Chemistry Reference
In-Depth Information
asymmetric
cis
- dihydroxylation of olefi ns, Que and coworkers found that Fe
II
complex
25
together with aqueous hydrogen peroxide as the oxidant can oxidize
trans
- 2 - heptene
to the epoxide (Scheme 11.31) [47]. Accompanying the
cis
- dihydroxylation product with
29% ee,
trans
-2-heptene oxide was obtained with 12% ee. Although the enantiomeric
excesses are modest, these results indicated a potential of iron-based complexes as cata-
lysts for asymmetric epoxidation.
O
12% ee
25
N
N
+ 50% H
2
O
2
+
Fe
2+
CH
3
CN
OH
N
N
(CF
3
SO
-
)
2
25
OH
29% ee
Scheme 11.31.
In 2007, Beller and coworkers achieved the fi rst highly enantioselective, iron-cata-
lyzed epoxidation using aqueous hydrogen peroxide as an oxidant (Scheme 11.32) [48].
Chiral amine-based ligands were examined in combination with achiral H
2
pydic ligand,
and
N
- benzyl -
N
- toluenesulfonyl - 1,2 - diphenylethylenediamine
26
was found to exhibit
high enantioselectivity in the epoxidation of
trans
- disubstituted aromatic olefi ns. The
authors noted the importance of the sulfonyl group for an intramolecular hydrogen
bonding. The method employs an inexpensive iron source, FeCl
3
· 6H
2
O, and the best ee
value of 97% is attained in the reaction of a substituted stilbene. Although the process
has a limitation on the substrates, the promising asymmetric epoxidation catalysis of
chiral iron-based complexes was proven. Spectroscopic and kinetic studies indicate that
the reaction proceeds through benzyl radical intermediates [49]. While a high-valent
Fe=O complex is proposed as the active species, further studies are necessary to ensure
the mechanism.
′
FeCl
3
•6H
2
O (5 mol
%)
H
2
pydic (5 mol)
26
(12 mol
%)
2-methylbutan-2-ol, RT
Ph
Ph
O
O
O
*
R
R
NH
HN
S
+ 30% H
2
O
2
(2 equiv.)
Ar
Ar
*
26
t
Bu
O
O
O
t
Bu
t
Bu
9
4
,
2
e
82%, 81% ee
4
9
e
Scheme 11.32.
