Sunlight promoted palladium catalysed Mizoroki-Heck, Suzuki-Miyaura and Sonogashira reactions

Article abstract of DOI:10.1016/j.tetlet.2012.08.139

The palladium catalysed Mizoroki-Heck, Suzuki-Miyaura and Sonogashira reactions were successfully carried out under irradiation with sunlight. The Heck reaction gives considerable amount of Z product due to photochemical isomerization of initially formed E alkenes. Reaction of methyl 2-iodobenzoate with acrylamide under solar condition furnished 2H-2-benzazepine-1,3-dione rather than the expected derivative of cinnamate while the same reaction with ethyl 2-iodobenzoate gave the desired cinnamide.

Full text of DOI:10.1016/j.tetlet.2012.08.139

Tetrahedron Letters 53 (2012) 6100–6103
Contents lists available at SciVerse ScienceDirect
Tetrahedron Letters
journal homepage: www.elsevier.com/locate/tetlet
Sunlight promoted palladium catalysed Mizoroki–Heck, Suzuki–Miyaura
and Sonogashira reactions
⇑
Anju R. Chaudhary, Ashutosh V. Bedekar
Department of Chemistry, Faculty of Science, M.S. University of Baroda, Vadodara 390 002, India
a r t i c l e i n f o
a b s t r a c t
Article history:
The palladium catalysed Mizoroki–Heck, Suzuki–Miyaura and Sonogashira reactions were successfully
carried out under irradiation with sunlight. The Heck reaction gives considerable amount of Z product
due to photochemical isomerization of initially formed E alkenes. Reaction of methyl 2-iodobenzoate
with acrylamide under solar condition furnished 2H-2-benzazepine-1,3-dione rather than the expected
derivative of cinnamate while the same reaction with ethyl 2-iodobenzoate gave the desired cinnamide.
Ó 2012 Elsevier Ltd. All rights reserved.
Received 8 August 2012
Revised 28 August 2012
Accepted 30 August 2012
Available online 7 September 2012
Keywords:
Sunlight
Renewable energy
Green chemistry
Coupling reactions
Homogeneous catalysis
One of the twelve principles of ‘Green Chemistry’ proposed by
Anastas and Warner refers to the use of energy efficient synthetic
processes.1 The sixth principle says ‘the energy requirements of
chemical processes should be recognized for their environmental
and economic impacts and should be minimized. If possible, syn-
thetic methods should be conducted at ambient temperature and
pressure.’ The solar energy is the greenest source of energy, it is
abundant, cheaper and a cleaner alternative to any process of gen-
eration of energy. It is the most powerful form of renewable
energy.
for the preparation of substituted alkynes are the three main reac-
tions of this class. These reactions are traditionally carried out in
the thermal conditions in the presence of catalysts and suitable
additives. Use of microwave or ultrasound irradiation to drive
these reactions has also been explored with varied success.12 In
this communication we present our efforts to perform these
important reactions under the direct solar irradiation without the
need of any special apparatus or conditions.
The intended driving force is the photothermal energy received
from sunrays. Recently we have screened a series of 1-(a-amino-
The use of solar energy to drive chemical transformations was
recognized very early2,3 and many efforts are made to explore dif-
ferent reactions under the natural sunlight. The direct sunlight or
concentrated sunlight is utilized either as a photon source for
photochemical or photothermal reactions. Some of the sunlight
induced reactions involve dimerization4 or cis–trans isomerization
of alkenes and the conjugated compounds5 and the related
reactions. Beside these a few other sunlight mediated reactions
have been developed over the years and are summarized in re-
cent articles.6,7 However, the focus of research has mostly been
directed to the chemistry of molecules which are capable of
absorbing light for the desired bond modifications leading to
chemical changes.
benzyl)-2-naphthols as phosphine free ligands for these three reac-
tions with reasonably good conversions at conventional heating
conditions13 (Fig. 1).
Initially, Mizoroki–Heck reaction of iodobenzene with styrene
was investigated under the conditions presented in Scheme 1.
The reaction was performed in a simple assembly of an Erlenmeyer
flask fitted with a guard tube and a magnetic stirrer. The reaction
was simply exposed to bright sunlight continuously for several
days and then the product was isolated. For better results the flask
Ph
N
Ph
N
The palladium catalysed coupling reactions are now well estab-
lished tools of modern synthetic chemists. Mizoroki–Heck reaction
to synthesize stilbenes or cinnamates,8,9 Suzuki–Miyaura reac-
tion10 for biaryls or substituted alkenes and Sonogashira reaction11
OH
OH
L-1
L-2
⇑
Corresponding author. Tel.: +91 0265 2795552.
E-mail address: avbedekar@yahoo.co.in (A.V. Bedekar).
Figure 1. Ligands for the present study.
0040-4039/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.tetlet.2012.08.139

A. R. Chaudhary, A. V. Bedekar / Tetrahedron Letters 53 (2012) 6100–6103
6101
Sunlight
Open Vessel
Ph
Ph
+
Ph
Ph
9 days
Ph
54 % Y
E-1
E-1
Z-1
Z-1
37 : 63
Pd(OAc)2 (0.5 %)
L-1 (0.6 %)
Ph
(Eq. 1)
+
Et3N (2 eq.)
DMA
PhI
5 days
36 % Y
Ph
Ph
+
Ph
(Eq. 2)
Sunlight
Dark Vessel
100 : 0
Scheme 1. Comparison of Mizoroki–Heck reaction under the solar irradiation
performed in open or dark vessels.
Figure 2. Isomerization of ethyl cinnamate under sunlight irradiation (j for E
isomer; N for Z isomer).
is placed on top of a simple mirror to utilize the reflected sunrays.
However, it was interesting to observe the formation of Z-stilbene
(ZÀ1) as a major product compared to E-stilbene (EÀ1) contrary to
the usual selectivity of the reaction (Eq. 1).
We also observed a very poor conversion when the same reac-
tion was performed at room temperature without exposure to the
sunlight (9% Y) or with only Pd(OAc)2 in sunlight in the absence of
the ligand (18% Y). The reaction proceeds mainly due to the photo-
thermal reaction as we have measured the reaction temperature to
reach about 56–58 °C in the day time when the brightest sunlight
falls on the reaction medium. However, the role of photochemical
cleavage of Ar–I bond for the SRN1 type substitution reaction as-
sisted by sunlight cannot be ruled out.15 This also accounts for
the observation of low reactivity of Ar–Br in this reaction under
the solar conditions. The reduction of Pd(II) to Pd(0) can also be as-
sisted under the photochemical conditions as postulated by Köh-
ler.16 This is the only known report on the beneficial effect of
artificial light on the Mizoroki–Heck reaction.
This could probably be due to the photochemical isomerization
of stilbene like compounds when exposed to light.14 Another
experiment was conducted under identical condition but the flask
was completely covered with a black paper to prevent exposure to
light (Eq. 2). This experiment gave exclusively trans-stilbene
(E À 1) as earlier observed for the same catalyst system for the
classical thermal conditions.13a Identical results were obtained
when iodobenzene was treated with acrylates to form Z/E cinna-
mates. Further evidence to the possibility that the initially formed
E olefin was getting isomerized to Z olefin over a period of sunlight
exposure was provided by a separate experiment. In this study the
pure E-ethyl cinnamate was gradually converted into Z isomer till
it reached the equilibrium or photostationary phase in about 56 h
of actual sunlight (Fig. 2).
Having reasonably established the conditions of optimized con-
version, a series of substrates were screened with moderate to
Table 1
Examples of photothermal Mizoroki–Heck reactions
No
1
ArX
Alkenea
Product
Yieldb/% [Z:E]c
COOEt
86
[25:75]
COOEt
C6H5l
C6H5
2
COOtBu
COOtBu
COOtBu
COOEt
85
[19:81]
2
3
C6H5l
C6H5
3
COOtBu
COOEt
41
[21:79]
4-ClC6H4Br
4-ClC6H4
4
84
[55:45]
4-RC6H4
4
5
6
4-RC6H4l
2-RC6H4l
2-RC6H4l
5
R = NHCOCH3
COOEt
COOEt
69
[70:30]
2-RC6H4
6
R = COOCH3
COOtBu
COOtBu
78
[78:22]
2-RC6H4
7
R = COOMe
COOEt
90
[80:20]
COOEt
CONH2
Ph
2-MeC6H4
7
8
2-MeC6H4l
C6H5l
8
CONH2
76
[48:52]
C6H5
9
Ph
78
[69:31]
2-RC6H4
9
2-RC6H4l
4-BrC6H4l
10
R = COOEt
COOEt
COOEt
83
[43:57]
4-BrC6H4
10
11
a
b
c
General conditions: L-1 (0.6%), Pd(OAc)2 (0.5%), Et3 N (2.0 equiv) in DMA, sunlight, 9 d.
Isolated yield in %.
Determined by H NMR analysis.

6102
A. R. Chaudhary, A. V. Bedekar / Tetrahedron Letters 53 (2012) 6100–6103
Reaction of ethyl 2-iodobenzoate 12a with acrylamide under
O
the same conditions afforded desired compound 13 in similar man-
ner, but notably the reaction of methyl 2-iodobenzoate 12b fur-
R
R
I
NH
67%
65%
O
nished
a
cyclized product, 2H-2-benzazepine-1,3-dione 14
(Scheme 2).26
CONH2
13 Z : E :: 67 : 33 12a; R = CO2Et
12b; R = CO2Me
14
The latter reaction when performed under thermal conditions
however, furnished the expected product 15 exclusively as the E
isomer (Scheme 3). This unexpected observation for the sunlight
induced Mizoroki–Heck reaction could be explained by the initial
isomerization of compound from E-15 to Z-15 followed by intra-
molecular cyclization. The less favourable cyclization on ethyl ester
as compared to methyl ester may be the reason for this observation
in the case of 12a and 12b. This hypothesis was confirmed when a
pure sample of E-15 was initially converted into Z-15 when ex-
posed to sunlight and later cyclized to 2H-2-benzazepine-1,3-
dione 14.
The present protocol of sunlight assisted coupling was further
extended to other two important coupling reactions. Preliminary
investigations for the Suzuki–Miyaura and Sonogashira reactions
are encouraging and the results are summarized in Table 2. The
outcome of the Suzuki–Miyaura reaction in the absence of light
was same as in open vessel indicating the sunlight is merely acting
as a heating source (Table 2, entry 1).
CH2=CH-CONH2, sunlight, Pd(OAc)2 (0.5 %), L-1 (0.6 %), Et3N (2 eq.), DMA, 9 d
Scheme 2. Unexpected cyclization in Mizoroki–Heck reaction in solar condition.
heating, 140 o
C
COOMe
I
COOMe
CONH2
Pd(OAc)2 (0.5 %)
L-1 (0.6 %)
Et3N (2 eq.), DMA
40 h, 41 % Y
CONH2
12b
E - 15 (only E isomer)
Sunlight
2 d
isomerisation
O
COOMe
NH
cyclization
O
CONH2
The outcome of all three reactions with either ligand is almost
14
Scheme 3. Probable route for the formation of 15.
15, Z : E :: 72 : 28
similar.13
The present findings offer a simple way to utilize sunlight as a
renewable energy source for palladium catalysed basic coupling
reactions without the need for any special apparatus or reaction
machinery.27–29
Table 2
Sunlight induced Suzuki–Miyaura and Sonogashira reactionsa
Acknowledgments
No
1
Reagents
Product(s)
Yieldb/%
C6H5l
C6H5B(OH)2
C6H5–C6H5 16
91
88c
83
82
77
55
We thank CSIR, New Delhi for the award of research fellowship
to ARC and Prof. B.V. Kamath, Head, Department of Chemistry for
the infrastructural facilities and constant encouragements.
2
3
4
5
C6H5l
C6H5l
C6H5l
C6H5l
2-MeC6H4B(OH)2
3-HOC6H4B(OH)2
3-HOCH2C6H4B(OH)2 3-HOCH2C6H4–C6H5 19
4-CHOC6H4B(OH)2 4-CHOC6H4–C6H5 20
2-MeC6H4–C6H5 17
3-HOC6H4–C6H5 18
References and notes
4-CHOC6H4–4-CHOC6H4 21 47
6
7
4-MeOC6H4l C6H5B(OH)2
C6H5l C6H5C„CH
4-MeOC6H4–C6H5 22
C6H5C„CC6H5 23
53
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4-NO2C6H4l C6H5C„CH
2-CO2EtC6H4l C6H5C„CH
4-NO2C6H4C„CC6H5 24
2-COOEtC6H4C„CC6H5 25 74d
a
General conditions for Suzuki reaction; L-2 (0.6%), Pd(OAc)2 (0.5%) K2CO3
(2 equiv) Dioxane-H2O (1:1), sunlight, 1 d.
Isolated yield in %. For Sonogashira reaction: L-2 (1.2%), Pd(OAc)2 (1.0%), base
(2.0 equiv), NMP, sunlight, 3 d.
b
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The reaction conducted in dark vessel.
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d
e
good results (Table 1). The stilbenes or cinnamic acid derivatives
formed were found to have a considerable amount of the Z isomer
in most of the cases. The methods available for the synthesis of Z
alkenes include selective hydrogenation of corresponding al-
kynes,17 from aldehydes by modified Wittig reactions,18 Favorskii
reaction,19 Negishi coupling,20 aldol reaction,21 Peterson reaction,22
photoisomerization of E alkenes23 etc. Special conditions for Mizor-
oki–Heck reaction for selective formation of Z alkene has also been
developed by some groups24 which involves the design of ligand or
other parameters. The similar olefinic moiety with Z stereochemis-
try is present in some bioactive molecules discovered recently.25
For practical application, this reaction was carried out on a multi
gram scale of iodobenzene (10.2 g, 0.05 mol) and ethyl acrylate
(0.075 mol) to afford the ethyl cinnamate 2 in high yield (90.5%
Y, Z:E ratio of 25:75).

A. R. Chaudhary, A. V. Bedekar / Tetrahedron Letters 53 (2012) 6100–6103
6103
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Z
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methyl)cinnamate (8) (Table 1, entry 7): To an oven-dried Erlenmeyer flask
equipped with a stirrer bar and guard tube was charged 2-iodotoluene (0.25 g,
1.147 mmol), triethyl amine (0.232 g, 2.293 mmol), palladium acetate
(1.29 mg, 0.006 mmol) and L-1 (2.18 mg, 0.007 mmol) in DMA (10 mL). To
this reaction mixture ethyl acrylate (0.172 g, 1.72 mmol) was added. The
reaction mixture was exposed to sunlight for 9 days. After completion of the
reaction, the mixture was quenched with water and extracted with ethyl
acetate (3 Â 25 mL). The combined organic phase was washed with water and
dried over anhydrous sodium sulfate. Solvent was removed under vacuum and
the crude product was purified by column chromatography on silica gel
(hexane) to afford ethyl (2-methyl)cinnamate 8 as a pale yellow oil (0.195 g,
90%) (Table 1, entry 7).% Yield: 90, Z:E 80:20.
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1H NMR (CDCl3, 400 MHz): d 8.0–7.1 (m, 5H, which also contains one set of
trans & cis olefinic protons), 6.38–6.34 & 6.04–6.01 & (2d, J = 16 Hz & J = 12 Hz,
1H, second set of trans & cis olefinic protons), 4.3–4.06 (2q, 2H, –CH2 of –COOEt
group of trans & cis isomer), 2.44 & 2.3 (2s, 3H, Ar-CH3 of trans & cis isomer),
1.36–1.13 (2t, 3H, –CH3 of –COOEt group of trans & cis isomer). MS (EI): M/Z,
(%): 190 (M)+ (11), 175 (5), 145 (56), 144 (63), 117 (29), 116 (59), 115 (100), 91
(25).
MS (EI): M/Z, (%): 190 (M)+ (11), 175 (5), 145 (56), 144 (63), 117 (29), 116 (59),
115 (100), 91 (25).
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(d) Hickson, C. L.; McNab, H. J. Chem. Res. 1989, 176; (e) Baag, M.; Kar, A.;
Argade, N. P. Tetrahedron 2003, 59, 6489; (f) Zhao, Y.-P.; Yang, L.-Y.; Liu, R. S. H.
IR (Neat): 3062, 2981, 1722, 1633, 1602, 1530, 1485, 1459, 1407, 1384, 1312,
1275, 1185, 1162, 1098, 1032, 982, 950, 839, 760, 730, 598 cmÀ1
.
29. On an average a day’s sunlight is equivalent to about 8 h of bright effective
exposure of solar rays. The results vary slightly with respect to the time of the
year and better conversions are observed in summer months.