New Pd(II) binuclear complexes as effective catalysts in oxidative-heck reaction using arylboronic acid derivatives

Article abstract of DOI:10.1080/15533174.2011.618480

A series of new binuclear Pd(II) complexes based on chelating diimines and bridging diphosphine or diimine ligands have been synthesized and characterized successfully. The formula of the new complexes are [Pd₂(mbyp) ₂(DPA)₂](PF₆)₄ (1), [Pd ₂(mbyp)₂(DPE)₂] (PF₆)₄ (2), [Pd₂(mbyp)₂(4,4-byp)₂] (PF ₆)₄ (3), [Pd₂(mbyp)₂(t-pye) ₂] (PF₆)₄ (4) [mbyp= 4,4-dimethyl-2,2- bipyridine, DPA= 1,2-bis(diphenylphosphino)acetylene, DPE= 1,2- bis(diphenylphosphino)ethylene, 4,4-byp= 4,4-bipyridine, and t-ype= trans 1,2-bis(4-pyridyl)ethylene)]. The catalytic activities of the new complexes have been investigated in the coupling of arylboronic acid derivatives to various olefins. Results obtained showed interesting catalytic activities and chemoselectivities for the new complexes in the coupling reactions to produce the conjugate addition and heck coupling products under free base or oxidant conditions.

Full text of DOI:10.1080/15533174.2011.618480

This article was downloaded by: [FU Berlin]
On: 21 October 2014, At: 03:31
Publisher: Taylor & Francis
Informa Ltd Registered in England and Wales Registered Number: 1072954 Registered office: Mortimer House,
37-41 Mortimer Street, London W1T 3JH, UK
Synthesis and Reactivity in Inorganic, Metal-Organic,
and Nano-Metal Chemistry
Publication details, including instructions for authors and subscription information:
http://www.tandfonline.com/loi/lsrt20
New Pd(II) Binuclear Complexes as Effective Catalysts
in Oxidative-Heck Reaction Using Arylboronic Acid
Derivatives
Rami Suleiman a , S. M. Shakil Hussain b , Mohammed Fettouhi b & Bassam El Ali b
a Center of Research Excellence in Corrosion , King Fahd University of Petroleum & Minerals
(KFUPM) , Dhahran , Saudi Arabia
b Chemistry Department , King Fahd University of Petroleum & Minerals (KFUPM) , Dhahran ,
Saudi Arabia
Accepted author version posted online: 18 Apr 2012.Published online: 11 Jul 2012.
To cite this article: Rami Suleiman , S. M. Shakil Hussain , Mohammed Fettouhi & Bassam El Ali (2012) New Pd(II) Binuclear
Complexes as Effective Catalysts in Oxidative-Heck Reaction Using Arylboronic Acid Derivatives, Synthesis and Reactivity in
Inorganic, Metal-Organic, and Nano-Metal Chemistry, 42:6, 850-856
To link to this article: http://dx.doi.org/10.1080/15533174.2011.618480
PLEASE SCROLL DOWN FOR ARTICLE
Taylor & Francis makes every effort to ensure the accuracy of all the information (the “Content”) contained
in the publications on our platform. However, Taylor & Francis, our agents, and our licensors make no
representations or warranties whatsoever as to the accuracy, completeness, or suitability for any purpose of the
Content. Any opinions and views expressed in this publication are the opinions and views of the authors, and
are not the views of or endorsed by Taylor & Francis. The accuracy of the Content should not be relied upon and
should be independently verified with primary sources of information. Taylor and Francis shall not be liable for
any losses, actions, claims, proceedings, demands, costs, expenses, damages, and other liabilities whatsoever
or howsoever caused arising directly or indirectly in connection with, in relation to or arising out of the use of
the Content.
This article may be used for research, teaching, and private study purposes. Any substantial or systematic
reproduction, redistribution, reselling, loan, sub-licensing, systematic supply, or distribution in any
form to anyone is expressly forbidden. Terms & Conditions of access and use can be found at http://
www.tandfonline.com/page/terms-and-conditions

Synthesis and Reactivity in Inorganic, Metal-Organic, and Nano-Metal Chemistry, 42:850–856, 2012
Copyright ꢀ Taylor & Francis Group, LLC
C
ISSN: 1553-3174 print / 1553-3182 online
DOI: 10.1080/15533174.2011.618480
New Pd(II) Binuclear Complexes as Effective Catalysts in
Oxidative-Heck Reaction Using Arylboronic Acid
Derivatives
Rami Suleiman,1 S. M. Shakil Hussain,2 Mohammed Fettouhi,2 and Bassam El Ali2
1Center of Research Excellence in Corrosion, King Fahd University of Petroleum & Minerals (KFUPM),
Dhahran, Saudi Arabia
2Chemistry Department, King Fahd University of Petroleum & Minerals (KFUPM), Dhahran,
Saudi Arabia
used in the olefin-methyl acrylate copolymerization.[4] Polynu-
A series of new binuclear Pd(II) complexes based on chelat-
ing diimines and bridging diphosphine or diimine ligands
have been synthesized and characterized successfully. The for-
mula of the new complexes are [Pd2(mbyp)2(DPA)2](PF6)4 (1),
[Pd2(mbyp)2(DPE)2] (PF6)4 (2), [Pd2(mbyp)2(4,4-byp)2] (PF6)4
(3), [Pd2(mbyp)2(t-pye)2] (PF6)4 (4) [mbyp = 4,4ꢀ-dimethyl-2,2ꢀ-
bipyridine, DPA = 1,2-bis(diphenylphosphino)acetylene, DPE =
1,2-bis(diphenylphosphino)ethylene, 4,4-byp = 4,4ꢀ-bipyridine,
and t-ype = trans 1,2-bis(4-pyridyl)ethylene)]. The catalytic ac-
tivities of the new complexes have been investigated in the coupling
of arylboronic acid derivatives to various olefins. Results obtained
showed interesting catalytic activities and chemoselectivities for the
new complexes in the coupling reactions to produce the conjugate
addition and heck coupling products under free base or oxidant
conditions.
clear palladium-dinitrogen complexes are receiving more in-
terest nowadays. One of the shortcomings in this research is
the lack of versatile and general precursors that allow di- and
polynuclear complexes to be constructed from mononuclear
sources. Square planar Pd(II) complexes have been used in con-
jugation with a range pyridine-containing ligands to generate
two- or three-dimensional arrays via self-assembly.[5] Palladium
dinitrogen-diphosphine complexes showed high catalytic activ-
ity in CO-ethylene polymerization.[6] However, reports on pal-
ladium dinitrogen-diphosphine bridged complexes are limited
in the literature.[7]
In this article, we wish to report our results of the synthe-
sis, characterization, and catalytic application of new Pd(II)
bimetallic mixed ligand complexes based on chelating diimines
and having bridging diphosphines or dimine ligands (Figure 1).
Keywords arylboronic acid, binuclear palladium, bipyridines,
chemoselectivity, coupling, olefins, phosphines
Experimental
INTRODUCTION
Materials and Methods
Palladium complexes based on pyridine-containing ligands
have been used effectively as catalysts in different organic and
polymeric reactions for the last years. For example, PdCl2(bpy)
complex shows high efficiency as a catalyst for Heck reac-
tion in glycerol-organic biphasic medium.[1] Pd(II)-catalyzed
intramolecular addition of vinylpalladium species to the nitrile
groups was achieved in the presence of 2,2ꢁ-bipyridine (bpy)
as a ligand.[2] Pd-2,2ꢁ-bipyridyl complex also catalyzed the ox-
idative carbonylation of phenol to diphenyl carbonate.[3] Palla-
dium complexes containing bulky dinitrogen ligands have been
Pd(OAc)2, diimine ligands, diphosphine ligands, triflu-
roacetic acid (TFA), ammonium hexafluorophospate (NH4PF6),
trans-cinnamate esters, and phenylboronic acid [PhB(OH)2] are
highly pure commercially available materials and were used
without any purification. Dry solvents have been used in all ex-
periments. 1H and 13C NMR spectra were recorded on 500 MHz
Joel NMR machine. IR spectra were recorded on Perkin-Elmer
16F PC FT-IR spectrometer. Melting points were determined
using Bu¨chi melting point apparatus. Elemental analysis was
carried out using Perkin-Elmer CHNS-O series 2 2400 ana-
lyzer. UV-Vis spectra were recorded in dichloromethane using
Perkin-Elmer Lambda EZ 210 spectrometer. The screening of
the reactants and products of the catalytic application part was
carried out using Agilent GC 6890 Series gas chromatograph
equipped with a split–splitless injector (split ratios of 20:1). The
temperature of the injector was 250◦C, with 10 psi constant pres-
sure. The column was an HP-5 column (30 m × 0.25 mm i.d.,
Received 8 April 2011; accepted 21 August 2011.
The authors thank King Fahd University of Petroleum and Minerals
(KFUPM-Saudi Arabia) for providing all support to this project.
Address correspondence to Bassam El Ali, Chemistry Department,
King Fahd University of Petroleum & Minerals (KFUPM), Dhahran
31261, Saudi Arabia. E-mail: belali@kfupm.edu.sa
850

PALLADIUM BINUCLEAR COMPLEXES AS CATALYSTS
851
FIG. 1. Pd(II) bimetallic mixed ligand complexes.
0.25 µm film thickness). Helium was the carrier gas at a flow nitrogen. The mixture was stirred and heated at 110◦C for 24 h.
rate of 1.5 mL/min and programmed temperature (50◦C, hold After cooling, the reaction mixture was filtered and a sample
2 min, then ramped at 10◦C/min to 140◦C, then finally ramped of this solution was immediately analyzed by GC and GC-MS.
at 20◦C/min to 250◦C, hold time 20 min). The detector was The solvent was then removed and the products were separated
flame ionization detector (FID), with hydrogen and air flow of by preparative TLC (30% EtOAc/petroleum ether 40–70◦C).
1
40.0 and 450.0 mL/min; respectively. Makeup flow was on with The products were identified by H and 13C NMR, FT-IR, and
45.0 mL/min of helium. The products of the reactions were also GC-MS analyses.
analyzed on GC-MS Varian Saturn 2000 equipped with 30 m
capillary column (HP-5). Helium was the carrier gas at a flow
Synthesis of the Complexes
rate of 1.0 mL/min and programmed temperature (50◦C, hold
Synthesis of Palladium precursor: (4,4ꢁ-Dimethyl-2,
2.0 min, then ramped at 10◦C/min to 140 ◦C, then finally ramped
2ꢁ-bipyridyl) bis(trifluroacetato)palladium(II) [Pd-prec]
at 20◦C/min to 250◦C, hold time 20 min). The temperatures of
The first step in the synthesis of binuclear palladium com-
injector, transfer line, and ionization source were 250, 225, and
plexes involves the synthesis of palladium precursor contain-
270◦C, respectively. NIST MS search was used for compound
ing chelating 4,4ꢁ-dimethyl-2,2ꢁ-bipyridine ligand according to
identification.
Scheme 1. This complex was then allowed to react with bridging
ligands in order to produce the new binuclear complexes.
To prepare Pd-prec, a literature method[8] was modified and
used. In 250 mL Erlenmeyer flask, 6.000 mmol (1.347 g) palla-
dium acetate were dissolved in 100 mL of anhydrous methanol.
General Procedure for the Conjugate Addition of
Phenylboronic Acid to Trans-Cinnamate Esters Catalyzed
by New Pd(II) Bimetallic Mixed Ligand Complexes
To a 45 mL glass liner tube, phenylboronic acid After stirring for 30 min, the orange-red mixture was filtered off
(0.50 mmol), trans-cinnamate ester (0.50 mmol), palladium to remove undissolved palladium acetate. 7.200 mmol (1.326 g)
complex (0.02 mmol), and solvent (10 mL) were added under of 4,4ꢁ-dimethyl-2,2ꢁ-bipyridine were then added to the filtrate

852
R. SULEIMAN ET AL.
[Pd2(mbyp)2(DPE)2] (PF6)4 (2)
Pd(OAc)2
F3CCOOH
N
N
N
N
N
N
Pd
Pd
AcO
OAc
TFA
TFA
Pd-prec
SCH. 1. Synthesis of [Pd-prec].
under nitrogen with constant stirring. The mixture was then
stirred for 30 min. The solution turned intense red. Subsequently,
12.3 mL of 100% trifluroacetic acid were added slowly to the
solution to precipitate the trifluroacetate salt (Scheme 1). Im-
mediately after the addition of trifluroacetic acid, a light green
creamy suspension was obtained. Stirring was continued for an-
other 30 min. The mixture was thoroughly washed with cold
methanol to remove excess of trifluroacetic acid. The yellow
precipitate was dried under vacuum (yield: 2.581 g, 83.2%).
(2)
Light yellow solid, Yield: 0.965 g (98.9%). M.p (◦C):
170–173. Anal. Calcd. for C76H70N4P8F24Pd2: C, 46.72; H,
3.51; N, 2.87. Found: C, 46.44; H, 3.59; N, 2.98%. UV-Vis l/nm
(e M−1cm−1): 250 (1698450), 301 (300000), 308sh (250000).
Selected IR bands (cm−1): 3057 w, 1617 m, 1436 m, 831 s,
553 m.
Synthesis of the binuclear complexes
All the complexes were prepared using a similar method. A
typical example is described below.
[Pd2(mbyp)2(4,4-byp)2] (PF6)4 (3)
[Pd2(mbyp)2(DPA)2] (PF6)4 (1)
0.500 mmol (0.257 g) of Pd-prec were dissolved in 50 mL
acetonitrile in 125 mL Erlenmeyer flask. Separately, 0.500 mmol
(0.197 g) of bis(diphenylphosphino)acetylene (DPA) were dis-
solved in 10 mL methanol. The two solutions were mixed and
stirred for 16 h at room temperature. The color of the mixed so-
lution changed from light orange to dark orange and finally it be-
came light yellow. The volume of the later solution was reduced
to 50 mL in vacuum, then a solution of NH4PF6 (3.0 mmol,
0.49 g) in 5 mL methanol was added. The resulted solution was
stirred at room temperature for 30 min. A light green precipitate
was formed. It was then washed with methanol and dried under
vacuum.
(3)
Light green solid, Yield: 0.725 g (98.4%). M.p (◦C):
180–183. Anal. Calcd. for C44H40N8P4F24Pd2: C, 35.86; H,
2.73; N, 7.60. Found: C, 35.67; H, 2.58; N, 8.04%. UV-Vis l/nm
(e M−1cm−1): 246 (10314539), 319 (437956), 330sh (291970).
Selected IR bands (cm−1): 3062 w, 1666 m, 1613 m, 1432 m,
825 s, 552 m.
[Pd2(mbyp)2(t-pye)2] (PF6)4 (4)
(4)
(1)
Light green solid, Yield: 0.758 g (99.4%). M.p (◦C):
181–184. Anal. Calcd. for C48H44N8P4F24Pd2: C, 37.79; H,
2.91; N, 7.34. Found: C, 37.88; H, 3.06; N, 7.65%. UV-Vis
l/nm (e M−1cm−1): 250 (27841815), 303 (19618320), 335sh
(13078880). Selected IR bands (cm−1): 3018 w, 1612 m, 1436 m,
828 s, 554 m.
Light green solid, Yield: 0.514 g (52.7%). M.p (◦C):
171–173. Anal. Calcd. for C76H64N4P8F24Pd2: C, 46.81; H,
3.31; N, 2.87. Found: C, 46.55; H, 3.45; N, 3.06%. UV-Vis l/nm
(e M−1cm−1): 253 (1443500), 304 (850000), 306sh (800000).
Selected IR bands (cm−1): 3063 w, 2019 w, 1435 m, 833 s,
554 m.

PALLADIUM BINUCLEAR COMPLEXES AS CATALYSTS
853
TABLE 1
cient e. These can be assigned to metal to ligand charge transfer
(MLCT) transitions.[8]
31P chemical shifts (d) (in DMSO-d6) relative to H3PO4 for
phosphorus-containing free ligands and their complexes
31P Chemical
1H and 31P NMR spectroscopy
Compound
31shift (d) ppm
The 31P chemical shift variation with respect to the free lig-
and is consistent with metal phosphorous binding. Down field
shifts of 65.71 and 43.91 d ppm have been observed for DPA-
containing complex (1) and DPE bridged complex (2), respec-
tively (Table 1).
DPA
DPE
(1)
–33.58
–9.13
32.13
34.78
(2)
For bridging diimine complexes, the change in the 1H NMR
chemical shifts of protons in the chelating and bridging diimine
ligands can be used to predict the formation of the new Pd-N
bonds in complexes (3) and (4). The complexation process in
the previously complexes resulted in an increase in the chemical
shifts of ligand protons by the order of 0.10–0.80 d ppm.
It is worth mentioning that our attempts to obtain good crystal
growth for the new complexes are in progress.
RESULTS AND DISCUSSION
Characterization of Complexes
The new Pd(II)-binuclear complexes (1–4) have been pre-
pared in good yields by reacting Pd-prec with diphosphines or
diimines bridging ligands in 1:1 ratio at room temperature. The
elemental analysis, infrared, UV-Vis, and NMR spectroscopic
data are supporting the formation of the new binuclear palladium Catalytic Applications of the New Complexes
complexes.
The use of mononuclear Pd(bpy) complexes as catalysts for
the conjugate addition and Heck coupling of arylboronic acid
to α,β-unsaturated esters,[10] encouraged us to investigate the
catalytic activity of the new binuclear Pd(II) complexes in the
coupling reactions of arylboronic acid derivatives with vari-
ous olefins. The conjugate addition of organometallic reagents
to olefins represents an example of transmetallation between
organometallic reagents and transition metals, which represents
a powerful tool for the construction of C-C bonds.[11]
Infrared spectra
The FT-IR spectra were recorded for free ligands and com-
plexes using KBr pellets in the range of 4000–400 cm−1
.
All spectra showed a strong absorption range from 820 to
830 cm−1 corresponding to aromatic C H out of plane bend-
ing modes. The spectra showed also a shift in the absorption in
the range 1350–1600 cm−1 after complexation. These shifts are
attributed to the change in electron density in the pyridine ring
when the non-bonding pair of electrons on the nitrogen atom is
donated to the metal ion.[9] The aromatic C C and C N stretch-
After the first report of Heck[12] and the further development
by Uemura[13] and Mori,[14] the transition metal-catalyzed cou-
pling of organoboronic acids and olefins, known as the oxidative
Heck reaction, had been extensively investigated,[15] mainly be-
cause boronic acids are stable, nontoxic, and easily available.
The methods developed thus far require the presence of an ox-
idant to reoxidize Pd(0), for example, Cu(OAc)2, quinone, or
O2, producing stoichiometric amounts of metal waste or being
associated with potentially hazardous handling and not suitable
for air-sensitive conditions. The development of efficient palla-
dium catalysts for the conjugate addition and Heck coupling of
arylboronic acid to olefins without adding any oxidant additives
is still a challenging area.
ing absorptions are found in the range of 1400–1650 cm−1
.
Electronic spectra of free ligands and complexes
The absorption spectra of all chelating and bridging ligands
used were recorded. The chelating 4,4ꢁ-dimethyl-2,2ꢁ-bipyridine
ligand has two specific absorption bands, one at 283 nm with
shoulder at 290 nm and the second one at 246 nm. DPA and DPE
as bridging ligands showed one absorption band at 246 nm and
244 nm, respectively, while the trans-1,2-bis(4-pyridyl)ethylene
ligand has three absorption bands at 300, 288, and 246 nm.
Ph
O
Ph
O
O
(3)
OMe
Eq. 1
OMe
+
OMe
+
PhB(OH)2
6a
T (oC), 16 h
Solvent, additive
8aa
7aa
5a
For complexes, the spectra showed absorption bands in the
region of 240–250 nm similar to those in free ligands but with
With any binuclear-bridged synthetic strategy, a certain
slight shift in lmax upon complexation. Most of the complexes amount of rigidity is necessary to generate active catalyst in
absorb in the region of 350–300 nm with high absorption coeffi- terms of selectivity and yield, although, in most cases, it is a

854
R. SULEIMAN ET AL.
TABLE 2
Coupling of arylboronic acid (6a) to 5a reaction. Effect of varying different solventsa
Products distribution (%)c
Entry
Solvent
Additive (mmol)
Conversion 5a (%)b
7aa
8aa
1
2
3
4
THF
CH2Cl2
THF/CH3COOH (1:1)
CH3OH
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
—
19
14
41
64
76
92
65
20
27
30
—
6
100
94
100
80
72
64
92
—
—
98
—
H2O 8.0
—
—
20
28
36
8
100
100
2
5
—
—
6d
7
p-TsOH 0.50
K2CO3 0.50
H2 (100 psi)
—
8
9e
10
CH3CN/i-PrOH (1:1)
aReaction conditions: (3) (0.02 mmol), 5a (0.50 mmol), 6a (1.0 mmol), Solvent (4 mL), 110◦C, 16 h. bDetermined by GC. cDetermined by
GC and 1H-NMR. dCatalyst = 0.04 mmol. e93% Hydrogenation product.
mistaken strategy to target only the most rigid structures, since
the proceeding of some elementary steps required in the catalytic palladium(II)-bridged complexes in the vinylation of aryl-
cycle will not be possible with rigid catalysts.[16] boronic acid in acetonitrile (4 mL) at 110◦C (Eq. 1) (Table 3).
We examined the catalytic activity of the new binuclear
The coupling reaction of phenylboronic acid (6a) with trans- The catalytic activity ranges from moderate to excellent with the
cinnamate ester (5a), adopted as model substrate, was carried formation of the heck-coupling product 8aa as a major product.
out using complex (3) as catalyst precursor (Eq. 1). This reaction
gave the conjugate addition (7aa) and the heck coupling (8aa) reaction of various arylboronic acids (6a–b) with differ-
as major products. ent olefins (5a–h) (Eq. 2). The results are summarized
Using the optimized conditions, we then examined the
The effect of varying different types of solvent on the yield in Table 4. Cinnamate esters 5a,b reacted efficiently with
and chemoselectivity of the reaction was carefully studied and 6a even in the absence of any base or oxidant as addi-
data are reported in Table 2. Low catalytic activities were ob- tives but with lower chemoselectivity in case of 5b (Ta-
tained with THF and CH2Cl2 solvents (Table 2, entries 1,2). ble 4, entries 1,2).[17] Comparable reaction conversion and
The use of glacial acetic acid[10] mixed with tetrahydrofuran chemoselectivity were obtained when 5a was allowed to re-
and in the presence of 8.0 mmol water gave a better conversion act with phenyboronic acid derivative 6b (Table 4, entry 3).
R''
R''
R'
R'
Catalyst (3)
R'
Eq. 2
+
R''B(OH)2
6a-b
+
CH3CN
110 oC, 16 h
R
R
R
8
5a-h
7
with complete chemoselectivity toward 8aa (Table 2, entry 3).
The use of methanol as a solvent resulted in a good activity
(Table 2, entry 4). Changing the reaction solvent to acetonitrile
significantly enhanced the activity of the catalyst producing the
heck-coupling product 8aa as a predominant product (Table 2,
entry 5). The conversion of 5a reached 92% by doubling the
amount of palladium catalyst (Table 2, entry 6). The addition
of acid as additive enhanced the formation of 8aa, while the
addition of base allows the formation of 7aa chemoselectively,
which suggests that the formation of 8aa and 7aa takes place via
two different catalytic pathways (Table 2, entries 7,8). The at-
tempts to increase the yield of 7aa by adding H2 or isopropanol
were not successful; the addition of H2 produced the hydro-
genated product of the starting material (Table 2, entry 9), and
isopropanol led to the Heck-coupling product 8aa in higher
selectivity (Table 2, entry 10).
TABLE 3
Coupling of arylboronic acid (6a) to 5a catalyzed by binuclear
palladium complexes (1–4)a
Products distribution
(%)c
Conversion 5a
Entry
Complex
(%)b
7aa
8aa
1
2
3
4
(1)
(2)
(3)
(4)
48
76
76
76
20
33
28
33
80
67
72
67
aReaction conditions: Complex (0.02 mmol), 5a (0.50 mmol),
6a (1.0 mmol), CH3CN (4 ml), 110◦C, 16 h. Determined by GC.
b
cDetermined by GC and 1H-NMR.

PALLADIUM BINUCLEAR COMPLEXES AS CATALYSTS
855
TABLE 4
Reaction of various olefins (5a–h) with arylboronic acids (6a–b)a
Products distribution (%)c
Entry
1
Olefin 5
Boronic acid 6
Conversion 5 (%)b
76
7
8
R = Ph, R’ = COOMe
R” = Ph
6a
28
7aa
40
72
8aa
60
5a
R = Ph, R’ = COOEt
5b
2
6a
75
7ba
8ba
Cl
B(OH)2
Cl
3
5a
72
32
68
6b
7ab
8ab
O
Ph
OMe
4
6a
6a
6a
6a
6a
6a
60
44
100
–
36
7ca
24
7da
91
64
8ca
76
8da
9
5c
5
R = H, R’ = COOMe
5d
6d
7
R = H, R’ = COCH2CH3
5e
7ea
–
8ea
–
R = Ph, R’ = CHO
5f
8
R = Ph, R’ = H
79
11
28
7ga
23
72
8ga
77
5g
R = H, R’ = CH2OPh
5h
9
7ha
8ha
aReaction conditions: Catalyst (3) (0.02 mmol), 5 (0.50 mmol), 6 (1.0 mmol), CH3CN (4 mL), 110◦C, 16 h. bDetermined by GC. cDetermined
by GC and 1H-NMR. d27% double conjugate addition products.
Acrylate esters 5c,d reacted moderately with 6a (Table 4, en-
The catalytic activity of the binuclear palladium complexes
tries 4,5). Excellent catalytic activity and chemoselectivity of reported herein in the absence of any base, water and oxidant ad-
the new palladium complex (3) was observed with alkene bear- ditives suggests some modifications on the mechanism already
ing ketone group (5e). This is expected since palladium(II) proposed in the literature.[19,20] Previous studies suggested that
catalysts promote 1,4-additions to unsaturated ketones more the regeneration of Pd(II) after each catalytic cycle is the key
efficiently than to unsaturated esters (Table 4, entry 6). This step for the Pd-catalyzed oxidative Heck reaction. Following the
is mainly due to a slow equilibration between C-enolate and release of the coupling product from Pd(II), the Ar-Pd-H inter-
water-sensitive O-enolate for ester derivatives.[18] Surprisingly, mediate might be intercepted by a hydrogen acceptor, such as
aldehyde 5f showed no reactivity with phenylboronic acid 6a solvent or substrate, thus regenerating the Pd(II) without form-
using the optimized catalyst system (Table 4, entry 7). For more ing Pd(0) and so circumventing the need for traditional oxidants
screening of different electron-rich and deficient olefins, we ex- or base.[17]
plored the reactivity of substrates 5g–h with 6a under optimized
reaction conditions. The alkene 5g was arylated effectively with
higher chemoselectivity (72%) forming the heck-coupling prod-
uct 8ga (Table 4, entries 8), while low conversion (11%) was
obtained with 5h (Table 4, entries 9). This can be explained by
the difference in electronic environment of the double bond in
the studied olefins.
CONCLUSION
In summary, new binuclear palladium complexes with chelat-
ing diimines and bridging diphosphines and diimines have been
synthesized and characterized. We have developed an efficient
protocol for the conjugate addition and oxidative Heck coupling

856
R. SULEIMAN ET AL.
of arylboronic acids with both electron rich and electron defi- 10. Lu, X.; Lin, S. J. Org. Chem. 2005, 70, 9651.
11. Rossiter, B.E.; Swingle, N. Chem. Rev. 1992, 92, 771.
12. Dieck, H.A.; Heck, R.F. J. Org. Chem. 1975, 40, 1083.
13. Cho, C.S.; Uemura, S.J. Organomet. Chem. 1994, 465, 85.
14. Du, X.; Suguro, M.; Hirabayashi, K.; Mori, A. Org. Lett. 2001, 3,
cient olefins using the new palladium complexes. The method
requires neither oxidant nor base to operate, broadening the
scope of palladium-catalyzed coupling reactions.
3313.
REFERENCES
1. Sangeeta, V.J.; Deshpande, R.M. Catal. Today 2008, 131, 353.
2. Zhao, B.; Lu, X. Organomet. 2006, 8, 5987.
3. Ishii, H.; Goyal, M.; Ueda, M.; Takeuchi, K.; Asai, M. Appl. Catal. A 2000,
201, 101.
4. Tian, G.; Boone, H.W.; Novak, B.M. Macromolecules 2001, 34, 7656.
5. Fujita, M.; Ogura, K. Coord. Chem. Rev. 1996, 148, 249.
6. Bianchini, C.; Lee, H.M.; Barbaro, P.; Meli, A.; Moneti, S. New J. Chem.
1999, 23, 929.
15. Crowley, J.D.; Ha¨nni, K.D.; Lee, A.L.; Leigh, D.A. J. Am. Chem. Soc.
2007, 129, 12092. (b) Lindh, J.; Enquist, P.A.; Pilotti, A.; Nilsson, P.;
Larhed, M. J. Org. Chem. 2007, 72 7957. (c) Yoo, K.S.; Park, C.P.; Yoon,
C.H.; Sakaguchi, S.; O’Neill, J.; Jung, K.W. Org. Lett. 2007, 9, 3933. (d)
Yoo, K.S.; Yoon, C.H.; Mishra, R.K.; Jung, Y.C.; Yi, S.W.; Jung, K.W.
J. Am. Chem. Soc. 2006, 128, 16384.
16. Gianneschi, C.N.; Masar, M.S.; Mirkin, C.A. Acc. Chem. Res. 2005, 38,
825.
17. Ruan, J.; Li, X.; Saidi, O.; Xiao, J. J. Am. Chem. Soc. 2008, 130, 2424.
18. Miyaura, N. Synlett. 2009, 13, 2039.
7. Janka, M.; Anderson, G.; Rath, N. Inorg. Chim. Acta 2004, 357, 2339.
8. Shemsi, A. Ph.D. Thesis, King Fahd University of Petroleum & Minerals,
Dhahran, Saudi Arabia, 2009.
19. Nishikata, T.; Yamamoto, Y.; Gridnev, I.D.; Miyaura, N. Organomet. 2005,
24, 5025.
20. Albe´niz, A.C.; Catalina, N.M.; Espinet, P.; Redo´n, R. Organomet. 1999,
18, 5571.
9. Durig, J.; Mitchell, B.R.; Sink, D.W. Jr.; Willis, N.; Wilson, A.S. Spec-
trochim. Acta 1967, 23A, 1121.