Chemistry Reference
In-Depth Information
O
O
O
O
N
N
N
N
N
N
Ru
Ru
R
R
R
R
O
O
O
O
N
N
O
O
O
O
Ru(pybox)(pydic)
16
Ru(pyboxazine)(pydic)
17
Figure 11.1.
O
O
17a
(5 mol
%)
AcOH (20 mol
%)
O
N
*
N
N
+ 30% H
2
O
2
*
Ru
2-methylbutan-2-ol, RT
O
O
N
91%, 84% ee
O
O
17a
Scheme 11.20.
Nishiyama's report that Ru(pybox)(pydic) complexes
16
(pydic = 2,6 - pyridinedicarbox-
ylate) promote epoxidation of
trans
- stilbene with PhI(OAc)
2
as oxidant [36], they
designed Ru(pyboxazine)(pydic) complexes
17
[pyboxazine = 2,2
- pyridine - 2,6 - diyl bis(5,6 - dihydro -
4
H
-1,3-oxazine)] for the reaction (Fig. 11.1). The ruthenium complex
17a
- bearing
2-naphthyl groups proved the most effective, and the highest ee value of 84% was
achieved in the epoxidation of 2 - methyl - 1 - phenyl - 1 - propene (Scheme 11.20 ). The addi-
tion of acetic acid improved the catalytic performances, and the authors suggested that
the additional acid stabilizes the active intermediate.
′
11.3.3. Titanium Catalyst
Katsuki and coworkers identifi ed a chiral titanium complex as a catalyst for asymmetric
olefi n epoxidation [37]. They found that di-
- oxo Ti(salalen) complex
18
(salalen = salen/
salan hybrid ONNO-type tetradentate ligand), which is readily prepared from Ti(O
i
Pr)
4
and the corresponding salen ligand
19
via an intramolecular Meerwein-Ponndorf-
Verley reduction, effi ciently promotes the epoxidation of unfunctionalized olefi ns in the
presence of one equivalent of 30% hydrogen peroxide as the oxidant (Scheme 11.21).
High yields and high enantioselectivities were obtained with 1 mol % of catalyst in
the reaction of conjugated olefi ns (Scheme 11.22). The epoxidation of styrene, which is
still a diffi cult substrate for asymmetric epoxidation with regard to both enantioselectiv-
ity and product selectivity, furnished styrene oxide with the high ee value of 93%, and
synthetically important indene oxide was obtained with 99% ee. The reaction of 1,2-
μ
