Asymmetric organocatalytic formal aza-Michael addition of ammonia to nitroalkenes

Article abstract of DOI:10.1002/chem.201002415

The formal addition of ammonia to nitroalkenes, affording optically active β-amino nitro compounds in high yields and enantioselectivities, is presented (see scheme). The approach is based on a novel thiourea-catalyzed aza-Michael reaction, by which benzo

Full text of DOI:10.1002/chem.201002415

DOI: 10.1002/chem.201002415
Asymmetric Organocatalytic Formal Aza-Michael Addition of Ammonia to
Nitroalkenes
Lennart Lykke, David Monge, Martin Nielsen, and Karl Anker Jørgensen*[a]
À
Stereogenic centers containing C N bonds are found in
many important biomolecules, such as amino acids[1] and
glycosamines.[2] Moreover, the activity of numerous pharma-
ceuticals depends on the absolute configuration of one or
more nitrogen-containing stereogenic centers.[3] Therefore,
low to moderate. Recently, Ooi et al.[6g] reported the aza-Mi-
chael addition of 2,4-dimethoxyaniline to nitroalkenes in ex-
cellent yields and enantioselectivities by Brønsted-acid cat-
alysis using chiral arylaminophosphonium barfates. A minor
drawback is that 2.5 equivalents of CAN (cerium ammoni-
um nitrate) were required for deprotection to afford the pri-
mary amine. The smallest nitrogen-based nucleophile, am-
monia, is conceptually the optimal nucleophile, concerning
atom economy,[12] but its volatile nature, high reactivity and
toxicity makes it difficult to work with in catalytic systems.
However, in the palladium-catalyzed Buchwald–Hartwig
coupling[13] the challenge of synthesizing anilines was solved
by applying imines as ammonia equivalents. The resulting
N-aryl imines are conveniently protected anilines, which are
deprotected by simple acidic work up.
À
the development of efficient stereoselective catalytic C N
bond forming reactions is of significant interest in organic
chemistry. Giving access to valuable compounds and struc-
tural complexity, the catalytic enantioselective conjugate ad-
dition of nitrogen-based nucleophiles to Michael acceptors[4]
using organocatalysts[5] has attracted considerable attention
in recent years.
In particular, the addition of ammonia equivalents[6] to ni-
troalkenes gives rise to the synthetically interesting b-
amino-nitro functionality (Scheme 1), which, due to the
unique chemistry of the nitro group,[7] can be transformed to
a variety of attractive compounds such as a-amino acids[8]
and 1,2-diamines.[9] Traditionally, the organocatalytic forma-
tion of b-amino nitro compounds is afforded by the nitro-
Mannich (aza-Henry) reaction,[10] while the aza-Michael ad-
dition to nitroalkenes has only received limited attention.
The first asymmetric organocatalytic aza-Michael addition
to nitroalkenes was demonstrated in 2006 by Wang et al.,[11]
employing aromatic heterocyclic benzotriazoles as nitrogen-
centred nucleophiles; however, the synthetic applicability of
the products are limited. Surprisingly, only two other exam-
ples of organocatalytic amination of nitroalkenes, giving
access to enantioenriched primary amines, have been report-
ed. In 2007, it was demonstrated that TMS-protected azide
can be employed as a masked ammonia equivalent in a high
yielding asymmetric base catalysed addition to aliphatic ni-
troalkenes;[6i] however, the enantioselectivity obtained was
Scheme 1. Concept for organocatalytic formal aza-Michael addition of
ammonia to nitroalkenes.
Inspired by the palladium-catalyzed amination protocol,
we envisioned that employing benzophenone imine 2a, as
an ammonia equivalent to react with nitroalkenes 1 under
organocatalytic conditions would allow us to formally pro-
duce optically active b-amino nitro compounds (Scheme 1).
Previous studies have shown that chiral thiourea-based cata-
lysts are prime specimens for conducting the activation of
nitroalkenes towards nucleophilic attack in a highly enantio-
selective manner.[14] Despite the abundance of numerous thi-
ourea catalyzed 1,4-additions, the enantioselective thiourea
catalyzed aza-Michael addition to nitroalkenes has to the
best of our knowledge not been reported. Hence, we per-
[a] L. Lykke, Dr. D. Monge, Dr. M. Nielsen, Prof. Dr. K. A. Jørgensen
Center for Catalysis, Department of Chemistry, Aarhus University
8000 Aarhus C (Denmark)
Fax : (+45)8919-6199
E-mail: kaj@chem.au.dk
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201002415.
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Chem. Eur. J. 2010, 16, 13330 – 13334

COMMUNICATION
formed our initial screening using different chiral hydrogen-
bond donating catalysts.[15]
liminary catalyst screening suggested that 3d[17] was a prom-
ising catalyst for the reaction.
We first examined the reaction between (E)-1-nitrohept-
1-ene 1a and benzophenone imine 2a catalyzed by different
chiral thioureas and squaramides, as the model system (see
Supporting Information) and Scheme 2 shows some selected
With the optimal catalyst in hand, nitrostyrene 1b was
tested as the aromatic model substrate for the reaction with
benzophenone imine 2a. The application of 1b gave a signif-
icant increase in enantioselectivity and provided 4b in up to
70% ee (Scheme 3).
Scheme 3. Screening for optimal ammonia equivalent structure.
In the next stage, the optimal structure of the ammonia
equivalent was investigated (Scheme 3). We envisioned that
bulkier substituents than the two phenyls in 2a would
enable the catalyst to enhance the face discrimination in the
addition to the nitroalkene. Unfortunately, 2,2,4,4-tetrame-
thylpentan-3-one imine (2b) was unreactive under the given
conditions. Replacing one phenyl group with a tert-butyl
group (2c) revealed a more reactive imine, but disappoint-
ingly, an enantioselectivity of only 36% ee was observed.
Another strategy was to introduce steric bulk by placing
substituents on the phenyl rings. Imine 2d bearing one
meta-methyl on both aryl groups needed prolonged reaction
time (48 h) with an enantioselectivity of 56% ee. Replacing
a phenyl group with a mesityl group afforded a reasonable
reactive imine (2 f); however, lower enantioselectivity (46%
ee) was observed. Finally, placing an electron-withdrawing
trifluoromethyl in one of the meta positions also had a nega-
tive effect on both reaction time (48 h) and enantioselectivi-
ty (30% ee). These investigations of the nucleophile re-
vealed that our attempts to optimize the structure resulted
in prolonged reaction times and lower enantioselectivities.
We therefore concluded that the benzophenone imine 2a
was the best ammonia source. Fortunately, the optimal nu-
cleophile 2a and catalyst 3d are both commercially avail-
able compounds, which make this ready-to-use amination
methodology very attractive, and enhance the synthetic ap-
plicability.
Scheme 2. Selected results from catalyst screening.
screening results. We were pleased to find that the thiourea
modified quinidine 3a efficiently catalyzed the reaction in
toluene but, unfortunately, no enantioenrichment of product
4a was found. The hydrogen-bonding donor squaramide
modified cinchonidine 3b was also investigated, affording
4a as a racemate. Catalysts 3a and 3b are both bifunctional
catalysts containing a Brønsted basic quinuclidine moiety,
which might be responsible for the lacking selectivity due to
non-stereoselective activation of 1a. The Jacobsen-type thio-
urea catalysts 3c and 3d, gave 4a in moderate enantioselec-
tivity and with full conversion (Scheme 2). The results for
3c and 3d showed that the latter catalyst is more active
compared to 3c, as a significant shorter reaction time was
required for 3d. In catalyst 3d, one nitrogen atom of the thi-
ourea is attached to an electron-deficient 3,5-bis(trifluoro-
methyl)-aryl substituent which provides stronger hydrogen-
bond donating ability.[16] Additionally, we applied thiourea
3e which contained the 3,5-bis(trifluoromethyl)-aryl func-
tionality; however, lower enantioselectivity was observed
(33% ee, ent-4a). Finally, the chiral squaramide 3 f was
tested but this catalyst showed no stereoselection. The pre-
Having found the optimal nucleophile and catalyst, we in-
itiated optimizing the reaction conditions for the aliphatic
nitroalkene 1a. First, we screened various solvents (Table 1,
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K. A. Jørgensen et al.
entries 1–9). Generally, full conversion was obtained in all
solvents studied; however, the enantioselectivity of 4a is sig-
nificantly dependent on the polarity of the solvent (0–69%
ee). Especially, saturated hydrocarbons proved to be promis-
ing solvents, with n-heptane as slightly superior (Table 1,
entry 8). By reducing the catalyst loading to 5 mol%, pro-
longed reaction time and decreased enantioselectivity of 4a
were observed (Table 1, entry 10). Secondly, we investigated
the effect of dilution (Table 1, entry 11). We were pleased to
observe that the enantioselectivity of 4a was enhanced to
81% ee, at a concentration of only 0.05m. It should be men-
tioned that lowering the temperature prolonged the reaction
time significantly, while affording no additional enantiose-
lectivity. Furthermore, at room temperature the enantiose-
lectivity decreases dramatically. Finally, it should be noted
that all experiments were performed in non-dried solvents
and under ambient conditions. We have also investigated
the effect of adding 10 mol% water to the reaction mixture
in the presence of 4 ꢁ molecular sieves (Table 1, entries 12
and 13). The addition of water had no effect on the results,
whereas, the presence of 4 ꢁ molecular sieves provided
30% ee of the opposite enantiomer of 4a. This suggests that
water might have a crucial role on the stereochemical out-
come of the reaction (see Scheme 5).
ties (78–89% ee). Aromatic nitroalkenes bearing both elec-
tron-withdrawing and -donating substituents (Table 2, en-
tries 2 and 6–9) also afforded the products, 4b and 4 f–i, in
high yields (79–88%), and high enantioselectivities (81–
92% ee).
Table 2. Scope of the reaction.
Entry
1, R
4, Product
Yield [%][b]
ee [%][c]
1
2
3
4
5
6
7
8
9
n-pentyl 1a
Ph 1b
Cy 1c
4a
4b
4c
4d
4e
4 f
4g
4h
4i
95
80
85
89
89
79
80
85
88
81
83 (S)
81 (S)
89
tBu 1d
(Z)-3-hexenyl 1e
p-Br-Ph 1 f
p-F-Ph 1g
p-OMe-Ph 1h
o-Cl-Ph 1i
78[d]
81
87 (S)
83 (S)
92
[a] All reactions performed on a 0.10 mmol scale (0.05m in heptane)
using 1 (0.20 mmol), 2 (0.10 mmol) and 3 (0.01 mmol) at À248C. [b] Iso-
lated by FC. [c] Determined by chiral stationary-phase HPLC. [d] Deter-
mined from the N-Boc-protected product, as the enantiomers of 4e could
not be separated by HPLC.
Table 1. Optimisation of reaction conditions.[a]
To demonstrate that the reaction is a formal addition of
ammonia, we have developed a one-pot asymmetric aza-Mi-
chael addition–acidic hydrolysis protocol. Thus, upon com-
pletion of the addition step, 2m HCl (aq) and Et2O were
added to the reaction mixture. Isolating the aqueous phase,
and subsequent removal of water as azeotrope with toluene,
furnished the hydrochloride salt of the primary b-amino
nitro compounds 5 (Scheme 4). Both aliphatic and aromatic
Entry
Solvent
c [m]
Conv. [%]
ee [%][b]
1
2
3
4
5
6
7
8
toluene
m-xylene
CH2Cl2
MeCN
THF
n-hexane
n-pentane
n-heptane
c-hexane
n-heptane
n-heptane
n-heptane
n-heptane
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.4
0.05
0.05
0.05
>95
>95
>95
>95
>95
>95
>95
>95
>95
90
51
62
7
0
2
68
64
69
62
63
81
81
À30
9[c]
10[d]
11
12[e]
13[f]
>95
>95
>95
[a] All reactions performed using 1 (0.20 mmol), 2 (0.10 mmol) and 3
(0.01 mmol) at À248C until full conversion was observed by TLC. Values
in bold indicate optimal conditions. [b] Determined by chiral stationary-
phase HPLC. [c] Reaction temperature: 58C. [d] Catalyst loading re-
duced to 5 mol%, reaction time 48 h. [e] 10 mol% water was added.
[f] Reaction conducted in presence of 4 ꢁ molecular sieves, under N2 at-
mosphere.
Scheme 4. One-pot aza-Michael addition and acidic hydrolysis protocol.
substituents were tolerated (5a–5e), giving good yields (60–
80%, 2 steps) and enantioselectivities (78–84% ee) consis-
tent with the results obtained for the addition products 4.
Remarkably, while no further purification of hydrochloride
salts 5 were required, benzophenone was recovered in 85%
yield from the organic phase. Benzophenone imine 2a is in-
dustrially produced in a patented process by condensation
of benzophenone and ammonia,[18] therefore protecting
To present the scope of this aza-Michael reaction, a repre-
sentative set of alkyl and aryl substituted nitroalkenes 1 was
reacted with benzophenone imine 2a under the optimal re-
action conditions. For nitroalkenes having linear, branched
and non-conjugated unsaturated substituents (Table 2, en-
tries 1 and 3–5), the products, 4a and 4c–e, were obtained in
high to excellent yields (85–95%), and high enantioselectivi-
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Asymmetric Organocatalytic Formal aza-Michael Addition
COMMUNICATION
group recovering and recycling, would minimize the envi-
ronmental impact of waste production in the present meth-
odology.
that benzophenone imine is an appropriate ammonia equiv-
alent to apply in combination with mild organocatalytic acti-
vations.
The absolute configurations of product 4b,c and 4g,h
were assigned to be S by chemical correlation with the cor-
responding described N-Boc-protected amines.[11b] The re-
maining configurations of adducts 4 and salts 5 are assumed
by analogy.
Extensive mechanistic studies of the role of this class of
chiral thiourea catalysts using kinetic and computational
work have previously been performed.[19] These investiga-
tions include, for example, the activation of nitroalkenes by
the thiourea moiety and the stereochemical outcome of the
nucleophilic approach to the activated substrate. Based on
these models and the role of water on the stereochemical
outcome of the present reaction, two working models are
suggested in Scheme 5. Model A accounts for the presence
Acknowledgements
This work was made possible by grants from Carlsberg Foundation and
OChemSchool. D.M. thanks the Ministerio de Ciencia e Innovaciꢂn of
Spain for a postdoctoral fellowship.
Keywords: alkenes
· Michael addition · nitroalkenes ·
organocatalysis
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Received: August 20, 2010
Published online: October 29, 2010
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Chem. Eur. J. 2010, 16, 13330 – 13334