Chemistry Reference
In-Depth Information
O
O
56
(5 mol
%)
EtOH, 0°C
+ UHP
(1.3 equiv.)
O
N
N
R
R
+
Co
75-78% ee
F
O
O
F
PF
6
−
F
F
56
Scheme 11.67.
O
O
L
N
N
57
(5 mol
%)
CH
2
Cl
2
, RT
Zr
+ UHP
O
O
O
Ph
L
R
R
Ph
81-87% ee
L = OPh
57
O
O
O
O
57
(8 mol
%)
PhCl, RT
+
+
+ UHP
O
O
R
17%, 94% ee
5
,
8
e
25%, >99% ee
Scheme 11.68.
Subsequently, Katsuki and coworkers reported the zirconium-catalyzed Baeyer-
Villiger reaction using UHP as oxidant (Scheme 11.68) [124]. In the presence of
Zr(salen) complex
57
, a range of 3-substituted cyclobutanone derivatives effi ciently
underwent oxidation to give the enantio-enriched lactones with high enantioselectivity.
Zr(salen) complex
57
also promoted parallel kinetic resolution of racemic cyclobuta-
nones, and the lactones and the recovered ketone were obtained in high enantiomeric
excesses. The authors proposed that the salen ligand of
57
adopts a
trans
- topology in
the resting state but that
cis
- β one in the transition state. The related hafnium(salen)
complex also catalyzed the reaction with the comparable enantioselectivity [125].
A palladium complex has been identifi ed by Ito and coworkers as a Baeyer-Villiger
oxidation catalyst. A cationic palladium
II
complex bearing
P
,
N
- ligand
58
promoted the
reaction of cyclobutanones with good enantioselectivity (Scheme 11.69) [126]. The com-
plete enantioselectivity was achieved in the reaction of a tricyclic ketone. Recently,
Malkov and Ko ˇ ovsk ý also reported chiral terpene - derived
P
,
N
- ligands for the palla-
dium-catalyzed reaction, and a good enantioselectivity of up to 81% ee was observed in
the reaction of 3 - substituted cyclobutanones [127] .
