Nucleophilic carbene-catalyzed redox-esterification reaction of α-halo-α,β-unsaturated aldehyde

Article abstract of DOI:10.1016/j.tet.2012.05.109

A nucleophilic carbene catalyzed redox esterification between α-halo-α,β-unsaturated aldehydes and various alcohols has been developed. Interestingly, the reaction provided α,β-unsaturated esters instead of the saturated α-halo substituted esters as the only product in good to high yield with excellent trans-selectivity, presumably via the umpolung-halo-elimination pathway.

Full text of DOI:10.1016/j.tet.2012.05.109

Tetrahedron 68 (2012) 6498e6503
Contents lists available at SciVerse ScienceDirect
Tetrahedron
journal homepage: www.elsevier.com/locate/tet
Nucleophilic carbene-catalyzed redox-esterification reaction
of a-halo-a,b-unsaturated aldehyde
*
Xiang-Bo Wang, Xiao-Lei Zou, Guang-Fen Du , Zhi-Yong Liu, Bin Dai
Key Laboratory for Green Processing of Chemical Engineering of Xinjiang Bingtuan and School of Chemistry and Chemical Engineering,
Shihezi University, Xinjiang 832000, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A nucleophilic carbene catalyzed redox esterification between
a
-halo-
a
,
b
-unsaturated aldehydes and
-unsaturated esters instead
-halo substituted esters as the only product in good to high yield with excellent trans-
selectivity, presumably via the umpolung-halo-elimination pathway.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 20 February 2012
Received in revised form 18 May 2012
Accepted 25 May 2012
various alcohols has been developed. Interestingly, the reaction provided
of the saturated
a,b
a
Available online 1 June 2012
Keywords:
Nucleophilic carbene
Redox-esterification
a
a
-Halo-
a,b-unsaturated aldehydes
,
b-Unsaturated esters
1. Introduction
2. Results and discussion
Our investigation commenced with the reaction of
Polarity reversal of novel functional group (umpolung) is a ver-
satile strategy in organic chemistry that allows transformations in
unconventional way.1 In the last decade, N-heterocyclic carbene
(NHC) mediated catalytic umpolung reactions have received con-
siderable attention.2 Based on this strategy, various reactions, such
as benzoin reaction3 and Stetter reaction,4 homoenolate trans-
formations and other reactions5 have been explored extensively.
a-bromo-
cinnamaldehyde and ethanol in the presence of catalytic amount of
1, 3-dimesitylimidazol-2-ylidene that generated from NHC pre-
cursor 1 and diisopropylethylamine in dry THF (Table 1, entry 1). To
our surprise, ethyl cinnamate 6 was obtained in very low yield after
40 h. We reasoned that HBr generated in the redox reaction may
neutralize the active NHC, thus, the reaction was stopped and gave
6 in low yield. In view of this, we then kept the catalyst loading and
increased base amount to 1.2 equiv, to our delight, 6 was isolated in
75% yield as the only product (Table 1, entry 2). Encouraged by this
result, several different types of NHCs generated from the corre-
sponding azolium salts and base were screened (Table 1,
entries3e7). It was found that all the NHCs screened for the re-
action can catalyze the redox reaction, but only in low efficiency.
After evaluation of other reaction parameters, such as base, solvents
and catalyst loading (Table 1, entries 8e18). optimal reaction con-
ditions were obtained: room temperature, 0.25 M in THF, 5 mol %
IMes, 1.2 equiv DBU (Table 1, entry 16).
Under standard reaction conditions, a variety of alcohols were
tested for the reaction and the results were summarized in Table 2.
Primary alcohols proceed smoothly to give the desired esters in
high yield with excellent stereoselectivity (entries 1e4). While
secondary alcohols need more time and afforded the corresponding
products in good yield (entries 5, 6). As anticipated, due to sterical
hindrance, no desired product was obtained with tert-butyl alcohol
And recently, redox esterification and amination of
a,b-unsaturated
aldehydes,6 -haloaldehydes7 and other functionalized aldehydes8
a
were developed through ‘extended umpolung’ of carbonyl com-
pounds, giving the corresponding products efficiently. However, in
contrast to saturated esters, NHC-mediated redox esterification to
unsaturated esters9 were farless examined. Recently, Zeitler10
reported that NHC can be used to promote the redox esterifica-
tion of alkynyl aldehydes to afford trans-a,b-unsaturated esters
with excellent E/Z stereoselectivity. As part of our continued efforts
on the development of NHC-catalyzed reactions,11 we found that
NHC can catalyze the redox esterification of
a-halo-a,b-unsaturated
-unsaturated esters in high
aldehydes12 efficiently, providing
a
,b
yield with excellent stereoselectivity. Herein, we would like to
disclose our preliminary results about this research.
* Corresponding author. E-mail address: duguangfen@shzu.edu.cn (G.-F. Du).
0040-4020/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tet.2012.05.109

X.-B. Wang et al. / Tetrahedron 68 (2012) 6498e6503
6499
Table 1
Evaluation of NHCs and conditions for redox esterificationa
10 mol% catalyst
C2H5OH (3 equiv)
CO2Et
O
Ph
Ph
Br
6
1.2 eq base
5a
(1.0 eq)
solvent, rt, 24h
Cl
Ar
Cl
Ar
N
N
N
N
Ar
Ar
2
1
1a
: Ar = 1, 3, 5-Me3C6H2
: Ar = 2, 6-(i-Pr)2C6H3
Ar = 2, 6-(i-Pr)2C6H3
1b
N
HO
X
N
Cl
Ph
N
S
N
R
4
3a
: R = Bn, X = Cl
3b
: R = Me, X = I
Entry
NHC
Solvent
Time (h)
Yield (%)b
Ratio E/Zc
1d
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16e
17f
18g
1a, DIPEA
1a, DIPEA
1b, DIPEA
2, DIPEA
3a, DIPEA
3b, DIPEA
4, DIPEA
1a, DBU
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
THF
Et2O
Toluene
CH2Cl2
THF
THF
40
24
24
24
24
24
24
17
40
40
40
18
24
18
24
17
17
18
<10
75
28
6
31
40
8
86
78
<5
68
18
75
75
45
84
24
61
/
>20:1
>20:1
/
>20:1
5:1
/
>20:1
>20:1
/
1a, Et3N
1a, Pyridine
1a, DMAP
1a, t-BuOK
1a, DBU
1a, DBU
1a, DBU
1a, DBU
1a, DBU
1a, DBU
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
>20:1
THF
The bold values highlight the standard reaction conditions for the reaction.
a
All reactions were conducted on a 0.5 mmol scale at 0.25 M in THF, with 1.2 equiv of diisopropylethylamine (DIPEA) as base.
b
c
d
e
f
Isolated yields after chromatography.
Determined by 1H NMR analysis of crude reaction mixtures.
Using 10 mol % base.
Using 5 mol % 1a.
Using 1 mol % 1a.
g
Using 1.5 equiv ethanol.
Table 2
Survey of nucleophiles in redox esterificationa
O
R1
R1
1b, 1.2 eq DBU
O
OR
X
ROH (3 eq)
THF, rt
5 (1 eq)
Entry
1
Aldehydes
Alcohols
Productb
O
O
O
CH3CH2OH
Br
6, 87%, E/Z > 20:1
O
O
O
O
2
3
CH3OH
Br
Br
7, 88%, E/Z > 20:1
O
O
CH3(CH2)2CH2OH
8, 85%, E/Z > 20:1
(continued on next page)

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X.-B. Wang et al. / Tetrahedron 68 (2012) 6498e6503
Table 2 (continued )
Entry
Aldehydes
Alcohols
Productb
O
O
O
O
O
O
O
4
BnOH
Br
Br
Br
Br
9, 81%, E/Z > 20:1
O
O
5
(CH3)2CHOH
Cyclohexanol
(CH3)3COH
PhOH
10, 61%, E/Z > 20:1
O
O
6
11, 74%, E/Z > 20:1
7c
/
O
O
8
Br
12, 74%, E/Z > 20:1
O
O
O
O
9
Allyl alcohol
Br
Br
13, 87%, E/Z > 20:1
O
O
10
Propargyl alcohol
14, 72%, E/Z = 10:1
O
O
O
11
12
Cinnamyl alcohol
Br
15, 70%, E/Z > 20:1
O
O
O
O
C2H5OH
Br
MeO
O2N
MeO
O2N
Cl
16, 91%, E/Z > 20:1
O
O
13d, e
C2H5OH
C2H5OH
C2H5OH
Br
Br
17, 80%, E/Z > 20:1
O
O
O
14d
Cl
18, 79%, E/Z > 20:1
O
O
O
15d, e
Br
NO2
NO2
19, 71%, E/Z > 20:1

X.-B. Wang et al. / Tetrahedron 68 (2012) 6498e6503
6501
Table 2 (continued )
Entry
Aldehydes
Alcohols
Productb
O
O
16
17
C2H5OH
C2H5OH
O
O
O
Br
20, 78%, E/Z > 20:1
O
NDf
Br
O
O
O
O
O
O
18
19
20
21
22g
C2H5OH
Cl
Cl
Cl
Cl
6, 85%, E/Z > 20:1
O
O
CH3OH
7, 80%, E/Z > 20:1
O
O
BnOH
9, 88%, E/Z > 20:1
O
O
(CH3)2CHOH
10, 58%, E/Z > 20:1
O
CHO
Br
C2H5OH
Br
5a, 80%
Br
O
O
6, 12%
O
O
CHO
Br
23h
C2H5OH
Br
6, 76%, E/Z > 20:1
a
All reactions were performed on a 0.5 mmol scale, the ratio of E/Z >20:1 for all the reactions based on the 1H NMR analysis of crude reaction mixtures.
Isolated yield.
Performed at 40 ꢀC.
Using 1.2 equiv Et3N instead of DBU.
Performed at 60 ꢀC.
No desired product was detected.
Using 1.2 equiv DBU.
Using 2.2 equiv DBU.
b
c
d
e
f
g
h
as nucleophile (entry 7). Interestingly, phenol was also proved to be
exhibit good conversion with high stereoselectivity in the reaction
(entry 16). However, when -Alkyl substituted -haloenal was ap-
good substrate, gave phenyl ester in 76% yield (entry 8). It is
worthwhile noting that alkene and alkyne groups were also well
tolerated in the reaction and provided the desired esters in high
yield (entries 9e11). On the other hand, we were pleased to find
b
a
plied in the redox esterification, no desired product was obtained
(entry 17). Chlorine substituted cinnamaldehyde were also found to
be very good electrophile for the reaction, which can react with
alcohols efficiently and afford the corresponding products in good
that the redox reaction to be general for a range of
a-halo-a,b-un-
saturated aldehydes.13 Both electron-donating and withdrawing
a
-
to high yield (entries 16e19). a,a
-Dibromoaldehyde14 was also
bromocinnamaldehydes were suitable candidates for the esterifi-
cation and higher yields were obtained for the former (entries
tested for the reaction and the elimination product 5a (80%), in-
stead of ester 6 (12%), was isolated as the major product (entry 20).
But when the base amount was increased to 2.2 equiv, ester 6 was
12e15).
b-Heteroaryl substituted a-halocinnamaldehyde also

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X.-B. Wang et al. / Tetrahedron 68 (2012) 6498e6503
CHO
Mes
N
X
R
O
R'O
OR'
R
IV
N
X
path A
Mes
base HX
OH Mes
N
Mes
N
H
O
O
R
R
X
N
N
X
OR'
O
R
Mes
III
Mes
I
NHC
R'O
OH Mes
N
Mes
N
path B
R
R'OH
base HX
R
N
X
N
Mes
H
Mes
V
II
Fig. 1. Possible catalytic cycle.
obtained in good yield as the only product, this maybe due to fur-
ther catalytic redox conversion of 5a to 6 under excess of base
(entry 21).
(200e300 mesh) and analytical TLC on silica gel 60-F254. 1H NMR
(400 MHz), 13C NMR (100 MHz) spectra were recorded on a Varian
Inova-400 spectrometer in CDCl3, with tetramethylsilane as an
Based on the pioneering work on NHC catalyzed redox re-
actions, one plausible mechanism is proposed and depicted in Fig.1.
Initially, the addition of NHC to aldehyde results in the generation
of ‘Breslow intermediate’ I,15 which isomerizes to homoenolate II
internal standard and reported in parts per million (d). EI mass
spectra were measured on Agilent 7890A/5975C GCeMS and
methanol or acetonitrile was used to dissolve the sample. All al-
cohols were distilled before to use. a-Bromo-a,b-unsaturated al-
and the activated carboxylate III will be formed after
b-protonation
dehydes prepared according to literature.1 Other starting materials
were obtained from commercial supplies and used as received.
Anhydrous THF, ether, toluene, Et3N were distilled from sodium.
Petroleum ether (PE), where used, has a boiling range of 60e90 ꢀC.
of II, and the subsequent addition of alcohol to give the saturated
a
-
haloester IV with release of NHC. Under basic reaction conditions,
the elimination of IV lead to the formation of the final product (path
A), this conclusion is confirmed by control experiment. The results
indicate that
action conditions or under simply basic conditions, which undergo
elimination reaction to afford -unsaturated ester efficiently
a-haloester IV is unstable under the above redox re-
4.2. General procedure for preparation of
esters
a,b-unsaturated
a,b
(Scheme 1). On the other hand, carboxylate III may also undergo
elimination directly to intermediate V, and after attack of alcohol to
afford the desired ester with release of NHC (path B).
To a stirred suspension of IMes$HCl (9 mg, 0.025 mmol) in anhyd
THF (2 mL) was added DBU (0.6 mmol, 88 L) via micro syringe
under N2. After the mixture was stirred at room temperature for
30 min, -halo- -unsaturated aldehyde (0.5 mmol) and alcohol
(1.5 mmol) were loaded, the reaction mixture was then stirred at
room temperature until full consume of the starting aldehyde in-
dicated by TLC. After concentration of the mixture under vacuum,
the crude product was purified through flash column chromatog-
raphy (silica gel, PE/EtOAc, 10:1e15:1) to give desired product.
m
a
a,b
O
O
5 mol% IMes HCl
1.2 eq DBU
OC2H5
OC2H5
(Eq. 1)
(Eq. 2)
C2H5OH (3 eq)
THF, rt
Br
Br
yield : 94%
O
O
4.2.1. Ethyl cinnamate (6).10 Slightly yellow oil, 77.1 mg (87%); 1H
1.2 eq DBU
THF, rt
OC2H5
OC2H5
NMR (400 MHz, CDCl3)
d
7.68 (d, J¼16.0 Hz, 1H), 7.51e7.46 (m, 2H),
7.37e7.32 (m, 3H), 6.42 (d, J¼16.0 Hz,1H), 4.25 (q, J¼7.1 Hz, 2H),1.32
(t, J¼7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 166.8, 144.5, 134.4,
yield : 96%
130.1, 128.8, 128.0, 118.2, 60.4, 14.3; GCeMS (EI): m/z 176 (Mþ).
Scheme 1. Elimination reaction of
a
-bromoester.
4.2.2. Methyl cinnamate (7).10 Colorless oil, 71.2 mg (88%); 1H NMR
3. Conclusions
(400 MHz, CDCl3)
d
7.70 (d, J¼16.0 Hz, 1H), 7.55e7.50 (m, 2H),
7.41e7.36 (m, 3H), 6.45 (d, J¼16.0 Hz, 1H), 3.81 (s, 3H); 13C NMR
In summary, we have demonstrated a NHC-catalyzed redox
esterification reaction of -halo- -unsaturated aldehydes with
(100 MHz, CDCl3)
d 167.4, 144.8, 134.3, 130.2, 128.8, 128.0, 117.8,
51.6; GCeMS (EI): m/z 162 (Mþ).
a
a,b
different alcohols to provide cinnamic esters in high yield with
excellent chemo- and stereoselectivity. Further study of the re-
action mechanism are currently under way in our laboratory.
4.2.3. Butyl cinnamate (8).16 Slightly yellow oil, 86.7 mg (85%); 1H
NMR (400 MHz, CDCl3)
d
7.69 (d, J¼16.0 Hz, 1H), 7.56e7.49 (m, 2H),
7.42e7.35 (m, 3H), 6.44 (d, J¼16.0 Hz, 1H), 4.21 (t, J¼6.7 Hz, 2H),
4. Experimental section
4.1. General methods
1.75e1.64 (m, 2H), 1.50e1.38 (m, 2H), 0.97 (t, J¼7.4 Hz, 3H); 13C
NMR (100 MHz, CDCl3)
d 167.1, 144.5, 134.5, 130.2, 128.9, 128.0,
118.3, 64.4, 30.8, 19.2, 13.8; GCeMS (EI): m/z 204 (Mþ).
Unless otherwise indicated, all reactions were conducted under
nitrogen atmosphere in oven- dried glassware with magnetic stir-
ring bar. Column chromatograph was performed with silica gel
4.2.4. Benzyl cinnamate (9).10 Slightly yellow oil, 96.2 mg (81%); 1H
NMR (400 MHz, CDCl3)
d
7.72 (d, J¼16.0 Hz, 1H), 7.52e7.47 (m, 2H),
7.43e7.32 (m, 8H), 6.48 (d, J¼16.0 Hz, 1H), 5.24 (s, 2H); 13C NMR

X.-B. Wang et al. / Tetrahedron 68 (2012) 6498e6503
6503
(100 MHz, CDCl3)
d
166.8, 145.2, 136.1, 134.4, 130.4, 128.9, 128.6,
1H), 8.06e8.02 (m, 1H), 7.67e7.63 (m, 2H), 7.58e7.53 (m, 1H), 7.37
(d, J¼15.8 Hz, 1H), 4.29 (q, J¼7.1 Hz, 2H), 1.35 (t, J¼7.1 Hz, 3H); 13C
128.3, 128.2, 128.1, 117.9, 66.4; GCeMS (EI): m/z 238 (Mþ).
NMR (100 MHz, CDCl3)
d 165.8, 148.3, 139.8, 133.5, 130.6, 130.3,
4.2.5. Isopropyl cinnamate (10).10 Yellowish oil, 58.0 mg (61%); 1H
129.1, 124.9, 123.3, 60.9, 14.3; GCeMS (EI): m/z 221 (Mþ).
NMR (400 MHz, CDCl3)
d
7.67 (d, J¼16.0 Hz, 1H), 7.53e7.49 (m, 2H),
7.39e7.35 (m, 3H), 6.41 (d, J¼16.0 Hz, 1H), 5.20e5.05 (m, 1H), 0.31
4.2.15. (E)-Ethyl-3-(furan-2-yl)acrylate (20).21 Slightly yellow oil,
(d, J¼6.2 Hz, 6H); 13C NMR (100 MHz, CDCl3)
d
166.4, 144.2, 134.5,
64.7 mg (78%); 1H NMR (400 MHz, CDCl3)
d 7.49e7.46 (m, 1H), 7.43
130.1, 128.8, 128.0, 118.8, 67.7, 21.9; GCeMS (EI): m/z 190 (Mþ).
(d, J¼15.8 Hz, 1H), 6.61e6.53 (m, 1H), 6.47e6.39 (m, 1H), 6.31 (d,
J¼15.8 Hz, 1H), 4.24 (q, J¼7.1 Hz, 2H), 1.32 (t, J¼7.1 Hz, 3H); 13C NMR
4.2.6. Cyclohexyl cinnamate (11).17 Slightly yellow oil, 72.0 mg
(100 MHz, CDCl3) d(100 MHz, CDCl3) d 167.00,150.93,144.63,130.92,
(63%); 1H NMR (400 MHz, CDCl3)
d
7.67 (d, J¼16.0 Hz, 1H),
115.94, 114.56, 112.20, 60.40, 11.29; GCeMS (EI): m/z 166 (Mþ).
7.54e7.50 (m, 2H), 7.39e7.34 (m, 3H), 6.43 (d, J¼16.0 Hz, 1H),
4.94e4.85 (m, 1H), 1.97e1.89 (m, 2H), 1.81e1.73 (m, 2H), 1.57e1.24
Acknowledgements
(m, 6H); 13C NMR (100 MHz, CDCl3)
d 166.4, 144.2, 134.5, 130.1,
128.8, 128.0, 118.8, 72.7, 31.7, 25.4, 23.8; GCeMS (EI): m/z 230 (Mþ).
We are gratefully acknowledge the financial support from Shi-
hezi university, we also thank Dr. Lin He and Dr. Cheng-Zhi Gu for
helpful discussions.
4.2.7. Phenyl cinnamate (12).17 White solid, 83.1 mg (74%); 1H NMR
(400 MHz, CDCl3)
d
7.86 (d, J¼16.0 Hz, 1H), 7.58e7.54 (m, 2H),
7.42e7.36 (m, 5H), 7.26e7.20 (m, 1H), 7.18e7.14 (m, 2H), 6.62 (d,
References and notes
J¼16.0 Hz,1H); 13C NMR (100 MHz, CDCl3)
d 165.4,150.8,146.5,134.2,
130.7, 129.4, 129.0, 128.3, 125.8, 121.6, 117.3; GCeMS (EI): m/z 224.
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d
7.71 (d, J¼16.0 Hz, 1H), 7.54e7.49 (m, 2H),
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Lett. 2007, 9, 371 for redox aminations see: (d) Vora, H. U.; Rovis, T. J. Am. Chem.
Soc. 2007, 129, 13796; (e) Bode, J.; Sohn, S. S. J. Am. Chem. Soc. 2007, 129, 13798.
7. (a) Reynolds, N. T.; Read de Alaniz, J.; Rovis, T. J. Am. Chem. Soc. 2004, 126, 9518;
(b) Reynolds, N. T.; Rovis, T. J. Am. Chem. Soc. 2005, 127, 16406; (c) Hyster, T. K.;
Rovis, T. J. Am. Chem. Soc. 2010, 132, 10565.
8. (a) Chow, K. Y.-K.; Bode, W. J. J. Am. Chem. Soc. 2004, 126, 8126; (b) Sohn, S. S.;
Bode, J. W. Angew. Chem., Int. Ed. 2006, 45, 6021; (c) Li, G.-Q.; Li, Y.; Dai, L.-X.;
You, S.-L. Org. Lett. 2007, 9, 3519; (d) Wang, L.; Thai, K.; Gravel, M. Org. Lett.
2009, 11, 891; (f) Li, G.-Q.; Dai, L.-X.; You, S. L. Org. Lett. 2009, 11, 1623.
9. Reviews for the synthesis of unsaturated esters see: (a) Beletskaya, I. P.; Che-
prakov, A. V. Chem. Rev. 2000, 100, 3009; (b) Van Staden, L. F.; Gravestock, D.;
Ager, D. J. Chem. Soc. Rev. 2002, 195.
7.40e7.35 (m, 3H), 6.46 (d, J¼16.0 Hz, 1H), 5.99 (ddt, J¼17.2, 10.4,
5.7 Hz, 1H), 5.37 (ddd, J¼17.2, 3.1, 1.5 Hz, 1H), 5.27 (ddd, J¼10.4, 2.6,
1.3 Hz, 1H), 4.71 (dt, J¼5.7, 1.5 Hz, 2H); 13C NMR (100 MHz, CDCl3)
d
166.6, 145.0, 134.4, 132.3, 130.3, 128.9, 128.1, 118.2, 117.9, 65.2;
GCeMS (EI): m/z 188 (Mþ).
4.2.9. Prop-2-ynyl cinnamate (14).18 Slightly yellow oil, 67.0 mg
(72%); 1H NMR (400 MHz, CDCl3)
d
7.74 (d, J¼16.0Hz,1H), 7.55e7.50 (m,
2H), 7.42e7.37 (m, 3H), 6.46 (d, J¼16.0 Hz, 1H), 4.81 (d, J¼2.5 Hz, 2H),
2.51 (t, J¼2.5 Hz, 1H); 13C NMR (100 MHz, CDCl3)
d 166.0, 146.0, 134.2,
130.6, 128.9, 128.2, 117.0, 77.8, 74.9, 52.1; GCeMS (EI): m/z 186 (Mþ).
4.2.10. Cinnamyl cinnamate (15).19 Slightly yellow oil, 92.4 mg
(70%); 1H NMR (400 MHz, CDCl3)
d
7.73 (d, J¼16.0 Hz, 1H),
7.54e7.48 (m, 2H), 7.42e7.37 (m, 3H), 7.37e7.34 (m, 2H), 7.34e7.24
(m, 3H), 6.70 (d, J¼15.9 Hz, 1H), 6.48 (d, J¼16.0 Hz, 1H), 6.35 (dt,
J¼15.9, 6.4 Hz, 1H), 4.86 (dd, J¼6.4, 1.2 Hz, 2H); 13C NMR (100 MHz,
CDCl3)
d 166.7, 145.1, 136.2, 134.4, 134.2, 130.3, 128.9, 128.6, 128.1,
128.0, 126.6, 123.3, 117.9, 65.1; GCeMS (EI): m/z 264 (Mþ).
4.2.11. (E)-Ethyl-3-(4-methoxyphenyl)acrylate (16).19 Slightly yel-
low oil, 94.2 mg (91%); 1H NMR (400 MHz, CDCl3)
d 7.64 (d,
J¼16.0 Hz, 1H), 7.48e7.44 (m, 2H), 6.91e6.87 (m, 2H), 6.30 (d,
J¼16.0 Hz, 1H), 4.25 (q, J¼7.1 Hz, 2H), 3.82 (s, 3H), 1.33 (t, J¼7.1 Hz,
10. Zeitler, K. Org. Lett. 2006, 8, 637.
3H); 13C NMR (100 MHz, CDCl3)
d 167.3, 161.3, 144.2, 129.7, 127.2,
11. (a) Du, G.-F.; He, L.; Gu, C.-Z.; Dai, B. Synlett 2010, 2513; (b) Cai, Z.-H.; Du, G.-F.;
He, L.; Gu, C.-Z.; Dai, B. Synthesis 2011, 13, 2073; (c) Zhang, J.; Du, G.-F.; Xu, Y.-K.;
He, L.; Dai, B. Tetrahedron Lett. 2011, 52, 7153; (d) Cai, Z.-H.; Du, G.-F.; Dai, B.; He,
L. Synthesis 2012, 5, 694; (e) Fan, Y.-C.; Du, G.-F.; Sun, W.-F.; Kang, W.; He, L.
Tetrahedron Lett. 2012, 53, 2231.
115.7, 114.3, 60.3, 55.3, 14.3; GCeMS (EI): m/z 206 (Mþ).
4.2.12. (E)-Ethyl-3-(4-nitrophenyl)acrylate
(17).10 White solid,
8.22e8.18 (m, 2H),
70.3 mg (64%); 1H NMR (400 MHz, CDCl3)
d
12. For NHCs catalyzed-reactions that involing a-haloenals see: (a) Sun, F.-G.; Sun,
L.-H.; Ye, S. Adv. Synth. Catal. 2011, 353, 3134; (b) Yao, C.-S.; Wang, D.-L.; Lu, J.; Li,
T.-J.; Jiao, W.-H.; Yu, C.-X. Chem.dEur. J. 2012, 18, 1914.
7.69e7.61 (m, 3H), 6.51 (d, J¼16.0 Hz,1H), 4.25 (q, J¼7.1 Hz, 2H),1.31
(t, J¼7.1 Hz, 3H); 13C NMR (100 MHz, CDCl3)
d 166.0, 148.5, 141.6,
13.
a-Bromo-a,b-unsaturated aldehydes can be easily prepared according to liter-
140.6, 128.6, 124.2, 122.6, 61.0, 14.3; GCeMS (EI): m/z 221 (Mþ).
ature: Allen, C. F. H.; Endens, C. O., Jr. Organic Syntheses; 1955; Collect. Vol. No. 3
731.
14. Rovis has reported chiral triazolium carbene catalyzed redox esterification of
4.2.13. (E)-Ethyl-3-(4-chlorophenyl)acrylate (18).19 Slightly yellow
dichlorosldehydes and got
see Ref. 8b.
a-chloroesters in high enantioselectivity, for details
oil, 83.3 mg (79%); 1H NMR (400 MHz, CDCl3)
d
7.62 (d, J¼16.0 Hz,
15. Breslow, R. J. Am. Chem. Soc. 1958, 80, 3719.
1H), 7.46e7.42 (m, 2H), 7.37e7.33 (m, 2H), 6.40 (d, J¼16.0 Hz, 1H),
4.26 (q, J¼7.1 Hz, 2H), 1.33 (t, J¼7.1 Hz, 3H); 13C NMR (100 MHz,
16. Calo, V.; Nacci, A.; Monopoli, A.; Ieva, E.; Cioffi, N. Org. Lett. 2005, 7, 617.
17. De Sarkar, S.; Grimme, S.; Studer, A. J. Am. Chem. Soc. 2010, 132, 1190.
18. Asit, K. C.; Bavneet, S.; Sunay, V. C.; Alpesh, R. P. J. Org. Chem. 2009, 74, 5967.
19. Naoya, S.; Yumi, I.; Katsuhiko, M.; Hideo, T. Tetrahedron 2011, 67, 951.
20. Raj, K. D.; Biswajit, S.; Wahidur, R. S. M.; Jitendra, K. B. Chem.dEur. J. 2010, 16,
14459.
CDCl3)
d 166.7, 143.1, 136.1, 132.9, 129.2, 129.1, 118.9, 60.6, 14.3;
GCeMS (EI): m/z 210 (Mþ).
4.2.14. (E)-Ethyl-3-(2-nitrophenyl)acrylate (19).20 Slightly yellow
21. Cao, P.; Li, C.-Y.; Kang, Y.-B.; Xie, Z.-W.; Sun, X.-L.; Tang, Y. J. Org. Chem. 2007, 72,
6628.
oil, 78.3 mg (71%); 1H NMR (400 MHz, CDCl3)
d
8.11 (d, J¼15.8 Hz,