C-N-palladacyclic-catalyzed Heck reaction in EGME/water: Rate and regioselectivity controlled by the solvents ratio

Article abstract of DOI:10.1016/j.ica.2008.03.017

Novel as well as known C,N-palladacyclic neutral complexes were tested as precatalyst in the Heck reaction between bromobenzene and styrene under aerobic conditions. The catalytic system Pd(II) complex/K₂CO₃/EGME-H₂O (EGME = ethylene glycol monomethyl ether) showed to be highly efficient. Best performances of the catalysts were achieved by controlling the amount of water: generally a H₂O content within the 25-50% v/v range resulted in the highest conversion of the substrates into trans-stilbene. As a matter of fact, bromobenzene and styrene can be converted quantitatively using only 0.01 mol% of precatalyst with very high regioselectivity for the trans product and with a TOF of 10 000 h^-1. In the absence of water, all complexes were less efficient and differences in their activity were found, while such a differentiation disappeared when water was added.

Full text of DOI:10.1016/j.ica.2008.03.017

Inorganica Chimica Acta 362 (2009) 97–104
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C–N-palladacyclic-catalyzed Heck reaction in EGME/water:
Rate and regioselectivity controlled by the solvents ratio
a
a
b
a
Alessandro Del Zotto a, , Francesca Iogna Prat , Walter Baratta , Ennio Zangrando , Pierluigi Rigo
*
a Dipartimento di Scienze e Tecnologie Chimiche, Università di Udine, via Cotonificio 108, I-33100 Udine, Italy
b Dipartimento di Scienze Chimiche, Università di Trieste, via L. Giorgieri 1, I-34127 Trieste, Italy
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel as well as known C,N-palladacyclic neutral complexes were tested as precatalyst in the Heck reac-
tion between bromobenzene and styrene under aerobic conditions. The catalytic system Pd(II) complex/
K2CO3/EGME–H2O (EGME = ethylene glycol monomethyl ether) showed to be highly efficient. Best per-
formances of the catalysts were achieved by controlling the amount of water: generally a H2O content
within the 25–50% v/v range resulted in the highest conversion of the substrates into trans-stilbene. As
a matter of fact, bromobenzene and styrene can be converted quantitatively using only 0.01 mol% of pre-
catalyst with very high regioselectivity for the trans product and with a TOF of 10000 hꢀ1. In the absence
of water, all complexes were less efficient and differences in their activity were found, while such a dif-
ferentiation disappeared when water was added.
Received 30 January 2008
Received in revised form 3 March 2008
Accepted 5 March 2008
Available online 18 March 2008
Keywords:
Palladium complexes
Cyclometalated ligands
Homogeneous catalysis
Heck reaction
Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
strated that cyclopalladated complexes exhibit a superior catalytic
efficiency in Heck and related reactions. However, the search for
The Heck reaction is one of the most powerful metal-catalyzed
processes for the synthesis of functionalised aryl-alkenes through
C–C bond formation [1–16]. However, despite great efforts mainly
focused on the use of palladium salts and complexes as catalyst,
only a few significative industrial applications of this process have
appeared [17–20]. Thus, Heck and related reactions, especially the
Suzuki aryl–aryl coupling, continue to stimulate the researchers in
both academia and industrial laboratories. Starting from the early
1990, the use of safe, low cost and (claimed) ‘‘environmentally
friendly” [21] protocols has found growing interest, and such is-
sues are nowadays of fundamental importance for the possible
industrial-scale applications. In this context, the use of water as
solvent is a challenging issue. Since the first studies on the Heck
reaction run in water in the late 1980s, this solvent has found wide
use in this type of catalyzed reaction, and several reports dealt
with the exclusive use of water as solvent [22]. Also, widely ex-
plored is the Heck reaction run in water/organic solvent mixtures
[22e,22h,22s,23]. Kinetic studies have outlined favourable [24] or
slightly unfavourable [25] effect played by water on the rate of
the reaction.
easily-handling, thermally robust and possibly low-cost catalysts
has addressed the research toward the development of phos-
phane-free palladacyclic complexes, for example, those bearing
C,N-, C,O- and C,S-coordinated ligands [27–30]. We herein report
the results of a study focused on the catalytic application of a series
of different C,N-cyclometalated Pd(II) complexes to the Heck reac-
tion with the aim to develop an operationally simple protocol,
starting from relatively cheap materials. Using different C,N-cyclo-
palladates as precatalysts, we have now found that the combina-
tion of H2O–ethylene glycol monomethyl ether (EGME)–K2CO3 is
effective in promoting the Heck reaction starting from non acti-
vated reagents such as bromobenzene and styrene. It is demon-
strated that a controlled water content in the organic solvent can
increase the reaction rate and improve the regioselectivity. Fur-
thermore, the catalytic reaction can be conveniently accomplished
in air using small amounts of the palladium precatalyst. Thus, a
nearly quantitative formation of trans-stilbene was obtained using
0.01 mol% of catalyst with a very high TOF value (10000 hꢀ1).
2. Experimental
Since the first report by Herrmann and co-workers dealing with
the use of tri(o-tolyl)phosphine [26], it has been clearly demon-
2.1. Materials and methods
* Corresponding author. Tel.: +39 432 558840; fax: +39 432 558803.
E-mail addresses: alessandro.delzotto@uniud.it (A. Del Zotto), ezangrando@
units.it (E. Zangrando).
All reagents were purchased from Aldrich and used without
further purification. Commercial solvents were dried according to
0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2008.03.017

98
A. Del Zotto et al. / Inorganica Chimica Acta 362 (2009) 97–104
standard methods and freshly distilled under argon before use. The
syntheses of the complexes were carried out in an atmosphere of
oxygen-free argon by using standard Schlenk techniques. The 1H
and 13C NMR spectra (at 200.13 and 50.32 MHz, respectively) have
been recorded on a Bruker AC 200 F QNP spectrometer. For both
nuclei the chemical shifts were referenced to SiMe4. The GC–MS
analyses, run to control the identity of the compounds obtained
in the catalytic trials, were carried out with a Fisons TRIO 2000 gas-
chromatograph–mass spectrometer working in the positive ion
70 eV electron impact mode. Injector temperature was kept at
250 °C and the column (SupelcoÒ SE-54, 30 m long, 0.25 mm i.d.,
coated with a 0.5 lm phenyl methyl silicone film) temperature
was programmed from 50 °C to 290 °C with a gradient of 10 °C/
min. The GC analyses were run on a Fisons GC 8000 Series gaschro-
matograph equipped with a SupelcoÒ PTA-5 column (30 m long,
0.53 mm i.d., coated with a 3.0 lm poly(5% diphenyl–95% dim-
ethylsiloxane) film) injector and column temperatures as indicated
above. The elemental analyses (C, H, N) have been carried out in
the microanalytical laboratory of our department.
the residue was dissolved in acetone (5 mL). Addition of n-hexane
(30 mL) afforded a yellow solid which was filtered off, washed with
n-hexane, and dried under vacuum. Yield 147 mg (92%). Anal. Calc.
for C38H38N2O4Pd2 (799.57): C, 57.08; H, 4.79; N, 3.50. Found: C,
57.03; H, 4.86; N, 3.45%. 1H NMR (CDCl3, 200 MHz): d = 7.87 (br
t, 1H, N@CH), 7.50 (dd, 1H, ArH), 7.44–6.95 (m, 8H, ArH), 5.34 (s,
1H, CH), 4.85 (br s, 2H, CH2), 2.04 (s, 3H, CH3), 1.96 (s, 3H, CH3);
13C{1H} NMR (CDCl3, 50 MHz): d = 188.0 (CO), 186.3 (CO), 174.6
(N@CH), 157.3 (Ci), 146.1 (Ci), 136.5 (Ci), 130.8 (CPh), 129.7 (CPh),
129.0 (Co), 128.6 (Cm), 127.0 (CPh), 126.7 (CPh), 124.2 (CPh), 100.4
(CH), 60.5 (CH), 52.1 (CH2), 27.8 (CH3), 27.5 (CH3).
2.6. Synthesis of complex 14
A mixture of palladium propionate (252 mg, 1.0 mmol) and 4
(215 mg, 1.1 mmol) in THF (20 mL) was stirred for 4 h at reflux.
The mixture was cooled to room temperature and passed through
a column of CeliteÒ to eliminate traces of palladium black. The
solution was concentrated to ca. 5 mL and cold n-hexane (30 mL)
was added affording a yellow solid, which was filtered off, washed
with cold n-hexane, and dried under vacuum. Yield 272 mg (73%).
Anal. Calc. for C34H34N2O4Pd2 (747.50): C, 54.63; H, 4.58; N, 3.75.
Found: C, 54.18; H, 4.76; N, 3.77%. 1H NMR (CDCl3, 200 MHz):
d = 7.29–7.20 (m, 3H, ArH), 7.17–6.98 (m, 4H, ArH), 7.09 (t,
4JH,H = 1.6 Hz, 1H, N@CH), 6.83–6.78 (m, 2H, ArH), 4.58 (dd,
2.2. Synthesis of (S)-(ꢀ)-N-benzylidene-a-methylbenzylamine (6)
Benzaldehyde (0.265 g, 2.5 mmol) and (S)-(ꢀ)-a-methylbenzyl-
amine (0.303 g, 2.5 mmol) were stirred at room temperature for
4 h in methanol (25 mL) in the presence of an excess of anhydrous
Na2SO4 (1.136 mg, 8.0 mmol). After elimination of the solid by fil-
tration, the solvent was removed under reduced pressure giving a
colorless oil, yield: 415 mg (79%); 1H NMR (CDCl3, 200 MHz):
d = 8.35 (s, 1H, N@CH), 7.70–7.83 (m, 2H, ArH), 7.16–7.47 (m, 8H,
4
2
2JH,H = 15.9 Hz, JH,H = 1.6 Hz, 1H, HCH), 3.95 (dd, JH,H = 15.9 Hz,
4JH,H = 1.6 Hz, 1H, HCH), 2.42 (q, 2H, CH2), 1.16 (t, 3H, CH3);
13C{1H} NMR (CDCl3, 50 MHz): d = 184.1 (CO), 171.8 (N@CH),
155.3 (Ci), 146.2 (Ci), 134.7 (Ci), 131.9 (CPh), 129.7 (Co), 129.5
(CPh), 128.7 (Cm), 128.0 (CPh), 126.6 (CPh), 123.9 (CPh), 61.4 (CH2),
31.0 (CH2), 10.9 (CH3).
3
3
ArH), 4.53 (q, JH,H = 6.6 Hz, 1H, CH), 1.59 (d, JH,H = 6.6 Hz, 3H,
CH3); 13C{1H} NMR (CDCl3, 50 MHz): d = 159.4 (N@CH), 145.1 (Ci),
136.3 (Ci), 130.50 (Cp), 128.5 (CPh), 128.4 (CPh), 128.2 (CPh), 126.8
(Cp), 126.5 (CPh), 69.7 (CH), 24.8 (CH3).
2.7. General procedure for the Heck reaction
2.3. Improved synthesis of complexes 10 and 11
A 10 mL Schlenk flask was charged with the appropriate
amount of palladium complex (5 lmol), bromobenzene
(0.5 mmol), styrene (0.6 mmol) and the base (0.6 mmol). After
addition of the solvent (2 mL), the flask was capped with a silicon
stopper and heated at 120 °C in an oil bath. When requested, all
operations were run under an argon atmosphere. The reaction mix-
ture was extracted from the flask by syringe and treated with a
small amount of water. The organic compounds were extracted
with dichloromethane, the solution was dried over Mg2SO4 and
analyzed by GC. This procedure was adopted for all catalytic trials
with 1 mol% palladium complex. Otherwise, suitable amounts of
solutions of the palladium complex prepared by dilution of a stan-
dard solution were used in all other trials with a lower precatalyst
charge (0.1–0.001%).
These complexes have been prepared in acetic acid by the
method described in detail for the synthesis of 12. Complexes 10
and 11 have been isolated as a yellow powder in 82% yield and
an off-white solid in 72% yield, respectively. The spectroscopic data
of 10 were identical to that reported in the literature [23i], while
11 is an insoluble material [31].
2.4. Synthesis of complex 12
A mixture of [Pd3(OAc)6] (337 mg, 0.50 mmol) and 6 (345 mg,
1.65 mmol) in 20 mL of acetic acid was stirred at 60 °C for 4 h.
The solution was then evaporated under vacuum leaving a yellow
gum which was dissolved in acetone (5 mL). Addition of n-hexane
(30 mL) afforded a yellow solid which was filtered off, washed with
n-hexane and little cold diethyl ether, and dried under vacuum.
Yield 439 mg (78%); Anal. Calc. for C34H34N2O4Pd2 (747.50): C,
54.63; H, 4.58; N, 3.75. Found: C, 53.94; H, 4.69; N, 3.87%. 1H
NMR (CDCl3, 200 MHz): d = 7.35–7.01 (m, 7 H, ArH), 6.96 (d,
4JH,H = 1.3 Hz, 1H, N@CH), 6.27 (m, 2H, ArH), 4.60 (dq, 2JH,H = 6.9 Hz,
2.8. Catalyst reuse check
A 10 mL Schlenk flask containing a magnetic stir bar was
charged with complex 10 (5 lmol), bromobenzene (0.5 mmol), sty-
rene (0.6 mmol) and K2CO3 (0.6 mmol). After addition of EGME
(1.5 mL) and H2O (0.5 mL), the flask was capped with a silicon
stopper and heated at 120 °C in an oil bath for 24 h. After cooling
to room temperature, bromobenzene (0.5 mmol) and styrene
(0.6 mmol) were added and the flask was heated at 120 °C. Further
workup as indicated above.
4JH,H = 1.3 Hz, 1H, CH), 2.19 (s, 3H, CH3); 1.75 (d, JH,H = 6.9 Hz, 3H,
2
CH3); 13C{1H} NMR (CDCl3, 50 MHz): d = 180.9 (CO), 170.4 (N@CH),
154.4 (Ci), 146.7 (Ci), 137.7 (Ci), 132.0 (CPh), 128.8 (Co), 128.4 (Cm),
127.9 (CPh), 126.8 (CPh), 126.5 (CPh), 124.0 (CPh), 64.4 (CH), 24.5
(CH3), 19.5 (CH3).
2.9. Catalyzed synthesis of trans-stilbene on a gram-scale
2.5. Synthesis of complex 13
A 250 mL three necked flask containing a magnetic stir bar was
charged in air with complex 10 (18 mg, 50 lmol) bromobenzene
(7.85 g, 50 mmol), styrene (6.25 g, 60 mmol) and K2CO3 (8.29 g,
60 mmol). After addition of EGME (150 mL) and H2O (50 mL), the
A mixture of complex 10 (144 mg, 0.20 mmol) and 2,4-pentane-
dione (50 mg, 0.50 mmol) in dichloromethane (20 mL) was stirred
for 2 h at room temperature. All volatiles were then removed and

A. Del Zotto et al. / Inorganica Chimica Acta 362 (2009) 97–104
99
mixture was heated under stirring at 120 °C in an oil bath for 30 h.
ligand
complex
The solvents were eliminated under reduced pressure overnight, at
60 °C, and the residual solid was purified on a silica gel column
using dichloromethane as the eluent affording 8.40 g (yield 93%)
of white crystalline trans-stilbene, whose purity (98%) was checked
by GC and 1H NMR.
1
2
3
4
5
6
7
8
9
10
11
12
C
N
N
C
O
O
O
O
Pd
Pd
2.10. Crystallographic structure determination
Fig. 2. The dimeric structure of complexes 7–12.
Diffraction data for the structure reported were collected at
room temperature on a Nonius DIP-1030H system (MoKa radia-
tion, k = 0.71073 Å). Cell refinement, indexing and scaling of the
data set were carried out using packages DENZO and SCALEPACK [32].
The structure was solved by direct methods and subsequent Fou-
rier analyses [33] and refined by the full-matrix least-squares
method based on F2 with all observed reflections [33]. The contri-
bution of H atoms at calculated position was introduced in the final
cycles of refinement. All the calculations were performed using the
WINGX system, Ver 1.70.01 [34].
in refluxing THF [23i], was isolated in 82% yield. Analogously, com-
plex 11 was obtained as a white insoluble material in 72% yield,
while the same compound was obtained from benzene at room
temperature in 52% yield [31]. Noticeably, it was reported that this
complex was isolated as a dark yellow solid from acetone at reflux
[37].
In addition to the acetato derivatives, the complexes [Pd(acetyl-
acetonate)(40)] (13) and [Pd2(propionate)2(40)2] (14) (40 represents
the deprotonated ligand 4) were also prepared (Fig. 3). Complex 13
was synthesized in good yield from precursor 10 by anion metath-
esis [38]. Attempts to prepare 13 directly from palladium(II) ace-
tylacetonate in refluxing THF or 2,4-pentanedione failed, in both
cases the unreacted starting Pd(II) complex was recovered from
the reaction mixture. The NMR characterization of complex 13
confirmed the mononuclear nature of this species [38]. In particu-
lar, couples of resonances for the CH3 groups in the 1H spectrum
and two resonances for the CO groups in the 13C NMR spectrum
are in agreement with trans C–Pd–O and N–Pd–O arrangements.
Complex 14 was obtained in good yield as yellow microcrystals
by reacting Pd(II) propionate and ligand 10 in 1:1.1 molar ratio
in refluxing THF. The 1H and 13C NMR spectra confirmed the forma-
tion of a single pure stereoisomer, whose chemical shift and cou-
pling constant values closely resemble that of complex 10 [23i].
Crystal data for 12: C34H34N2O4Pd2, M = 747.43, monoclinic,
space group P21, a = 11.571(3), b = 19.696(4), c = 13.746(3) Å,
b = 90.20(2)°, V = 3132.7(12) Å3, Z = 4, Dcalc = 1.585 g/cm3, l(MoKa) =
1.188 mmꢀ1, F(000) = 1504, hrange = 1.48–27.10°. Final R1 = 0.0459,
wR2 = 0.1067, S = 0.934 for 762 parameters and 38636 reflections,
11172 unique [Rint = 0.0366], of which 8614 with I > 2r(I), max
positive and negative peaks in DF map 1.860 and ꢀ0.465 e Åꢀ3
.
3. Results and discussion
3.1. Synthesis and properties of the cyclometalated complexes
The ligands 1–6 used in this study (Fig. 1) have the common fea-
ture to form a cyclometalate 5-membered ring via deprotonation
of a phenyl ring. The imine (S)-(ꢀ)-N-benzylidene-a-methylben-
zylamine (6) was synthesized in high yield as an oily liquid by
the reaction between benzaldehyde and (S)-(ꢀ)-a-methylbenzyl-
amine in 1:1 molar ratio. The Pd(II) cyclometalated complexes 7–
11 (Fig. 2) bearing the ligands 1–5 in the deprotonated form are
known compounds [23i,35–37], while complex 12 was isolated
in good yield by reacting Pd3(OAc)6 and 6 in 1:3 molar ratio, in
pure acetic acid at 60 °C for 4 h. It was characterized by elemental
analysis and 1H and 13C NMR spectroscopy, and its solid state
structure was established by an X-ray diffraction analysis. The
crystallographic investigation evidenced a trans stereochemistry
(Fig. 2), as found for the closest analogue 10 [23i].
3.2. Single crystal X-ray structure determination of complex 12
The crystal structure of 12 evidences the formation of a dimeric
cyclometalated species and Fig. 4 reports an Ortep view of the two
crystallographic independent molecules, while a selection of bond
lengths and angles is reported in Table 1. The palladium centres
adopt approximately square planar coordination geometry,
achieved through two oxygens from each of the two bridging ace-
tate anions, the imine nitrogen donor and the ortho-carbon atom of
phenyl supporting the carbon atom of the imine group. The two
independent complexes, having a pseudo (non-crystallographic)
C2 symmetry and thus a trans configuration, present a close compa-
rable geometry, not only in bond lengths and angles but also in the
overall conformation. The configuration is closely related to that of
By using the acetic acid route, the synthesis of complexes 10
and 11 was improved. The former, previously obtained in 68% yield
N
N
N
N
1
2
3
N
N
N
H
4
5
6
Fig. 1. The nitrogen ligands used in this work.

100
A. Del Zotto et al. / Inorganica Chimica Acta 362 (2009) 97–104
Table 1
Selected coordination bond lengths (Å) and angles (°) for complex 12
Molecule A
Molecule B
Pd(1)–N(1)
Pd(1)–C(5)
Pd(1)–O(1)
Pd(1)–O(3)
Pd(2)–N(2)
Pd(2)–C(20)
Pd(2)–O(4)
Pd(2)–O(2)
2.009(7)
1.976(9)
2.051(6)
2.139(7)
2.021(6)
1.962(9)
2.044(6)
2.159(7)
2.857(1)
82.2(4)
172.1(3)
94.4(3)
92.6(4)
175.6(3)
90.5(3)
81.1(3)
Pd(3)–N(3)
Pd(3)–C(45)
Pd(3)–O(7)
Pd(3)–O(5)
Pd(4)–N(4)
Pd(4)–C(60)
Pd(4)–O(6)
Pd(4)–O(8)
2.003(7)
1.991(12)
2.038(6)
2.129(8)
2.027(7)
1.981(10)
2.046(7)
2.159(7)
2.883(1)
81.5(4)
170.5(3)
93.5(3)
93.5(4)
174.9(4)
91.6(3)
81.4(4)
N
O
O
O
O
O
O
Pd
Pd
Pd
N
N
Pd(1)–Pd(2)
Pd(3)–Pd(4)
13
14
N(1)–Pd(1)–C(5)
N(1)–Pd(1)–O(1)
N(1)–Pd(1)–O(3)
C(5)–Pd(1)–O(1)
C(5)–Pd(1)–O(3)
O(1)–Pd(1)–O(3)
N(2)–Pd(2)–C(20)
N(2)–Pd(2)–O(4)
N(2)–Pd(2)–O(2)
C(20)–Pd(2)–O(4)
C(20)–Pd(2)–O(2)
O(2)–Pd(2)–O(4)
N(3)–Pd(3)–C(45)
N(3)–Pd(3)–O(7)
N(3)–Pd(3)–O(5)
C(45)–Pd(3)–O(7)
C(45)–Pd(3)–O(5)
O(7)–Pd(3)–O(5)
N(4)–Pd(4)–C(60)
N(4)–Pd(4)–O(6)
N(4)–Pd(4)–O(8)
C(60)–Pd(4)–O(6)
C(60)–Pd(4)–O(8)
O(6)–Pd(4)–O(8)
Fig. 3. Novel complexes bearing the deprotonated ligand 4.
173.0(3)
97.2(3)
92.8(3)
177.3(3)
88.7(3)
171.4(3)
97.8(3)
92.3(4)
178.1(4)
88.6(3)
the other by an angle of ca. 17.2° in molecule A and 9.6° in B (aver-
age values of C–Pd–Pd–N torsion angles). All distances and angles
within the acetate and the aromatic groups are normal, the two
acetate bridges are separated by a dihedral angle of 84.7(4) and
89.7(4)° in the two complexes. The Pd–Pd distance in the two com-
plexes is 2.857(1) and 2.883(1) Å, a value that does not exclude a
possible metal–metal interaction, being shorter than the sum of
palladium van der Waals radii. As a matter of fact, an indication
of metal–metal interaction in 12 can be inferred from the observa-
tion that each metal is displaced by 0.06 Å from its mean coordina-
tion plane towards the other metal. It should be noted that a longer
Pd–Pd distance (3.226 Å) was measured in the dimeric imine com-
plex with metals bridged by trifluoroacetate anions [43]. The crys-
tal packing does not evidence any significant p–p interaction
between aromatic rings. The geometrical parameters are very
similar to those found in complex 10 and in the correspondent
N-phenyl analogue [23i], with the exception of the intramolecular
metal–metal separation (2.999 and 2.940 Å) and of the interplanar
coordination plane angle (33.0°, 37.1°), which differ significantly in
the cited complexes [23i].
3.3. Catalysis
The initial catalytic study involved complexes 7–11, whose
catalytic activity was checked in the standard reaction between
bromobenzene and styrene (Scheme 1). The catalytic potential of
complexes 7 [44] and 10 [23i] in the Heck coupling has been
previously investigated in N,N-dimethylacetamide (DMA). The
reaction was initially performed in EGME, under argon at 120 °C
and in the presence of Cs2CO3. The choice of the solvent was based
on our previous studies which demonstrated the superior
performance of a Pd(II) cationic complex in EGME with respect to
N,N-dimethylformamide (DMF) and 1-methyl-2-pyrrolidinone
(NMP) [45]. As almost identical results were obtained working in
air, the catalytic trials were conveniently run under aerobic condi-
tions. Selected data among those collected within the 2–20 h range
using 1 mol% of complex are shown in Table 2 (entries 1–5).
Complexes 7–11 behaved as active precatalysts in EGME, gener-
ally showing a very good regioselectivity for trans-stilbene. Among
all, complex 10 exhibited a superior efficiency, in fact trans-stil-
bene yield after 20 h was about 89%, and the selectivity was higher
than 97% (the main byproduct was 1,2-diphenylethene). A com-
monly observed feature was the high trans-stilbene yield detected
Fig. 4. ORTEP drawing (35% probability ellipsoids) of complexes 12-A (top) and 12-
B (bottom) with atom labelling scheme.
10 [23i] and other cis-bis(l-carboxy1ato)-dipalladium complexes
bearing a C,N-coordinated imine [39–42].
The Pd–N and Pd–C bond distances vary in a range from
2.003(7) to 2.027(7) Å and from 1.962(9) to 1.991(12) Å, respec-
tively, while the Pd–O bond lengths having nitrogen donors in
trans position (2.038(6)–2.051(6) Å range) are significantly shorter
than those trans to aromatic carbons (2.129(8)–2.159(7) Å), for the
trans-lengthening influence of the r-bonded carbon. The Pd–N and
Pd–C bonds form the basis for a five-membered chelate ring; the
‘‘bite” angles average to 81.5°. The metal coordination planes form
a dihedral angle of ca. 29.3° (mean of the two complexes), and for
interligand repulsion each plane is slightly tilted with respect to

A. Del Zotto et al. / Inorganica Chimica Acta 362 (2009) 97–104
101
[Pd], base
Br
+
+
+
- baseH+Br-
Scheme 1.
Table 2
Comparison of the catalytic activity of complexes 7–14a
The influence of the base on the catalytic efficiency of 10 in
EGME was also investigated. The results are reported in Table 4.
The activity decreased in the following order: Cs2CO3 > NaOAc >
K2CO3 > K3PO4 ꢁ dimethylcyclohexylamine. Thus, Cs2CO3 is the
base of choice in combination with EGME, most probably for its
ready complete solubilization in this solvent.
The following conclusions can be drawn for the first part of the
investigation: (i) the imine palladacycle 10 seems to behave as the
best precatalyst, (ii) the protic solvent EGME is superior to other
solvents commonly used in the Heck reaction, (iii) best perfor-
mances of the catalysts in EGME are obtained using Cs2CO3 as
the base.
Entry
Complex
trans-Stilbene yieldb (%)
2 h
4 h
20 h
1
2
3
4
5
6
7
8
9
7
8
9
10
11
12
13
14
10c
72.1 [74.3]
80.9 [83.1]
61.4 [64.0]
79.1 [81.0]
62.8 [65.2]
71.6 [72.9]
71.5 [74.1]
68.1 [70.0]
61.7 [63.7]
76.6 [78.8]
82.6 [84.9]
63.0 [66.3]
83.6 [85.5]
65.4 [66.9]
74.8 [76.3]
73.6 [76.5]
70.4 [72.4]
62.4 [64.3]
79.1 [81.7]
85.3 [87.7]
64.7 [68.5]
88.7 [91.0]
69.4 [71.5]
82.1 [84.2]
75.3 [78.7]
73.3 [75.5]
67.8 [70.7]
In several reports on the Heck reaction, the crucial kinetic role
played by water added to a given organic solvent was pointed
out [22c,22e,23a–d,23f,23g,23k,23l,23n,23o,24]. Furthermore, in a
case of intramolecular Heck annulation it was found that also the
regiochemistry of the reaction was influenced by the presence of
small amounts of water [23e]. Starting from these observations,
the effects of water addition to EGME were analyzed. Selected data
among those collected in the 0.5–4 h range, using complex 10 as
the precatalyst, are reported in Table 5. As a matter of fact, a small
amount of H2O added to EGME such as 5% v/v (entry 2) is sufficient
to strongly accelerate the rate of the reaction, which was found to
a
Reaction conditions: bromobenzene (0.5 mmol), styrene (0.6 mmol), Cs2CO3
(0.6 mmol), complex (5 lmol, 1 mol%), EGME (2 mL), t = 120 °C, on air.
b
Bromobenzene conversion (%) in brackets.
0.5 lmol (0.1 mol%) complex.
c
after 2 h, which only poorly increased within 20 h owing to cata-
lyst decomposition. On the basis of the very good catalytic behav-
iour of complex 10, we decided to examine new related
cyclopalladated species. Thus, complexes 12 (Fig. 2), 13 and 14
(Fig. 3) were prepared and explored as precatalyst to establish
whether modifications in the structure of the cyclometalated li-
gand or the O,O-donor anion could affect the catalytic activity of
the Pd(II) complex. Complexes 12–14 were shown to be slightly
less efficient than 10.
Table 4
Influence of the base on the Heck coupling of bromobenzene with styrene in EGME
using complex 10a
Giving these results, next catalytic experiments were performed
using complex 10 only. The activity of this complex in EGME was
dramatically lowered below 120 °C, thus only ca. 40% and 9%
trans-stilbene was formed after 20 h at 100 and 80 °C, respectively,
using 1 mol% of complex. The catalytic efficiency of complex 10 in
other solvents such as DMF, 1,4-dioxane, NMP, 1,1,1-trifluorotolu-
ene, and protic n-amyl alcohol and 2,20-thiodiethanol was also
examined. The data (Table 3) show that EGME is the solvent of
choice for the Heck coupling of bromobenzene with styrene in
the presence of Cs2CO3. It was reported that complex 10 can con-
vert almost quantitatively bromobenzene and styrene into trans-
stilbene, at 140 °C in DMA/NaOAc within 24 h, however with low
regioselectivity (90%) [23i].
Base
trans-Stilbene yieldb (%)
2 h
4 h
20 h
Cs2CO3
K2CO3
NaOAc
K3PO4
DMCAc
79.1 [81.0]
25.0 [27.0]
64.3 [73.9]
13.4 [15.8]
13.0 [14.8]
83.6 [85.5]
28.4 [31.0]
66.0 [75.4]
13.7 [16.0]
17.3 [19.4]
88.7 [91.0]
29.4 [31.5]
68.3 [77.9]
15.1 [18.0]
19.5 [22.3]
a
Reaction conditions: bromobenzene (0.5 mmol), styrene (0.6 mmol), base
(0.6 mmol), 10 (5 lmol, 1 mol%), EGME (2 mL).
b
Bromobenzene conversion (%) in brackets.
DMCA = dimethylcyclohexylamine.
c
Table 5
Influence of the EGME/H2O ratio on trans-stilbene yield and regioselectivity using
Table 3
complex 10a
Influence of the solvent on the Heck coupling of bromobenzene with styrene using
Entry
Solvent volume (mL)
trans-Stilbene yieldb (%)
complex 10a
EGME
H2O
0.5 h
1 h
4 h
Solvent
trans-Stilbene yield (%)
1
2
3
4
5
6
7c
2.0
1.9
1.5
1.0
0.8
0.5
1.5
–
52.4 [53.4]
71.2 [74.7]
88.4 [89.6]
88.3 [89.9]
81.6 [83.7]
48.4 [51.2]
91.0 [91.9]
68.7 [70.1]
88.2 [89.9]
95.2 [96.2]
94.1 [95.8]
87.8 [90.2]
64.2 [68.1]
95.5 [96.4]
83.6 [85.5]
95.5 [96.7]
98.4 [99.5]
97.8 [99.0]
92.1 [95.1]
80.3 [85.6]
98.7 [99.8]
2 h
4 h
20 h
0.1
0.5
1.0
1.2
1.5
0.5
EGME
DMF
79.1
39.3
38.0
0.9
14.0
22.0
0.7
83.6
64.5
50.8
1.1
20.3
24.3
1.2
88.7
74.6
53.7
1.4
25.9
28.9
1.7
1,4-Dioxane
NMP
C6H5CF3
n-C5H11OH
S(CH2CH2OH)2
a
Reaction conditions: bromobenzene (0.5 mmol), styrene (0.6 mmol), Cs2CO3
(0.6 mmol), 10 (5 lmol, 1 mol%), total solvent (2 mL).
a
b
Reaction conditions: bromobenzene (0.5 mmol), styrene (0.6 mmol), Cs2CO3
(0.6 mmol), 10 (5 lmol, 1 mol%), solvent (2 mL).
Bromobenzene conversion (%) in brackets.
Using K2CO3 (0.6 mmol) instead of Cs2CO3.
c

102
A. Del Zotto et al. / Inorganica Chimica Acta 362 (2009) 97–104
Table 6
increase when the water content progressively raised. The highest
values of product yield were found using amounts of water in the
25–50% v/v range (entries 3 and 4). However, when the volume of
H2O exceeded that of EGME, the reaction rate diminished (entries 5
and 6). Noteworthy, a high water content such as 75% v/v gave an
opposite effect on the reaction rate, in fact trans-stilbene yield was
lower than that obtained using pure EGME (entry 6 versus entry 1).
Furthermore, it should be noted that also regioselectivity slightly
benefits by the presence of 5–50% v/v H2O. As well shown in
Fig. 5, the minimum difference between bromobenzene conversion
and trans-stilbene yield is reached when the amount of water is in
the 5–50% v/v range, (entries 2–4), i.e., higher the conversion high-
er the regioselectivity. Thus, selectivity, likewise yield, is markedly
reduced using high water amounts (entries 5 and 6). Notably, for
3:1 EGME/H2O solutions, the use of K2CO3 instead of Cs2CO3 did
not affect the catalytic activity of 10 (entry 7 versus entry 3). This
is indeed a clear economic advantage in the case of application of
our catalytic system to a large-scale production of arylalkenes via
Heck reaction.
Like 10, all other complexes were tested as precatalysts in 3:1
EGME/H2O solutions. The results (Table 6) show that a dramatic
increase of product yield is obtained for all complexes when water
is present (see data reported in Table 2). In this case the activity of
the catalyst is scarcely affected by the nature (ligand, coordinated
anion, nuclearity) of the cyclopalladate precursor. The very high
conversions and yields obtained are in line with the experimental
finding that no visible palladium black was formed in all catalytic
experiments run in EGME/H2O. Interestingly, when methanol was
used instead of water (entry 9) no marked differences were
observed with respect to the reaction run in pure EGME (Table 2,
entry 4), thus confirming the peculiar kinetic role played by water.
When ligand-free Pd3(OAc)6 was used as the precatalyst, a very
low reaction rate was observed and, notably, it decreased further
upon addition of water (entries 10 and 11). The activity of palla-
dium acetate became comparable with that of complexes 7–14
only when an excess of nBu4Br was added to the EGME/H2O
solution (Jeffery’s protocol) [22d,22g] (entry 12). A comparison
between the catalytic activity of 10 and Pd3(OAc)6 has been done
also by adding the catalyst (1 mol%) to the hot reaction mixture
Catalytic activity of complexes 7–14 and Pd3(OAc)6 in 3:1 EGME/watera
Entry
Complex
trans-Stilbene yieldb (%)
0.5 h
2 h
20 h
1
2
3
4
5
6
7
8
9
7
8
9
10
11
12
13
88.4 [89.4]
87.8 [91.1]
80.5 [83.1]
91.0 [91.9]
81.8 [84.2]
85.3 [88.0]
91.3 [93.7]
86.1 [89.2]
63.0 [65.4]
19.1 [20.0]
35.3 [36.5]
72.4 [75.2]
93.4 [94.5]
95.9 [97.8]
93.6 [95.0]
98.1 [99.1]
92.4 [93.8]
94.9 [96.8]
96.4 [97.8]
94.4 [96.0]
72.4 [74.5]
32.9 [34.7]
38.9 [40.2]
92.9 [94.4]
98.1 [99.2]
98.0 [99.1]
95.7 [97.3]
99.0 [99.9]
95.8 [97.4]
98.2 [99.2]
98.7 [99.9]
97.8 [99.5]
93.6 [94.9]
39.5 [41.4]
44.6 [46.5]
98.1 [99.4]
14
10c
d
e
f
10
11
12
Pd3(OAc)6
Pd3(OAc)6
Pd3(OAc)6
a
Reaction conditions: bromobenzene (0.5 mmol), styrene (0.6 mmol), catalyst
(5 lmol, 1 mol%), K2CO3 (0.6 mmol), EGME (1.5 mL), water (0.5 mL), t = 120 °C.
b
Bromobenzene conversion (%) in brackets.
EGME (1.5 mL), CH3OH (0.5 mL).
In 3:1 EGME/water mixture.
In pure EGME and using Cs2CO3 instead of K2CO3.
c
d
e
f
In 3:1 EGME/water mixture, in the presence of nBu4Br (nBu4Br/Pd molar
ratio = 10:1).
Pd3(OAc)6 the reaction proceeded slower (79.4% trans-stilbene
after 10 min). In the former case a clear green solution formed,
on the contrary some palladium black was observed using
Pd3(OAc)6.
Having established that addition of water to EGME in the 5–50%
v/v range resulted in a neat increase of activity of the catalyst, fur-
ther measurements were run in order to establish the concentra-
tion limit at which complex 10 still gives acceptable activity in
3:1 EGME/H2O. The data are shown in Table 7. After 20 h, both
trans-stilbene yield and regioselectivity did not appreciably fade
on lowering the precatalyst amount from 1 to 0.01 mol%. On the
contrary, the reaction proceeded slowly when complex 10 load
was further diminished to 0.001 mol%, as ca. 20% trans-stilbene
only was formed after 20 h. As already reported [9,24,46,47], the
TOFs (Table 7, TOF = moles of substrate per mole of complex per
hour, at 50% conversion) clearly show that catalyst efficiency
increases in diluted solutions. This feature can be ascribed to the
less probable aggregation of metallic palladium particles in diluted
solutions. The TOF value of 10000 hꢀ1 obtained using 0.01 mol% of
10 is a very high value indeed for a non activated starting material
such as bromobenzene. In this regard, in Table 8 is shown a com-
parison between the activity of 10 and that, to the best of our
knowledge, of the closely related species reported in the literature.
In order to confirm the effectiveness of our catalytic method
applied to the bromobenzene/styrene system, trans-stilbene was
prepared on a gram-scale (8.40 g) in 93% isolated yield, using
18 mg (0.1 mol%) of complex 10. Furthermore, a reuse of the cata-
lyst (see Section 2 for details) was possible with still good yield and
(bromobenzene
(0.5 mmol),
styrene
(0.6 mmol),
Cs2CO3
(0.6 mmol), EGME (1.5 mL), water (0.5 mL), t = 120 °C). A higher
conversion of bromobenzene (94.7%) to products (92.8% trans-stil-
bene) within 10 min was observed using 10, while in the case of
100
95
90
Table 7
trans-Stilbene yield at different loadings of complex 10a
c
Mol% 10
trans-Stilbene yield (%)b
TOF (hꢀ1
)
85
0.5 h
2 h
20 h
1
0.1
0.01
0.001
91.0 [91.9]
79.5 [82.7]
50.1 [51.5]
2.7 [3.6]
97.8 [99.1]
87.1 [89.5]
77.6 [79.4]
8.4 [9.6]
98.8 [99.9]
97.6 [99.3]
98.2 [99.9]
19.8 [20.4]
950
2 800
10 000
–
a-tbyld/cversion(%)
80
0
5
10 15 20 25 30 35 40 45 50 55 60 65 70 75
volume % water
a
Reaction conditions: bromobenzene (0.5 mmol), styrene (0.6 mmol), K2CO3
Fig. 5. Effect of water addition on trans-stilbene yield (N) and bromobenzene co-
nversion (j). Reaction conditions: bromobenzene (0.5 mmol), styrene (0.6 mmol),
Cs2CO3 (0.6 mmol), 10 (5 lmol, 1 mol%), solvent (EGME + H2O) 2 mL. The reported
values have been measured after 4 h.
(0.6 mmol), complex 10, EGME (1.5 mL), water (0.5 mL).
b
Bromobenzene conversion (%) in brackets.
Turnover frequency (moles of substrate per mole of complex per hour) at 50%
c
conversion.

A. Del Zotto et al. / Inorganica Chimica Acta 362 (2009) 97–104
103
Table 8
Comparison of the activity of C,N-cyclopalladated complexes in the reaction between bromobenzene and styrene
Complex (amount)
Solvent
Temperature (°C)
Yield (%)
TOFa (hꢀ1
)
Reference
10 (0.01 mol%)
A (0.0014 mol%)c
B (0.1 mol%)d
EGME/H2Ob
NMP
DMA
NMP
DMF
120
140
140
150
135
98.2
93
38
86.6
>99
10 000
511
5.3
2 988
66
This work
[43]
[44]
[48]
[49]
C (0.001 mol%)
D (0.5 mol%)e
OAc
Cl
Cl
Cl
Pd
N
Pd
N
Pd
Pd
N
N
2
2
2
2
O
OH
A
B
C
D
a
b
c
Calculated at 50% conversion only in this work.
In 3:1 v/v ratio.
Using methyl acrylate.
In the presence of nBu4NBr.
Using 4-bromoacetophenone.
d
e
selectivity. Thus, in the second use of the catalytic system trans-
stilbene was formed in 87% yield after 20 h, while regioselectivity
remained unchanged.
owing to its strong ligating ability, to the displacement of the
organic ligand giving the requested low-ligated palladium
catalyst [6].
Although a mechanistic study was not undertaken, we tenta-
tively suggest that a mechanism similar to that proposed for the
high-temperature Heck reaction starting from bromoaryls should
be effective. In accord with such a mechanism, Pd(0) nanoparticles
(in the form of colloids) are formed [9,12,15,47,50–53], which are
in equilibrium with the supposed effective catalyst. This hypothe-
sis is well supported by the TOF/precatalyst concentration trend
(Table 7). By contrast, a monitoring of the early stages of the reac-
tion (Fig. 6) did not evidence an induction period, which is typical
of the formation of Pd colloids [50], though their formation within
an extremely short time cannot be ruled out. This fact is not
unprecedented, as it was found that the induction period observed
using dry DMA disappeared in DMA/water solution [24]. For our
systems, the presence of water in the 25–50% v/v range can prob-
ably help in the stabilization of palladium nanoparticles of suitable
size [51,54], not too large to induce palladium black formation. On
the other hand, it has been claimed that water, which certainly
favours the solubilization of the inorganic base, can also contribute,
4. Conclusions
The catalyst system here described, based on the combination
C,N-cyclopalladate complex/EGME/H2O/M2CO3 (M = K or Cs), has
shown to be highly efficient for promoting the Heck reaction start-
ing from bromobenzene. The key-tool to maximize the perfor-
mance of the catalyst is the use of a controlled amount of water,
generally in the 25–50% v/v range. Under optimized conditions,
bromobenzene and styrene can be converted quantitatively to
trans-stilbene (yield > 98%) using 0.01 mol% of precatalyst 10 only.
It has been recognized that water improves both rate of conversion
and regioselectivity. Furthermore, the difference of catalytic activ-
ity shown by complexes 7–14 in pure EGME disappears when
water is added. In other words, water seems to possess the prop-
erty of enhancing the activity of the C,N-cyclopalladated precata-
lysts to a unique superior level. The activity of precatalyst 10 is
higher than that exhibited by palladium acetate in the same exper-
imental conditions, and only adopting the so-called Jeffery’s proto-
col does the efficiency of palladium acetate parallel that of a
C,N-cyclopalladated precatalyst. Thus, as already reported, the
presence of ions in solution is essential for obtaining high perfor-
mances of the catalyst system, most probably they can stabilize
the palladium(0) colloidal nanoclusters, which are commonly sup-
posed to act as the source of the true catalyst. It is likely that water
could give an important contribution to the stability of the colloids,
reducing thereby the formation of inactive palladium black.
100
90
80
70
60
50
40
30
20
10
0
Supplementary material
CCDC 664411 contains the supplementary crystallographic data
for this paper. These data can be obtained free of charge from The
Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/
data_request/cif.
Acknowledgements
0
5
10
15
20
25
30
time (min)
This work was financially supported by the Ministero dell’Uni-
versità e della Ricerca (MiUR), Rome. F.I.P. is grateful to the Regione
Autonoma Friuli-Venezia Giulia for a grant. We wish to thank
Mr. P. Polese for carrying out elemental analyses.
Fig. 6. trans-Stilbene yield in the early stages of the reaction. Reaction conditions:
bromobenzene (0.5 mmol), styrene (0.6 mmol), K2CO3 (0.6 mmol), catalyst 10
(1 mol%), solvent: EGME (1.5 mL) + H2O (0.5 mL), t = 120 °C.

104
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