Catalytic activity of Pd(II) and Pd(II)/DAB-R systems for the Heck arylation of olefins

Article abstract of DOI:10.1016/S0022-328X(03)00375-9

Palladium-catalyzed reactions of aryl bromides with various olefins involving Pd(II)/diazabutadiene (DAB-R) systems have been investigated. The scope of a coupling process using Pd(II) sources and an α-diimine as ligand in the presence of Cs₂CO₃ as base was tested using various substrates. The Pd(OAc)₂/ DAB-Cy (1, DAB-Cy=1,4-dicyclohexyl-diazabutadiene) system presents the highest activity with respect to electron-neutral and electron-deficient aryl bromides in coupling with electron rich olefins. The synthesis and X-ray characterization of a Pd(II)-diazabutadiene ligand is reported. Extensive optimization experiments showed that another Pd(II) source, Pd(acac)₂ (acac=acetylacetonate), proved to activate aryl bromides at high temperatures, low catalyst loadings when the appropriate concentration of ⁿBu₄NBr additive was employed. The effect of the DAB-Cy ligand is important at very low catalyst loadings and high temperatures. Pd(acac)₂ and Pd(acac)₂ /DAB-Cy precatalysts were very effective for the arylation of various olefins with aryl bromides with respect to reaction rate, catalyst loadings, and functional group tolerance.

Full text of DOI:10.1016/S0022-328X(03)00375-9

Journal of Organometallic Chemistry 687 (2003) 269ꢁ279
/
www.elsevier.com/locate/jorganchem
Catalytic activity of Pd(II) and Pd(II)/DAB-R systems for the Heck
arylation of olefins
Gabriela A. Grasa, Rohit Singh, Edwin D. Stevens, Steven P. Nolan *
Department of Chemistry, University of New Orleans, New Orleans, LA 70148-2820, USA
Received 26 February 2003; received in revised form 1 April 2003; accepted 9 April 2003
Abstract
Palladium-catalyzed reactions of aryl bromides with various olefins involving Pd(II)/diazabutadiene (DAB-R) systems have been
investigated. The scope of a coupling process using Pd(II) sources and an a-diimine as ligand in the presence of Cs2CO3 as base was
tested using various substrates. The Pd(OAc)2/DAB-Cy (1, DAB-Cyꢀ1,4-dicyclohexyl-diazabutadiene) system presents the highest
/
activity with respect to electron-neutral and electron-deficient aryl bromides in coupling with electron rich olefins. The synthesis and
X-ray characterization of a Pd(II)-diazabutadiene ligand is reported. Extensive optimization experiments showed that another
Pd(II) source, Pd(acac)2 (acacꢀacetylacetonate), proved to activate aryl bromides at high temperatures, low catalyst loadings when
/
the appropriate concentration of nBu4NBr additive was employed. The effect of the DAB-Cy ligand is important at very low catalyst
loadings and high temperatures. Pd(acac)2 and Pd(acac)2/DAB-Cy precatalysts were very effective for the arylation of various
olefins with aryl bromides with respect to reaction rate, catalyst loadings, and functional group tolerance.
# 2003 Elsevier Science B.V. All rights reserved.
Keywords: Heck coupling; Diazabutadienes; Palladium
1. Introduction
ployed as ancillary ligands [4] in controlling the reactiv-
ity and selectivity in the Heck cross-coupling reaction
[1]. The main role of tertiary monophosphine ligands is
to stabilize the zerovalent palladium as PdL3 or PdL4
species, which can enter the catalytic cycle and conse-
quently prevent the formation of inactive palladium
black. Important examples of the use of phosphines are
found in the work of Fu [5], Hartwig [6] and Beller [7]
who have made use of sterically demanding, electron
rich tertiary phosphines as catalyst modifiers in the
Heck aryl olefination reactions. The ligand properties in
these cases make possible the activation of the less
reactive aryl bromides and aryl chlorides as coupling
partners in the Heck reaction. However, despite their
effectiveness in controlling reactivity and selectivity
transition-metal catalyzed transformations [4], tertiary
monophosphines and their palladium complexes are
The Heck cross-coupling reaction of organohalides
and pseudohalides with olefins has proven to be a
powerful synthetic method for CÃ
/
C bond formation [1],
with wide applications from natural product [2] to fine
chemicals synthesis [3]. In addition to its versatility, and
unlike Kumada, Hiyama, Stille and other Pd-catalyzed
cross-coupling reactions, which often employ expensive
reagents, the Heck reaction presents the advantage of
using cheap and easy available olefins as coupling
partners.
Although Heck arylation and alkylation of olefins
have proven to be one of the most versatile methods for
CÃ
/
C bond formation, various catalytic systems are used
with a myriad of procedures involving different support-
ing ligands, metal source, solvent, additives or substrates
often air-sensitive and are subject to PÃ
/
C bond degra-
[1]. Palladiumꢁphosphine complexes have been em-
/
dation at elevated temperatures [8], commonly em-
ployed under Heck conditions. Hence, higher
phosphine concentrations are often needed with direct
consequences on large-scale applications since tertiary
electron-rich phosphines are unrecoverable from reac-
* Corresponding author. Tel.: ꢂ
6860.
E-mail address: snolan@uno.edu (S.P. Nolan).
/
1-504-286-6311; fax: ꢂ1-504-280-
/
0022-328X/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved.
doi:10.1016/S0022-328X(03)00375-9

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G.A. Grasa et al. / Journal of Organometallic Chemistry 687 (2003) 269ꢁ279
/
tion mixtures and sometimes more expensive than
palladium. A notable exception is the very thermally
stable palladacycle formed by the reaction of Pd(OAc)2
and tris(o-tolyl)phosphine, which represents a very
active catalyst for aryl bromides and activated aryl
chlorides, albeit under harsh conditions [9]. To this
point, bidentate chelating phosphines can form more
tion, catalyst loading, and temperature on the cross-
coupling of olefins with aryl halides.
2. Results and discussions
The a-diimines or 1,4-diazabutadienes represent a
class of ligands of interest not only due to their s-
donating and low p-accepting abilities [6], but also due
to cost-effective issues. These ligands are easily acces-
sible by one step syntheses from the corresponding a-
diketo compounds and various amines [19]. Further ring
closure can afford the synthesis of various imidazolium
salts, precursors of NHC ligands [20]. Alternatively,
Pd(II)-a-diimine complexes can be easily synthesized
from Pd(OAc)2 in ether at room temperature with the
formation of square planar Pd(DAB-R)(OAc)2 com-
plexes (Scheme 2).
stable palladium complexes (Pd/Lꢀ1/1) for Heck reac-
/
tions and therefore an excess of ligand and higher
palladium loadings can be avoided. To date, few
catalytic systems using electron-rich bidentate phos-
phines 1,4-bis(diisopropylphosphino)butane and bis(dii-
sopropylphosphino)propane [10] as catalyst modifiers
have been reported effective for the Heck reaction of
aryl chlorides.
Nucleophilic N-heterocyclic carbenes (NHC) [11], the
imidazol-2-ylidenes, have attracted extensive attention
due to their similar electronic and steric properties to
basic phosphines. Unlike many phosphines, NHC
ligands show a considerable stabilizing effect in orga-
nometallic systems [12] with respect to heat, moisture
and air. As a result, NHC ligands have been employed
efficiently for the cross-coupling of aryl halides with
olefins [13].
We previously reported a general and efficient meth-
odology for the SuzukiꢁMiyaura cross-coupling reac-
/
tion of aryl bromides and activated aryl chlorides with
aryl boronic acids based on the Pd(OAc)2/DAB-R
complexes generated in situ (Scheme 3). The
Pd(OAc)2/DAB-R system is very efficient in the
SuzukiꢁMiyaura cross-coupling reaction of aryl bro-
/
Alternative catalytic systems represent the less ex-
mides with aryl boronic acids in terms of reactivity,
reaction temperature, reaction time and air-stability
plored palladiumꢁnitrogen containing ancillary ligand
/
complexes. A Pd(II) cyclometallated imine catalyst for
the Suzuki and Heck reactions of aryl iodides and
mainly activated aryl bromides has been reported by
Milstein [14,15], but the system requires high reaction
temperatures and reaction times. Ligands containing the
1,4-diaza-1,3-butadiene skeleton (DAB-R, Scheme 1)
have attracted much interest. The coordination versati-
lity of these ligands, a consequence of the flexibility of
the NCCN backbone and the s-donating and low p-
accepting properties, reflects the very interesting proper-
[21a]. Similar systems employing Pdꢁ
/
6 (6ꢀbis(arylimi-
/
no)acenaphthene or Ph-BIAN) have been also reported
to catalyze the coupling of alkyl or allyl halides or aryl
iodides with organomagnesium, tin and zinc reagents
[21b,21c].
2.1. Choice of DAB-R supporting ligands for Heck
reaction of 4-bromotoluene with n-butylacrylate
Since the use of DAB-R as supporting ligand for the
ties of DAB-Rꢁmetal complexes [16]. However, rela-
/
SuzukiꢁMiyaura cross-coupling reaction represents an
/
tively less investigated is the influence of DAB-R as a
supporting ligand in various catalytic processes. Few
interesting alternative to existing catalytic systems based
on the use of tertiary phosphine ligands, we were
interested to determine whether the Heck coupling of
aryl bromides with olefins could be affected by Pd(II)-
diimine complexes. We observed that the coupling of 4-
bromotoluene and 1.5 equivalents of the activated olefin
n-butylacrylate, in the presence of 3 mol% of Pd(OAc)2,
3 mol% DAB-Cy (1) and 1.5 equivalents Cs2CO3 in
N,N-dimethylacetamide (DMAc) at 100 8C proceeded
to give 4-methylstyrene in 93% yield (Table 1, entry 2).
The beneficial ligand effect was proven by running the
control reaction in the absence of ligand (Table 1, entry
1), since it is known that naked palladium can catalyze
the Heck reaction of olefins with activated aryl bromides
[22].
Pdꢁdiimine-catalyzed reactions, such as olefin polymer-
/
ization [17] and alkyne co-cyclotrimerization [18] have
been reported. We now wish to report an extended study
on the use of these ligands as alternative catalyst
modifiers along with the influence of various factors
such as base, solvent, Pd(II) source, additive concentra-
Investigation of other diazabutadiene ligands led to
the observation that alkyl-diazabutadienes (Table 1,
entries 3 and 8) are superior supporting ligands for the
Scheme 1. Diazabutadiene (DAB-R) ligands.

G.A. Grasa et al. / Journal of Organometallic Chemistry 687 (2003) 269ꢁ
/
279
271
Scheme 2. Synthesis of a Pd(DAB-R)(OAc)2 complex.
Pd-catalyzed Heck reaction compared to aryl-diazabu-
tadienes (Table 1, entries 4ꢁ7). This reactivity trend is in
/
agreement with the stronger donating ability of alkyl
substituents making the ligand more electron rich.
However, it is important to note that the steric bulk of
the ligand (usually very significant in the reductive
elimination step in other coupling reactions), is not
very important in this case, with the least sterically
demanding DAB-Cy ligand being the most active ligand
in the alky-substituted DAB-R series. Moreover, this
tendency is in agreement with the general observation
that the steric bulk of the ligand does not significantly
affect the outcome of Pd-free phosphine-catalyzed Heck
reactions [1a].
Scheme 3. Pd(OAc)2/DAB-R catalytic systems for SuzukiꢁMiyaura
reaction.
/
Table 1
Influence of DAB-R ligand on the Heck coupling of 4-bromotoluene
with n-butylacrylate
2.2. Reaction conditions optimization of Pd(OAc)2/
DAB-Cy-catalyzed Heck reaction
An investigation of the influence of the base suggested
that the strong base Cs2CO3 was the reagent of choice.
K2CO3 and KOAc also affect the Heck coupling of 4-
bromotoluene with n-butylacrylate leading to moderate
yields in 5 h. (Table 2). NaOAc, an effective base in
combination with the phosphine palladacycle catalyst
for the Heck reaction [9], showed no activity under our
conditions (Table 2, entry 5). Moreover, Cy2NMe, a
very effective additive for the Pd2(dba)3/P(tBu)3-cata-
lyzed Heck reaction [5b], also proved to be less effective
in the present system.
A survey of other solvents as reaction media showed
that the polar aprotic DMAc is the most suitable solvent
in combination with the Pd(OAc)2/DAB-Cy system. The
more polar solvent N-methylpyrrolidinone (NMP), used
also as medium for Heck reactions [9c,23], led to low
conversions (Table 2, entry 7). As expected ethereal
solvents, which represent more suitable media for Pd/
phosphine catalytic systems [5], did not influence the
outcome of the model reaction (Table 2, entry 8). This
reaction could be performed in air as well, but requires
longer reaction times to lead to a moderate yield since
(Table 2, entry 2).
(a) Reaction conditions: 1 mmol 4-bromotoluene, 1.5 mmol n-
butylacrylate, 3 ml DMAc, 3 mol% Pd(OAc)2, 3 mol% DAB-Cy, 1.5
mmol Cs2CO3, 5 h. (b) GC yields (diethyleneglycol di-n-butlyl ether
used as internal standard, average of two runs).
It is well known that quaternary ammonium salts
show a beneficial effect on the coupling of aryl halides
with olefins [24]. We determined that 5 mol% tetra-n-

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/
Table 2
Influence of the base and solvent on the Heck coupling of 4-
bromotoluene with n-butylacrylate
The effect of the olefin substrates in the Heck
reactions was also examined. Unlike methylacrylate,
which led to low conversion (Table 3, entry 3), we found
that the less active styrene leads to 97% isolated yield in
only 1 h. Para-substituted styrenes led to excellent yields
of the desired products (Table 3, entries 6ꢁ9 and 14),
/
while sterically hindered substrates led to low conver-
sions, even under prolonged reaction times (Table 3,
entries 12 and 13). Attempts to couple 4-bromotoluene
with meta-substituted styrenes like 3-nitrostyrene re-
sulted in low yields. The lower reaction rate may be
explained in terms of the different electronic properties
of meta-substituted styrenes [27].
This catalytic system was very active for more
challenging olefins such as 1-hexene, which led to 1-
aryl substituted hexene as the major product in only 2 h.
Heck reactions involving the less active ethene usually
require high pressure [3b]. We found that the Pd(OAc)2/
DAB-Cy system activates ethene in the reaction with 4-
bromotoluene at atmospheric pressure with the forma-
tion of 4-methylstyrene in 58% isolated yield. However,
since this system is very active for the coupling of aryl
bromides with styrenes, a part of the resulting styrene
participates also in reaction with 4-bromotoluene with
the formation of p-substituted stilbene (Table 3, entry
5). Efforts to couple activated aryl chlorides were not
successful when this catalytic system was employed.
Entry Solvent Base
Time (h) Yield(%) a
1
2
3
4
5
6
6
7
8
DMAc Cs2CO3
DMAc Cs2CO3
DMAc Cs2CO3/5% n Bu4NBr
DMAc K2CO3
DMAc NaOAc
5
24
3
93
57 b
95
5
49
5
NR c
47
DMAc KOAc
DMAc Cy2NMe
5
5
15
30
NMP
DME
Cs2CO3
Cs2CO3
5
6
12
Reaction conditions: 1 mmol 4-bromotoluene, 1.5 mmol n-butyla-
crylate, 3 ml solvent, 3 mol% Pd(OAc)2, 3 mol% DAB-Cy, 1.5 mmol
base.
a
GC yields (diethyleneglycol di-n-butlyl ether used as internal
standard, average of two runs).
b
Reaction performed in the air.
NR, no reaction.
c
butylammonium bromide (nBu4NBr) gave a higher
conversion in a shorter reaction time for the model
reaction (Table 2, entries 1 and 3). This result is in
agreement with studies performed by Amatore and
Jutand [25], who have shown that halide anionic species
Pd0L2Xꢃ could act as active intermediates in the Heck
reaction catalyzed by underligated/phosphine-free sys-
tems. Higher amounts of nBu4NBr (10 and 15 mol%) led
to significantly lower yields. This could be due to the
2.4. Synthesis and structural characterization of
Pd(DAB?-Mes)(OAc)2
In order to gain some insight with regard to the
behavior of the Pd(OAc)2/DAB-R systems in the Heck
arylation of olefins we synthesized a Pd-a-diimine
complex by the reaction of Pd(OAc)2 with diimine 6
(DAB?-Mes). A thermal stability study showed that this
complex decomposes slowly in CDCl2 at 40 8C.
Although, we were not able to obtain the oxidative
addition product of this complex with 4-bromoaceto-
phenone, probably due to its instability, we assume that
the acetate ligands are thermally decoordinated and the
resulting species are reduced in the presence of the base
or olefin [1] to highly active Pd(0)(DAB-R)ꢃ-solvent
stabilized species that activate the aryl bromide towards
oxidative addition [28]. Moreover, the activity and
lifetime of these active species could be improved by
the stabilization effect brought by halide anions [9].
To unequivocally establish the structure of Pd(DAB?-
Mes)(OAc)2 (8) complex, single-crystals suitable for X-
ray diffraction were grown by slow diffusion of a diluted
Pd(OAc)2/THF solution into a diluted DAB?-Mes (6)/
THF solution. The ORTEP diagram of Pd(DAB?-Me-
s)(OAc)2 (8) is presented in Fig. 1 as are selected bond
lengths and angles. As shown in the ORTEP structure
(Fig. 1), 8 adopts a slightly distorted square planar
coordination geometry around the palladium center (the
formation of coordinatively saturated Pdꢁbromide ionic
/
species. Optimization experiments performed on the
influence of the amount of the base and olefin on the
model reaction, showed that the optimum combination
base/olefin is 1.5 equivalents Cs2CO2 and 1.5 equivalents
olefin [26].
2.3. Functional group tolerance of the Pd(OAc)2/DAB-
Cy-catalyzed Heck reaction of aryl bromides with olefins
As illustrated in Table 3, the palladium-catalyzed
Heck reaction of aryl bromides with various olefins with
the assistance provided by the DAB-Cy ancillary ligand
(1) proved active and highly regioselective, with the
formation of the terminally substituted olefin (ꢀ99:1 Z
/
selectivity). While neutral and activated aryl bromides
were converted to the corresponding trans-n-butylcin-
namates in high isolated yields using the optimized
reaction conditions (Table 3, entries 1 and 2), unac-
tivated aryl bromides such as 4-bromoanisole led to low
conversions.

G.A. Grasa et al. / Journal of Organometallic Chemistry 687 (2003) 269ꢁ
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279
273
Table 3
Functional group tolerance of the Pd(OAc)2/DAB-Cy-catalyzed Heck reaction of aryl bromides with olefins
(a) Reaction conditions: 1 mmol 4-bromotoluene, olefin, 3 ml DMAc, 3 mol% Pd(OAc)2, 3 mol% DAB-Cy, 5% n Bu4NBr, 1.5 mmol Cs2CO3. (b)
Isolated yield, average of two runs. (c) 10% 4,4?-dimethyl-trans-stilbene was isolated. (d) GC conversion. (e) E:Zꢀ
20:1 as determined by 1H-NMR,
terminal:internalꢀ3:1.
/
/
sum of the bond angles around Pd is 360.148 with
inequivalent PdÃN and, respectively PdÃO bonds. Also,
the NÃPdÃN bite angle in the rigid five-membered
palladacycle is smaller than 908 due to the pressure
imposed by the bulky mesityl substituents.
plane free, possibly favoring the migratory insertion of
the olefin at the end position with high selectivity for the
formation of the terminally substituted olefin [29]. This
is in agreement with theoretical studies performed on
cationic phenylpalladium(II)diimine catalysts and on
the influence of the electronic and steric effects of the
ligand on the selectivity of the Heck reactions, which
/
/
/
/
This observation is important, since the chelating
planar ligand leaves the space below and above the

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G.A. Grasa et al. / Journal of Organometallic Chemistry 687 (2003) 269ꢁ279
/
with styrene proceeds with a lower catalyst loading at
lower temperature (Schemes 4 and 5).
2.6. Reaction conditions optimization for the Pd(acac)2/
DAB-Cy-catalyzed Heck reaction
Optimization experiments showed that the reaction of
4-bromotoluene with styrene can be run with as low as 1
n
mol% Pd(acac)2 using 5 mol% Bu4NBr in a very short
reaction time and with high conversion (Table 5, entry
9). nBu4NBr additive has a significant effect on the
coupling of 4-bromotoluene with styrene. When no
nBu4NBr was used tꢁ
he reaction rate decreases signifi-
cantly (Table 5, entries 4, 6 and 7), probably due to slow
generation of Pd(0) species [33].
Running the control reaction (no ligand), we observed
that Pd(acac)2 is also an active catalyst at higher
loadings and higher temperatures with slightly longer
reaction times (Table 4, entries 2, 5 and 6).
This is not surprising, since it is known that non-
ligated Pd(0) species are highly active and therefore able
to activate aryl halides with respect to oxidative addition
[25,34]. In this case, the Pd(0) species generated from
Pd(acac)2 appear to be very active and sufficiently stable
at high temperatures in order to support oxidative
addition. However, at 60 8C, the activity of the Pd(a-
cac)2 catalyst decreases significantly, while at the same
temperature we observed a significant ligand effect
(Table 4, entries 11 and 12). As a general trend, the
ligand effect increases as the reaction temperature
decreases; that is the Pd(acac)2 catalyst is thermally
activated at high temperature and requires the assistance
provided by the supporting ligand DAB-Cy in order to
form the active catalytic species under milder condi-
tions.
/
Fig. 1. ORTEP diagram of Pd(DAB?-Mes)(OAc)2 (8). Thermal ellip-
˚
soids are drawn at 50% probability. Selected bond lengths (A) and
angles (8): PdÃO(1), 2.0027(9); PdÃO(3), 2.0079(9); PdÃN(1),
2.0278(4); PdÃN(2), 2.0183(10); O(1)ÃPdÃO(3), 89.97(4); O(1)ÃPdÃ
N(2), 92.79(4); O(3)ÃPdÃN(2), 175.07(4).
/
/
/
/
/
/
/
/
/
/
showed that while electronic effects are minor, steric
effects have a major influence on the regiochemistry of
the migratory insertion step [30].
2.5. Influence of the Pd source on the Heck coupling of 4-
bromotoluene with styrene
Given the fact that the Pd(OAc)2/DAB-Cy system is
not very efficient for hindered and electron-rich aryl
bromides at high temperatures and high catalyst load-
ing, we were interested to determine whether other Pd
sources would improve the yields obtained. Pd(dba)2
and Pd(C6H5CN)2Cl2/DAB-Cy systems gave low con-
versions using 3 mol% catalyst, 5 mol% nBu4NBr,
Cs2CO3 as base and DMAc at 100 8C. We were pleased
to find out that using Pd(acac)2 (acacꢀ/acetylacetonate)
as precatalyst [31,32] the reaction of 4-bromotoluene
Scheme 4. Formation of active Pd(0) species.
Scheme 5. Pd(II)/DAB-Cy-catalyzed Heck reaction of 4-bromotoluene with styrene.

G.A. Grasa et al. / Journal of Organometallic Chemistry 687 (2003) 269ꢁ
/
279
275
Table 4
Reaction conditions optimization of the Heck reaction of 4-bromotoluene with styrene
Entry
T (8C)
Pd(acac)2/DAB-Cy
0.03/0.03
n Bu4NBr
Time (h)
Yield (%) a
1
2
100
0.05
0.05
1
1.5
97
95
0.03/ꢁ
/
3
4
80
0.03/0.03
0.02/0.02
0.05
0.05
0.05
1
1
3
2
95
93
5
0.02/ꢁ
/
93
6
0.02/0.02
ꢁ/
NR
93
7
18
8
0.015/0.015
0.01/0.01
0.005/0.005
0.05
0.05
0.05
2
2.5
98
97
10 b
9
10
20
11
12
60
0.02/0.02
0.05
0.05
6
9
94
22
0.02/ꢁ
/
13
RT
0.02/0.02
0.05
24
NR
Reaction conditions: 1 mmol 4-bromotoluene, 1.5 mmol styrene, 3 ml DMAc, Pd(acac)2, DAB-Cy, n Bu4NBr, 1.5 mmol Cs2CO3.
a
GC yields(diethyleneglycol di-n-butlyl ether used as internal standard, average of two runs).
Reaction conditions: 2 mmol 4-bromotoluene, 3 mmol styrene, 6 ml DMAc, 3 mmol Cs2CO3.
b
Table 5
Influence of n Bu4NBr concentration on the Heck reaction of 4-bromotoluene with styrene
Entry
T (8C)
0.005 mmol DAB-Cy
n Bu4NBr (mmol)
Time (h)
Yield (%) a
1
2
3
4
5
100
0.05
0.10
0.15
0.20
0.10
24
1
30
92
75
40
90
1
24
1
No DAB-Cy
6
7
8
9
80
60
0.10
0.10
3
9
3
9
43
95
48
96
No DAB-Cy
10
11
12
13
14
15
0.10
0.10
0.15
0.15
0.20
0.20
24
24
24
24
24
24
74
62
90
86
44
50
No DAB-Cy
No DAB-Cy
No DAB-Cy
16
17
120
0.001 Pd(acac)2/0.001 DAB-Cy
0.001 Pd(acac)2
0.10
0.10
24
24
70
43
Reaction conditions: 2 mmol 4-bromotoluene, 3 mmol styrene, 6 ml DMAc, 3 mmol Cs2CO3.
a
GC yields(diethyleneglycol di-n-butlyl ether used as internal standard, average of two runs).

276
G.A. Grasa et al. / Journal of Organometallic Chemistry 687 (2003) 269ꢁ279
/
Table 6
Functional group tolerance of the Heck coupling
(a) Reaction conditions: 1 mmol aryl halide, 1.5 mmol olefin, 3 ml DMAc, 10 mol% n Bu4NBr, 1.5 mmol Cs2CO3. (b) Isolated yield, average of two
runs. (c) GC conversion. (d) E:Zꢀ 4/1. (e) E:Zꢀ
20:1 as determined by 1H-NMR, terminal/internalꢀ 20:1 as determined by 1H-NMR,
/
/
/
terminal:internalꢀ/8:1. (f) 1 mol% Pd(acac)2 was used. (g) 24 h.
n
2.7. Influence of Bu4NBr additive
concentration at 100 and 80 8C is 10 mol%, with no
significant ligand effect at these temperatures.
Since the reaction of 4-bromotoluene with styrene
turns using as little as 0.5 mol% Pd(acac)2 as precatalyst,
we decided to investigate the influence of the additive
concentration. We observed that the optimum bromide
Decreasing the temperature to 60 8C and using as low
as 0.5 mol% Pd(acac)2 the optimum concentration of
nBu4NBr increased to 1 mol%. The reaction rate
decreases with the temperature perhaps due to higher

G.A. Grasa et al. / Journal of Organometallic Chemistry 687 (2003) 269ꢁ
/
279
277
concentrations of coordinatively saturated species at
lower temperatures. We believe that at lower tempera-
tures, besides its activation quality, nBu4NBr serves also
as transfer reagent for the insoluble base/solvent/sub-
strates heterogeneous system [24,35]. Moreover, the
concentration of nBu4NBr additive is also important,
since higher bromide concentrations can lead to co-
ordinative saturation, and hence deactivation of Pd(0)
species. A significant ligand effect was observed when
the catalyst loading was reduced to 0.1 mol%. The role
of nBu4NBr additive concentration is threefold: (i)
activation of Pd(0) species with the formation of anionic
species; (ii) stabilization low coordinated Pd(0) species;
(iii) phase transfer catalyst for the inorganic base/polar
solvent/organic substrates/product phases [35].
Pd(acac)2 proved to be a very active and efficient
catalyst precursor with respect to reaction rate, catalyst
loadings, and functional group tolerance in the coupling
of a wide range of aryl bromides with both electron-
deficient or electron-rich olefins. Extensive optimization
experiments showed that using the more active Pd(a-
cac)2 salt the reaction can be performed in certain cases
without the support provided by the DAB-R ligand.
Using the proper concentration of additive, Heck
olefination can be performed in mild conditions (im-
portant factors for thermally labile substrates), under
low catalyst loadings and with high conversions, which
are important factors for industrial scale applications.
Moreover, we observed that optimization of additive
concentration is more important than the role of the
ancillary ligand, which sometime is overestimated in
coupling chemistry. However, at lower catalyst loadings,
the DAB-Cy ligand shows a beneficial effect on the
activity and productivity of the Pd(acac)2 precatalyst.
To this end, we can conclude that generally, unlike other
Pd-catalyzed couplings, good results for the Heck
reaction of aryl halides with olefins are often the
consequence of the appropriate overlapping of a multi-
tude of factors, such as olefin, base, and additive
concentration, solvent, Pd source and supporting ligand.
2.8. Functional group tolerance of the Pd(acac)2/DAB-
Cy-catalyzed Heck reaction of arylbromides with olefins
Since the coupling of styrene with 4-bromotoluene is
accelerated at 100 8C, we decided to investigate the
functional group tolerance at this temperature. Cou-
pling of electron-neutral and activated aryl bromides
with styrenes proceeded with as little as 0.5 mol%
Pd(acac)2, in very short reaction times, giving the
corresponding trans-stilbenes in excellent isolated yields
(Table 6, entries 6ꢁ8). Acrylates proved to be poorer
/
coupling partners under these reaction conditions, when
either Pd(acac)2 or Pd(acac)2/DAB-Cy catalysts were
employed (Table 6, entries 1 and 2). Surprisingly,
unactivated olefins like 1-hexene and 1-octene lead to
excellent yields and high selectivity in terminal Z olefin
in very short reaction times when either the Pd(acac)2 or
Pd(acac)2/DAB-Cy system was employed as the catalyst.
More difficult substrates such as 4-bromoanisole and
sterically hindered substrates were converted to the
corresponding products in high yields and with short
reaction times using 1 mol% Pd(acac)2, with no sig-
nificant change when DAB-Cy was used as a supporting
ligand.
4. Experimental
4.1. General information
All reactions were carried out under an atmosphere of
dry argon using standard Schlenk techniques or in a
MBraun glovebox containing less than 1 ppm of oxygen
and water. NMR spectra were recorded using Varian
400 or 300 MHz spectrometers. GC analyses were
performed on a Hewlett-Packard HP 5890 II equipped
with a FID and a HP-5 column. Diazabutadiene ligands
1ꢁ7 were synthesized according to the literature meth-
/
ods [18,19].
3. Conclusions
4.2. Synthesis of Pd(DAB?-Mes)(OAc)2 (8)
In summary, optimization studies of the Pd(II)-
catalyzed arylation of olefins have shown that, gener-
ally, there is no good or universal formula for this
reaction when diazabutadiene ligands are employed.
Pd(OAc)2 in combination with the DAB-Cy ligand
showed high activity with respect to coupling of
electron-neutral and electron-deficient aryl bromides
with a series of olefins, with excellent activity and
selectivity for styrenes. Square planar Pd(II)(DAB-
R)(OAc)2 complexes are excellent precatalysts for
Pd(0)(DAB-R)ꢃ ionic species generated in the polar
solvent DMAc.
Pd(OAc)2 (10 mg, 0.045 mmol) was dissolved in 5 ml
diethyl ether and added to a 5 ml diethyl ether solution
of DAB?-Mes (6). The deep red solution was stirred at
room temperature for 2 h and the resulting precipitate
filtered, washed with cold diethyl ether and dried under
vacuo (microcrystalline deep red powder: 26 mg, 0.04
1
mmol, 89%); H-NMR (CD2Cl2, 400 MHz), d (ppm):
8.15 (d, Jꢀ
/
8.4 Hz, 2H), 7.54 (t, Jꢀ/8 Hz, 2H), 7.04 (s,
4H), 6.88 (d, Jꢀ
/
7.2 Hz, 2H), 2.46 (s, 12H), 2.35 (s, 6H),
1.31 (s, 6H); 13C-NMR (CD2Cl2, 400 MHz), d (ppm):
177.4, 174.3, 148, 140.3, 139, 132.8, 131.9, 130.7, 130,
129.8, 125.5, 125.1, 21.9, 21.4, 18.4%.

278
G.A. Grasa et al. / Journal of Organometallic Chemistry 687 (2003) 269ꢁ279
/
4.3. X-ray diffraction measurements
compound 8. Copies of this information may be
obtained free of charge from the Director, CCDC, 12
Single crystals of 8 were obtained by slow diffusion of
a diluted THF solution of DAB?-Mes (6) into a diluted
THF solution of Pd(OAc)2. A single crystal was placed
in a capillary tube and mounted on a Bruker SMART
CCD X-ray diffractometer. Data were collected using
Union Road, Cambridge, CB2 1EZ UK (Fax: ꢂ44-
1223-336033; e-mail: deposit@ccdc.cam.ac.uk or www:
http://www.ccdc.cam.ac.uk).
/
MoꢁKa radiation at 150 K. The structures were solved
/
Acknowledgements
using direct methods (SHELXS-86) and refined by full-
matrix least-squares techniques. Selected bond distances
and angles are shown in Table 4.
We gratefully acknowledge financial support from the
National Science Foundation, the Petroleum Research
Fund administered by ACS and the Louisiana Board of
Regents. We thank OMG for the generous gift of
Pd(acac)2.
4.4. Pd(OAc)2/DAB-R cross-coupling reactions of aryl
halides olefins
General procedure: Under an atmosphere of argon
DMAc (3 ml), aryl halide (1 mmol), Cs2CO3 (1.5 mmol),
n
5 mol% Bu4NBr and olefin (1.5 mmol) were added in
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