CHEMCATCHEM
COMMUNICATIONS
observed that slight variations in the substituents of the li-
gave low conversions but high selectivity (Table 3, entries 19
and 20).
[
6g]
gands drastically affect the activity of the catalyst.
In view of these results, ligands 1 and 4 were used in the
Fe-catalyzed epoxidation of several alkene substrates. The re-
sults are presented in Table 3. For substituted styrene sub-
In all cases, very similar results were obtained with both li-
gands, and this indicates that these ligands possess similar be-
havior under these conditions. The isolated yields were also
determined for most substrates and showed the synthetic utili-
ty of this methodology (Table 3, entries 1, 3, 5, 7, 9, 13, and
17).
Table 3. Fe-catalyzed epoxidation of various alkenes by using ligands
1
and 4.
The catalytic systems bearing tridentate ligands 1 and 4 are
therefore highly active and selective for the transformation of
aromatic olefins, whereas lower conversions and selectivities
were obtained with aliphatic olefin substrates.
[
b]
[b,c]
Entry
Substrate
L
Conv. [%]
Sel. [%]
Yield [%]
In conclusion, we have developed and tested Fe-based cata-
lysts bearing tridentate nitrogen donor ligands 1–4 in the se-
lective epoxidation of olefins by using hydrogen peroxide as
the oxidant. The optimum L/Fe ratio was revealed to be 1:2,
and this suggests the formation of dimeric iron species bearing
one tridentate ligand. However, further work is required to de-
termine the role of this species in the catalysis system and is
currently ongoing in our laboratory. The presence of substitu-
ents at the central N atom of the ligands drastically lowers the
1
2
3
4
5
6
7
8
9
4-Me-styrene
4-Me-styrene
4-MeO-styrene
4-MeO-styrene
4-tBu-styrene
4-tBu-styrene
4-Cl-styrene
4-Cl-styrene
4-F-styrene
1
4
1
4
1
4
1
4
1
4
1
4
1
4
1
4
1
4
1
4
93
94
95
75
93
93
76
81
55
70
10
13
88
100
39
43
68
65
7
85
90
77
80
87
87
87
81
89
71
57
78
98
97
30
35
71
68
75
66
70
–
65
–
76
–
60
–
46
–
–
–
72
–
1
0
4-F-styrene
1
1
3-NO
3-NO
2
-styrene
-styrene
activity and selectivity of the system. C -symmetric ligands
2
12
13
14
15
16
17
18
19
20
2
1
and 4 form efficient catalytic systems for the epoxidation of
trans-stilbene
trans-stilbene
cis-stilbene
cis-stilbene
cis-cyclooctene
cis-cyclooctene
1-octene
aromatic olefins. Lower conversions and selectivities were ob-
tained for aliphatic olefin substrates.
[
[
d]
d]
–
–
40
–
–
1-octene
6
–
Experimental Section
[a] General conditions: FeCl
3
·6H
2
O (0.01 mmol), ligand (0.005 mmol), ace-
(0.6 mmol, added dropwise
tonitrile (3 mL), substrate (0.2 mmol), H
2
O
2
Materials and synthesis
during the reaction), t=30 min. [b] Determined by HPLC or GC (for non-
aromatic olefin) by comparing with real samples and by using m-xylene
as an internal standard. [c] The only byproduct detected was the corre-
sponding aldehyde, except for the disubstituted olefins cis- and trans-stil-
bene and cis-cyclooctene, for which the corresponding ketone was
formed. [d] The byproducts were the corresponding ketone (ꢂ40%) and
the trans-stilbene oxide (ꢂ30%).
All reagents were purchased from commercial suppliers (Sigma–Al-
drich, Fluka, Merck) and used without further purification. Ultra-
pure iron sources were purchased from Aldrich [for example, iron-
(III) chloride hexahydrate puriss. p. a., Reag. Ph. Eur., ꢀ99%; total
impurities: ꢁ0.001%]. Ligand 1 (2,2’-dipicolylamine, DPA) was also
purchased from Sigma–Aldrich. Ligands 2–4 were synthesized ac-
cording to reported procedures and their identities and purities
were confirmed by comparison with their published NMR
[11–13]
data.
strates bearing electron-donating groups, excellent conver-
sions and selectivities were obtained for both catalytic systems
(Table 3, entries 1–6). The presence of electron-withdrawing
substituents such as chloride, fluoride, and NO2 resulted in
lower values with the formation of the corresponding alde-
hyde as the byproduct (Table 3, entries 7–12). If trans-stilbene
was used, excellent conversions and selectivities were also ob-
tained (Table 3, entries 13 and 14). However, if the cis isomer of
stilbene was the substrate, lower conversions were obtained
and a strong decrease in selectivity was observed (Table 3, en-
tries 15 and 16). In these cases, 2-phenylacetophenone was de-
tected as a byproduct, and for cis-stilbene as the substrate,
about 40% of the trans epoxide was also formed. This observa-
tion is in agreement with the results described in the litera-
Instrumentation
1
13
1
H NMR and C{ H} NMR spectra were recorded with a Varian Mer-
cury VX 400 (400 and 100.6 MHz, respectively) or a Varian 400-MR
spectrometer in CDCl with chemical shifts (d) referenced to CDCl
3
3
1
13
(7.26 ppm for H, 77.23 ppm for C) or TMS (0.00 ppm). HPLC anal-
yses were performed with an Agilent Technologies series 1200 in-
strument equipped with a UV detector with a ZORBAX Eclipse
XDB-C18 4.6ꢄ150 mm, 5 mm column. GC analyses were performed
with an Agilent 6850 Series GC with FID detection equipped with
a column HP-INNOWAX (30 m lengthꢄ0.25 mm I.D.ꢄ0.25 mm film).
GC–MS analysis were carried out with a Agilent 7890A with a MS
[
6d,16]
ture.
In the case of cis-cyclooctene, somewhat intermedi-
5
0
975C detector by using a HP5-MS column (30 m, 0.25 mm,
.25 mm). ESI-TOF analyses were run with an HPLC series 1200, Agi-
ate activities and selectivities were obtained (Table 3, entries 17
and 18). In this case, the corresponding ketone was the by-
product. Interestingly, the use of 1-octene as the substrate
lent Technologies coupled with a Time-of-Flight series 6210, Agi-
lent Technologies detector.
ꢃ 2013 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 2013, 5, 1092 – 1095 1094