Effect of microwaves in the chiral switching asymmetric Michael reaction

Article abstract of DOI:10.3390/80200256

Highly enantioselective Michael reactions of malonates with cyclic enones are achieved in remarkably less time under microwave irradiation using newly developed heterobimetallic catalysts.

Full text of DOI:10.3390/80200256

Molecules 2003, 8, 256-262
molecules
ISSN 1420-3049
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Effect of Microwaves in the Chiral Switching Asymmetric
Michael Reaction†
S. Narasimhan* and S. Velmathi
No. 88, Centre for Natural Products, SPIC Science Foundation, Mount Road, Guindy, Chennai – 600
032, India
† This paper is dedicated to Prof H.C. Brown, a pioneer in Physical Organic Chemistry and
Asymmetric Synthesis on the occasion of his 90th birthday
*Author to whom correspondence should be addressed; e-mail: narasim3_@hotmail.com
Received: 20 May 2002; in revised form: 20 February 2003 / Accepted: 21 February 2003 / Published:
28 February 2003
Abstract: Highly enantioselective Michael reactions of malonates with cyclic enones are
achieved in remarkably less time under microwave irradiation using newly developed
heterobimetallic catalysts.
Keywords: Michael reaction, Al - complex, chiral switch, microwave irradiation
Introduction
Earlier, we have synthesized from readily available salicylaldehyde and L-valine the novel chiral
catalysts 1 and 2a-d, which displayed an interesting phenomenon of enantiomer switching in
asymmetric reductions of prochiral ketones [1] and asymmetric Michael reactions [2] (Scheme 1).
Catalytic asymmetric Michael reactions using chiral amines [3], alkaloids [4], polymer bound alkaloids
[5], chiral crown ethers and bases [6], optically active Co (II) complexes [7], natural proteins [8] and
amino alcohols [2] as catalysts have been extensively studied. A breakthrough in catalytic asymmetric
Michael addition reactions was achieved with the introduction of heterobimetallic catalysts. In a series
of reports Shibasaki and coworkers [9] elaborated the utility of such heterobimetallic catalysts of

Molecules 2003, 8
257
BINOL –La (Al)-alkali metals in bringing about highly enantioselective Michael addition reactions,
however, the Michael reaction usually requires prolonged reaction times.
It is well known that ultrasound or microwaves [10] can accelerate the reaction rates in normal
reactions. We were interested in exploring such techniques in the Michael addition reaction and their
effect on enantioselectivity. Accordingly we modified a domestic microwave oven to allow the
reactions to be carried out in a flask attached to a reflux condenser using a wide variety of solvents as
the fire hazard is significantly reduced compared to an open vessel system.
Scheme 1
R
R
R
N
H
N
N
H
H
OCH3
HO
OH
HO
O
OH
OH
2
2a-d
1a-d
LiAlH4
1 eq.BH3
LiAlH4
Complex
Complex
(3)
Complex
(4)
1) 1 eq. BH3
2) prochiral ketone
Prochiral ketone
1.Enone
1.Enone
2.Malonate
2.Malonate
R-Enantiomer
R-Enantiomer S-Enantiomer
S-Enantiomer
90 % ee
60 % ee
> 80 % ee
60 % ee
1a R = isopropyl, 1b R= iso butyl, 1c R= sec butyl, 1d R = benzyl
When the asymmetric Michael reaction of cyclohexenone and diethyl malonate was carried out
with the aluminium complex (3), the reaction was complete in 4 hours yielding the S-isomer in more
than 80% ee, whereas the aluminium complex (4) gave the R – isomer in 60 % ee [2].
R
R
O
NH
Al
N
(4)
O
Al
O
O
O CH3
+Li
2
(3)

Molecules 2003, 8
258
Interestingly the microwave assisted asymmetric Michael reaction using the aluminium complexes
3 and 4 was remarkably accelerated without altering the chiral switching nature (Scheme 2). Thus,
cyclic enones when treated with various dialkyl malonates under microwave irradiation produced the
adducts in less than 5 minutes. The results are presented in Table 1
Scheme 2
O
O
O
CO2
CO2
R
R
H
4
3
+
(n )
THF,MW
H
THF,MW
(n)
CH (CO2R)2
(n)
CH (CO2R)2
R - isomer
S - isomer
Table 1. Asymmetric Michael additions of cyclic enones with malonates under microwave irradiation.
Time
M W
Yield (%)
ee (%)b
0
a
[
]
D
Malonate
(R)
Catalyst
Enone
(c=5 in
CHCl3)
RT
(h)
RT M W RT M W
(Min)
-25.8
-2.9 c
-26.9
-2.72
n = 1
n = 1
n = 1
n = 2
n = 2
n = 2
n =2
Ethyl
Isopropyl
Benzyl
Ethyl
4
6
5
4
5
4
4
6
5
2
3
2
2
3
2
2
3
2
80
74
85
82
70
86
79
75
80
85
75
88
82
70
85
80
70
85
73
-
78
-
3
3
3
3
3
3
4
4
4
82
80
43
84
60
37
55
80
78
40
80
60
40
50
d
-2.1
Isopropyl
Benzyl
Ethyl
-0.95
+2.14
n=2
Isopropyl
Benzyl
+2.05
+0.59
n =2
MWI was carried out in pulses of 1minute duration with 30 sec intervals; (b) ee
was determined based on the optical rotation values available in the literature [9];
a
(c) Literature [ ]D was not available; (d) See [3a]
The results indicate that using microwaves comparable enantioselectivity could be achieved in a
shorter period of time (2 min) in excellent yields. Also, chiral switching which occurred under room
temperature conditions was reproduced under microwave conditions.

Molecules 2003, 8
259
a b
The study was extended to include acyclic , -unsaturated ketones and aldehydes (Scheme 3).
The reaction times and yields are given in Table 2. Although the reaction was complete in a shorter
period, resulting in 90 % yield of the Michael adduct, the expected enantioselectivity could not be
achieved. This may be attributed to the loss of rigidity in the acyclic system, since the same reaction
when carried out under normal conditions also resulted in a racemic product.
Scheme 3
O
O
O
R"
CH(CO2R)2
OR
OR
3 or 4
THF, MW
H2C
R"
H
+
R'
R'
O
a b
Table 2, Michael additions of acyclic , -unsaturated enone with dialkyl malonates
under microwave irradiation.
Time
Yield (%)
Enone
Malonate (R)
RT (h) MW (Min) RT MW
R'=R"=Ph
R'=R"=Ph
Ethyl
Isopropyl
Benzyl
Ethyl
6
7
5
4
3
7
7
8
7
70
65
75
75
72
80
75
72
68
70
70
72
75
72
R'=R"=Ph
10
5
R'=Ph, R"=H
R'=Ph, R"=H
R'=CH3, R"=H
R'=CH3, R"=H
Benzyl
Ethyl
7
4
Benzyl
6
In order to prove the versatility of the catalyst and the application of microwaves, thiols were also
used as Michael donors, which is a useful methodology for C-S bond formation (Scheme 4). The
Michael adducts are obtained in very good yield within remarkably reduced reaction times (1 minute).
Results are presented in Table 3.
Scheme 4
O
O
R"
R"
3 or 4
THF, MW
+ HS
R
H
R'
R'
S
R

Molecules 2003, 8
260
a b
Table 3. Michael additions of thiols with acyclic and cyclic , -unsaturated enones
Time (Min)
Yield (%)
S.No
Enone
Thiol (R)
RT
MW
RT M W
1
2
3
4
5
R'=R"=Ph
R'=R"=Ph
R'=R"=Ph
Cyclohexenone
“
H
2
1
90
88
95
92
90
88
90
95
90
92
CH3
2
2
CH3COSH
30 sec
30 sec
CH3
H
1
2
1
1
The fact that the products obtained in the microwave assisted Michael reactions exhibit no change
in the ee when compared with the same reaction carried out under normal conditions, indicates the
“isoentropic” nature of these reactions.
Conclusions
The newly developed heterobimetallic catalysts along with the microwave technology provide
better methodology for asymmetric Michael reaction for a variety of substrates in lesser reaction time.
Acknowledgements
One of the authors (S.V) thanks CSIR for financial assistance
Experimental
General
The solvents and chemicals used in this work were obtained commercially and were purified using
conventional methods. All reactions were carried out in a modified domestic Microwave oven (IFB
model 750W). The NMR spectra were recorded on BRUKER 200 MHz instrument. Optical rotations
°
were recorded using an Autopol polarimeter using CHCl as solvent at 23 C using the sodium D-line
3
°
monochromator (2580 A )
Procedure for the modification of the domestic microwave oven
The commercial domestic microwave oven used in our laboratory was provided with a facility for
inserting a reflux condenser and magnetic stirrer (Table 4).

Molecules 2003, 8
261
Table 4 . Modifications effected in the domestic microwave oven
Domestic Microwave oven
1. Rotating plate
Modified Microwave oven
1. Removed and magnetic stirrer is fitted
2. Provision made for inserting a reflux
condenser (B19 size)
2. Leak proof top cover
3. Microwave source (both from the
side and top)
3. Microwave source from one side only.
General experimental procedure for Michael addition reaction using Al – Complex 3
In an oven dried side arm flask closed with a septum and containing LiAlH4 (0.038 g, 1 mmol) in
dry THF (2 mL) was added 3 (0.237 g, 1 mmol) in dry THF (3 mL) at room temperature. The resulting
mixture was stirred for one hour. To the stirred solution 2-cyclohexenone (0.490g, 5 mmol) and diethyl
malonate, (0.800 g, 5 mmol) were added and the mixture was irradiated in a microwave oven for the
stipulated period of time. The reaction was quenched with dil. HCl and extracted with ethyl acetate
(3x10 mL). The organic layer was washed with water, followed by saturated. NaHCO3 solution. The
combined organic layers were dried over anhydrous Na2SO4 and the solvent were removed under
o
reduced pressure to yield the product. (1.1 g. 90 % yield, [ o]=-2.3 78 % ee ). All the products were
a
D
identified and characterized by their H and 13C-NMR spectral data and found identical with the
1
literature reported values [11].
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