An expedient approach to enhance Mizoroki-Heck coupling reaction by infrared irradiation using palladacycle compounds

Article abstract of DOI:10.1002/aoc.3331

An alternative and environmentally friendly strategy to promote the Mizoroki-Heck cross-coupling reaction by the use of infrared irradiation using palladacycles as precatalysts is reported. Coupling products are obtained in high yield and short reaction time. A comparison with the classical use of reflux conditions, and commercial sources of palladium complexes, shows the advantages of this new alternative for promoting coupling reactions.

Full text of DOI:10.1002/aoc.3331

Full paper
Received: 16 February 2015
Revised: 10 April 2015
Accepted: 16 April 2015
Published online in Wiley Online Library: 1 June 2015
(wileyonlinelibrary.com) DOI 10.1002/aoc.3331
An expedient approach to enhance Mizoroki–
Heck coupling reaction by infrared irradiation
using palladacycle compounds
Fernando Ortega-Jiméneza*, Francisco X. Domínguez-Villab,
Alfredo Rosas-Sánchezc, Guillermo Penieres-Carrilloa, José G. López-Cortésb
and M. Carmen Ortega-Alfaroc*
An alternative and environmentally friendly strategy to promote the Mizoroki–Heck cross-coupling reaction by the use of infrared
irradiation using palladacycles as precatalysts is reported. Coupling products are obtained in high yield and short reaction time. A
comparison with the classical use of reflux conditions, and commercial sources of palladium complexes, shows the advantages of
this new alternative for promoting coupling reactions. Copyright © 2015 John Wiley & Sons, Ltd.
Additional supporting information may be found in the online version of this article at the publisher’s web-site.
Keywords: palladacycles; Mizoroki–Heck reaction; infrared irradiation
and moisture; also phosphinite palladacycles exhibit better activity
Introduction
than structurally similar phosphite and phosphine complexes.14a)
Methodologies based on non-conventional sources of heating
such as microwaves, ultrasound, flow chemistry and micro-
reactors have attracted growing attention in recent years.[1] In
particular, microwave irradiation under controlled conditions is
an invaluable technology that has enormous applications in
various areas, including academic and industrial research.[2] This
methodology has been applied in organic synthesis and
catalysis,[3] and cross-coupling reactions are no exception.[4] The
Mizoroki–Heck coupling reaction involving palladium complexes
as catalysts is by far one of the most powerful synthetic strategies
for generating new C–C bonds.[5] This coupling reaction has a wide
variety of applications including total synthesis of natural
products,[6] fine chemicals syntheses,[7] bioorganic chemistry,[8]
material science and industrial applications,[9] among others. One
of the principal features of this reaction is the great tolerance to a
diversity of functional groups such as amines, hydroxyls, aldehydes,
ketones, carboxylic acids, esters, nitriles, etc., without protection–
Consequently, the search for easily handled, active, thermally stable
and low-cost catalysts has caused the development of phosphine-
free palladium catalysts such as palladium complexes bearing
nitrogen-donor ligands.[20] In addition, extensive studies have
shown the efficiency and robustness of palladacycles in performing
Mizoroki–Heck coupling reaction.[21]
Infrared irradiation is an energy source typically used for spectro-
scopic applications and its use as non-conventional heating has
been scarcely explored in comparison to microwaves. Some exam-
ples in organic synthesis show that infrared irradiation efficiently
promotes condensation reactions,[22] oxidation reactions,[23]
heterocyclic compound syntheses[24] and Diels–Alder reactions,[25]
among others. To the best of our knowledge, there is no precedent
in the literature regarding the use of infrared irradiation in C–C
coupling reactions.
deprotection methodologies.[10]
A diversity of catalyst or
precatalyst systems based on palladium compounds with a broad
range of structural features exists, with emphasis on properties
such as thermal and moisture stability[11] and always enhancing
catalytic aspects related to efficiency and activity.
*
Correspondence to: Fernando Ortega-Jiménez and M. Carmen Ortega-Alfaro
Departamento de Ciencias Químicas, Facultad de Estudios Superiores
Cuautitlán-UNAM, Campo 1, Avenida 1 de Mayo s/n, Cuautitlán Izcalli, CP
54740 Estado de México, México; Instituto de Ciencias Nucleares, UNAM,
Circuito Exterior, Cd. Universitaria, México 04360, DF, México. E-mail: fdo.
ortega@unam.mx; carmen.ortega@nucleares.unam.mx
In this context, cyclopalladated complexes are important starting
materials in organometallic chemistry.[12] Palladacycles have
attracted great interest due to their applications in many areas,
including organic synthesis,[13,14] material science,[15] biologically
active compounds[16] and as building blocks in macromolecular
chemistry.[17] Cyclopalladated complexes exhibit a superior cata-
lytic efficiency in cross-coupling reactions.[18] In particular, the
phospha-palladacycles have found application in a wide variety of
palladium-catalyzed reactions.[19] Phosphine ligands, especially
the electron-rich phosphines, are often toxic and sensitive to air
a Departamento de Ciencias Químicas, Facultad de Estudios Superiores Cuautitlán-
UNAM, Campo 1, Avenida 1 de Mayo s/n, Cuautitlán Izcalli, CP 54740 Estado de
México, México
b Instituto de Química UNAM, Circuito Exterior, Ciudad Universitaria, México 04360
DF, México
c
Instituto de Ciencias Nucleares, UNAM, Circuito Exterior, Cd. Universitaria, México
04360, DF, México
Appl. Organometal. Chem. 2015, 29, 556–560
Copyright © 2015 John Wiley & Sons, Ltd.

Mizoroki–Heck reaction by infrared irradiation using palladacycles
We herein report a comparative study of the Mizoroki–Heck reac-
tion promoted by this non-conventional heating, using various
air-stable palladacycle complexes.
Microwave irradiation experiments were performed using an
Anton Paar Monowave 300 single-mode microwave reactor. The
reaction temperature was monitored using an internal fiber-optic
temperature probe (ruby thermometer) protected by a borosili-
cate immersion well inserted directly into the reaction mixture.
Reaction times refer to the hold time at the desired set temper-
ature and not to the total irradiation time. Pressure sensing was
achieved using a hydraulic sensor integrated in the swiveling
cover of the instrument. A reusable 10 ml Pyrex vial was sealed
with poly(ether ether ketone) (PEEK) snap caps and standard
polytetrafluoroethylene (PTFE)-coated silicone septa. Reaction
cooling was performed using compressed air automatically after
the heating period had elapsed.
Experimental
General considerations
All operations were carried out in open atmosphere. Column chro-
matography was performed using 70–230 mesh silica gel. All re-
agents and solvents were obtained from commercial suppliers
and used without further purification. All compounds were charac-
terized using IR spectra, recorded with a Perkin-Elmer 283B or 1420
spectrophotometer, by means of film and KBr techniques, and all
data are expressed in wavenumbers (cmꢀ1). Melting points were
obtained with a Melt-Temp II apparatus and are uncorrected.
NMR spectra were measured with a Varian Eclipse +300 and Varian
+500 MHz, using CDCl3 and DMSO-d6 as solvents. Chemical shifts
are in ppm (δ), relative to tetramethylsilane. MS-FAB and MS-EI
spectra were obtained with a JEOL SX 102A. The values of the
signals are expressed in mass/charge units (m/z), followed by the
relative intensity with reference to a 100% base peak.
The equipment used for irradiation with IR energy was created
by employing an empty cylindrical metal vessel in which an
Osram lamp (bulb model Thera-Therm, 250 W, 125 V) was inserted
(Fig. 1). This lamp is a special short-wave IR lamp (IR-A) for use in
body care and wellness applications, with a maximum radiation at
a wavelength of about 1100 nm. The lamp instantly emits a full
thermal output as soon as it is switched on. For controlling the
temperature, a Digi-Sense variable-time power controller was
used. This time controller turned the output load on and off and
then repeated the cycle. Although all the reactions were per-
formed in open atmosphere, this arrangement also allows the
use of inert conditions.
General experimental methods
Synthesis of ligand 1
A mixture of N,N-diphenylhydrazine hydrochloride (570 mg, 2.6
mmol) and 2-pyridinecarboxaldehyde (280 mg, 2.6 mmol) in 10
ml of methanol was stirred for 24 h at room temperature. The
resulting solution was evaporated to dryness and the crude prod-
uct was chromatographed on silica gel. Elution with hexane–AcOEt
(99:1 v/v) allowed the product to be recovered. The pure product
was obtained as a yellow solid. Yield 640 mg (84%); m.p. 94–96°C.
Selected FT-IR (ν, cmꢀ1): 3052 and 3006 (H-Csp2), 1568 (C¼N), 1492
(C¼CAr). MS-EI m/z (%): 273 [M]+ (77), 195 [M ꢀ C6H6]+ (23), 168
[M ꢀ C12H10N]+ (100). 1H NMR (300 MHz, CDCl3, δ, ppm): 7.10 (dd,
1H, H-4, JH4H3 =8.1 Hz, JH4H5 = 2.7 Hz), 7.18 (d, 4H, H-8, JH8H9 =5.1 Hz),
7.21 (d, 2H, H-10, JH10H9 = 5.1 Hz), 7.30 (s, 1H, H-6), 7.41 (dd, 4H,
H-9, JH9H8 = 5.1 Hz, JH9H10 = 5.1 Hz), 7.68 (dd, 1H, H-3, JH3H4 = 8.1 Hz,
J
J
H3H2 = 4.8 Hz), 8.09 (d, 1H, H-2, JH2H3 = 4.8 Hz), 8.44 (d, 1H, H-5,
H5H4 = 2.7 Hz). 13C NMR (75 MHz, CDCl3, δ, ppm): 119.4 (C-2), 122.2
(C-4), 122.4 (C-8), 124.9 (C10), 129.8 (C-9), 135.8 (C-6), 136.1 (C-3),
143.0 (C-7), 148.9 (C-5), 155.3 (C-1).
Synthesis of complex 2
A mixture of PdCl2 (64 mg, 0.36 mmol) and NaCl (42 mg, 0.72 mmol)
in 10 ml of methanol was stirred for 1 h. Ligand 1 (100 mg, 0.36
mmol) was added to the solution and the mixture was stirred at
room temperature for 72 h, to give an orange solid, which was re-
covered by filtration, washed with cold methanol and finally dried
in air. The pure product was obtained as an orange solid. Yield
120 mg (90%); m.p. 280°C. Selected FT-IR (ν, cmꢀ1): 3055 (H-Csp2),
1590 (C¼N), 1489 (C¼CAr). MS-FAB+ m/z (%): 413 [M]+ (3), 378
[Mꢀ Cl]+ (15), 273 [M ꢀ PdCl]+ (15), 195 [Mꢀ C6H5PdCl]+ (5), 168
[Mꢀ C6H5N2PdCl]+ (58). 1H NMR (500 MHz, DMSO-d6, δ, ppm):
5.70 (d, 1H, H-11, JH11H10 = 6.5 Hz), 6.59 (dd, 1H, H-10, JH10H11 =6.5
Hz, JH10H9 = 6.0 Hz), 6.78 (dd, 1H, H-9, JH9H10 =6.0 Hz, JH9H8 = 7.5 Hz),
7.25 (d, 1H, H-8, JH8H9 = 7.5 Hz), 7.35 (s, 1H, H-6), 7.53 (d, 1H, H-16,
JH16H15 = 7.5 Hz), 7.55 (d, 2H, H-14, JH14H15 = 6.5 Hz), 7.62 (d, 1H, H-2,
JH2H3 = 7.5 Hz), 7.66 (dd, 1H, H-4, JH4H3 =7.5 Hz, JH4H5 =4.5 Hz), 7.73
(dd, 2H, H-15, JH15H14 =6.5 Hz, JH15H16 = 7.5 Hz), 7.95 (dd, 1H, H-3,
JH3H2 =7.5 Hz, JH3H4 = 7.5 Hz), 8.31 (d, 1H, H-5, JH5H4 =4.5 Hz). 13C
NMR (175 MHz, DMSO-d6, δ, ppm): 109.9 (C-5), 120.7 (C-3), 124.7
(C-2), 125.2 (C-11), 125.5 (C-10), 128.8(C-15), 130.6 (C-8), 131.2
(C-14, C-16), 133.9 (C-6), 134.4 (C-12), 135.4 (C-9), 140.3 (C-1),
148.4 (C-5), 156.5 (C-7), 157.1 (C-13).
Synthesis of palladium complexes 3b and 3b
The palladium complexes 3b and 3b were synthesized and charac-
Figure 1. Infrared equipment.
terized as described in the literature.[26]
Appl. Organometal. Chem. 2015, 29, 556–560
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

F. Ortega-Jiménez et al.
Table 1. Evaluation of catalytic conditions for Mizoroki–Heck cross-
coupling of 4-iodotoluene with methyl acrylate using complex 2
Table 2. Evaluation of catalytic conditions for Mizoroki–Heck cross-
coupling of 4-iodotoluene with methyl acrylate using complexes 2, 3a
and 3b
Entrya
[Pd]
(mol%)
Time
Base
Yield
(%)c
TON
TOF
Entrya Complex
Type of
heating
Time
Yield TON
(%)c
TOF
(min)b
(min)b
1
2
3
4
5
6
7
0.1
60
60
60
60
60
60
60
K3PO4
K3PO4
K3PO4
Na3PO4
Li3PO4
K2CO3
AcOK
90
97
77
90
50
86
87
900
1940
7700
1800
1000
1720
1740
900
1940
7700
1800
1000
1720
1740
1
2
3
4
5
6
7
8
2
Conventionald
Conventionald
Conventionald
Infrarede
60
360
720
15
97
90
85
98
96
87
94
89
1940
1800
1700
1960
1920
1740
1940
300
0.05
0.01
0.05
0.05
0.05
0.05
3a
3b
2
141
7840
960
3a
3b
2
Infrarede
120
180
10
Infrarede
Microwavef
Microwavef
580
1880 11280
1780 1186
aAll reactions were performed with 2 mmol of 4-iodotoulene, 3.3 mmol
of methyl acrylate, DMF (5 ml), 2.5 mmol of base at 140°C.
bReaction time based on total consumption of aryl iodide determined by
3a
90
aAll reactions were performed with 2 mmol of 4-iodotoulene, 3.3 mmol
of methyl acrylate, DMF (5 ml), 2.5 mmol of K3PO4 and [Pd] = 0.05 mol%.
bReaction time based on total consumption of aryl iodide determined by
TLC.
cIsolated yields after extraction with hexane.
TLC.
cIsolated yields after extraction with hexane.
dReaction conducted at 140°C.
General procedure for Mizoroki–Heck coupling reactions under conventional
eReaction conducted at 140°C, under infrared irradiation using an Osram
lamp (bulb model Thera-Therm, 250 W, 125 V). For controlling the
temperature, a Digi-Sense variable-time power controller was used.
fReaction conducted under microwave irradiation at 160°C using an
Anton Paar Monowave 300 single-mode microwave reactor. For
further information see the Experimental section.
thermal heating
In a 50 ml round-bottomed flask, 4-iodotoluene (2 mmol), methyl
acrylate (3.3 mmol) and base (2.5 mmol) were placed in 5 ml of
DMF, and then the palladium complex 2 was added. The reaction
mixture was refluxed for the time stated in Table 1 at 140°C. The re-
action mixture was poured into water (10 ml) and extracted with
hexane (3× 10 ml). The combined organic layers were dried over
anhydrous sodium sulfate. The solvent was removed in vacuo, to
give (E)-methyl p-methylcinnamate in yields stated in Table 1.
Table 3. Scope of Mizoroki–Heck cross-coupling using complexes 2
and infrared irradiation
General procedure for Mizoroki–Heck coupling reactions under infrared
irradiation
In a 50 ml round-bottomed flask, a mixture of aryl iodide (2 mmol),
methyl acrylate (3.3 mmol) and base (2.5 mmol) was placed in 5 ml
of DMF, and then the palladium complex was added. The reac-
tion mixture was irradiated using an Osram lamp (bulb model
Thera-Therm, 250 W, 125 V, see Fig. 1) for the time stated in
Tables 2–4 at 140°C. The reaction mixture was poured into water
(10 ml) and extracted with ether or hexane (3 × 10 ml). The com-
bined organic layers were dried over anhydrous sodium sulfate.
The crude product was finally purified by flash column chroma-
tography on silica-gel to give the isolated products in yields
stated in Tables 2–4.
Entrya
R
Time (min)b
Yield (%)c
TON
TOF
1
2
3
4
5
6
7
8
9
10
NH2
15
15
99
99
98
91
98
99
95
99
98
98
1980
1980
1960
1820
1960
1980
1900
1980
1960
1900
7920
7920
7840
3640
7840
1980
3800
792
CH3O
CH3
15
N-Pyrrole
H
30
60
Br
60
CH3COO
CF3
30
150
60
NO2
1960
760
CH3CO
150
General procedure for Mizoroki–Heck coupling reactions under microwave
aAll reactions were performed with 2 mmol of 4-aryl iodide, 3.3 mmol of
methyl acrylate, DMF (5 ml), 2.5 mmol of base and [complex 2] = 0.05
mol% at 140°C.
irradiation
A 10 ml microwave-transparent process vial was filled with 4-
iodotoluene (2 mmol), methyl acrylate (3.3 mmol), base (2.5 mmol),
5 ml of DMF and the palladium complex. The vial was sealed with
PEEK snap caps and standard PTFE-coated silicone septa. The reac-
tion mixture was then exposed to microwave heating for the time
stated in Table 2 at 160°C. The reaction mixture was poured into
water (10 ml) and extracted with hexane (3× 10 ml). The combined
organic layers were dried over anhydrous sodium sulfate. The sol-
vent was removed in vacuo, to give (E)-methyl p-methylcinnamate
in yields stated in Table 2.
bReaction time based on total consumption of aryl iodide determined by
TLC.
cIsolated yield after purification.
The purified product was identified by means of determination
of melting point and from 1H NMR and 13C NMR spectra. The data
obtained are consistent with those of the literature.[27]
wileyonlinelibrary.com/journal/aoc
Copyright © 2015 John Wiley & Sons, Ltd.
Appl. Organometal. Chem. 2015, 29, 556–560

Mizoroki–Heck reaction by infrared irradiation using palladacycles
Table 4. Mizoroki–Heck reaction between methyl acrylate and 4-
iodotoluene promoted by infrared irradiation and commercial palla-
dium sources
Once the catalyst load and base were established, we evaluated
complexes 3a and 3b under the same catalytic conditions (Table 2,
entries 1–3). The yields of coupling product are similar across the
series, but complex 2 exhibits a better performance with turnover
number (TON) and turnover frequency (TOF) of around 2 × 103.
These results also reveal the good catalytic activity of 2 in
Mizoroki–Heck cross-coupling reactions, compared with other
palladacycles.14a) In all cases, reactions were conducted in open
vessels and the coupling product was purified by simple extraction
with hexane, yielding almost pure methyl trans-cinnamate. With
the aim of testing the effectiveness of infrared irradiation for
Mizoroki–Heck reaction, we performed this coupling reaction under
infrared conditions using complexes 2, 3a and 3b and the same
reaction conditions, refluxing DMF and K3PO4 as base (Table 2,
entries 4–6). We observe an important decrease in reaction time
for all cases, obtaining the coupling product in excellent yields.
Comparing the activity and efficiency between complexes 2 and
3a (Table 2, entries 4 and 5), we observe a similar activity with TONs
of around 1.9× 103, but complex 2 is more efficient as revealed in
the TOF values. This behavior can be explained by the presumable
hemilability of pyridine fragment of ligand 1 to palladium atom in
complex 2, providing open coordination sites[28] for the possible
olefin coordination that could favor the precatalysis step giving
the active specie responsible, Pd(0), of the coupling reaction.[29]
In the case of complexes 3a and 3b, they involve a similar coor-
dination pattern, complex 3a being a more rigid system. These
palladacycles are very stable and the ligands strongly coordinate
to palladium atom as was previously demonstrated.[26,30] We have
compared the catalytic performance of complexes 2 and 3a under
microwave irradiation using the same reaction conditions (Table 2,
entries 7 and 8). The results obtained no show significant differ-
ences in TON values between infrared and microwave irradiation,
which means that infrared irradiation can be considered as a new
alternative methodology to promote coupling reaction.
To evaluate the scope of these complexes as catalytic precursors
in the Mizoroki–Heck coupling reaction assisted by infrared irradia-
tion, a variety of activated and deactivated aryl iodides with methyl
acrylate using complex 2 as a precatalyst were examined (Table 3).
The results obtained show that complex 2 is highly active and effi-
cient in catalyzing this cross-coupling reaction producing the cou-
pling product with yields ranging from 91 to 99%. In general, the
complex tested displays a good performance, being more active
when aryl iodides are para-substituted by electron-releasing
groups. This study shows that the Mizoroki–Heck reaction can be
carried out using infrared radiation as a heating source, reducing re-
action times and positively increasing the efficiency of this reaction.
As a further step, we also compared these findings with known
palladium systems, in order to demonstrate the effectiveness of in-
frared irradiation in promoting the Mizoroki–Heck cross-coupling
reaction (Table 4). Thus, we have tested various commercial palla-
dium compounds in combination with triphenylphosphine (entries
5 and 6) as the most popular ligand used in this kind of reaction,
using the same model reaction. These results show that infrared ir-
radiation assisted this coupling reaction, and the catalytic system
formed by Pd(OAc)2/PPh3/K3PO4 is the most efficient catalytic
system.
Entrya Catalytic system
Time (min)b Yield (%)c TON TOF
1
2
3
4
5
6
Pd(AcO)2/K3PO4
15
15
15
15
5
52
89
82
88
97
86
1040 4160
1780 7120
1640 6560
1760 7040
1940 24250
1720 21500
Pd(AcO)2/Et3N
PdCl2(PPh3)2/K3PO4
PdCl2(PPh3)2/Et3N
Pd(AcO)2 + PPh3/K3PO4
Pd(AcO)2 + PPh3/Et3N
5
aAll reactions were performed with 2 mmol of 4-iodotoulene, 3.3 mmol
of methyl acrylate, DMF (5 ml), 2.5 mmol of base and [Pd] = 0.05 mol% at
140°C.
bReaction time based on total consumption of aryl iodide determined by
TLC.
cIsolated yield after purification.
Note. The entireties of the round flasks used in each coupling re-
action were meticulously cleaned with aqua regia to avoid the pres-
ence of unseen palladium catalyst.
Results and discussion
The palladacycle complexes 3a and 3b were synthesized following
procedures in the literature.[26] In the case of new palladacycle com-
plex 2, it was prepared by reaction between ligand 1 and Na2PdCl4
generated in situ, to give an orange crystalline solid in 90% yield
(Scheme 1). This complex was fully characterized by means of con-
ventional spectroscopic techniques.
Having obtained palladacycles 2, 3a and 3b, we started to ex-
plore their catalytic properties as catalytic precursors in the
Mizoroki–Heck cross-coupling reaction using conventional thermic
conditions (Table 1). Initially, we evaluated the effect of the concen-
tration of the palladium complex on the Mizoroki–Heck reaction
between methyl acrylate and 4-iodotoluene. We chose complex 2
as a model precatalyst. The coupling reaction was conducted in
refluxing DMF (5 ml) for 1 h using different concentrations of
precatalyst 2. We obtain good yields within 1 h, when 0.1 and
0.05 mol% of 2 are used (Table 1, entries 1 and 2). The influence
of a base was also evaluated and five experiments were conducted
using K3PO4, Na3PO4, Li3PO4, K2CO3 and AcOK. The first of these
(Table 1, entry 2) gives the best results.
Conclusions
We report for the first time the use of an infrared-assisted method-
Scheme 1. Palladacycle complexes.
ology for conducting the Mizoroki–Heck cross- coupling reaction.
Appl. Organometal. Chem. 2015, 29, 556–560
Copyright © 2015 John Wiley & Sons, Ltd.
wileyonlinelibrary.com/journal/aoc

F. Ortega-Jiménez et al.
[15] J. Buey, P. Espinet, J. Organometal. Chem. 1996, 507, 137.
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The results obtained show that this energy source can be con-
sidered as an excellent, economical and accessible alternative to
promote this coupling reaction, showing advantages such as short
reaction times and good yields, facilitating access to a clean, simple
and economic methodology comparable to those involving
microwaves.
[18] a) I. P. Beletskaya, A. V. Cheprakov, J. Organometal. Chem. 2004, 689,
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Acknowledgments
The authors thank CONACYT 153059 and DGAPA-IA201112
projects.
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