X.L. Yan et al. / Polyhedron 147 (2018) 75–79
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2.3. Catalysis studies
To develop an optimal catalytic system, we investigated 4 as a
precatalyst for the Friedel–Crafts acylation reaction of anisole with
cinnamoyl chloride (Scheme 2 and Table 2). A solution of anisole
(2 mmol) and cinnamoyl chloride (6 mmol) in 1,2-dichloroethane
(3.5 mL) was heated in the presence of the precatalyst and differ-
ent amounts of o-chloranil under an argon atmosphere. Using
refluxing 1,2-dichloroethane, the main para products was
obtained. The isolated yield reached 66.7% with higher para selec-
tivity when the reaction time was 24 h and the ratio of 4 to o-chlo-
ranil was 1:4 (Table 2, entry 6). Upon increasing the proportion of
oxidant, the isolated yield has not improved (Table 2, entries 16
and 18). With an extended reaction time, the yield has not
improved also. The experiment was carried out under the absence
of 4 or o-chloranil (Table 2, entries 9 and 10), the results showed
that no products were obtained, which suggested that elec-
trophilic-substitution mechanism was involved in the complex 4/
o-chloranil catalytic system.
In order to test the capability of Friedel–Crafts acylation reac-
tions (Scheme 3) catalyzed by these monobridged bis(cyclopenta-
dienyl)molybdenum carbonyl complex, influencing factors such
as the reaction time, yield, economic considerations etc., the fol-
lowing experimental conditions were chosen for further work:
1,2-dichloroethane as solvent; a molar ratio of aromatic substrates
and acylation reagents of 1:3; the molar ratio of catalyst to oxidant
was 1:4; reaction time 24 h; temperature 80 °C. Three aromatic
substrates were chosen under the optimized conditions and six
acylation reagents were employed, as shown in Table 3. Complex
4 proved to be capable of catalyzing Friedel–Crafts acylation reac-
tions. The yields% were found to vary with the different reactions
as indicated in Table 3. The structure and characterization of cat-
alytic products were presented in the Supporting Information.
Among six acylation reagents, benzoyl chloride could be used as
acylation reagent in these reactions and the corresponding acyl
products were obtained with high selectivity for the para-products
Scheme 3. Complex 4 catalyzed Friedel–Crafts acylation reactions.
in all cases, this reason may be due to the fact that benzoyl acylium
ion is relatively stable. The order of increasing reactivity was found
to be: 2-bromoanisole < anisole < 2-methylanisole, the results
showed that the reaction was favored by the presence of elec-
tron-donating groups in the aromatic substrate. It is consistent
with the characteristics of the aromatic electrophilic substitution
reaction [27]. Further studies will be needed to elucidate the mech-
anism of these catalytic reactions.
With the above catalytic studies of 4, 5 and 6 with o-chloranil
were further investigated under the optimal experimental condi-
tions, the results are shown in Table 4. Both complexes 5 and 6
proved to be capable of catalyzing Friedel–Crafts acylation reac-
tions also; moreover, the catalytic yields had only a small change
with the different catalysts used. The results showed that the cat-
alytic behavior was not obviously affected by the different ligand
bridges.
3. Conclusions
The reactions of monobridged biscyclopentadiene ligands
(C5H5)R(C5H5) [R = C(CH3)2 (1), Si(CH3)2 (2), C(CH2)5 (3)] with Mo
(CO)6 in refluxing xylene result in the formation of monobridged
bis(cyclopentadienyl) dimolybdenum carbonyl complexes. A new
Friedel–Crafts acylation catalyst is reported in this study, consist-
ing of various monobridged bis(cyclopentadienyl) dimolybdenum
carbonyl complexes and an organic oxidant, namely o-chloranil.
Compared with traditional catalysts, the present catalyst system
has a significant practical advantage: both of the catalyst precur-
sors [(g
5-C5H4)2R][Mo(CO)3]2 and o-chloranil are stable, easy to-
handle solids and they do not exhibit acidity unless mixing in an
aromatic solvent. Therefore, directly employing a harmful strong
acid reagent can be avoided. Besides, the catalyst system could
Scheme 2. Complex 4 catalyzed Friedel–Crafts acylation reaction of anisole with
cinnamoyl chloride.
Table 2
Optimization of Friedel–Crafts acylation of anisole using 4.
Entry
4 [mol%]
o-Chloranil [mol%]
Temp. [°C]
t [h]
Isolated yielda [%]
1
2
3
4
5
6
7
8
2.5
10
60
12
24
12
24
12
24
12
24
24
24
12
24
12
24
12
24
12
24
28.6
40.2
30.3
43.7
35.9
66.7
32.1
50.3
0
2.5
2.5
2.5
10
10
10
70
80
90
80
9
0
2.5
2.5
10
0
5
10
11
12
13
14
15
16
17
18
0
25.4
29.3
30.4
40.0
33.2
49.8
33.4
49.8
2.5
2.5
2.5
7.5
12.5
15
a
Isolated yield was determined by column chromatography.