1958
M. Guerrero et al. / Journal of Organometallic Chemistry 695 (2010) 1957e1960
In this paper we describe the study of the catalytic activity of
several PdII complexes in the Heck reaction with the main objective
a
L1
L2
N
N
of checking the influence of the linkers between the pyrazole rings.
O
N
O
N
N
2. Experimental
All experiments were carried out under an inert atmosphere
using standard Schlenk-type techniques. The quantification of the
catalytic reaction was carried out using a Hewlett Packard HP5890
gas chromatograph equipped with a flame ionization detector
(FID), and a Hewlett Packard HP-5 column (30 m long, 0.32 mm
internal diameter and 0.25 mm film thickness). The stationary
phase consists of 5% diphenyl/95% dimethyl polysiloxane.
Complexes 1 [37], 2, 3, and 4 [36] were prepared according to the
literature methods. NMR spectra were recorded on a Bruker DPX-
300 spectrometer. Chemical shifts are referenced to an internal
Me4Si standard for 1H and 13C NMR.
N
O
N
O
N
L3
L4
N
N
O
N
O
N
N
N
O
N
O
N
2.1. General procedure for the Heck-type coupling reactions
O
O
b
R
Prescribed amount of base (1.4 equiv), alkene (1.5 equiv), aryl
halide (1.0 equiv) and decane (GC internal standard) were placed in
a round bottom flask under a dry nitrogen atmosphere with
a magnetic stirring bar. A solution of the palladium complex (2 mL)
was added through a rubber septum and the resulting mixture was
heated to the prescribed temperature until reaction completion.
The reaction mixture was then cooled to room temperature. After
extraction with CH2Cl2 (3 ꢀ 20 mL), the combined organic phases
were dried over MgSO4. The solvent was evaporated and a crude
product analyzed by GC. The cross-coupling products were char-
acterized by their 1H NMR or GC analysis.
R = ethyl (1)
R = o-xylyl (2)
R = m-xylyl (3)
R = p-xylyl (4)
N
N
N
N
Cl
Pd
Cl
Scheme 1.
active catalyst for the Heck reactions when aryl iodides are used). It
is worthy to note that the normally inactive styrene can also
proceed the cross-coupling reaction smoothly under the same
reaction conditions to afford the cross-coupled products in quan-
titative yields (entries 5e6). However, the reaction time has to be
prolonged to get a complete conversion. Otherwise, the use of
bromobenzene as reagent yields slightly lower results (entry 7e8),
especially for complex 2 (entry 9e10). However, it is remarkable
that complex 1 has a better catalytic activity than complex 2. This
behaviour may be explained by the higher flexibility exhibited by
L1 (its square planar PdII complex is less distorted) [37].
For several years, aryl bromides and iodides have been preferably
used as substrates in such reactions; aryl chlorides are transformed
very sluggishly by standard palladium catalysts due to the strength
of the CeCl bond. There has been a growing interest in finding
catalytic systems that can successfully catalyze cross-coupling
reactions with aryl chlorides [39,40]. Therefore, in order to check the
activity of complexes 1e4 and try to optimize the conditions of the
reaction, we have chosen the reaction of chlorobenzene with tert-
butyl acrylate as olefinic counterpart. Interestingly, we have found
that complex 1 shows the bests catalytic results (entries 11e12) and
no marked differences in the reactivity between complexes 2, 3, and
4 have been observed (entries 13e18). All these observations are in
agreement with those published by our group for L1. This ligand has
a major flexibility and versatility than their analogous because L1 is
able toaccommodate awide range of metal coordination geometries
(tetrahedral, cis/trans-square planar, or octahedral) [37]. In conse-
quence, the presence of the phenyl group (linker) in the alkyl chain
of the complex provokes an important decreasing effect in the
catalytic activity. Moreover, there is no significant difference
between the relative positions of the substituents in the phenyl ring
(ortho, meta and para). In spite of that, complex 1 exhibits good
catalytic results in the Heckreaction evenforless reactiveolefin such
as styrene (entries 19e20) and these results are better than those
obtained for complex 2 (entries 21e22).
3. Results and discussion
We have previously reported the synthesis and characterization
of the ligands 1,8-bis(3,5-dimethyl-1H-pyrazol-1-yl)-3,6-dioxaoc-
tane (L1) [37],1,2-bis[4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-oxabutyl]
benzene (L2), 1,3-bis[4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-oxabutyl]
benzene (L3), and 1,4-bis[4-(3,5-dimethyl-1H-pyrazol-1-yl)-2-oxa-
butyl]benzene (L4) [36] (Scheme 1a). These ligands and their cor-
responding complexes [PdCl2(L)] (L ¼ L1 (1), L2 (2), L3 (3) and L4 (4))
were prepared in good yields using the method previously described
in the literature by our research group (Scheme 1b) [36,37].
Complexes 1e4 have been used as pre-catalysts in the Heck
reaction between phenyl halides and tert-butyl acrylate. A charac-
teristic of these complexes is the thermal stability, which makes it
possible to perform the reactions even at temperatures above
140 ꢁC. In some cases, the reaction has also been studied using
styrene as the olefin. The reaction progress was analyzed by gas
chromatography (GC). The use of complexes 1e4 for the Heck
olefination of aryl halides gives rise exclusively to the formation of
trans-acrylic acid esters (1H NMR). These complexes were not
sensitive to oxygen or moisture; no change in their efficiencies was
observed even if the Heck coupling reactions were carried out
under aerobic conditions. During the reaction, a black solid
precipitated from the reaction mixture. This solid was identified as
Pd0 through the mercury poisoning test [38]. The results of the
catalytic Heck olefination of aryl iodides, bromides and chlorides
using complexes 1e4 are summarized in Table 1.
Preliminary catalytic studies of complexes 1 and 2 in the Heck
reaction between iodobenzene and tert-butyl acrylate at 140 ꢁC
(bath temperature) and with triethylamine (Et3N) as base in
dimethylformamide (DMF) showed that they have similar effi-
ciencies (entries 1e4). We have found that complexes 1 and 2 are
more effective catalysts than classical PdII salts (even PdCl2 is an
Guerrero, Miguel
