Asymmetric Oxidation of Alcohols and Phenol Derivatives with Air as Oxidant - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 232

20.9% by volume, and the use of air as oxidant provides other advantages in
cost and safety. Hence the asymmetric catalysis of oxidation reactions using
air as the oxidant has been actively investigated. Generally, oxygen activation
requires electron transfer and redox-active transition metal complexes can
be ecient catalysts for aerobic oxidation, and various ecient transition
metal catalysts have recently been developed for asymmetric aerobic oxi-
dation. 2 In this chapter, asymmetric oxidation of both alcohol and phenol
derivatives using air as oxidant is described with a focus on ruthenium-,
iridium- and iron-based catalysts.

d n 4 r 4 n g | 1

9.2 Asymmetric Oxidation of Alcohols with Air
Asymmetric oxidation of alcohols is roughly classified into two types,
(i) oxidative kinetic resolution (OKR) of racemic secondary alcohols and
(ii) oxidative desymmetrization of meso-diols (Scheme 9.1).
Optically active secondary alcohols are versatile chiral building blocks
and their synthesis continues to be of growing importance. Although
many methods for the asymmetric synthesis of enantioenriched secondary
alcohols have been reported, 3 few methods are available for obtaining
optically pure or almost optically pure secondary alcohols, especially
aliphatic alcohols, except for enzymatic processes that show (almost)
complete enantioselectivity, albeit generally with narrow substrate scopes. 4
OKR of racemic secondary alcohols, most of which are readily available in
bulk, is a route to enantiopure alcohols, 5

although at least half of the

racemic alcohols are consumed. 6
Asymmetric oxidative desymmetrization of meso-diols is a potent tool
for obtaining optically active hydroxy ketones, lactols and lactones, which
are useful synthetic intermediates for various natural product syntheses.

.

(i) Oxidative kinetic resolution of racemic alcohols

cat.
air

OH OH O

+

R 1 R 2 R 1 R 2 R 1 R 2

racemic

(ii) Oxidative desymmetrization of meso -diols

cat.
air

OH OH OH O

R R R R

n n

meso

Scheme 9.1 Examples of typical asymmetric oxidation of alcohols.