Catalytic asymmetric β-peroxidation of nitroalkenes

Article abstract of DOI:10.1002/adsc.200800387

The first example of an asymmetric β-peroxidation of nitroalkenes is disclosed. The reaction is promoted by catalytic loadings of a commercially available diaryl-2-pyrrolidinemethanol derivative and tert-butyl hydroperoxide as the oxidant. A synthetically useful class of peroxides is obtained in good yield and enantioselectivity (up to 84% ee).

Full text of DOI:10.1002/adsc.200800387

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DOI: 10.1002/adsc.200800387
Catalytic Asymmetric b-Peroxidation of Nitroalkenes
Alessio Russoa and Alessandra Lattanzia,*
a
Dipartimento di Chimica, Università di Salerno, Via Ponte don Melillo, 84084 Fisciano, Italy
Fax : (+39)-089-969-603; e-mail: lattanzi@unisa.it
Received: June 23, 2008; Published online: August 19, 2008
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/adsc.200800387.
Abstract: The first example of an asymmetric b-per-
oxidation of nitroalkenes is disclosed. The reaction
is promoted by catalytic loadings of a commercially
available diaryl-2-pyrrolidinemethanol derivative
and tert-butyl hydroperoxide as the oxidant. A syn-
thetically useful class of peroxides is obtained in
good yield and enantioselectivity (up to 84% ee).
Despite the synthetic importance of epoxides or b-
hydroxy compounds, achievable through asymmetric
conjugate addition to nitroalkenes, the strong basicity
and poor nucleophilicity of oxygen nucleophiles
render this approach problematic. Indeed, only a few
examples of stereoselective oxa-Michael reactions
were reported by the groups of Enders,[8] Dixon[9] and
Jørgensen.[10]
Keywords: asymmetric catalysis; Michael addition;
nitroalkenes; organocatalysis; peroxides
We have recently developed a simple methodology
for the highly enantioselective epoxidation of a,b-
enones catalyzed by diaryl-2-pyrrolidinemethanol
compounds and tert-butyl hydroperoxide (TBHP) as
the oxidant.[11] The same organocatalysts were found
to be effective in the Michael addition of malonate
esters to nitroalkenes although with only a moderate
Catalytic asymmetric conjugate additions are funda- degree of asymmetric induction.[12] Encouraged by
mental reactions in organic synthesis, widely used to these results and the small number of asymmetric ap-
form carbon-carbon and carbon-heteroatom bonds.[1] proaches based on oxa-Michael addition to nitroal-
An impressive number of Michael addition reactions, kenes, we envisaged to exploit our organocatalytic
either using metal-based chiral complex catalysis[2] or system for their epoxidation.[13] Interestingly, perox-
small chiral molecules as organocatalysts[3] have been ides 3 were formed as the result of the addition [Eq.
developed. Among the different acceptors, easily ac- (1)]. To the best of our knowledge, compounds 3
cessible nitroalkenes have been intensively investigat-
ed as being compounds of a highly reactive nature.[4]
Moreover, the nitro group can undergo a number of
further transformations, after the addition, giving rise
to a variety of enantioenriched compounds.[5] In this
context, organocatalysts such as Cinchona alkaloids
and thioureas, besides enamine activation of ketones
and aldehydes as donors,[6] can promote efficient
asymmetric conjugate additions of different acidic
pronucleophiles to nitroalkenes.[7] In these examples, have been previously suggested as intermediates in
a bifunctional type of catalysis has been proposed, the oxidation of trans-b-nitrostyrenes to a-nitroaceto-
that involves activation of the acceptor via hydrogen phenones using TBHP and n-BuLi and one example
bonding between the nitro moiety and a Brønsted (3a) was isolated.[14] Herein we report on the develop-
acid functionality of the catalyst, whereas the pronu- ment of the first asymmetric variant of this relatively
cleophile is deprotonated by a basic group of the cata- unexplored reaction, which represents a straightfor-
lyst. Hence, a synergic network of hydrogen bonding ward entry to functionalized enantioenriched acyclic
and ion pairing interactions engaged between the re- peroxides. Indeed, chiral non-racemic acyclic perox-
actants and the active sites of the chiral promoter ides are extremely difficult to obtain and the rare ex-
assure a well-organized transition state leading to amples reported are based on a modified version of
high stereocontrol.
the asymmetric metal-catalyzed Kharasch–Sosnovsky
Adv. Synth. Catal. 2008, 350, 1991 – 1995
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
1991

Alessio Russo and Alessandra Lattanzi
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oxidation.[15] Moreover, the great potential of com-
In toluene or p-xylene, peroxide 3a was isolated in
pounds 3 can be recognized as synthetic precursors of moderate yield and encouraging ee values (entries 1
enantioenriched b-amino alcohols, which are products and 2). In hexane, the conversion increased, although
of pharmaceutical interest[16] as well as widely em- the enantioselectivity slightly decreased (entry 3).
ployed chiral building blocks and ligands for asym- Moreover, polymeric by-products were observed. Re-
metric synthesis.[17]
actions carried out in hexane with more hindered
Based on our earlier work,[11,12] we initially investi- cumene hydroperoxide led to the corresponding prod-
gated commercially available l-a,a-diphenylprolinol uct 3b in low yield and 18% ee (entry 4). Hence,
2a, in catalytic amounts, as promoter of the reaction TBHP was chosen for further optimization of the re-
of trans-b-nitrostyrene 1a and TBHP in different sol- action in hexane as the solvent. Cinchona alkaloids
vents at room temperature (Table 1).
such as quinine, quinidine, cinchonine and cinchoni-
dine gave disappointing results.[18] The reaction car-
ried out with the O-methylated catalyst 2b (Figure 1)
yielded the product in poor enantioselectivity
(entry 5).
Table 1. Optimization of the oxa-Michael addition of TBHP
to 1a.[a]
This proved the key role exerted by the hydroxy
group in the control of the asymmetric induction,
likely via hydrogen bonding with the nitro group of
the alkene. The tertiary N-methylated catalyst 2c
proved to be ineffective, thus demonstrating that the
secondary amine is crucial for the reaction to succeed
(entry 6). Different readily accessible diaryl-2-pyrroli-
dinemethanols were then screened for their catalytic
ability to promote the reaction (entries 7–11). Com-
mercially available compound 2g turned out to be the
most effective and to our delight, a significant im-
provement of the ee was observed (entries 10 and 12).
A better solubilization of the nitroalkene was ach-
ieved using methylcyclohexane as the solvent
(entry 13). The rate of conversion increased, which
helped to reduce catalyst loading and when the reac-
tion was performed at À188C, 3a was isolated in good
yield and 84% ee (entry 14).
Having estabilished an optimal reaction protocol,
we explored the scope and limitations of our system
for the b-peroxidation of trans-nitroalkenes (Table 2).
We first studied the influence of steric hindrance in
the b-nitrostyrenes (entries 2–5). Both para and meta
substitution with alkyl substituents were well tolerat-
ed and the corresponding peroxides were recovered
in good yields and fairly good ee values. The introduc-
Entry
2
Solvent
Time
[h]
Yield
[%][b]
ee
[%][c]
1
2
2a toluene
2a p-xylene
2a hexane
2a hexane
2b hexane
2c hexane
2d hexane
2e hexane
2f hexane
2g hexane
2h hexane
16
17
15
18
18
46
17
19
16
16
20
26
38
38
58
30
52
5
37
60
trace
60
10
58
53
72
51
55
44
18
-6
3
4[d]
5
6
7
7[e]
8[e]
9[e]
10[e]
11[e]
48
45
n.d.[f]
64
54
78
68
84
12[e,g] 2g hexane
13[e]
2g methylcyclohexane 6
14[e,g,h] 2g methylcyclohexane 60
[a]
Molar ratios: 1a/TBHP/2=1.0/1.5/0.3. Reaction carried
out at c=0.5M of 1a.
Yield of isolated product after flash chromatography.
Determined by HPLC analysis on a Daicel Chiralcel
OD-H column.
Cumene hydroperoxide as the oxidant.
Reaction carried out at c=0.2M of 1a.
Not determined.
[b]
[c]
[d]
[e]
[f]
[g]
[h]
At À188C.
Using 20 mol% of 2g.
Figure 1. Various diaryl-2-pyrrolidinemethanol derivatives tested.
1992
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Catalytic Asymmetric b-Peroxidation of Nitroalkenes
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Table 2. Asymmetric b-peroxidation of trans-nitroalkenes
catalyzed by 2g/TBHP system.[a]
the peroxides in moderate yields, but the asymmetric
induction was maintained (entries 10–12).[19] Interest-
ingly, the aliphatic cyclohexyl nitroalkene 3n was con-
verted into the peroxide in good yield and 72% ee
(entry 13). Unfortunately, the acyclic derivative 3o
gave the product in low yield (entry 14).
To demonstrate the synthetic utility of peroxides 3,
we studied their transformation into 1,2-amino alco-
hols. The b-peroxidation, carried out on model nitro-
alkenes 1a, 1c and 1n at a 1.5-mmol scale, afforded
compounds 3 with comparable or improved yields
(Scheme 1).
Entry
R
3
Yield [%][b]
ee [%][c]
1
Ph
3a
3c
3d
3e
3f
3g
3h
3i
3j
3k
3l
3m
3n
3o
72
59
70
83
62
60
4
80
4
38
37
4
84
83
83
83
43
78
2[d]
4-MeC6H4
3,4-Me2C6H3
tBuC6H4
2-MeC6H4
4-CF3C6H4
4-BrC6H4
4-ClC6H4
3-MeOC6H4
2-naphthyl
2-furyl
3
44
5
-
À
6
One-pot cleavage of the O O bond and reduction
of the nitro group could be easily accomplished by hy-
7[d]
1
0
82
8[d]
84
drogenation. 1,2-Amino alcohols 4 were obtained in
9[d,e]
10[d,e]
11[d,e]
12[d,e]
13
80
quantitative yield and only a slight erosion of the
80
81
enantioselectivity was observed. The S absolute con-
figuration of the stereogenic centre and ee values of
3-furyl
cyclohexyl
5
83
compounds 4a,c were determined on the correspond-
ing N-Boc derivatives 5a,c (Boc=tert-butylcarboxy)
by comparison of their optical rotations and HPLC
data with those reported in the literature.[20] Aliphatic
compound 5n showed to be enriched in the R-enan-
tiomer.[20c] The optical purity of 1,2-amino alcohols 4
can be eventually improved to high levels by a single
crystallization as demonstrated for compound 4a
whose derivative 5a was isolated in 93% ee. Hence,
the synthetic sequence illustrated in Scheme 1 served
also to establish the absolute configuration of perox-
ides 3.[21]
73
20
72
14pentyl
n.d.[f]
[a]
Molar ratio: 1/TBHP/2g=1.0/1.5/0.2. Reaction carried
out at c=0.15M of 1 at 0.15 mmol scale for 48–130 h.
Yield of isolated product after flash chromatography.
Determined by HPLC analysis on chiral columns.
Using a 3:1 (v/v) mixture of methylcyclohexane and tolu-
ene.
[b]
[c]
[d]
[e]
[f]
Using 30 mol% of 2g.
Not determined.
Upon analysis of the data and in agreement with
tion of an ortho-methyl group significantly decreased our previous reports,[11,12] we suggest a bifunctional
the enantioselectivity, presumably on steric grounds, type of catalysis with the formation of the contact ion
although a good conversion was observed (entry 5). b- pair A,[22] after deprotonation of TBHP by amino al-
Nitrostyrenes having electron-withdrawing groups af- cohol 2g (Scheme 2).
forded the products in fairly good enantioselectivity
In transition state B, the intermolecular hydrogen
and in some examples slightly reduced yields, which bonding between the catalyst OH group and the nitro
can be ascribed to the lower solubility of the starting moiety appears to preferentially orientate the ap-
materials in the reaction mixture (entries 6–9). 2- proach of the nitroalkene, favoring the attack of tert-
Naphthyl and heteroaromatic derivatives furnished butyl peroxy anion to its Si face. Finally, protonation
Scheme 1. Synthetic elaboration of peroxides 3 to 1,2-amino alcohols.
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Alessio Russo and Alessandra Lattanzi
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Scheme 2. Proposed catalytic cycle for the b-peroxidation of nitroalkenes.
of the anionic intermediate forms the product and 3 (which is in general slightly more mobile on TLC than the
corresponding nitroalkene) could be removed by precipita-
tion in hexane at low temperature.
catalyst 2g is regenerated.
In conclusion, we have developed the first example
of an asymmetric b-peroxidation of nitroalkenes,
which gives access to optically active peroxides 3 in
moderate to good yields and synthetically useful
levels of enantioselectivity. This reaction is notewor-
thy for its simplicity, easy access of starting com-
pounds and more importantly, commercial availability
in both enantiomeric forms, of diaryl-2-pyrrolidineme-
thanol 2g as the catalyst. The utility of the asymmetric
b-peroxidation was demonstrated by a straightforward
and efficient conversion of peroxides into 1,2-amino
alcohols, showing an interesting development of
TBHP as a synthon for the hydroxy group. Further in-
vestigations on the full scope of the b-peroxidation
and applications of ion pair catalysis provided by
diaryl 2-pyrrolidinemethanol derivatives are under-
way.
Acknowledgements
MIUR is gratefully acknowledged for financial support.
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