X. Li et al. / Applied Catalysis A: General 462–463 (2013) 15–22
19
step occurring at a relative pressure (P/P ) ranging from 0.50 to
and 7 vs. 1). However, the amount of benzenediazonium tetrafluo-
roborate apparently affected this reaction yield. With the increase
of the amount, the yield improved gradually (Table 2, entries 1, 8
and 9) and the best yield (99%) was obtained when 1.5 equiv of
benzenediazonium tetrafluoroborate was used (Table 2, entry 9).
In summary, extensive screening showed that the optimal reaction
conditions were 0.2 mmol styrene, 1.5 equiv of benzenediazonium
tetrafluoroborate and 2.0 mol% of Pd under air atmosphere in 1.0 mL
0
0.98, which was characteristic of ordered mesoporous materi-
als with large and uniform mesopores. It has a BET surface area
2
3
of 393 m /g, a pore volume of 0.83 cm /g, and an average pore
size of 8.5 nm calculated from the adsorption branch using the
Barrett–Joyner–Halenda (BJH) model (Table 7, run 0).
As a palladium nanoparticles catalyst, the catalyst 1 (see
Scheme 1) has showed excellent catalytic activity in our recent
studies on Suzuki–Miyaura cross-coupling reactions of arenedia-
zonium tetrafluoroborate salts [48]. Encouraged by these results,
we decided to extend the application of this palladium catalyst 1
and sought to take advantage of its catalytic abilities for the Heck
coupling reactions of arenediazonium tetrafluoroborate salts with
various olefins.
◦
of EtOH at 25 C for 24 h (Table 2, entry 9).
To demonstrate the efficiency and generality of this nanoparti-
cles palladium catalyst 1 and this process, an examination of the
scope of the cross-coupling reactions of a series of arenediazo-
nium tetrafluoroborate salts with methyl acrylate was performed
under the optimized experimental conditions and the results are
presented in Table 3. Whatever the electronic or steric nature
of the cross-coupling partners was, most reactions proceeded
smoothly with methyl acrylate and gave the corresponding cou-
pling products in good to excellent yields under mild conditions.
2-Nitro-substituted benzenediazonium tetrafluoroborate provided
93% yield which is quite significant since this salt failed in several
Heck reactions with acrylates and it was very important that 3-
methyl-substituted diazonium salt gave up to 91% yield which was
higher than that Yang [37] obtained (Table 3, entry 3) because it is
much more sensitive to decomposition than the other salts. Surpris-
ingly, 3-nitro-substituted salt failed to participate in this coupling
reaction and para-substituted benzenediazonium tetrafluorobo-
rates provided the lower yields than ortho-, meta-substituted ones
(Table 3, entries 4, 5 and 7 vs. 2, 3, 6 and 8). In addition, it was found
that 2,6-dichlorobenzenediazonium tetrafluoroborate was also a
good substrate providing 90% yield in terms of methyl acrylate
(Table 3, entry 9).
We began with the examination of the catalytic activity and
catalyst loading after the coupling reaction of benzenediazonium
tetrafluoroborate with styrene was chosen as a model case in the
first optimization studies. Reactions were carried out in the pres-
◦
ence of different catalyst loading under air atmosphere at 35 C and
the results are listed in Table 1. 83% yield was obtained when as low
as 2.0 mol% Pd was utilized (Table 1, entry 3). Either increasing or
decreasing the amount of the catalyst 1 failed to increase the reac-
tion yields (Table 1, entries 1, 2 and 4 vs. 3). The investigation of
◦
reaction temperature indicated that 25 C was the most suitable
for this reaction (Table 1, entry 5 vs. 3 and 6).
The exploration of several frequently-used solvents in cross-
coupling reactions such as MeOH, EtOH, dioxane, DMF and DMSO
revealed that EtOH was the most appropriate solvent (Table 2, entry
1
). MeOH and dixone provided somewhat lower yields (Table 2,
entries 2 and 3 vs. 1). In sharp contrast, DMF and DMSO gave no
product (Table 2, entries 4 and 5). Subsequently, the solvent dosage
was also screened. Unfortunately, neither increasing nor reducing
the amount of EtOH could improve the yields (Table 2, entries 6
In terms of acrylic acid n-butylester, the electronic effect of the
substituted groups of benzenediazonium tetrafluoroborate had no
Table 4
Substrate scope of the Mizoroki–Heck cross-coupling reactions of acrylic acid n-butylester with arenediazonium tetrafluoroborate salts using Pd catalyst 1.a
R2
R2
Pd Catalyst 1
N BF
+
COOn-Bu
2
4
o
.
EtOH, 25 C
COOn-Bu
1.5 equiv
1.0 equiv
Entry
Arenediazonium tetrafluoroborate salt
Product
Yieldb (%)
98
COOn-Bu
1
2
COOn-Bu
COOn-Bu
85
97
3
4
COOn-Bu
COOn-Bu
95
98
NO2
N BF
2
4
5
Cl
Cl
a
Reaction conditions: acrylic acid n-butylester (0.2 mmol), arenediazonium tetrafluoroborate salt (0.3 mmol, 1.5 equiv.), Pd catalyst 1 (2.0 mol% Pd, 62.0 mg), EtOH (1.0 mL),
C, 24 h.
Isolated yield.
◦
2
5
b