Synthesis of new pyrrolidine-based organocatalysts and study of their use in the asymmetric Michael addition of aldehydes to nitroolefins

Article abstract of DOI:10.3762/bjoc.13.59

New pyrrolidine-based organocatalysts with a bulky substituent at C2 were synthesized from chiral imines derived from (R)-glyceraldehyde acetonide by diastereoselective allylation followed by a sequential hydrozirconation/iodination reaction. The new compounds were found to be effective organocatalysts for the Michael addition of aldehydes to nitroolefins and enantioselectivities up to 85% ee were achieved.

Full text of DOI:10.3762/bjoc.13.59

Synthesis of new pyrrolidine-based organocatalysts and
study of their use in the asymmetric Michael addition of
aldehydes to nitroolefins
Alejandro Castán, Ramón Badorrey*, José A. Gálvez and María D. Díaz-de-Villegas*
Full Research Paper
Open Access
Address:
Beilstein J. Org. Chem. 2017, 13, 612–619.
Instituto de Síntesis Química y Catálisis Homogénea (ISQCH), CSIC -
Universidad de Zaragoza, Departamento de Química Orgánica, Pedro
Cerbuna 12, E-50009 Zaragoza, Spain
doi:10.3762/bjoc.13.59
Received: 26 October 2016
Accepted: 09 March 2017
Published: 27 March 2017
Email:
Ramón Badorrey* - badorrey@unizar.es;
María D. Díaz-de-Villegas* - loladiaz@unizar.es
This article is part of the Thematic Series "Strategies in asymmetric
catalysis".
* Corresponding author
Guest Editor: T. P. Yoon
Keywords:
enantioselective synthesis; Michael addition; organocatalysis;
pyrrolidines; synthetic methods
© 2017 Castán et al.; licensee Beilstein-Institut.
License and terms: see end of document.
Abstract
New pyrrolidine-based organocatalysts with a bulky substituent at C2 were synthesized from chiral imines derived from
(R)-glyceraldehyde acetonide by diastereoselective allylation followed by a sequential hydrozirconation/iodination reaction.
The new compounds were found to be effective organocatalysts for the Michael addition of aldehydes to nitroolefins and enantiose-
lectivities up to 85% ee were achieved.
Introduction
In the first decades of the 21st century, the enantioselective organocatalysts for a wide variety of chemical transformations
organocatalysis has witnessed a tremendous development [1-4] [11].
and it is now considered to be the third pillar of enantioselec-
tive catalyses together with metal complex-mediated catalysis In the course of our research we have been involved in the
and biocatalysis. Among the different structures usually found synthesis of new tuneable catalytic motifs to be used in
in organocatalysis, the five-membered secondary amine struc- organocatalysis starting from the chiral pool. Highly modular
ture of pyrrolidine has proven to be a privileged motif [5] with a chiral aminodiol derivatives were obtained by the addition of
powerful capacity in aminocatalysis [6-10]. In this context organometallic reagents to chiral imines derived from (R)-gly-
diarylprolinol silyl ethers have proven to be extremely efficient ceraldehyde – which is easily accessible from D-mannitol – and
612

Beilstein J. Org. Chem. 2017, 13, 612–619.
these were evaluated as chiral organocatalysts in the enantiose- scribed methodology [15]. The configuration at C2 of the
lective α-chlorination of β-ketoesters, with excellent results ob- pyrrolidine ring would be determined in the diastereoselective
tained after optimisation of the organocatalyst structure [12].
allylation of the starting chiral imines.
In an effort to identify new, easily accessible and tuneable
organocatalysts with the privileged pyrrolidine motif from the
chiral pool, we have now focused on the synthesis of new chiral
pyrrolidines capable of creating a sterically demanding environ-
ment due to the presence of a bulky 2,2-disubstituted-1,3-dioxo-
lan-4-yl moiety at C2 from chiral imines derived from (R)-gly-
ceraldehyde. The Michael addition of aldehydes to nitroolefins
was selected as a model reaction to evaluate the effectiveness of
the new pyrrolidine-based organocatalysts in aminocatalysis.
Figure 1: Strategy for the preparation of 2-substituted pyrrolidines C.
The homoallylic amine 1 with syn-configuration was obtained
Results and Discussion
by the reaction of the corresponding imine with allylmagne-
We reasoned that pyrrolidines of type C with a bulky 2,2-disub- sium bromide as previously described [15]. The amine 1 was
stituted-1,3-dioxolan-4-yl moiety at C2 could provide the reacted with the Schwartz reagent in CH2Cl2 at room tempera-
appropriate environment to lead to high levels of enantioselec- ture to afford the hydrozirconated intermediate, which was
tivity in asymmetric transformations in which enamine interme- immediately treated with iodine to yield N-benzylpyrrolidine 2
diates are formed. The substituent R1 in the 1,3-dioxolane in 69% isolated yield. The subsequent exposure of compound 2
moiety in pyrrolidines C could be varied to modulate the reac- to molecular hydrogen in the presence of Pd(OH)2/C as a cata-
tivity and selectivity of the new organocatalysts.
lyst afforded the desired organocatalyst OC1 in 73% yield
(Scheme 1).
The sequential hydrozirconation/iodination of chiral homo-
allylic amines has been described as a straightforward approach The same reaction sequence led to organocatalyst OC2 in 52%
to enantiomerically pure 2-substituted pyrrolidines [13,14]. overall yield starting from homoallylic amine 3 having anti-
Therefore we decided to test this methodology to gain access to configuration, which was obtained by reaction of the corre-
the pyrrolidine ring in compound C. The required chiral homo- sponding BF3·OEt2 pre-complexed imine with allylmagnesium
allylic amines B can be easily obtained by the addition of allyl- bromide as previously described [15] (Scheme 2). It is worth
magnesium bromide to imines derived from (R)-glyceralde- mentioning that the starting homoallylic amines 1 and 3 can be
hyde acetonides A (Figure 1) according to our previously de- obtained on a multigram scale from the chiral pool.
Scheme 1: Synthesis of the new organocatalyst OC1.
Scheme 2: Synthesis of new organocatalyst OC2.
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Beilstein J. Org. Chem. 2017, 13, 612–619.
In order to obtain a series of new organocatalysts with substitu- In addition another new organocatalyst, OC11, with a
ents of different sizes and stereoelectronic properties in the different bulky substituent at C2 in the pyrrolidine moiety
dioxolane moiety, the following reaction sequence was applied was prepared. Reacting diol 7 with 1,3-dichlorotetraiso-
to compounds 2 and 4: a) N-deprotection by hydrogenolysis of propyldisiloxane in the presence of imidazole and
the benzylic group with molecular hydrogen using Pd(OH)2/C subsequent hydrogenolysis of the benzylcarbamate with
as a catalyst in the presence of trifluoroacetic acid, b) reprotec- molecular hydrogen in the presence of catalytic Pd/C
tion of the amino group as benzylcarbamate by treatment of the (Scheme 5) afforded OC11 in 43% overall yield for the two
crude reaction mixture with benzyl chloroformate in the pres- steps.
ence of diisopropylethylamine, c) hydrolysis of the dioxolane
moiety with trifluoroacetic acid, d) reconstruction of the dioxo- With this series of pyrrolidines at hand, the well-established
lane moiety by reaction of the diol with the corresponding Michael addition of aldehydes to nitroolefins [16-18] was
dimethoxyacetal in the presence of SnCl2 and e) N-deprotec- selected as a benchmark reaction to study their behaviour as
tion of the pyrrolidine by exposure of the benzylcarbamate to organocatalysts. Compounds with a related structure prepared
molecular hydrogen in the presence of catalytic Pd/C. In this from proline by Diez et al. have proven to work well as organo-
way organocatalysts OC3–OC10 were obtained (Scheme 3 and catalysts in the Michael addition of cyclohexanones to nitro-
Scheme 4).
styrenes [19,20].
Scheme 3: Synthesis of new organocatalyst OC3.
Scheme 4: Synthesis of new organocatalysts OC4–OC10.
Scheme 5: Synthesis of new organocatalyst OC11.
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Beilstein J. Org. Chem. 2017, 13, 612–619.
We first tested organocatalysts OC1–OC4 in the reaction of selectivites were ee ≈ 68% for the syn-adducts and ee = 44–63%
trans-β-nitrostyrene with 3-phenylpropionaldehyde in order to for the anti-adducts. The stereochemistry of the major com-
determine the influence that the relative configuration of the pound depended on the stereochemistry of the organocatalyst
pyrrolidine had on the results (Table 1). The reaction was and Michael adducts of opposite configuration were obtained on
initially carried out at room temperature in the presence of using syn or anti-pyrrolidines with similar levels of enantiose-
10 mol % of the catalyst and using CH2Cl2 as solvent. Under lectivity for the major syn-diastereoisomer.
these conditions the yield of the Michael adducts was 95–99%
within 7 hours. The diastereoselectivity was moderate (dr = Next, the effect of the solvent and temperature was studied
70:30–78:22) in favour of the syn-diastereoisomer and enantio- using OC4 as the organocatalyst (Table 2). The best results
Table 1: Initial screening of catalysts for the Michael addition of 3-phenylpropionaldehyde to trans-β-nitrostyrene.a
Catalyst
t (h)
Yieldb (%)
syn:antib
ee sync (%)
ee antic (%)
OC1
OC2
OC3
OC4
7
7
7
7
95
97
99
96
70:30
78:22
74:26
77:23
−68
68
−63
46
−68
66
−44
44
aReaction performed in CH2Cl2 (2 mL) at room temperature using 0.2 mmol of β-nitrostyrene, 0.4 mmol of 3-phenylpropionaldehyde and 10 mol % of
catalyst. bDetermined from the crude reaction mixture by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as internal standard. cDetermined by
chiral HPLC.
Table 2: Optimization of the reaction conditions for the Michael addition of 3-phenylpropionaldehyde to trans-β-nitrostyrene using catalyst OC4.a
Solvent
T (°C)
t (h)
Yieldb (%)
syn:antib
ee sync (%)
ee antic (%)
CH2Cl2
THF
rt
rt
7
7
96
86
82
89
85
87
96
87
93
90
85
82
87
84
77
77:23
80:20
84:16
78:22
76:24
86:14
87:13
77:23
89:11
92:8
66
71
74
57
62
81
63
57
78
80
76
76
85
80
85
44
51
44
43
21
67
35
23
75
62
74
73
58
39
40
toluene
rt
7
CHCl3
rt
7
EtOH
rt
7
cyclohexane
MTBE
rt
7
rt
7
MeCN
rt
7
CF3C6H4
C6F6
rt
7
rt
7
C6F11CF3
C10F8
rt
7
68:32
73:27
92:8
rt
7
methylcyclohexane
toluene
0
24
24
24
0
86:14
94:6
methylcyclohexane
−20
aReaction performed using 0.2 mmol of β-nitrostyrene, 0.4 mmol of 3-phenylpropionaldehyde and 10 mol % of OC4 in the given solvent (2 mL).
bDetermined from the crude reaction mixture by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as internal standard. cDetermined by chiral
HPLC.
615

Beilstein J. Org. Chem. 2017, 13, 612–619.
were obtained with methylcyclohexane as the solvent at 0 °C hand, the presence of thiourea additives could activate nitro-
reaction temperature. Under these conditions after 24 h the reac- alkenes when used as substrates by double hydrogen bonding,
tion yield was high (87%), the observed diastereoselectivity was which lead to improved reactivities [24]. Based on these find-
92:8 in favour of the syn-adduct and the enantioselectivity ings, we decided to explore the effect of a Brønsted acid or an
reached 85% ee for the major syn-adduct. A further decrease in achiral thiourea as additive on the reaction between trans-β-
temperature did not improve these results substantially but did nitrostyrene and 3-phenylpropionaldehyde promoted by
diminish the reaction yield (Table 2).
OC4 (Table 4). When thioureas were used as additives the
reaction was performed in toluene in order to improve the solu-
The organocatalysts OC1–OC11 were then screened to reveal bility.
the influence of the substituent R attached to the dioxolane
moiety on the reaction outcome and the results are collected in The addition of benzoic or acetic acid increased the reactivity
Table 3. However, the variation of this substituent did not result and anti-enantioselectivity but it was detrimental for the dia-
in any significant improvement of the diastereo- or enantiose- stereoselectivity and syn-enantioselectivity. On the other hand,
lectivity and based on these results organocatalyst OC4 was in the presence of trifluoroacetic acid the reaction proceeded
found to be the most efficient and stereoselective organocata- slowly and the diastereoselectivity decreased to some extent.
lyst. It is worth mentioning that on reducing the catalyst loading The presence of an achiral thiourea did not improve the results.
to 5 mol % the reactivity remained good and the diastereoselec-
tivity and enantioselectivity were only slightly affected.
Finally, we considered the possibility of accelerating the forma-
tion of the enamine intermediate and simultaneously activating
It has been reported that additives present in the reaction the nitroalkene by using a combination of organocatalyst OC4,
medium can lead to improved results without changing other a Brønsted acid and an achiral thiourea. Thus the reaction was
reaction conditions [21]. For example, in secondary amine- repeated in the presence of a combination of benzoic acid and
catalysed asymmetric reactions a Brønsted acid additive was N,N'-diphenylthiourea (TU1) as additives. The enantioselectivi-
found to accelerate the formation of the enamine intermediate ty of both syn and anti-adducts reached quite good values
and thus to improve not only the reactivity but also the dia- (87% ee and 91% ee, respectively) but the diastereoselectivity
stereoselectivity and enantioselectivity [22,23]. On the other dropped to 67:33.
Table 3: Screening of organocatalysts OC1–OC11 for the Michael addition of 3-phenylpropionaldehyde to trans-β-nitrostyrene.a
Catalyst
Yieldb (%)
syn:antib
ee sync (%)
ee antic (%)
OC1
OC2
OC3
OC4
OC5
OC6
OC7
OC8
OC9
OC10
OC11
OC4d
OC4e
84
77
91
87
99
86
90
91
93
83
72
81
23
84:18
94:6
77
81
77
85
84
80
83
73
76
85
63
82
82
72
50
78:22
93:7
65
58
92:8
63
88:12
92:8
61
57
93:7
70
93:7
n.d.
n.d.
n.d.
59
93:7
87:13
89:11
78:22
45
aReaction performed using 0.2 mmol of β-nitrostyrene, 0.4 mmol of 3-phenylpropionaldehyde and 10 mol % of catalyst in methylcyclohexane (2 mL)
at 0 °C for 24 h. bDetermined from the crude reaction mixture by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as internal standard.
cDetermined by chiral HPLC. dCatalyst loading 5 mol %. eCatalyst loading 2 mol %.
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Beilstein J. Org. Chem. 2017, 13, 612–619.
Table 4: Screening of additives for the Michael addition of 3-phenylpropionaldehyde to trans-β-nitrostyrene using catalysts OC4.a
Acidb
TUc
Solvent
Yieldd (%)
syn:antid
ee syne (%)
ee antie (%)
none
PhCO2H
AcOH
TFA
none
none
none
none
none
TU1
TU2
TU1
TU1
TU2
TU2
methylcyclohexane
methylcyclohexane
methylcyclohexane
methylcyclohexane
toluene
87
93
98
32
84
92
87
94
90
94
85
93:7
75:25
60:40
76:24
86:4
85
77
75
83
80
72
61
87
80
83
77
58
83
83
82
39
63
24
91
58
80
36
none
none
toluene
76:24
87:13
67:33
80:20
74:26
90:10
none
toluene
PhCO2H
AcOH
PhCO2H
AcOH
toluene
toluene
toluene
toluene
aReaction performed using 0.2 mmol of β-nitrostyrene, 0.4 mmol of 3-phenylpropionaldehyde, 10 mol % of OC4 and 10 mol % of additive in the
given solvent (2 mL) at 0 °C for 24 h. bAcOH = acetic acid, TFA = trifluoroacetic acid. cTU1 = N,N'-diphenylthiourea, TU2 = N,N'-bis[3,5-di(trifluoro-
methyl)phenyl]thiourea. dDetermined from the crude reaction mixture by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as internal standard.
eDetermined by chiral HPLC.
The aryl group of benzoic acid was varied (Table 5) in an effort with β-nitrostyrene to provide the Michael adducts in
to improve the diastereoselectivity. The best results in terms good yields when the reaction was conducted at room tempera-
of diastereoselectivity were obtained with the combination ture. The diastereoselectivity was moderate to good (dr =
p-methoxybenzoic acid/N,N'-diphenylthiourea.
79:21–95:5) in favour of the syn-diastereoisomer and enantio-
selectivites ranged from 75–84% ee. The reaction of butyr-
aldehyde with other trans-β-nitroolefins at room temperature
also provided the Michael adducts with moderate to
good distereoselectivity (dr = 74:26–92:8) in favour of
the syn-diastereoisomer and enantioselectivites from
72–84% ee.
Finally, with the most efficient organocatalyst OC4 at hand we
surveyed the scope of this transformation with respect to the
aldehyde and the nitroolefin (Table 6). Other aliphatic
aldehydes were less reactive and the reaction temperature
had to be increased. Linear aliphatic aldehydes reacted
Table 5: Screening of benzoic acids as additives for Michael addition of 3-phenylpropionaldehyde to trans-β-nitrostyrene using catalysts OC4.a
Acid
Yieldb (%)
syn:antib
ee sync (%)
PhCO2H
94
89
91
90
96
92
67:33
63:37
61:39
63:37
72:28
80:20
87
86
90
87
85
85
4-MeC6H4CO2H
4-NO2C6H4CO2H
4-FC6H4CO2H
4-ClC6H4CO2H
4-MeOC6H4CO2H
aReaction performed using 0.2 mmol of β-nitrostyrene, 0.4 mmol of 3-phenylpropionaldehyde, 10 mol % of OC4, 10 mol % of N,N'-diphenylthiourea,
and 10 mol % of the given benzoic acid in toluene (2 mL) at 0 °C for 24 h. bDetermined from the crude reaction mixture by 1H NMR spectroscopy
using 1,3,5-trimethoxybenzene as internal standard. cDetermined by chiral HPLC.
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Beilstein J. Org. Chem. 2017, 13, 612–619.
Table 6: Scope of Michael addition of aldehydes to trans-β-nitroolefins using catalyst OC4.a
Product
R1
R2
T (ºC)
t (h)
Yieldb (%)
syn:antib
ee sync (%)
9a
9b
9b
9c
9d
9e
9f
Bn
n-Pr
Ph
Ph
0
0
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
rt
24
48
48
22
20
24
70
24
44
46
24
22
31
33
87
18
93:7
66:34
85:15
77:23
79:21
86:14
95:5
85
72
75
82
83
83
84
72
80
84
85
82
84
82
n-Pr
Ph
100
100
100
94
Et
Ph
Me
Ph
n-Hex
CH2=CH(CH2)7
n-Pr
Ph
Ph
100
88
9g
9h
9i
2-furyl
4-MeOC6H4
4-MeC6H4
4-ClC6H4
4-BrC6H4
3-BrC6H4
2-BrC6H4
74:26
87:14
89:11
88:12
89:11
93:7
n-Pr
100
100
100
92
n-Pr
9j
n-Pr
9k
9l
n-Pr
n-Pr
100
100
9m
n-Pr
92:8
aReaction performed using 0.2 mmol of nitroolefin, 0.4 mmol of aldehyde, 10 mol % of OC4, in methylcyclohexane (2 mL). bDetermined from the
crude reaction mixture by 1H NMR spectroscopy using 1,3,5-trimethoxybenzene as internal standard. cDetermined by chiral HPLC.
Conclusion
Acknowledgements
In conclusion, we have prepared 2-substituted pyrrolidines Financial support from the Government of Aragón (GA E-102)
using a hydrozirconation/iodination reaction of chiral homo- is acknowledged. A. C. thanks the Government of Aragón for a
allylic amines. The latter were obtained on a multigram scale pre-doctoral fellowship.
from imines derived from glyceraldehyde. These easily avail-
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