Enantioselective Mukaiyama-Michael reaction of silyl enol ethers to 2-enoylpyridine N-oxides catalyzed by copper-bis(oxazoline) complex

Article abstract of DOI:10.1002/adsc.201200631

A catalytic enantioselective Mukaiyama-Michael reaction of 2-enoylpyridine N-oxides has been developed using a simple bis(oxazoline)-copper complex. A variety of silyl enol ethers undergo smooth Michael addition with 2-enoylpyridine N-oxides to furnish the corresponding Michael adducts in high yields with high enantioselectivities (up to 97% ee).

Full text of DOI:10.1002/adsc.201200631

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DOI: 10.1002/adsc.201200631
Enantioselective Mukaiyama–Michael Reaction of Silyl Enol
Ethers to 2-Enoylpyridine N-Oxides Catalyzed by Copper-
Bis(oxazoline) Complex
Jimil Georgea and Basi V. Subba Reddya,*
a
Natural Product Chemistry, CSIR–Indian Institute of Chemical Technology, Hyderabad – 500007, India
Fax : (+91)-40-2716-0512; e-mail: basireddy@iict.res.in
Received: July 18, 2012; Revised: October 25, 2012; Published online: January 16, 2013
This paper is dedicated to Dr. J. S.Yadav on the occasion of his birthday.
Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/adsc.201200631.
Abstract: A catalytic enantioselective Mukaiyama–
Michael reaction of 2-enoylpyridine N-oxides has
been developed using a simple bis(oxazoline)-
copper complex. A variety of silyl enol ethers un-
dergo smooth Michael addition with 2-enoylpyri-
dine N-oxides to furnish the corresponding Michael
adducts in high yields with high enantioselectivities
(up to 97% ee).
system for the enantioselective Mukaiyama–Michael
reaction still remains as a challenging task. Recently,
Pedro et al. have explored 2-enoylpyridine N-oxides
as efficient bidentate substrates for various enantiose-
lective transformations[14] including the Friedel–Crafts
reaction and Michael addition reactions of dialkylmal-
onates.[15] More recently, Faita et al. have developed
an elegant approach for the addition of cyclic enol
silyl ethers to 2-alkenoylpyridine N-oxides using
Cu(II)-bis(oxazoline) complexes.[16]
Because of the electron-withdrawing nature of the
pyridine N-oxide moiety, it is reactive and effective in
conjugate addition reactions. Inspired by the inherent
properties of enoylpyridine N-oxides, we were inter-
ested to explore these substrates for the Mukaiyama–
Michael reaction with less reactive silyl enol ethers.
Keywords: asymmetric catalysis; bis(oxazoline) li-
gands; copper complexes; Michael reaction; silyl
enol ethers
The Mukaiyama–Michael addition is one of the most Furthermore, pyridine N-oxides can easily be cleaved
À
simple C C bond forming reactions to generate the into the corresponding carboxylic acids under mild
1,5-dicarbonyl compounds, which are important build- conditions.[17]
ing blocks for the synthesis of various biologically
Following our interest in asymmetric synthesis
active molecules.[1] Therefore, the Michael reaction using bis-oxazoline ligands,[18] we herein report a cata-
has emerged as a powerful synthetic route for con- lytic enantioselective Mukaiyama–Michael reaction of
structing various molecules.[2] Consequently, a large 2-enoylpyridine N-oxides. Initially, we attempted the
number of organocatalysts has been developed for Michael addition of the silyl enol ether derived from
this transformation.[3] In most cases, an unmodified acetophenone 5a with 2-enoylpyridine N-oxide 4a
ketone has been used as substrate. Among several using the bis(oxazoline)-CuACTHNUTRGNEUNG(OTf)2 complex. To opti-
Michael acceptors, nitrostyrenes,[4] alkylidene malo- mize the reaction, a set of chiral bis(oxazoline) li-
nates,[5] vinyl phosphonates,[6] vinyl sulfones[7] vinyl gands was tested (Figure 1). Most of the bis(oxazo-
ketones,[8] and a,b-unsaturated thiol esters are the line)-Cu
ACTHUNTRGNE(UGN OTf)2 complexes were found to catalyze the
most widely used substrates.[9] Although several cata- reaction at room temperature in excellent yields but
lysts are known for the direct Michael reaction, only with low to moderate enantiomeric excess. Among
few catalysts are reported for the Mukaiyama–Mi- them, the ligands 1a and 1b in combination with
chael reaction of silyl enol ethers with enones.[10,11] In Cu
ACHTUNGTRENNUNG
AHCTUNGTRENNUNG
AHCTUNGTRENNUNG
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Table 2. Screening of various parameters in the enantioselec-
tive Mukaiyama–Michael reaction of 4a with silyl enol ether
5a.[a]
Figure 1. Bisoxazoline ligands.
Table 1. Screening of various bis(oxazoline) ligands in Mu-
kaiyama–Michael reaction between silyl enol ether 5a and
enoylpyridine N-oxide 4a.[a]
[a]
All the reactions were carried out using 1.0 mmol 4a and
2.0 mmol of 5a in the presence of 10 mol% 1d-CuACHTUNGTRENNUNG(OTf)2
in 2.0 mL solvent.
Yield after purification.
The ees were determined by HPLC analysis
5 mol% of the catalyst was used.
2 mol% of the catalyst was used.
[b]
[c]
[d]
[e]
3b) also gave the product with low ee values (Table 1,
entries 6 and 7).
[a]
All the reactions were carried out using 1.0 mmol of 4a
To improve the ee, various reaction parameters
were screened and the results are presented in
Table 2. Initially, we have examined the effect of reac-
tion temperature on enantioselectivity. By lowering
the reaction temperature from 08C and À508C, a con-
siderable enhancement in enantiomeric excess was
observed from 80 to 96% ee (Table 2, entry 3). No im-
provement in enantiomeric excess was observed by
and 2.0 mmol of 5a in the presence of 10 mol% of the
catalyst in 2 mL of dichloromethane at room tempera-
ture.
Yield after purification.
The ees were determined by HPLC analysis.
[b]
[c]
selectivity (70% ee, Table 1, entry 4). To our surprise, further lowering the reaction temperature. Among
aminoindanol-derived ligand 2 and Cu (OTf)2 was found
A
ACHTUNGTRENNUNG
gave the product with very low ee (Table 1, entry 5). to be the best choice for the Mukaiyama–Michael re-
Similarly, sugar-derived glucoBOX ligands (3a and action to obtain good yields and enantioselectivity.
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Enantioselective Mukaiyama–Michael Reaction of Silyl Enol Ethers to 2-Enoylpyridine N-Oxides
Table 3. Scope of various 2-enoylpyridine N-oxides and silyl
Among various solvents (chloroform, tetrahydrofur-
an, diethyl ether and toluene) tested, dichlorome-
thane gave the best results in terms of yields and
enantioselectivity (Table 2, entry 3 vs. entries 4–9). As
shown in Table 2, 5 mol% of the catalyst is optimal to
provide the best results (Table 2, entry 10).
enol ethers in the enantioselective Mukaiyama–Michael re-
action.[a]
After optimizing the reaction conditions, we ex-
tended this method to various enoylpyridine N-oxides
and silyl enol ethers (Table 3). Accordingly, we at-
tempted to study the effect of substituents on the aro-
matic ring of the enoylpyridine N-oxides. Interesting-
ly, all the p-substituted arylidenoylpyridine N-oxides
reacted effectively with silyl enol ether 5a to afford
the corresponding products in good yields with high
enantioselectivities (Table 3, entries 2–6). Among p-
substituted enoylpyridine N-oxides, the best ee value
of 96% was obtained with the substrate 4d (Table 3,
entry 4). However, o-substituted enoylpyridine N-
oxides 4g and 4h gave the corresponding products
with lower enantioselectivities than the para counter-
part (Table 3, entries 7 and 8, 84% and 82%, respec-
tively).
Next an attempt was made to examine the reactivi-
ty of meta-substituted arylidenoylpyridine N-oxides.
In the case of meta-substituted substrates, the ee was
slightly higher than with para-substituted benzene de-
rivatives. The substrates 4i and 4j reacted effectively
with enol ether 5a and the corresponding products 6i
and 6j were obtained with high ee values of 97% each
(Table 3, entries 9 and 10). However, the reaction of
2-furyl-substituted enoylpyridine N-oxide 4k with 5a
gave the product 6k in relatively low yield and enan-
tiomeric excess (80% yield with 83% ee, Table 3,
entry 11). Notably, a sterically bulky substrate, i.e., 2-
naphthyl-substituted N-oxide 4l gave the Michael
adduct 6l with excellent ee (93%, Table 3, entry 12).
Furthermore, we examined the scope of the Mu-
kaiyama–Michael reaction of alkylidenoyl pyridine N-
oxides. The reaction of tert-butyl substituted enoyl N-
oxide 4m with the silyl enol ether 5a gave the product
in 88% yield with 91% ee (Table 3, entry 13). To our
surprise, the reaction of cyclohexyl-substituted N-
oxide 4n with enol ether 5a was found to be sluggish
and only a trace amount of the product was obtained
after a long reaction time (Table 3, entry 14). Further-
more, 1,4-addition of enol ether 5a with enoylpyridine
N-oxide 4o gave the product 6o in 80% yield with
72% ee (Table 3, entry 15). It is noteworthy to men-
tion that the conjugated substrate 4o underwent
a smooth 1,4-addition exclusively rather than 1,6-ad-
dition under the present reaction conditions.
[a]
All the reactions were carried out on a 1.0-mmol scale in
2.0 mL solvent using 5 mol% of the catalyst with
2.0 equiv. of silyl enol ether.
After purification.
Determined by HPLC analysis
Subsequently, we examined the scope of various
silyl enol ethers in the Mukaiyama–Michael reaction
with 4a. It was clearly observed that the silyl enol
ethers derived from electron-rich acetophenones only
participated in this enantioselective Mukaiyama–Mi-
chael reaction. For example, silyl enol ethers such as
[b]
[e]
[d]
nd=not determined.
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Jimil George et al.
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Scheme 1. Determination of absolute stereochemistry of compound 6a.
5b, 5c and 5d reacted well with 4a to afford the corre-
sponding products 6p, 6q and 6r in good yields with
ee values of 82, 80 and 75%, respectively (Table 3, en-
tries 16, 17 and 18).
Furthermore, triethylsilyl enol ether 5e and tert-bu-
tyldimethylsilyl enol ether 5f also reacted with enoyl-
pyridine N-oxide 4a under similar reaction conditions.
But the reactions were slow in comparison with those
of trimethylsilyl enol ethers. For instance, the reaction
of 5e with 4a gave the product 6a in good yield with
91% ee (Table 3, entry 19). However, TBS enol ether
5f was found to be less reactive under the optimized
conditions and the product 6a was obtained in low
yield 35% with 85% ee (Table 3, entry 20). It is note-
worthy to mention that the reactivity of various silyl
enol ethers is in the order of TMS>TES>TBS.
Subsequently, we focused on the determination of
the absolute stereochemistry of the Mukaiyama–Mi-
chael products. Accordingly, we converted the prod-
uct 6a to a known ester 7. The cleavage of the pyri-
dine N-oxide moiety of product 6a with KOH gave
the acid which was then esterified with methyl iodide
to furnish the methyl ester 7 (Scheme 1). The optical
rotation of the methyl ester 7 was then compared
with that of a known ester reported in the litera-
ture.[20]
Figure 2. Plausible transition state models
Next an attempt was made to formulate a transition top face but the product forms predominately as an
state model to explain the stereochemical outcome of (S)-isomer (Figure 2b). The stereochemical outcome
the enantioselective Mukaiyama–Michael reaction of the reaction clearly indicates that the reaction pro-
(Figure 2). From the stereochemical outcome, it is ceeds through a tetrahedral transition state as depict-
clear that the product was obtained as an (R)-isomer. ed in Figure 2a. Conversely, the Cu(II)-BOX-cata-
As per Jørgensen et al.ꢁs observation, the Ph-BOX- lyzed enantioselective Mukaiyama–Michael reaction
copper complex is not exactly square planar but of enol silanes also proceeds through a cyclic transi-
rather shows a static equilibrium between square tion state which would transform into the product as
planar and tetrahedral geometry.[21] The flexibility of shown in Figure 2a.
the Cu(II)-Ph-BOX complex from a square-planar to
In conclusion, we have developed a catalytic enan-
a tetrahedral intermediate plays a crucial role in de- tioselective Mukaiyama–Michael reaction of silyl enol
termining the stereochemistry of the product. Thus, ethers with 2-enoylpyridine N-oxides using the Cu-
we assume that the Cu(II)-Ph-BOX complex co-ordi-
ACHTUNGRTEN(NUNG OTf)2-bis(oxazoline) complex. The reaction was suc-
nated to the substrate is not in a square planar but cessful with a large number of enoylpyridine N-oxides
rather in almost tetrahedral geometry (Figure 2a). In and silyl enol ethers. Further applications of bis(oxa-
the case of tetrahedral geometry, the silyl enol ether zoline) ligands for asymmetric synthesis are being
attacks from the top face which leads to the formation studied in our laboratory.
of a product with (R)-configuration. If the substrate
co-ordinates with Cu(II)-Ph-BOX in a square-planar
geometry, the silyl enol ether still attacks from the
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Adv. Synth. Catal. 2013, 355, 383 – 388

Enantioselective Mukaiyama–Michael Reaction of Silyl Enol Ethers to 2-Enoylpyridine N-Oxides
sell, S. Lee, D. W. C. MacMillan, Chem. Sci. 2010, 1,
Experimental Section
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Typical Procedure for Enantioselective Mukaiyama–
Michael Reaction
A solution of a ligand 1d (17 mg, 0.050 mmol) and CuACTHNUTRGNEUNG(OTf)2
(18 mg, 0.050 mmol) in dry dichloromethane (2.0 mL) was
stirred at room temperature for 1 h under a nitrogen atmos-
phere. To this solution, 2-enoylpyridine N-oxide 4a
(1.0 mmol) was added. The resulting mixture was stirred at
room temperature for 10 min and then cooled to À508C. To
this mixture, a solution of silyl enol ether 5a (2.0 mmol) in
0.5 mL dichloromethane was added slowly and then allowed
to stir at À508C until completion of the reaction (as judged
by TLC analysis). After completion of the reaction, TBAF
(1.0 mmol) was added slowly to the reaction mixture at the
same temperature and then the mixture was stirred for an-
other 30 min at the same temperature. The mixture was
then allowed to warm to room temperature. The solvent was
removed and the resulting mixture was purified by column
chromatography on silica gel using hexane:ethyl acetate
mixture as eluent to afford the pure product 6a as a solid;
yield: 93%; 96% ee; mp 102-1048C; [a]2D5: +49.2 (c 0.4,
CHCl3). The optical purity was determined by chiral HPLC
(Daicel Chiralpak OD-H column, hexane/i-PrOH=70:30,
flow rate 1.0 mLminÀ1, 254 nm): tR =20.29 min (R) and
1
22.96 min (S); H NMR (500 MHz; CDCl3): d=3.34 (dd, J=
6.6, 17.0 Hz. 1H), 3.40 (dd, J=6.6, 17.0 Hz, 1H), 3.62 (dd,
J=6.6, 10.5 Hz, 1H), 3.70 (dd, J=6.6, 10.5 Hz, 1H), 4.20 (t,
J=5.2 Hz, 1H), 7.10 (dd ~t, J=6.6, 10.5 Hz, 1H), 7.18 (dd ~
t, J=6.6, 10.5 Hz, 3H), 7.26 (dd, J=7.8, 5.2 Hz, 3H), 7.38 (t,
J=7.9 Hz, 4H), 7.50 (dd, J=6.6, 7.9 Hz, 1H), 7.88 (d, J=
7.9 Hz, 2H); 13C NMR (75 MHz; CDCl3): d=36.8, 44.9,
48.7, 126.5, 127.4, 128.4, 132.9, 136.7, 140.3, 143.5, 146.5,
146.9, 196.1, 198.1; IR (neat): u=3448.4, 2924.5, 1683.6,
1429.5, 1258.6, 1158.0, 1032.1, 995.5, 760.02 cmÀ1; HR-MS
(ESI): m/z=368.1259, calcd. for C22H19NO3Na: 368.1263.
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