Heterogeneous bisoxazoline/copper complex: A green catalyst for the enantioselective reaction of nitromethane with substituted benzaldehydes

Article abstract of DOI:10.1002/ejoc.201100702

Polystyrene-supported chiral 2,2′-methylenebis[(4S)-4-tert-butyl-2- oxazoline] is a promising heterogeneous catalyst for the enantioselective Henry reaction of nitromethane with substituted benzaldehydes. The process is carried out in ethanol as a green s

Full text of DOI:10.1002/ejoc.201100702

SHORT COMMUNICATION
DOI: 10.1002/ejoc.201100702
Heterogeneous Bisoxazoline/Copper Complex: A Green Catalyst for the
Enantioselective Reaction of Nitromethane with Substituted Benzaldehydes
[a]
[b]
[a]
[a]
Raimondo Maggi,* Daniela Lanari, Chiara Oro, Giovanni Sartori, and
Luigi Vaccaro[
b]
Dedicated to Professor Gianfranco Scorrano on the occasion of his 72nd birthday
Keywords: Aldol reactions / Asymmetric catalysis / Copper / Heterogeneous catalysis
Polystyrene-supported chiral 2,2Ј-methylenebis[(4S)-4-tert-
butyl-2-oxazoline] is a promising heterogeneous catalyst for
the enantioselective Henry reaction of nitromethane with
substituted benzaldehydes. The process is carried out in eth-
anol as a green solvent, and the corresponding chiral β-nitro-
alcohols are obtained in 56–95% yield with 49–88%ee.
Introduction
nitromethane to aldehydes (89–94%ee); in this case, the
–
moderately Brønsted basic ligand AcO facilitates the de-
The coupling of carbonyl compounds with alkyl nitro
compounds bearing α-hydrogen atoms, namely, the Henry
reaction, is one of the most basic and widely utilized meth-
ods for the construction of carbon–carbon bonds and the
preparation of β-nitroalcohols, and as such a variety of
valuable building blocks can be easily accessed. The
Henry reaction involves the formation of one or two asym-
metric centers at the new carbon–carbon junction, de-
pending on the complexity of the reactants. The stereoselec-
[
7]
protonation of nitromethane. A dinuclear zinc(II) com-
plex with a chiral semi-azacrown ligand was shown by Trost
to give high levels of enantioselection (78–93%ee).[ Some
other chiral copper complexes have also been shown to act
8]
[
9]
as useful catalysts, such as CuCl /sparteine, CuCl/chiral
[
1]
2
[
10]
1
,2-cyclohexane diamine derivatives,
DAT chiral (1R,2R)-N,NЈ-bis(2,2Ј-bithiophen-5-yl-
methyl)cyclohexane-1,2-diamine],
Cu(OAc) /DAT
2
[
=
[
11]
Cu(OTf) /tetrahydro-
2
[
12]
[13]
salen,
and chiral tetraaminophosphonium salts.
Two
[
2]
tive Henry reaction, first described by Shibasaki in 1992,
short reviews have been published on the catalytic asym-
metric Henry reaction, showing two general strategies,
namely, the preactivation of the nitroalkane through forma-
tion of silyl nitronates and the more convenient, direct reac-
tion of unmodified nitroalkanes and carbonyl com-
has remained less studied in comparison to the great
number of studies published on the stereocontrol of the al-
[
3]
dol reaction. This gap can be ascribed to both the diffi-
culty in preparing suitable chiral auxiliaries and the revers-
ibility of many Henry reactions together with the relative
ease of racemization of the carbon stereocenter located at
[
4,14]
pounds.
Despite the excellent synthetic results achieved with reac-
tions performed under homogeneous catalysis, to the best
of our knowledge only a few examples of the stereoselective
Henry reaction have been reported to be carried out under
heterogeneous catalysis, including the use of lanthanum–
[4]
the α-position with respect to the nitro group.
However, some interesting results on the stereoselective
Henry reaction carried out under homogeneous catalysis
have recently been published.[5] For example, Jørgensen re-
ported the reaction of α-keto esters with nitromethane cata-
lithium–BINOL chiral complex covalently linked to MCM-
lyzed by Cu(OTf) /(S)-tert-butylbisoxazoline (BOX) in the
2
[15]
4
1 mesoporous silica (57–84%ee),
polythiophene salen–
presence of triethylamine (57–94%ee).[6] Shortly thereafter,
[
16]
chromium complex derivatives (51–77%ee),
a chiral
Evans demonstrated the effectiveness of Cu(OAc) /inda-
BOX in promoting a base-free stereoselective addition of
2
BOX copper complex immobilized onto magnetically sepa-
[
17]
rable mesoporous silica (55–86%ee), and chiral diamines
[
18]
linked to zeolites.
In these cases promising results were
[
[
a] “Clean Synthetic Methodologies Group”,
Dipartimento di Chimica Organica e Industriale
dell’Università,
achieved, but catalyst deactivation, low enantioselectivities,
or leaching phenomena were often encountered.
An interesting paper reported the five-step preparation
of a recoverable PEG-supported copper catalyst to be uti-
lized as a chiral solid catalyst in the stereocontrolled ni-
troaldol condensation. The optically active nitroalcohols
Parco Area delle Scienze 17A, 43124 Parma, Italy
Fax: +39-0521-905472
E-mail: raimondo.maggi@unipr.it
b] “Laboratory of Green Synthetic Organic Chemistry, CEMIN”,
Dipartimento di Chimica dell’Università,
Via Elce di Sotto 8, 06123 Perugia, Italy
Eur. J. Org. Chem. 2011, 5551–5554
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
5551

R. Maggi, D. Lanari, C. Oro, G. Sartori, L. Vaccaro
SHORT COMMUNICATION
were obtained with excellent conversions (65–90%) and Table 1. Activity of BOX/Cu(OAc)
2
·H
2
O complexes in the reaction
[a]
enantioselectivities (83–95%).[
19]
between para-chlorobenzaldehyde and nitromethane.
Also, a heterobimetallic
catalyst composed of Nd O(OiPr) , an amide-based li-
5
13
gand, and sodium hexamethyldisilazide was reported to ef-
ficiently promote the same reaction at –40 °C with good
diastereoselectivities (3.4:1Ǟ40:1) and enantioselectivities
[
20]
(77–95%).
With our research we have been contributing to the prep-
aration, characterization, and use of organic and organo-
metallic catalysts supported on different materials[ to re-
alize chemically and environmentally efficient synthetic pro-
21]
Yield [%][b]
ee [%][b,c]
Entry
Ligand
1
2
3
4
1a
1b
1c
1d
30
57
66
65
34
47
49
30
[
22]
cedures.
In the present contribution, we have evaluated
the activity of a polystyrene-supported chiral BOX/copper
complex as a heterogeneous catalyst able to promote the
Henry reaction for the production of enantiopure β-nitro- [a] Reaction conditions: para-Chlorobenzaldehyde (1 mmol),
2 2 2
MeNO (10 mmol), BOX/Cu(OAc) ·H O (5% with respect to para-
alcohols.
chlorobenzaldehyde), EtOH (2 mL). [b] Determined by HPLC
analysis (Chiracel OD-H column, hexane/iPrOH = 85:15). [c] Ab-
solute configuration of the major enantiomer: R.
The best result in terms of yield and enantioselectivity
for the production of nitroalcohol 4a was obtained with the
complex derived from ligand 1c (66% yield, 49%ee;
Table 1, Entry 3). These results are undoubtedly not excel-
lent, and this behavior is probably due to the possible for-
mation of a dimeric complex of BOX with copper, as al-
Results and Discussion
Our approach was divided into three steps: First, we pre-
pared and tested a set of chiral BOX ligands with different
substituents at the 4Ј- and 5Ј-positions. Second, we heterog-
enized the best ligand on polystyrene through a copolymer-
ization approach. Third, we tested the resulting solid cata-
lyst in the model reaction between para-chlorobenzaldehyde
and nitromethane, evaluating catalytic activity, metal leach-
ing, recovery, and reuse.
[
24]
ready observed for cyclopropanation reactions, which of
course could decrease both catalyst reactivity and stereose-
lectivity. Usually to overcome this drawback, BOX ligands
bearing two substituents at the methylene bridge are uti-
[
4]
lized; nevertheless, it must be taken into account that the
formation of a dimeric complex could be greatly reduced,
due to site isolation, when the ligand is anchored onto a
solid support.
The strategy followed for the preparation of chiral BOX
ligands 1 is based on the well-known reaction of malono-
[
23]
nitrile with optically active amino alcohol (2 equiv.); the
important feature of this approach is that the amino alcohol
configuration is retained in the final BOX ligand. All four
BOX ligands prepared (Figure 1) carried an unsubstituted
methylene bridge, with the aim of using this moiety to an-
chor them onto an insoluble support through an additional
spacer.
For these reasons we selected ligand 1c as a good candi-
date for its immobilization onto a polystyrene matrix. The
monofunctionalization of the methylene bridge was chosen
as the best approach on the basis that the C symmetry of
2
the ligand is not crucial to obtain good enantioselection
in the Henry reaction, contrary to that observed for the
[
23]
cyclopropanation process;
in addition, a single spacer
could give greater mobility of the ligand and, therefore, bet-
ter availability of the catalytic site. The synthetic strategy
followed for the preparation of functionalized ligand 5c[
is reported in Scheme 1.
25]
Figure 1. BOX ligands prepared.
Scheme 1. Ligand 1c functionalization.
The prepared ligands were complexed in situ with
Cu(OAc) ·H O and tested in the model reaction between
Successively, the monofunctionalized ligand was copoly-
2
2
para-chlorobenzaldehyde (2a) and nitromethane (3) in eth- merized with styrene and divinylbenzene in the presence of
[
26]
anol at room temperature for 24 h (Table 1).
AIBN in toluene at reflux for 20 h
to obtain heterogen-
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Eur. J. Org. Chem. 2011, 5551–5554

Enantioselective Reaction of Nitromethane with Substituted Benzaldehydes
ized ligand 6c, whose loading value, calculated by elemental Table 3. Enantioselective Henry reaction.[a]
–
1
analysis, was 0.6 mmolg .
Polymer 6c was complexed with Cu(OAc) ·H O, and so-
2
2
obtained solid chiral catalyst 7c was tested in the model
reaction; chiral nitroalcohol 4a was obtained in 90% yield
with 75%ee (Table 2, Entry 2). This promising result
prompted us to prepare catalyst 7c with different loading
–
1
values, ranging from 0.4 to 1.0 mmolg , to evaluate the
effect of the concentration of the active site on the reaction
efficiency by using the same active sites/para-chlorobenz-
aldehyde 5% ratio; a constant value of styrene/divinyl-
benzene ratio was utilized to obtain polymers with the same
cross-linking.
Yield [%][b]
ee [%][b,c]
Entry
R
1
2
3
4
5
6
7
4-Cl
3-Cl
2-Cl
H
4-Br
4-OMe
95
56
80
94
92
91
97
86
79
88
85
87
85
49
Table 2. Heterogeneous catalyst 7c activity in the model reaction.[a]
4-NO2
–1
Yield [%][b]
ee [%][b,c]
[a] Reaction conditions: Benzaldehyde (1 mmol), MeNO
10 mmol), 6c/Cu(OAc) ·H O (5% with respect to benzaldehyde),
2
Entry
Loading [mmolg ]
(
2
2
1
2
3
4
0.4
0.6
0.8
1.0
75
90
95
95
70
75
86
85
EtOH (2 mL). All the products were characterized and their spectra
compared with those reported.[9,12,17,28] [b] Determined by HPLC
analysis (Chiracel OD-H column, hexane/iPrOH = 85:15). [c] Ab-
solute configuration of the major enantiomer: R.
[
a] Reaction conditions: para-Chlorobenzaldehyde (1 mmol),
MeNO O (5% with respect to para-
chlorobenzaldehyde), EtOH (2 mL), r.t., 24 h. [b] Determined by
2
2 2
(10 mmol), 6c/Cu(OAc) ·H
electron-withdrawing and electron-donating groups gave
similar results (Table 3, Entries 4–6); the only exception was
para-nitrobenzaldehyde, which afforded a very high yield
but a low enantiomeric excess (Table 3, Entry 7). This be-
HPLC analysis (Chiracel OD-H column, hexane/iPrOH = 85:15).
[c] Absolute configuration of the major enantiomer: R.
–1
The optimum loading was found to be 0.8 mmolg : with havior can be ascribed to the fact that the reaction with this
this catalyst the yield increased to 95% and a good level of very activated benzaldehyde can proceed also in the absence
enantioselectivity was achieved (86%ee; Table 2, Entry 3).
of a catalyst; in fact, a blank experiment carried out simply
The real heterogeneity of the reaction was then evaluated by stirring a solution of para-nitrobenzaldehyde and nitro-
through the well-known Sheldon test[ by filtering the re- methane in ethanol afforded the racemic nitroalcohol in
action mixture after 9 h (50% yield): no increase in yield 38% yield. This uncatalyzed contribution, on the contrary,
was observed by stirring the filtrate for an additional 15 h. was not observed with other substituted benzaldehydes
In addition, the recovered solid catalyst, washed with etha- such as para-chloro- and para-bromobenzaldehyde.
nol and dried under vacuum, was reused under the same
27]
conditions, affording chiral alcohol 4a without any activity
loss over at least four cycles (Figure 2).
Conclusions
In conclusion we have reported that the immobilization
of chiral 2,2Ј-methylenebis[(4S)-4-tert-butyl-2-oxazoline]
onto a polystyrenic support results in a robust ligand that
after complexation with Cu(OAc) ·H O affords an efficient
2
2
catalyst able to promote the enantioselective Henry reaction
of a variety of benzaldehydes with nitromethane in good
yield and enantioselectivity. The catalyst can be utilized for
at least five runs without any loss in activity.
Experimental Section
Figure 2. Reusability of heterogeneous catalyst 7c.
Procedure for the Homogeneous Henry Reaction: In a typical pro-
2 2
cedure Cu(OAc) ·H O (0.05 mmol) and 2,2Ј-methylenebis[(4S)-4-
tert-butyl-2-oxazoline] (0.05 mmol) were dissolved under an atmo-
sphere of nitrogen in EtOH (2 mL). After 1 h, the selected benzal-
The reaction was then applied to some substituted aro-
matic aldehydes to test the applicability of the method
(Table 3).
dehyde (1 mmol) was added together with MeNO
meta-Chlorobenzaldehyde, being less activated than the the mixture was stirred at room temperature for 24 h. Then, the
corresponding para isomer, showed a lower yield and a solvent was evaporated, and the crude mixture was purified by flash
2
(10 mmol), and
2 2
lower level of enantioselectivity (Table 3, Entry 2); in the chromatography (CH Cl ). The obtained colorless oil was analyzed
by HPLC.
same way, when the chloride substituent is at the ortho posi-
tion, its steric hindrance lowered the reaction rate but main- Procedure for the Preparation of Heterogenized Ligand 6c: To a
tained a good enantiomeric excess (Table 3, Entry 3). Both stirred solution of ligand 5c (0.57 g, 0.0015 mol) in dry toluene
Eur. J. Org. Chem. 2011, 5551–5554
© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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5553

R. Maggi, D. Lanari, C. Oro, G. Sartori, L. Vaccaro
SHORT COMMUNICATION
(
0
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Received: May 19, 2011
Published Online: August 24, 2011
7, 2046–2049; k) J.-J. Jiang, M. Shi, Tetrahedron: Asymmetry
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