Chemistry Reference
In-Depth Information
2
-hydroperoxide as the active species. It is noteworthy
that the catalyst promotes the reaction under solvent-free conditions, and the exception-
ally high turnover number of the catalyst is observed without erosion of the high enan-
tioselectivity (Scheme 11.63 ) [113] .
[112] . The author proposed a
η
53
(0.002 mol %)
pH 7.4 phosphate buffer
O
−
S
S
+
+
30% H
2
O
2
(1.1 equiv.)
Solvent-free
92%, 96% ee
TON 46,000
Scheme 11.63.
11.5. ASYMMETRIC BAEYER - VILLIGER OXIDATION
The Baeyer-Villiger oxidation is a powerful method for obtaining esters from the car-
bonyl compounds [114,115] . The confi guration of the migrating group is retained through
the process, and the reaction is stereospecifi c at the migrating carbon. The predictable
order of migrating groups and the broad tolerance of functionalities also offer a great
advantage. In 1994, the Strukul and Bolm groups independently reported the fi rst
examples of the asymmetric Baeyer-Villiger oxidation using metal complexes as catalyst
[116,117]. Strukul and coworkers discovered that chiral platinum catalyst
54
promotes
the oxidative kinetic resolution of racemic mixture of cyclic ketones with aqueous hydro-
gen peroxide as oxidant and achieved ee values of up to 58% (Scheme 11.64). On the
other hand, the Bolm's group identifi ed chiral copper catalyst
55
for the reaction of
racemic 2-aryl cycloalkanones and obtained the lactones with up to 69% ee (Scheme
11.65). The method employs molecular oxygen as oxidizing agent in the presence of
pivalaldehyde as a sacrifi cial reductant (the Mukaiyama condition) [118].
Since the initial reports on the metal-catalyzed asymmetric Baeyer-Villiger oxida-
tions, signifi cant advances have been seen in the fi eld of the reaction. Especially, Bolm
and coworkers have dedicated their research efforts to this area and found several useful
catalysts. For example, an aluminum/BINOL complex was found to be an effective cata-
lyst for the reaction [119]. Although a substoichiometric amount of the catalyst was
required, 3-substituted cyclobutanones underwent in good enantiomeric excesses of up
to 73% ee (Scheme 11.66). The parallel kinetic resolution of the racemic bicyclooctane
O
Ph
P
h
O
O
O
54
P
n
C
5
H
11
O
n
C
5
H
11
+ 35% H
2
O
2
+
Pt
P
O
n
C
5
H
11
Ph
Ph
OMe
58% ee
54
Scheme 11.64.
