under reduced pressure. The
residue was purified by flash column chromatography on silica gel
Scheme 2 Synthesis of catalyst 4.
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Table 1 Variation in base and temperature for reaction of cyclohexanone
with trans-nitrostyrenea
Table 2 Screening of the Additivea
Entry Time [h] Adductb [10 mol%]
Yieldc [%] drd
eee [%]
1
2
3
4
5
6
7
8
71
71
71
71
71
71
71
71
4-ClC6H4COOH
4-CF3C6H4CH2COOH 98
2-NO2C6H4COOH 84
4-NO2C6H4CH2COOH 98
99
94 : 6
97 : 3
86 : 13 87
98 : 2
97 : 3
96 : 4
94 : 6
94 : 6
85
86
87
89
92
94
94
4-HOC6H4COOH
4-CH3C6H4COOH
C6H5COOH
87
94
64
92
Entry Temp.[◦C] Time [h] KOHb [mmol] Yieldc [%] drd
eee [%]
4-NO2C6H4COOH
1
2
3
4
5
6
7
r. t.
r. t.
r. t.
r. t.
r. t.
50
115
115
57
57
57
19
19
0
n. d.
trace
85
75
94
94
89
0.025
0.050
0.075
0.100
0.050
0.075
a Unless otherwise noted, all the reactions were carried out at room
temperature by using trans-b-nitrostyrene (0.25 mmol, 1.0 equiv) and
cyclohexanone (5 mmol, 20 equiv) in the presence of 50 mL of 1 M KOH
solution (0.05 mmol). b The amount of additive is 10 mol%. c Isolated yield.
88 : 12 90
96 : 4 86
94 : 6 88
90 : 10 83
92 : 8 82
d Determined by H NMR of the crude mixture. e Determined by chiral
1
50
HPLC analysis (Chiralpak AS-H, hexane/2-propanol = 85/15, 1.0 mL
min-1).
a Unless otherwise noted, all the reactions were carried out by using trans-
b-nitrostyrene (0.25 mmol, 1.0 equiv) and cyclohexanone (5 mmol, 20
equiv) in the presence of 0.05 mmol of catalyst (20 mol%). b Concentration
of KOH is 1 M. c Isolated yield. d Determined by 1H-NMR of the
crude mixture. e Determined by chiral HPLC analysis (Chiralpak AS-H,
hexane/2-propanol = 85/15, 1.0 mL min-1).
of the product, p-nitrobenzoic acid was the best choice as the acid
cocatalyst. Next, we investigated the influence of catalyst loading
on the reaction outcome (Table 3, entry 1–8). A reduction in the
amount of catalyst 4 from 20 to 5 mol% resulted in a relatively slow
reaction but still gave good chemical yield and excellent dr and ee
(Table 3, entry 8). Increasing the amount of 1 M KOH aqueous
solution with 5 mol% catalyst 4 and 10 mol% p-nitrobenzoic acid
was predominantly beneficial for chemical yield with a longer
reaction time (Table 3, entry 5–8). Under the optimal reaction
conditions (5 mol% catalyst, 10 mol% p-nitrobenzoic acid, 50
mL 1 M KOH (0.050 mmol), at r.t., solvent effect was taken
into consideration. Unfortunately, all these experiments were
disappointing giving poor conversions. Therefore all reactions
were carried out in neat solution of cyclohexanone in large excess
(20 eq).
decomposed slowly on storage as the free base, it was kept as its
hydrochloric acid salt and converted into the free base in situ for
catalysis by addition of a base such as 1 M KOH solution.
The screening of the catalytic condition
With the catalyst to hand, a preliminary examination was carried
out on the reaction of b-nitrostyrene with cyclohexanone. First,
the result of variations in base and reaction temperature is shown
in Table 1. The addition of 1 M KOH aqueous solution was found
to be an essential factor for this reaction (Table 1, entry 1–5). It
could be seen that addition of 50 mL 1 mol L-1 KOH (0.050 mmol)
gave out the best enantioselectivity (ee), diastereoselectivity (dr)
and reasonable chemical yield (Table 1, entry 3). In this case, the
amount of KOH just neutralized the acidity of itself 20 mol%
catalyst, so it could accelerate the Michael addition smoothly.
There was no any benefit to the ee value by increasing the amount
of inorganic base because the KOH used in large excess could
also promote cyclohexanone coupling with the nitroolefins even
without the assistance of catalyst in the free base. Interestingly,
when the base was not enough to neutralize the acidity of catalyst,
no reaction progress was detected even to prolong its reaction
time (Table 1, entry 1–2). In order to shorten the reaction time
and keep good chemical yield of our product, we tried to push the
temperature from room temperature to 50◦ C, however, it was a
pity that the ee value of product dropped from 90% to 83% sharp.13
It is worth mentioning that, thus far, for some reported nitro-
Michael additions, a suitable Brønsted acid must be used as a
cocatalyst.14 A survey of eight acidic cocatalysts revealed that the
acidic cocatalyst had an important influence on the reaction (Table
2, entry 1–8).
We also had a test to lower the molecular ratio of cyclohexanone
from 20 equivalents to 10 equivalents. The reaction was elongated
to an unreasonable time. And the yield of this reaction became
very low. So it is inadvisable to reduce the use of cyclohexanone
(20 equivalents).3,13a,15
The scope of the substrate of this catalytic reaction
With optimized reaction conditions established, we probed the
scope of the reaction with a variety of substituted trans-b-
nitrostyrenes to establish the broader utility of this asymmetric
transformation. The results are summarized in Table 4.
The results (Table 4) show that the nature of the substituents
on the aryl group slightly influenced the enantioselectivity and
diastereoselectivity except the yield. For nitrostyrenes with methyl
group at different position of benzene ring, the reaction proceeded
smoothly to afford Michael adduct (Table 4, entry 2–4) in good
enantio-(92–96% ee) and diastereoselectivity (syn/anti = 92 : 8),
especially for methyl group at ortho- and meta-position.
The reactions of the benzene ring substituents at para-position
were also studied systematically. When substituted for the strong
electron-donating groups such as methoxyl group, the entire
catalytic reaction would greatly reduce the yield even that no
reaction occurs in this situation. When substituted for the strong
electron-withdrawing group such as fluorine and chlorine atom
(Table 4, entry 5–6), both the yield and the ee value would be
greatly improved. Unfortunately, the point was that this series
The addition of 10 mol % carboxylic acid as a cocatalyst
significantly promoted the Michael addition and enhanced the
selectivity relative to the catalyst in the absence of an acidic
cocatalyst (Table 2, entry 6–8). Higher enantioselectivity was
observed for p-nitrobenzoic acid (Table 2, entry 8, 94% ee), benzoic
acid (Table 2, entry 7, 94% ee), and p-methylbenzoic acid (Table 2,
entry 6, 92% ee). Considering both the yield and enantioselectivity
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Table 3 Screening of the loading of catalyst 4a
Entry
Cat.[mmol%]
Time [h]
KOHb [mmol]
Yieldc [%]
drd
eee [%]
1
2
3
4
5
6
7
8
20
15
10
5
5
5
71
71
71
71
159
159
159
159
0.05
92
98
79
46
74
89
84
82
94 : 6
94
81
92
95
94
94
95
96
0.0375
0.025
0.0125
0.0125
0.025
0.0375
0.050
85 : 15
89 : 11
89 : 11
92 : 8
90 : 10
91 : 9
5
5
92 : 8
a Unless otherwise noted, all the reactions were carried out at room temperature by using trans-b-nitrostyrene (0.25 mmol, 1.0 equiv) and cyclohexanone
(5 mmol, 20 equiv) in the presence of 10 mol % 4-NO2C6H4COOH. b The concentration of KOH is 1 M. c Isolated yield. d Determined by 1H NMR of the
crude mixture. e Determined by chiral HPLC analysis (Chiralpak AS-H, hexane/2-propanol = 85/15, 1.0 mL min-1)
Table 4 Michael addition reactions of neat cyclohexanone to trans-
nitroolefins using catalyst 4a
Entry
Ar
Time [h]
Yieldb [%]
drc
eed [%]
1
2
3
4
5
6
7
8
9
Ph
3a
3b
3c
3d
3e
3f
3g
3h
3i
7d
82
88
91
87
99
87
94
91
93
92 : 8
92 : 8
92 : 8
93 : 7
96
92
96
95
4-MePh
3-MePh
2-MePh
4-FPh
4-ClPh
2-ClPh
2-BrPh
Furanyl
17d
17d
20d
17d
7d
7d
7d
Scheme 3 Reaction of the other ketones.
90 : 10 94
92 : 8
99 : 1
99 : 1
91
96
96
first reacts with the carbonyl group of cyclohexanone to form
an enamine with the help of the acidic cocatalyst. Subsequently,
the proton of the hydroxyl group would orientate the nitro group
through hydrogen-bonding interaction so that enamine acted as a
nucleophile and attacked the nitroolefin from different faces to give
the corresponding enantio- and diastereo-selective products. The
relative energies of the different transition states or intermediates
are given in Fig. 1.
20d
82 : 18 91
a Unless otherwise noted, all the reactions were carried out at r.t. using
nitrostyrene (0.25 mmol, 1.0 equiv) and cyclohexanone (5 mmol, 20
equiv) in the presence of 10 mmol% 4-NO2C6H4COOH and 50 mL1 M
KOH. b Isolated yield. c Determined by 1H NMR of the crude mixture.
d Determined by chiral HPLC analysis (Chiralpak AS-H, hexane/2-
propanol = 85/15, 1.0 mL min-1).
The lowest energy transition states leading to the four products
from the reactions of the enamine of cyclohexanone with the (E)-
(2-nitrovinyl)benzene were shown in Fig. 1. The transition states
involving the Re attack on the anti-enamine (anti-SSts, anti-SRts)
were lower in energy than that involving Si attack on the syn-
enamine (syn-RRts, syn-RSts) as shown in Fig. 1. Favourable hy-
drogen bonding interaction between the hydroxyl group of catalyst
4 and the nitro group of (E)-(2-nitrovinyl)benzene contributed to
further electrostatic stabilization of all of the transition states.
Transition state anti-SSts was slightly destabilized due to the
eclipsed conformation from the forming C–C bond (see the
Newman projections in the ESI, Figure S2†), and the phenyl group
easily adopted a relatively sterically hindered arrangement in this
eclipsed conformation. Fortunately, this transition state composed
of substituted substrate did not get the best ee value. Ortho-
substituted electron-withdrawing groups such as 3-chloro- and
3-bromo- substituted substrates (Table 4, entry 7–8), could be a
very good dr value of products with ee values. Finally, substituted
heterocyclic substrate (Table 4, entry 9) can also get good ee values.
The asymmetric additions of other ketones to nitrostyrene
2a with the use of 4 as a catalyst were also preliminarily
investigated. As shown in Scheme 3, other cyclic ketones such
as tetrahydrothiopyran-4-one 1b and cyclohexane-1,4-di-one mo-
noethyleneacetal 1c also reacted smoothly with 2a at room
temperature, and the diastereoselectivity of these products were
maintained although the ee value (85%) of the product was greatly
reduced.
˚
the strongest hydrogen bond with the shortest distance 1.668 A to
stabilize the orientation of b-nitroolefins.
However, in transition state anti-SRts (see Fig. 2, Figure S1†),
the conformation was changed from eclipsed to staggered, which
sufficiently released the steric repulsion of the phenyl group. So the
activation energy dropped to 4.9 kcal mol-1, which was the lowest
barrier of all the transition states. That meant its corresponding
compound was the main product of our asymmetric catalytic
The computational study of the catalytic mechanism
On the basis of the experimental results, a mechanism based upon
catalyst 4 is proposed to account for the observed diastereo-
and enantioselectivity. As shown in Fig. 1, the free base of our
catalyst behaves as a bifunctional catalyst. The pyrrolidine ring
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Fig. 1 Transition state geometries for the reaction of syn- and anti-enamine with (E)-(2-nitrovinyl)benzene.
bisfunctional organocatalyst to promote the asymmetric Michael
addiction of ketones to nitrostyrenes. The reaction takes place
smoothly with perfect diastereo- (up to >99 : 1 dr) and high
enantioselectivity (up to >96% ee) in the presence of a low
loading of this catalyst (5 mol%). This can provide a potential
useful method for the preparation of enantiomerically enriched
g-nitroketones. Further investigations on the application of this
catalyst in asymmetric catalysis are in progress.
Acknowledgements
We would like to acknowledge financial supports from the Major
Program of National Natural Science Foundation of China (No.
20732004), the National Natural Science Foundation of China
(No. 20805037 and No. 20972130).
Fig. 2 Schematic energy profile of four stereoselective products (RS, RR,
SS, SR) for this organocatalytic mechanism.
reaction. This explanation was consistent with the experimental
results. It was worth noting that the pathways involving the anti-
enamine were both exothermic reactions thermodynamically. On
the contrary, both the reactions involving the syn-enamine were
endothermic, which meant the products were energy-unfavorable
according to the computed geometries (see Fig. 2). In the second
hand, the activation energy of both transition states were quite
high (10.0 kcal mol-1 and 18.0 kcalmol-1) purely due to the steric
hindrance of phenyl group though their conformation are both
staggered (see Figure S3 and S4†).
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Conclusion
We have developed a novel type of pyrrolidine-based catalyst
bearing chiral phosphoproline functions, which works well as a
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