Highly enantioselective organocatalysis of the Michael addition of benzyloxyacetaldehyde to nitroolefins

Article abstract of DOI:10.1016/j.tetasy.2017.09.018

A novel category of di(N,N-dimethylbenzylamine)prolinol silyl ether catalyst, which when used in conjunction with an acidic co-catalyst, generates an ammonium salt supported organocatalyst. This catalytic system is shown to be very effective for the Micha

Full text of DOI:10.1016/j.tetasy.2017.09.018

Tetrahedron: Asymmetry xxx (2017) xxx–xxx
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Tetrahedron: Asymmetry
journal homepage: www.elsevier.com/locate/tetasy
Highly enantioselective organocatalysis of the Michael addition of
benzyloxyacetaldehyde to nitroolefins
⇑
Sripragna Burugupalli, Yupu Qiao, Allan D. Headley
Department of Chemistry, Texas A&M University-Commerce, Commerce, TX 75429-3011, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel category of di(N,N-dimethylbenzylamine)prolinol silyl ether catalyst, which when used in con-
junction with an acidic co-catalyst, generates an ammonium salt supported organocatalyst. This catalytic
system is shown to be very effective for the Michael reaction of benzyloxyacetaldehyde and various
nitroolefins in isopropanol. Excellent enantioselectivities (up to 99%) and diastereoselectivities (syn/anti
of 75:25) and short reaction times were obtained. The presence of the bulky OTMS group combined with
the presence of two large N,N-dimethylbenzyl ammonium ion groups accounts for the effectiveness of
this catalytic system.
Received 28 July 2017
Revised 20 September 2017
Accepted 26 September 2017
Available online xxxx
Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction
applied to a wide range of organic transformations, in which asym-
metric products are obtained with high stereoselectivities.9
The catalyzed asymmetric Michael reaction of aldehydes with
nitroolefins1 is an especially important type reaction in organic
synthesis. This type reaction is one of the most powerful tools for
the formation of carbon–carbon bonds and can also afford synthet-
Recently, our research efforts have focused on the development
of ionic liquid and ammonium salt supported organocatalysts that
are effective for a wide range of reactions.10 A major advantage of
this category of organocatalysts is that they are highly soluble in
polar protic solvents, such as water and alcohols, and are also very
effective for the asymmetric Michael addition of aldehydes to
nitroolefins in which high diastereo- and enantioselectivities are
obtained.11
Herein, we report the results of further investigation of our cat-
alytic system involving the reaction of benzyloxyacetaldehyde
with various nitroalkenes. This type reaction is very important
since recently an aldehyde, 3-pentyloxyacetaldehyde, was utilized
in the synthesis of oseltamivir and a Michael addition was the key
step.12 Due to the importance of oseltamivir, which is also known
as Tamiflu, to the pharmaceutical community, we were prompted
to apply our organocatalytic system to study the key step in its
synthesis. The catalysts and co-catalysts considered are shown in
Figure 1; the Michael reaction that served as the model reaction
to gain the optimized set of reaction conditions, catalyst, co-cata-
lyst, catalyst loading, temperature, and solvent is shown in Figure 2,
where R is the phenyl group.
ically useful c-nitro carbonyl compounds with excellent diastere-
oselectivities both in Nature and in modern organic synthesis.2
Oxyaldehydes are extremely important compounds in organic syn-
thesis. The auto-aldol reaction of oxyaldehydes has been used for
the synthesis of carbohydrates,3 and benzyloxyacetaldehyde,
which is used in the Mannich reaction for the synthesis of the side
chain of Paclitaxel.4
The use of organocatalysts to catalyze asymmetric reactions has
received much attention, and different types of organocatalysts
have been developed over the past decade that result in high enan-
tioselectivities for asymmetric transformations.5 Organocatalysts
are highly effective, relatively non-toxic, environmentally friendly,
and require mild reaction conditions. As a result, they have become
an integral component of asymmetric catalysis.6 In 2008, List et al.
and Hayashi et al. independently reported for the first time the
asymmetric Michael addition in which
a diarylprolinol silyl
ether-based catalyst was used.7 Subsequently, renewed interest
has been generated in the use of secondary amines as
organocatalysts.8
For the optimization, separate reactions were carried out using
catalyst 1 in methanol and isopropanol in which benzoic acid was
used as a co-catalyst (entries 1 and 2 of Table 1), but the yields,
diastereoselectivities, and enantioselectivities were only moderate,
even though the reactions were completed within 2 h. The next
solvents considered were water and brine (entries 3 and 4). Brine
was considered since our research has shown it to be a very
The development of ionic liquids as asymmetric organocatalysts
is a growing area of research and many have been developed and
⇑
Corresponding author.
E-mail address: allan.headley@tamuc.edu (A.D. Headley).
https://doi.org/10.1016/j.tetasy.2017.09.018
0957-4166/Ó 2017 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Burugupalli, S.; et al. Tetrahedron: Asymmetry (2017), https://doi.org/10.1016/j.tetasy.2017.09.018

2
S. Burugupalli et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
-
PF6
Me2N
N
N
N
N
NMe2
n-Bu
OTMS
OTMS
N
N
H
H
COOH
ILS-PhCO2H
Catalyst 1
Catalyst 2
Figure 1. Catalysts and co-catalyst used for the optimization of the asymmetric Michael reaction of benzyloxyacetaldehyde and nitrostyrenes.
O
NO2
Catalyst 2 (5 mol%)
O
R
O
R
+
R
OBn
PhCOOH (50 mol%)
H
+
NO2
NO2
H
H
iso-PrOH (0.5 ml), r.t.
(0.4 mmol)
(0.8 mmol)
(syn)
OBn
(anti)
OBn
Figure 2. Reaction involving benzyloxyacetaldehyde and various substituted nitroolefins.
Table 1
Results for the optimization of catalysts, co-catalysts and reaction conditions for the asymmetric Michael reaction of benzyloxyacetaldehyde and nitrostyrene at 25 °C
Entry
Cat
(mol%)
Solventa
Time (h)
AcidX mol %
Yield (%)b
dr
%ee
(syn/anti)c
(syn/anti)d
1
1 (5)
1 (5)
1 (5)
1 (5)
1 (5)
1 (5)
1 (5)
2 (5)
2 (5)
2 (5)
2 (5)
2 (5)
2 (5)
2 (3)
MeOH
2
2
PhCO2H
50 mol %
PhCO2H
50 mol %
PhCO2H
50 mol %
PhCO2H
50 mol %
PhCO2H
50 mol %
PhCO2H
50 mol %
ILS-PhCO2H
50 mol %
PhCO2H
50 mol %
PhCO2H
50 mol %
PhCO2H
50 mol %
PhCO2H
50 mol %
PhCO2H
50 mol %
PhCO2H
30 mol %
PhCO2H
50 mol %
37
41
35
31
39
45
43
57
68
53
40
46
48
40
65/35
70/30
75/25
73/27
69/31
74/26
74/26
66/34
75/25
76/24
73/27
78/22
70/30
67/33
93/95
92/90
99/90
94/77
95/91
99/85
99/88
98/95
99/97
99/93
99/91
96/96
99/95
98/85
2
i-PrOH
3
H2O
5
4
Brine
3
5
MeOH/H2O (1:1)
i-PrOH/H2O (1:1)
i-PrOH/H2O (1:1)
MeOH
5
6
3
7
4
8
6
9
i-PrOH
7
10
11
12e
13
14
H2O
5
Brine
5
i-PrOH
53
96
120
i-PrOH
i-PrOH
a
b
c
Using 0.5 ml of solvent.
Determined by 1H NMR.
Determined by 1H NMR.
Determined by chiral HPLC.
Carried out at 4 °C.
d
e
effective solvent when used in conjunction with this type cata-
lyst.13 Water was also considered due to its unique solvation prop-
erties and its desirability as a solvent in general. As shown in
Table 1, the yields, diastereoselectivities, and enantioselectivities
were only moderate, and the reaction in water took a longer time
to complete (5 h). The next set of solvents considered were aque-
ous mixtures of methanol and isopropanol (entries 5 and 6). It
was observed that the reaction time in aqueous methanol was
longer, compared to that in the isopropanol/water mixture. In an
effort to improve the outcome of the reaction, another type of ionic
liquid supported co-catalyst, which was developed in our lab, was
considered (entry 7). This newly developed ionic liquid supported
benzoic acid14 however, showed similar results as those in which
benzoic acid was used as co-catalyst.
Next, the reaction was screened using catalyst 2. The reaction in
methanol showed only a slight improvement in stereochemical
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S. Burugupalli et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
3
Table 2
Scope for the asymmetric Michael reaction involving benzyloxyacetaldehyde and various substituted nitroolefins
Entry
R
Time
Yield (%)
syn/anti
%ee
1
2
3
4
5
6
Ph
7
24
24
20
18
18
68
70
67
60
79
79
75/25
80/20
90/10
75/25
76/26
79/21
99/97
92/89
92/93
96/97
98/92
95/96
4-F-C6H4
2-F-C6H4
4-NO2-C6H4
4-OMe-C6H4
4-Me-C6H4
outcome, compared to catalyst 1 (entry 8). In isopropanol however,
catalyst 2 showed a dramatic improvement in the enantioselectiv-
ity under similar reaction conditions (entry 9), compared to cata-
lyst 1. Encouraged by these results for catalyst 2, we next tested
this catalytic system in aqueous media (entries 10 and 11), but
there was not a significant improvement in these media, compared
to that in isopropanol. Next, the effect of temperature was studied
on the reaction (entries 12). It is obvious that at a lower tempera-
ture, there was no effect on the stereochemical outcome of the
reaction and the yield was in fact reduced. Next, the concentration
of the co-catalyst was changed and the results examined. It is obvi-
ous that a reduction in the concentration of the co-catalyst from
50 mol % did not improve the reaction outcome (entry 13). It
appears that a high concentration of the acidic co-catalyst is
needed in order to favor the formation of ammonium ions. Next,
the amount of catalyst was decreased to 3 mol %, but it was
observed that the reaction took a much longer time, 120 h, and
there was no real improvement in the yield or diastereomeric ratio
(entry 14).
The reaction mixture was stirred until complete conversion of the
starting materials (monitored by TLC). The solvent was removed
and the product was purified by flash column chromatography (sil-
ica gel, hexane/AcOEt) to afford the Michael adduct. Percentage
yields and syn/anti ratios were determined by 1H NMR spec-
troscopy. Racemates were synthesized using morpholine as a cata-
lyst in order to identify enantiomers. Enantiomeric excess
determinations were made based on comparisons with previously
reported literature for determinations.16 Similar chiral HPLC condi-
tions were used for the separation of the enantiomers for each
reaction and based on the retention times, NMR and IR data, the
identity of each enantiomer was determined.
3.1.1. 2-(Benzyloxy)-4-nitro-3-phenylbutanal
1H NMR (400 MHz, CDCl3): d = 9.51, 9.41 (d, J = 1.5 Hz, 1H),
7.21–7.39 (m, 10H), 4.85 (m, 1H), 4.77 (m, 1H), 4.65 (m, 1H),
4.52 (m, 1H), 4.09 (m, 1H), 3.99 (m, 1H) ppm. 13C NMR
(100 MHz, CDCl3): d = 202.7, 200.3, 136.6, 136.4, 135.3, 134.2,
128.5, 84.1, 82.6, 76.3, 76.2, 74.0, 73.5, 45.3, 44.8 ppm. HPLC (AD-
i
These results show that the optimum set of reaction conditions
are those shown in entry 9 and were used to study the reaction
scope in which various substituted nitroolefins were considered,
and the results are shown in Table 2.
H, PrOH/n-hexane = 5:95, 1 mL/min, k = 217 nm): tR = 21.1 (anti-
major), 18.4 (syn-minor), 18.0 (anti-minor), 14.8 (syn-major) min.
3.1.2. 2-(Benzyloxy)-3-(4-fluorophenyl)-4-nitrobutanal
The results in Table 2 indicate that the reactions proceeded effi-
ciently affording the products in relatively high yields (67–79%),
with excellent ee (up to 98:92) for both electron withdrawing
and electron-donating substituted styrenes. The high enantioselec-
tivities of catalyst 2 for the Michael additions can be explained by
the formation of a polar transition state,15 which would be favored
in the polar medium of this reaction. Also, the presence of the
bulky OTMS group, combined with two N,N-dimethylbenzyl
ammonium ion groups, which exist in the acidic medium serves
to make this catalyst effective.
1H NMR (400 MHz, CDCl3): d = 9.54, 9.48 (d, J = 1.5 Hz, 1H),
7.11–7.49 (m, 9H), 4.80 (m, 1H), 4.77 (m, 1H), 4.60 (m, 1H), 4.42
(m, 1H), 4.19 (m 1H), 3.90 (m, 1H) ppm. 13C NMR (100 MHz,
CDCl3): d = 203.5, 200.3, 138.6, 136.4, 135.0, 134.4, 128.5, 80.1,
i
78.6, 76.3, 75.2, 74.2, 73.5, 45.0, 44.2 ppm. HPLC (AD-H, PrOH/n-
hexane = 5:95, 1 mL/min, k = 217 nm): tR = 26.5 (anti-major), 21.1
(syn-major), 19.3 (anti-minor), 16.0 (syn-minor) min.
3.1.3. 2-(Benzyloxy)-3-(2-fluorophenyl)-4-nitrobutanal
1H NMR (400 MHz, CDCl3): d = 9.70, 9.51 (d, J = 1.5 Hz, 1H),
7.11–7.49 (m, 9H), 4.80 (m, 1H), 4.77 (m, 1H), 4.60 (m, 1H), 4.42
(m, 1H), 4.18 (m 1H), 4.10 (m, 1H) ppm. 13C NMR (100 MHz,
CDCl3): d = 202.6, 200.4, 137.6, 135.4, 134.3, 130.2, 128.7, 83.1,
2. Conclusion
i
82.8, 77.3, 76.5, 75.0, 73.4, 38.3, 37.8 ppm. HPLC (AD-H, PrOH/n-
In conclusion, a novel di(N,N-dimethylbenzylamine)prolinol
silyl ether catalyst, which when used in conjunction with an acidic
co-catalyst generates an ammonium salt organocatalyst. This cat-
alytic system is shown to be very effective for the Michael reaction
of benzyloxyacetaldehyde and various substituted nitroolefins.
Excellent enantioselectivities (up to 99%) and diastereoselectivities
(syn/anti of 75:25) were obtained. Further studies are being carried
out on a broader scope of reaction substrates and other types of
reaction and the results will be reported in due course.
hexane = 5:95, 1 mL/min, k = 217 nm): tR = 18.0 (anti-major), 17.5
(syn-major), 16.7 (anti-minor), 16.3 (syn-minor) min.
3.1.4. 2-(Benzyloxy)-3-(4-nitrophenyl)-4-nitrobutanal
1H NMR (400 MHz, CDCl3): d = 9.52, 9.48 (d, J = 1.5 Hz, 1H),
7.11–7.49 (m, 9H), 4.80 (m, 1H), 4.77 (m, 1H), 4.60 (m, 1H), 4.42
(m, 1H), 4.19 (m 1H), 4.1 (m, 1H) ppm. 13C NMR (100 MHz, CDCl3):
d = 202.6, 201.3, 137.6, 136.5, 132.3, 130.2, 128.0, 83.1, 82.6, 77.5,
i
76.2, 74.3, 73.8, 45.7, 45.0, 44.8 ppm. HPLC (AD-H, PrOH/n-hex-
ane = 5:95, 1 mL/min, k = 217 nm): tR = 40.2 (syn-minor), 36.8
(syn-major), 31.7 (anti-minor), 30.1 (anti-major) min.
3. Experimental
3.1. General procedure for the Michael addition reaction
3.1.5. 2-(Benzyloxy)-3-(4-methoxyphenyl)-4-nitrobutanal
1H NMR (400 MHz, CDCl3): d = 9.50, 9.9.41 (d, J = 1.5 Hz, 1H),
7.11–7.60 (m, 9H), 4.82 (m, 1H), 4.70 (m, 1H), 4.60 (m, 1H), 4.50
(m, 1H), 4.10 (m 1H), 3.86 (m, 1H) ppm. 13C NMR (100 MHz,
CDCl3): d = 204.2, 202.3, 135.6, 130.4, 129.3, 129.7, 128.0, 83.1,
The synthesis of catalysts 1 and 2,13 along with ILS-PhCO2H14 is
described elsewhere. For the Michael reactions herein, benzyloxy-
acetaldehyde (0.8 mmol) was added to a solution of the catalyst
(0.02 mmol, 5 mol %), nitroolefin (0.4 mmol) and benzoic acid
(0.2 mmol, 50 mol %) in isopropanol (0.5 mL) at room temperature.
i
82.2, 75.0, 74.2, 73.0, 72.5, 45.0, 44.5 ppm. HPLC (AD-H, PrOH/n-
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S. Burugupalli et al. / Tetrahedron: Asymmetry xxx (2017) xxx–xxx
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hexane = 5:95, 1 mL/min, k = 217 nm): tR = 19.6 (anti-major), 18.4
(syn-major), 17.7 (anti-minor), 16.5 (syn-minor) min.
3.1.6. 2-(Benzyloxy)-4-nitro-3-(p-tolyl)butanal
1H NMR (400 MHz, CDCl3): d = 9.51, 9.46 (d, J = 1.5 Hz, 1H),
7.11–7.49 (m, 9H), 4.80 (m, 1H), 4.77 (m, 1H), 4.60 (m, 1H), 4.42
(m, 1H), 4.19 (m, 1H), 3.9 (m, 1H) ppm. 13C NMR (100 MHz, CDCl3):
d = 201.7, 200.2, 137.6, 137.42, 131.3, 130.2, 128.5, 84.1, 82.8, 82.3,
76.2, 74.0, 73.5, 45.3, 44.6 ppm. HPLC (AD-H, iPrOH/n-hex-
ane = 5:95, 1 mL/min, k = 217 nm): tR = 19.0 (syn-major), 16.0
(anti-minor), 14.1 (anti-major), 13.1 (syn-minor) min.
Acknowledgements
We are grateful to the National Science Foundation for financial
support of this research (CHE-1213287) and the Robert A. Welch
Foundation (T-0014).
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Please cite this article in press as: Burugupalli, S.; et al. Tetrahedron: Asymmetry (2017), https://doi.org/10.1016/j.tetasy.2017.09.018