Organocatalytic "difficult" michael reaction of ketones with nitrodienes utilizing a primary amine-thiourea based on Di-tert-butyl aspartate

Article abstract of DOI:10.1002/ejoc.201101402

The primary amine-thiourea based on di-tert-butyl (S)-aspartate and (1R,2R)-1,2-diphenylethylene-1,2-diamine has been successfully employed in the "difficult" Michael reaction between aromatic ketones or acetone with nitrodienes. The high stability and re

Full text of DOI:10.1002/ejoc.201101402

FULL PAPER
DOI: 10.1002/ejoc.201101402
Organocatalytic “Difficult” Michael Reaction of Ketones with Nitrodienes
Utilizing a Primary Amine–Thiourea Based on Di-tert-butyl Aspartate
Michail Tsakos[a] and Christoforos G. Kokotos*[a]
Keywords: Amino acids / Michael addition / Nitrodienes / Organocatalysis / Thiourea
The primary amine–thiourea based on di-tert-butyl (S)-
aspartate and (1R,2R)-1,2-diphenylethylene-1,2-diamine has
been successfully employed in the “difficult” Michael reac-
tion between aromatic ketones or acetone with nitrodienes.
The high stability and reactivity of the catalyst led to im-
proved reaction conditions. Thus, the desired products were
obtained in high to excellent yields (up to 100%) and excel-
lent enantioselectivities (up to 99%) even at elevated tem-
peratures.
aldehydes and nitrodienes,[13] and Xu et al. have shown the
synergistic effect of an organocatalyst with a chiral thiourea
for the reaction of cyclic ketones with nitrodienes.[14] How-
ever, the Michael addition of aromatic methyl ketones to
nitrodienes is still a great challenge (Scheme 1).[10,15,16] Wu
et al. have utilized a 30 mol-% catalyst loading of thiourea
1 to obtain the desired product after a reaction time of 4 d
in reasonable to good yields.[15] Although Ma et al. required
a longer reaction time, the catalyst loading was reduced to
15 mol-%, which led to excellent enantioselectivities and
good to high yields.[16] Xu et al. have reported that their
catalytic system in cooperation with an acid cocatalyst led
to excellent results after 3 d. However, a sole example was
demonstrated without providing any further details.[10] In
this work, we present the application of organocatalyst 4,
which is based on di-tert-butyl (S)-aspartate and (1R,2R)-
1,2-diphenylethylene-1,2-diamine, to this transformation to
provide the products in high yields and selectivities in a
shorter reaction time and with lower catalyst loading.
Introduction
Since the dawn of the 21st century, organocatalysis has
grown at such a dramatic pace that is considered to be the
third branch of asymmetric catalysis.[1,2] Among the numer-
ous enantioselective C–C and C–X bond-forming reactions
in the literature, the asymmetric Michael reaction consti-
tutes one of the most important.[3] In particular, the reac-
tion between carbonyl compounds and nitrostyrenes is
among the most intensively studied as the products of such
additions can be transformed into a number of different
useful organic intermediates.[4] Two of the most challenging
reactions in this field are the Michael addition of acetone
to nitro olefins (benchmark catalysts such as proline lead
to low enantioselectivities) and the reaction between aceto-
phenone and nitrostyrenes (low yields are usually ob-
tained). To this end, molecules that combine a primary
amine with a thiourea moiety have emerged as very power-
ful organocatalysts.[5] Among the first to demonstrate the
unique catalytic properties of this class of organocatalysts
were the groups of Jacobsen,[6] Tsogoeva,[7] Ma[8] and us.[9]
Recently, Xu et al. have developed a new thiourea for the
same transformation.[10] Our interest in the field of organ-
ocatalysis is mainly targeted in studying the building aspects
that are required in the structure of a catalyst in order to
possess increased reactivity and deliver increased selectiv-
ity.[11] Thus, we have recently evaluated a variety of catalysts
for the same transformation and identified a superior cata-
lyst.[12] An even more “difficult” transformation is the
Michael addition of carbonyl compounds to nitrodienes.
Recently, Alexakis et al. have reported the reaction between
[a] Laboratory of Organic Chemistry, Department of Chemistry,
University of Athens, Panepistimiopolis,
Athens 15771, Greece
Fax: +30-210-7274761
E-mail: ckokotos@chem.uoa.gr
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejoc.201101402.
Scheme 1. “Difficult” Michael addition of acetophenone to a
phenyl-nitrodiene.
576
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 576–580

Organocatalytic “Difficult” Michael Reaction of Ketones with Nitrodienes
Results and Discussion
Based on our previous experience, we postulated that the
use of a bifunctional catalyst such as 4 could lead to high
catalytic activity, and our initial experiments were success-
ful (Table 1, Entry 1). The replacement of (1R,2R)-1,2-di-
phenylethylene-1,2-diamine with (1S,2S)-1,2-diphenylethyl-
ene-1,2-diamine led to the formation of the other enantio-
mer in a lower yield and enantioselectivity (Table 1, En-
try 2; Figure 1). Directly attaching a phenyl group to the α-
carbon atom of the acid (phenylglycine) instead of the as-
partic acid side-chain led to slightly worse results (Table 1,
Entry 3 vs. 1). When the 1,2-diphenylethylene moiety was
replaced by a cyclohexyl group, the ee remained high, but
the yield dropped significantly (Table 1, Entry 4). Both the
primary amine functionality and thiourea moiety are re-
quired for excellent catalytic reactivity as no reaction took
place when (1R,2R)-diphenylethylene-1,2-diamine (8) was
used as the catalyst (Table 1, Entry 5). Once the optimum
catalyst was found, a survey of the reaction solvent was
undertaken (Table 2). Chlorinated solvents generally led to
good yields and excellent selectivities (Table 2, Entries 1–3).
The best results were obtained in CHCl3, where the best
yield and enantioselectivity among the various solvents
were observed (Table 2, Entry 3). Nonpolar solvents, where
hydrogen bonding is favored, led to good yields and selec-
tivities, whereas polar solvents did not afford better results
(Table 2, Entries 4–12). When water was used as a solvent,
the desired product was isolated in 41% yield, which show-
cases the possibility for the development of an environmen-
tally friendly procedure. Under neat reaction conditions, the
product was obtained in a good yield similar to that of non-
polar solvents (Table 2, Entry 13).
Figure 1. Organocatalysts used in this study.
Table 2. Michael reaction between acetophenone and phenyl-nitro-
diene with 4 in various solvents.[a]
Entry
Solvent
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
CH2Cl2
CH2ClCH2Cl
CHCl3
toluene
benzene
xylene
53
40
60
57
52
50
33
26
–
98
98
Ͼ 99
99
98
95
97
95
–
96
Et2O
MeCN
THF
10
11
12
13
EtOAc
DMSO
H2O
27
–
41
53
–
99
Ͼ 99
neat
[a] Reaction conditions: catalyst (0.03 mmol), phenyl-nitrodiene
(0.20 mmol), and acetophenone (2.00 mmol) in solvent (1.0 mL).
[b] Isolated yield. [c] The ee was determined by chiral HPLC.
Table 1. Michael reaction between acetophenone and phenyl-nitro-
diene with various catalysts.[a]
acids, such as trifluoroacetic acid (TFA) and camphorsulf-
onic acid (CSA), completely stops the catalytic activity of
4, which is probably due to undesired salt formation of the
primary amine of the catalyst with the acid (Table 3, En-
tries 7 and 8).
In an effort to further optimize the reaction conditions,
a study on the nitrodiene concentration was undertaken
(Table 4, Entries 1–4). The yield and enantioselectivity were
dependent on the concentration of the nitrodiene. Ex-
tending the reaction time had a high impact on the reaction
yield and led to a quantitative yield after 5 d (Table 4, En-
tries 5 and 6). Similar results were obtained when the cata-
Entry
Catalyst
Yield [%][b]
ee [%][c]
1
2
3
4
5
4
5
6
7
8
53
35
39
15
–
98
–94
98
95
–
[a] Reaction conditions: catalyst (0.03 mmol), phenyl-nitrodiene
(0.20 mmol), and acetophenone (2.00 mmol) in CH2Cl2 (1.0 mL).
[b] Isolated yield. [c] The ee was determined by chiral HPLC.
The use of an acidic additive to improve the catalytic lyst loading was decreased to 10 mol-% (Table 4, Entry 7),
activity has been well documented.[11b,11c,17] Initially, benz- and the ratio of ketone/nitrodiene can be further reduced
oic acid with water and benzoic acid alone were tested successfully (Table 4, Entries 8 and 9). Finally, when the re-
(Table 3, Entries 2 and 3). Contrary to the findings of Ma action took place in a pressure vessel at elevated tempera-
et al.,[16] the presence of acid additives led to inferior results ture (80 °C), the desired product was isolated in a quantita-
with 4. Various acids of medium acidity led to decreased tive yield with no loss of the excellent enantioselectivity
yields, with the exception of acetic acid, where similar yields (Table 4, Entry 10). This concept of high reactivity and
and selectivities were obtained to when no additive was selectivity at elevated temperatures is not unique as Soos et
used (Table 3, Entries 4–6). Furthermore, the use of strong al. have observed similar trends.[18]
Eur. J. Org. Chem. 2012, 576–580
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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577

M. Tsakos, C. G. Kokotos
FULL PAPER
Table 3. Michael reaction between acetophenone and phenyl-nitro-
Table 5. Michael reaction between ketones and phenyl-nitrodiene
diene with 4 and various acid additives.[a]
with 4.[a]
Entry
Additive[b]
Yield [%][c]
ee [%][d]
1
2
3
4
5
6
7
8
–
PhCOOH
PhCOOH, H2O
4-NBA
60
50
40
51
49
58
–
Ͼ 99
Ͼ 99
Ͼ 99
99
99
99
4-CBA
AcOH
TFA
CSA
–
–
–
[a] Reaction conditions: catalyst (0.03 mmol), additive (0.03 mmol),
phenyl-nitrodiene (0.20 mmol), and acetophenone (2.00 mmol) in
CHCl3 (1.0 mL). [b] 4-NBA = 4-nitrobenzoic acid, 4-CBA = 4-
cyanobenzoic acid. [c] Isolated yield. [d] The ee was determined by
chiral HPLC.
Table 4. Michael reaction between acetophenone and phenyl-nitro-
diene with 4.[a]
Entry Nitrodiene con- Time Temperature
centration
Yield
[%][b]
ee
[m]
[d]
[°C]
[%][c]
1
2
3
4
5
0.4
0.2
0.1
0.05
0.2
0.2
0.2
0.2
0.2
0.2
2
2
2
2
3
5
2
2
2
3
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
r.t.
80
61
60
45
41
67
99
55
67
58
100
98
Ͼ 99
96
96
Ͼ 99
Ͼ 99
Ͼ 99
Ͼ 99
99
6
7[d]
8[e]
9[f]
10[e,g]
99
[a] Reaction conditions: catalyst (0.03 mmol), phenyl-nitrodiene
(0.20 mmol), and ketone (1.00 mmol) in CHCl3 (1.0 mL) were
stirred in a pressure vessel at 80 °C for 72 h. [b] Isolated yield. [c]
The ee was determined by chiral HPLC.
[a] Reaction conditions: catalyst (0.03 mmol), phenyl-nitrodiene
(0.20 mmol), and acetophenone (2.00 mmol) in CHCl3. [b] Isolated
yield. [c] The ee was determined by chiral HPLC. [d] 10% catalyst
was used. [e] Ratio of ketone/nitrodiene = 5:1. [f] Ratio of ketone/
nitrodiene = 2:1. [g] The reaction was performed in a pressure ves-
sel.
To further demonstrate the high enantiocontrol and reac-
Once the optimum reactions conditions were found, the tivity that is provided by our catalyst, a variety of substi-
scope and limitations of this methodology were explored. tuted nitrodienes were used in the reaction with either ace-
A variety of substituted aromatic methyl ketones were used tophenone or acetone (Table 6). It is well established that
successfully under this protocol (Table 5). Electron-with- such reactions suffer from decreased yields in the case of
drawing groups (p-nitro, p-cyano), halogens (p-fluoro), and acetophenone and low enantioselectivities in the case of
methyl substituents furnished the desired products in high acetone. Although acetophenone required 3 d in order for
to excellent yields and excellent enantioselectivities (Table 5, the reaction to reach completion when phenyl-nitrodiene
Entries 2–5). p-Acetoxyacetophenone and electron-rich was used (Table 6, Entry 1), acetone needed only 48 h to
substrates were also well tolerated (Table 5, Entries 6–9), afford the product in quantitative yield and excellent
and heterocyclic aromatic moieties, such as furyl, led to enantioselectivity (Table 6, Entry 2). When electron-donat-
quantitative yields and high selectivities (Table 5, Entry 10). ing substituents such as p-methoxy were used, high yields
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© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Eur. J. Org. Chem. 2012, 576–580

Organocatalytic “Difficult” Michael Reaction of Ketones with Nitrodienes
were obtained for both acetophenone and acetone, which
were accompanied by high enantioselectivities (Table 6, En-
tries 3–5). In the case of acetone, along with the desired
product, the product of a second Michael addition on the
free methyl group of the product (double Michael reaction)
was isolated in 11% yield (Table 6, Entry 4). In order to
suppress the second Michael reaction, 10 mol-% of an
acidic additive (AcOH) was used, and the desired product
was obtained in excellent yield (Table 6, Entry 5). The use
of electron-withdrawing substituents on nitrodiene afforded
the desired product in good to high yield and always with
a high enantioselectivity, although prolonged reaction times
were required in some cases (Table 6, Entries 6–12). When
acetone was used with (2-nitrophenyl)- or (4-nitrophenyl)-
nitrodiene, apart from the desired adduct, the product of a
second Michael addition was observed (around 30%,
Table 6, Entry 7). To avoid such double addition, again
10 mol-% of AcOH was used, which afforded higher yields
of the desired product.
Figure 2. Proposed transition state of the Michael addition of aryl
methyl ketones to phenyl-nitrodiene.
Taking into account the high catalyst loading used by
Wu et al.,[15] the prolonged reaction time used by Ma,[16]
and the single example from Xu,[10] our protocol, which
uses 10–15 mol-% catalyst loading and a 3 d reaction time,
constitutes the method of choice for such a reaction.
Table 6. Michael addition reaction between ketones and nitrodienes
with 4.[a]
Conclusions
A low-cost, primary amine–thiourea, which is based on
di-tert-butyl aspartate, was used for the “difficult” Michael
reaction between aryl methyl ketones and nitrodienes to af-
ford the products in high to excellent yields and enantio-
selectivities. This protocol provides the best results among
the few others that exist in the literature when the catalyst
loading, reaction time, molar ratio between ketone and
nitrodiene, substrate scope, yield, and selectivity are taken
into account.
Entry
R1
R2
Time
[h]
Yield
[%][b]
ee[c]
[%]
1
2
3
4
5
6
7
8
9
Ph
Me
Ph
Me
Me
Ph
Me
Me
Ph
Me
Ph
Ph
Ph
72
48
96
24
24
120
24
24
72
48
100
99
99
98
99
99
99
94
92
92
94
96
92
98
4-MeOC6H4
4-MeOC6H4
4-MeOC6H4
2-O2NC6H4
2-O2NC6H4
2-O2NC6H4
4-O2NC6H4
4-O2NC6H4
4-ClC6H4
4-ClC6H4
72
77[d]
92[e]
71
58[f]
95[e]
77
Experimental Section
General Procedure for the Michael Reaction of Ketones with Nitro-
dienes: A solution of nitrodiene (0.2 mmol), ketone (1 mmol), and
4 (15 mg, 0.03 mmol) in chloroform (1 mL) was placed in a pres-
sure vessel equipped with a stirring bar, and the mixture was heated
at 80 °C for the stated time. The solvent was evaporated, and the
crude product was purified by flash column chromatography elut-
ing with the appropriate mixture of petroleum ether (40–60 °C)/
EtOAc to afford the desired product.
10
11
12
94[e]
79
120
48
Me
86
[a] Reaction conditions: catalyst (0.03 mmol), nitrodiene
(0.20 mmol), and ketone (1.00 mmol) in CHCl3 (1.0 mL) were
stirred in a pressure vessel at 80 °C for the stated time. [b] Isolated
yield. [c] The ee was determined by chiral HPLC. [d] The product
of double Michael addition was isolated in 11% yield. [e] 10 mol-
% AcOH was added. [f] The product of double Michael addition
was isolated in 30% yield.
(S,E)-3-(Nitromethyl)-1,5-diphenylpent-4-en-1-one:[16] Table 4, En-
try 1. White solid (59 mg, 100%). M.p. 104–105. [α]2D0 = +12.1 (c =
1.0, CHCl3). 1H NMR (200 MHz, CDCl3): δ = 3.29 (d, J = 6.5 Hz,
2 H, COCH2), 3.66–3.83 (m, 1 H, CH), 4.60 (dd, J = 12.2 and
7.4 Hz, 1 H, CHHNO2), 4.72 (dd, J = 12.2 and 6.0 Hz, 1 H,
CHHNO2), 6.16 (dd, J = 16.0 and 8.6 Hz, 1 H, =CH), 6.58 (d, J
= 16.0 Hz, 1 H, =CH), 7.20–7.35 (m, 5 H, ArH), 7.43–7.63 (m, 3
H, ArH), 7.96 (d, J = 8.0 Hz, 2 H, ArH) ppm. 13C NMR (50 MHz,
CDCl3): δ = 37.2, 40.2, 78.7, 126.4, 126.5, 127.9, 128.0, 128.5,
128.7, 133.3, 133.6, 136.1, 136.3, 197.0 ppm. MS (ESI): m/z (%) =
313 (100) [M + NH4]+, 296 (15) [M + H]+. 99% ee determined
by HPLC analysis Daicel Chiralpak AD-H, iPrOH/hexane (5:95),
1.0 mLmin–1, tR = 16.25 min (minor), 24.22 min (major).
In all other cases, the product of the double Michael ad-
dition was either not observed or isolated in yields lower
than 5%.
In order to account for the excellent stereoselectivity of
the reaction, the following transition state is proposed (Fig-
ure 2). The primary amine of the catalyst activates the
methyl ketone through the formation of an enamine. Simul-
taneous with the ketone activation, the hydrogen bonding
between the thiourea moiety of the catalyst and the nitro
group of the nitrodiene is responsible for the electrophilic
activation, which guides the Michael addition from one face
and leads to high enantiocontrol.
Supporting Information (see footnote on the first page of this arti-
cle): Full experimental details on the synthesis of nitrodienes, cata-
lyst, and substrates and NMR and HPLC data.
Eur. J. Org. Chem. 2012, 576–580
© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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579

M. Tsakos, C. G. Kokotos
FULL PAPER
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