Asymmetric oxidations of electron-poor alkenes promoted by the β-amino alcohol/TBHP system

Article abstract of DOI:10.1055/s-0029-1216638

The asymmetric oxyfunctionalization of alkenes is a fundamental process in synthetic organic chemistry. In this contribution, we review our findings on the enantioselective organocatalyzed oxidation of electron-poor alkenes. Readily or commercially available β-amino alcohols displayed catalytic activity in the asymmetric epoxidation of α,β-enones and β-peroxidation of nitroalkenes with tert-butyl hydroperoxide (TBHP) as the oxidant. The corresponding epoxides and peroxides were isolated in good to high yield and enantioselectivity. Georg Thieme Verlag Stuttgart.

Full text of DOI:10.1055/s-0029-1216638

SPECIAL TOPIC
1551
Asymmetric Oxidations of Electron-Poor Alkenes Promoted by the b-Amino
Alcohol/TBHP System
Asymmetric
O
rga
n
l
ocataly
e
z
ed
O
xida
s
tions of
E
s
i
or
A
lk
o
Dipartimento di Chimica, Università di Salerno, Via Ponte don Melillo, 84084 Fisciano, Italy
Fax +39(89)969603; E-mail: lattanzi@unisa.it
Received 16 February 2009
Abstract: The asymmetric oxyfunctionalization of alkenes is a fun-
damental process in synthetic organic chemistry. In this contribu-
tion, we review our findings on the enantioselective
organocatalyzed oxidation of electron-poor alkenes. Readily or
commercially available b-amino alcohols displayed catalytic activ-
ity in the asymmetric epoxidation of a,b-enones and b-peroxidation
of nitroalkenes with tert-butyl hydroperoxide (TBHP) as the oxi-
dant. The corresponding epoxides and peroxides were isolated in
good to high yield and enantioselectivity.
OH
OH
Ar
H2N
Ar
Ar
Ar
O
O
N
O
R1
2
H
R3
R2
R3
R2
1
4
3
5
OOt-Bu
NO2
TBHP, hexane
NO2
R4
R4
6
Key words: a,b-enones, epoxides, peroxides, a,a-diaryl-L-proli-
nols, nitroalkenes
Scheme 1 Asymmetric epoxidation of a,b-enones 3 and b-peroxi-
dation of nitroalkenes 5 catalyzed by the b-amino alcohol 1, 2/TBHP
system (R1 = alkyl)
The development of novel oxidative systems able to cata-
lyze the enantioselective epoxidation of alkenes is one of
the most important research areas of synthetic organic
chemistry. Metal-catalyzed asymmetric systems have
played a prominent role in alkene epoxidation,1 but lately,
organocatalyzed methodologies have rapidly progressed.2
Very recently, improvements and, most of all, important
new achievements have been attained in the epoxidation
and b-oxyfunctionalization of electron-poor alkenes by
using readily accessible organic promoters.3 Indeed, the
first examples of enantioselective epoxidation of chal-
lenging compounds such as enals4 and cyclic a,b-enones5
have been successfully realized as well as the asymmetric
b-peroxidation6 and hydroperoxidation7 of a,b-unsaturat-
ed ketones using secondary and primary amines as the or-
ganocatalysts.
Enantioselective Epoxidation of a,b-Enones Cata-
lyzed by the b-Amino Alcohol/TBHP System
The epoxidation of a,b-enones has been accomplished ei-
ther by using asymmetric metal-catalyzed protocols or by
employing organocatalyzed oxidative versions.10 Poly-
leucine/hydrogen peroxide11 and cinchona quaternary am-
monium salts/sodium hypochlorite12 systems are illustra-
tive of the most effective organocatalytic systems. These
methodologies, originally discovered more than two de-
cades ago, have been recently the subject of renewed in-
terest.13
Stimulated by novel concepts on the activation of com-
pounds offered by functional groups residing in small or-
ganic molecules,14 we targeted these to develop a simple
oxidative system for the nucleophilic asymmetric epoxi-
dation of a,b-enones. According to the mechanism of the
Weitz–Scheffer15 epoxidation, a bifunctional system was
thought to be able to promote the epoxidation. In particu-
lar, the presence of a chiral base and an additive with hy-
drogen-bonding donating groups would simultaneously
activate the oxidant and the enone, respectively. After
checking a variety of different combinations, we disclosed
that the epoxidation of trans-a,b-enones 3a–f, catalyzed
In this context, we recently found that commercially or
readily accessible diarylprolinols 1 and primary b-amino
alcohols 2 catalyze the epoxidation of a,b-enones in the
presence of tert-butyl hydroperoxide (TBHP) as the oxy-
gen source.8 Moreover, this simple catalytic system could
be applied to realize an unprecedentedly reported asym-
metric b-peroxidation of nitroalkenes9 (Scheme 1). Ep-
oxides 4 and peroxides 6 could be obtained in good to high
yield and enantioselectivity (70–92% ee).
Our findings on the enantioselective epoxidation of a,b- by commercially available a,a-diphenylprolinol (1a) with
unsaturated ketones and the b-peroxidation of nitroalk- tert-butyl hydroperoxide, proceeded in an enantioselec-
enes catalyzed by b-amino alcohols and tert-butyl hydro- tive way at room temperature in hexane to give epoxides
peroxide as the oxygen source are reviewed.
4a–f (Scheme 2).8a
The epoxides 4 were isolated in good yield and enantio-
selectivity. Polar, protic, halogenated, and ethereal media
proved to be unsuitable solvents. The reaction did not pro-
ceed when using hydrogen peroxide as the oxidant. The
SYNTHESIS 2009, No. 9, pp 1551–1556
Advanced online publication: 14.04.2009
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x
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x
x
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0
0
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DOI: 10.1055/s-0029-1216638; Art ID: C00709SS
© Georg Thieme Verlag Stuttgart · New York

1552
A. Russo, A. Lattanzi
SPECIAL TOPIC
O
OH
Ph
O
O
catalyst (30 mol%)
TBHP, hexane, r.t.
Ph
Ph
Ph
Ph
N
Ph
O
O
H
O
1a (30 mol%)
R2
R1
R2
R1
4a
3a
TBHP, hexane, r.t.
4
3
OH
OH
Cl
R1 = Ph, R2 = Ph
4a
4b
4c
4d
4e
4f
72%, 75% ee
70%, 78% ee
46%, 80% ee
73%, 74% ee
87%, 63% ee
52%, 79% ee
R1 = 3-MeC6H4, R2 = Ph
R1 = Ph, R2 = 4-MeOC6H4
R1 = Ph, R2 = 4-ClC6H4
R1 = Ph, R2 = Me
N
N
H
t-Bu
H
Cl
R1 = Me, R2 = Ph
Cl
Cl
t-Bu
1b
26%, 81% ee
1c
Scheme 2 Enantioselective epoxidation of trans-a,b-enones 3a–f
catalyzed by the 1a/TBHP system
79%, 82% ee
epoxidation carried out with modified compound 1a de-
void of the hydroxy group furnished poor results in terms
of activity and enantioselectivity, thus showing the crucial
role played by this group in the catalysis.
OH
N
H
The investigation on nonlinear effects16 in the epoxidation
of trans-chalcone (3a) employing enantiomerically en-
riched 1a, showed a linear correlation, which suggested
that one molecule of catalyst 1a is likely to have been in-
volved in the enantiodiscriminating step.
1d
99%, 88% ee
Scheme 4 Enantioselective epoxidation of trans-chalcone (3a) ca-
talyzed by modified the prolinol 1b–d/TBHP system
A dual activation of the reagents provided by organocata-
lyst 1a has been proposed as illustrated in Scheme 3.
electron-donating groups 1c,d, catalyzed the reaction with
significantly higher efficiency when compared to catalyst
1a.8b
Ar
Ar
Prolinol 1d was chosen as the most effective promoter to
develop an improved methodology. The formation of the
epoxides 4a,c–g in good to high yield and enantioselectiv-
ity was realized under reduced catalyst loading at 4 °C
(Scheme 5).
N
O
R3OO
H
H
H
O
R2
R1
O
Ar
O
O
O
Ar
O
O
O
R2
R1
1d (20 mol%)
+ R3OH
R2
R1
R2
R1
R2
R1
N
H
TBHP, hexane, 4 °C
H
4
O
3
H
R3O
R1 = Ph, R2 = Ph
4a
4c
4d
4e
4f
90%, 91% ee
70%, 90% ee
81%, 92% ee
92%, 75% ee
60%, 87% ee
70%, 74% ee
R1 = Ph, R2 = 4-MeOC6H4
R1 = Ph, R2 = 4-ClC6H4
R1 = Ph, R2 = Me
Scheme 3 Mode of dual activation of reacting partners in the nu-
cleophilic epoxidation mediated by b-amino alcohol 1
R1 = Me, R2 = Ph
R1 = Me, R2 = (CH2)4Me
4g
After deprotonation of the pronucleophile, the resultant
ammonium ion would remain coordinated to the peroxy-
anion, while the enone would be activated and properly
orientated for the conjugate addition via hydrogen bond-
ing with the hydroxy group. Then, ring closure to the ep-
oxide occurs with the elimination of the tert-butoxy anion,
which deprotonates the ammonium ion regenerating the
catalyst.
Scheme 5 Enantioselective epoxidation of trans-a,b-enones 3a,c–g
catalyzed by the 1d/TBHP system
It is interesting to note that this system can be successfully
employed for the epoxidation of challenging compounds
such as alkyl, aryl, and dialkyl ketones.
A deeper investigation, carried out to refine catalyst activ-
ity by modifications on the phenyl rings, showed that
ortho-phenyl substituted prolinols were ineffective pro-
moters.8c Sterically encumbered tert-butyl groups at meta
positions completely inhibited the catalyst activity. The
phenyl-trisubstituted compound 1e proved to be more ac-
tive than promoter 1d and it could be conveniently used at
10 mol% loading at room temperature (Scheme 6).
Next our efforts were directed to improving the perfor-
mance of the epoxidizing system. To this end, different
aryl-substituted prolinols 1b–d, readily synthesized start-
ing from L-proline,17 were checked under standard condi-
tions in the epoxidation of trans-chalcone (3a)
(Scheme 4).
The presence of electron-withdrawing groups was detri-
mental to the catalyst activity, while compounds with
Synthesis 2009, No. 9, 1551–1556 © Thieme Stuttgart · New York

SPECIAL TOPIC
Asymmetric Organocatalyzed Oxidations of Electron-Poor Alkenes
1553
and tertiary b-amino alcohols 9c–e were unsuitable pro-
moters.
OH
Me
N
OMe
The epoxidation carried out with catalysts 1d and 9d in
the presence of different acids as co-catalysts did not pro-
ceed. These findings supported our mechanistic hypothe-
sis based on the formation of an ion pair as the reactive
species, rather than the generation of an iminium ion of
the enone as an intermediate.
H
Me
Me
Me
O
OMe
O
O
1e (10 mol%)
TBHP, hexane, r.t.
R2
R1
R2
R1
4
3
R1 = Ph, R2 = Ph
93%, 89% ee
4a
4c
4d
4f
O
O
R1 = Ph, R2 = 4-MeOC6H4 61%, 87% ee
catalyst (30 mol%)
TBHP, hexane, r.t.
O
R1 = Ph, R2 = 4-ClC6H4
R1 = Me, R2 = Ph
88%, 87% ee
60%, 83% ee
Ph
Ph
Ph
Ph
4a
3a
H2N
HO
Scheme 6 Enantioselective epoxidation of trans-a,b-enones
3a,c,d,f catalyzed by the 1e/TBHP system
H2N
HO
i-Pr
HO
i-Pr
Ph
Ph
Me
N
Ph
H2N
H
HO
Ph
Having optimized the phenyl substitution pattern, we
turned our attention to the influence of the amine ring size
on the catalyst performance.8d Six- and four-membered-
ring compounds 7 and 8 were synthesized and used in the
epoxidation of model compound 3a (Scheme 7).
30%, 34% ee
18%, racemic 41%, 22% ee
37%, 19% ee
9c
2a
9b
9a
Ph
OMe
H2N
Ph
O
O
catalyst (30 mol%)
TBHP, hexane, r.t.
O
HO
HO
Ph
Ph
Ph
Ph
21%, racemic
H2N
H2N
3a
4a
9d
i-Pr
i-Pr
Ph
OH
OH
OMe
Me
OH
N
N
H
N
OMe
58%, 35% ee
21%, 52% ee
Me
Ph
H
OMe
2b
2c
5%,13% ee
9e
OMe
OMe
Scheme 8 Enantioselective epoxidation of trans-chalcone (3a) ca-
talyzed by the b-amino alcohols 9 and 2/TBHP system
8
7
48%, 75% ee
33%, 85% ee
Scheme 7 Enantioselective epoxidation of trans-chalcone (3a) ca-
talyzed by the modified secondary amino alcohol 7, 8/TBHP system
The different catalytic activity showed by primary, sec-
ondary, and tertiary b-amino alcohols in the epoxidation
of a,b-enones can be explained by taking into account fac-
tors affecting the formation and the stability of the ion
pair. In an apolar medium, hydrogen bonding interactions
play a fundamental role in the stabilization of polar spe-
cies. Stronger localized ammonium ions are generated
starting from primary b-amino alcohols, which can more
efficiently stabilize the tert-butyl peroxyanion. Ion pair
destabilization increases when going from secondary to
tertiary b-amino alcohols, whose sterically more crowded
ammonium ions less effectively stabilize the anion, giving
rise to a looser ion pair. Intramolecular hydrogen bonding
interaction between the NH and the vicinal oxygen of the
OH group are known to stabilize charged ammonium ions
derived from b-amino alcohols and block their conforma-
tions.18 The structures of the most stable ammonium ion
conformers of promoters 1e and 2c were then calculated
in the gas-phase by using B3LYP density functional theo-
ry (Figure 1).
In both cases, their efficiency was lower than that ob-
served for compound 1e. Indeed, the activity was marked-
ly reduced, although the asymmetric induction was
slightly affected.
In order to have a comprehensive idea of the effectiveness
of b-amino alcohols as catalysts in the enantioselective
epoxidation of a,b-enones, easily accessible primary, sec-
ondary, and tertiary b-amino alcohols 9a–e and 2a–c and
were checked in the epoxidation of 3a (Scheme 8).
Primary b-amino alcohols were able to catalyze the epoxi-
dation of 3a in modest yield and low enantioselectivity.
Interestingly, as previously observed when using a,a-di-
arylprolinols 1, the activity and the enantioselectivity of
primary b-amino alcohols 2a–c, readily accessible from
a-amino acid esters, could be significantly tuned by mod-
ifying the substitution pattern of the phenyl rings. The pri-
mary amine 9d, devoid of the hydroxy group, showed
itself to be a poor catalyst, thus confirming the importance
of the hydroxy moiety in the catalysis. Acyclic secondary
Most stable conformer A of catalyst 1e shows an intramo-
lecular hydrogen bond between the oxygen of the OH
group with the N–H bond from the same side of the ring
Synthesis 2009, No. 9, 1551–1556 © Thieme Stuttgart · New York

1554
A. Russo, A. Lattanzi
SPECIAL TOPIC
Enantioselective b-Peroxidation of trans-Nitroalk-
enes Catalyzed by b-Amino Alcohols/TBHP System
Among conjugate additions,19 the oxa-Michael addition
represents one of the most difficult process to realize. The
major problems associated with this transformation are
the low nucleophilicity and strong basicity of the oxygen
nucleophiles as well as the reversibility of the conjugate
addition. Moreover, the polymerization of the alkene can
be easily observed. Indeed, only a few examples of stereo-
selective oxa-Michael addition have been successfully ac-
complished.20
We thought it would be interesting to investigate the ap-
plication of the diarylprolinols/tert-butyl hydroperoxide
system in the epoxidation of nitroalkenes. Unexpectedly,
the model reaction carried out on trans-nitrostyrene 5a
with catalyst 1a and tert-butyl hydroperoxide afforded the
enantiomerically enriched b-peroxide 6a (Scheme 10).9
OOt-Bu
1a (30 mol%)
NO2
NO2
Ph
Ph
Figure 1 DFT-calculated ammonium conformers of catalysts 1e
and 2c
TBHP, hexane, r.t.
5a
6a
38%, 51% ee
Scheme 10 Enantioselective b-peroxidation of trans-nitrostyrene
5a catalyzed by the 1a/TBHP system
plane. Conformer C, having the intramolecular hydrogen
bond with the N–H bond on the opposite side of the ring
plane, proved to be 3.6 kJ/mol less stable than A.
Peroxide 6a has been previously isolated as an intermedi-
ate in the oxidation of nitrostyrenes to a-nitroacetophe-
nones.21 Our reaction represents the first asymmetric route
to this type of compounds. Enantiomerically enriched
acyclic peroxides are very difficult to be obtained and
only a few examples have been reported.22
In the case of the acyclic catalyst 2c, conformers B and D
are almost comparable since they have an energy differ-
ence of 0.6 kJ/mol. The energy differences observed sug-
gest that conformer A would be likely involved in the ion
pair during the epoxidation, thus affording a univocal ste-
reochemical control. On the other hand, conformers B and
D would be reasonably active in the epoxidation, which
would give rise to an average result in terms of enantiose-
lectivity. The result achieved in the epoxidation of 3a pro-
moted by catalyst 1f, having the protected hydroxy group,
could be explained by taking into account the formation of
the intramolecular hydrogen bond in the ammonium ion
(Scheme 9).
After screening of the reaction parameters and catalysts
activity, compound 1d was found to be the most effective
in methylcyclohexane as the solvent at –18 °C
(Scheme 11).
OOt-Bu
1d (20 mol%), –18 °C
NO2
NO2
R
R
5
TBHP, methylcyclohexane
6
OMe
6a
R = Ph
72%, 84% ee
70%, 83% ee
80%, 84% ee
45%, 83% ee
73%, 72% ee
6b R = 3,4-(Me)2C6H3
6c R = 4-ClC6H4
6d R = 3-furyl
Ph
N
Ph
H
1f (30 mol%)
O
O
O
6e R = cyclohexyl
Ph
Ph
Ph
Ph
TBHP, hexane, r.t.
4a
Scheme 11 Enantioselective b-peroxidation of trans-nitroalkenes 5
catalyzed by the 1d/TBHP system
3a
14%, 63% ee
Scheme 9 Enantioselective epoxidation of trans-chalcone (3a) ca-
talyzed by the 1f/TBHP system
Peroxides 6 were recovered in moderate to good yield and
fairly good asymmetric induction. Compounds 6 can be
synthetically exploited to produce enantioenriched b-ami-
no alcohols 10, which are widely used building blocks and
ligands in asymmetric synthesis as well as pharmaceutical
targets.23 Catalytic hydrogenation of products 6 allowed
direct isolation of b-amino alcohols 10 in high yield and
only slightly decreased enantioselectivity (Scheme 12).
Although a dramatic reduction of the catalyst activity was
observed, the ammonium ion involved in the ion pair
would be conformationally blocked, thus leading to an al-
most comparable control of the enantioselectivity with re-
spect to catalyst 1a.
Synthesis 2009, No. 9, 1551–1556 © Thieme Stuttgart · New York

SPECIAL TOPIC
Asymmetric Organocatalyzed Oxidations of Electron-Poor Alkenes
1555
gel 60 (particle size 0.040–0.063 mm). All commercially available
reagents were purchased from Aldrich.
OH
OH
OOt-Bu
Pd/C, H2
NO2
(Boc)2O
Et3N
NHBoc
NH2
R
R
R
MeOH, r.t.
11
6
10
>90%
Epoxidation of trans-a,b-Enones; General Procedure
A sample vial was charged with trans-a,b-enone (0.150 mmol), the
b-amino alcohol (0.045 mmol) and hexane (0.300 mL) at r.t. TBHP
(5–6 M soln in decane; 37 mL, 0.21 mmol) was then added. After
monitoring by TLC, the crude reaction mixture was directly puri-
fied by flash chromatography on silica gel (PE–Et2O, 99:1) to pro-
vide the epoxy ketone 4. All epoxides are known compounds, their
R = Ph (S)-11a
94%, 78% ee
R = cyclohexyl (R)-11e 85%, 61% ee
Scheme 12 Synthetic elaboration of peroxides 6
These findings can be framed under the previously sug- analytical data were identical to those reported in the literature.8
Enantiomeric excess was determined by chiral HPLC using Daicel
gested mechanistic pathway illustrated for the epoxida-
tion. Catalyst 1d serves as bifunctional promoter, via
Chiralcel OD and Daicel Chiralpak AD columns.8
deprotonation of tert-butyl hydroperoxide to generate the
contact ion pair. Then, the intermolecular hydrogen bond-
ing between the catalyst hydroxy group and the oxygen of
nitro group would orientate the nitroalkene favoring the
peroxyanion attack to its Si face (Scheme 13).
b-Peroxidation of trans-Nitroalkenes; General Procedure
A sample vial was charged with trans-nitroalkene (0.15 mmol), cat-
alyst 1d (9.3 mg, 0.03 mmol) and methylcyclohexane (or methylcy-
clohexane–toluene, 3:1) (1.0 mL). TBHP (5–6 M soln in decane;
40 mL, 0.23 mmol) was then added to the stirring solution at –18 °C.
The reaction was monitored by TLC. Unsoluble polymeric by-prod-
ucts were formed during the reaction. The solvent was removed un-
der reduced pressure and the crude reaction mixture was directly
purified by flash chromatography on silica gel, eluting with mix-
In conclusion, easily accessible b-amino alcohols have
been showed to be useful promoters in the catalytic asym-
metric epoxidation of a,b-enones and the b-peroxidation
of nitroalkenes, affording the products in good to high tures of PE and PE–Et2O (98:2) to provide peroxide 6. Residual ni-
troalkene isolated with the peroxide 6 (which is in general slightly
more mobile on TLC than the corresponding nitroalkene) was re-
moved by precipitation in hexane at low temperature. Peroxides 6
are known compounds, their analytical data were identical to those
reported in the literature.9 Enantiomeric excess was determined by
chiral HPLC using Daicel Chiralcel OD-H and Daicel Chiralpak
AS-H columns.9
yield and enantioselectivity. A bifunctional activation of
the reagents provided by b-amino alcohols has been pro-
posed. It is interesting to point out that they operate via
noncovalent catalysis in contrast to covalent catalysis, via
enamine/iminium intermediates formation, generally dis-
played by L-proline derivatives.24 An analogy can be rath-
er found with the mechanism of action provided by
cinchona alkaloids in Michael addition reactions pio-
neered by Winberg over 20 years ago.25
Acknowledgment
MIUR is gratefully acknowledged for financial support.
All reactions requiring dry or inert conditions were conducted in
flame-dried glassware under a positive pressure of nitrogen. Reac-
tions were monitored by thin-layer chromatography (TLC) on Mer-
ck silica gel plates (0.25 mm) and visualized by UV light, by
exposure to iodine vapor, or by phosphomolybdic acid/ethanol
spray test. Flash chromatography was performed on Merck silica
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N
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t-BuOOH
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Ph
OOt-Bu
NO2
Ph
N
O
O
H
H
O
H
O
O
N
Ph
Scheme 13 Proposed catalytic cycle for the b-peroxidation
Synthesis 2009, No. 9, 1551–1556 © Thieme Stuttgart · New York

1556
A. Russo, A. Lattanzi
SPECIAL TOPIC
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Synthesis 2009, No. 9, 1551–1556 © Thieme Stuttgart · New York