Homogeneous, Heterogeneous and Nanocatalysis - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 5

of alcohol substrates. Together, these catalytic systems provide compelling
aerobic alternatives to traditional alcohol oxidation methods. This Cu( I )/
ABNO protocol is also suitable for larger-scale applications; 10-50 mmol
(1-9 g) of selected alcohols produced the desired products in Z90% yield in
an open-reaction flask at room temperature within 1-3 h. On the other hand,
several drawbacks due to catalyst deactivation require further attention
(reactions with a 1 mol% MeObpy/Cu loading suggest that the catalyst de-
activates before reaching full conversion of the substrate).
Following the route indicated by the rational design well defined in the
work of Stahl and co-workers, 16 in this first section devoted to homogeneous
catalysis, the most recent examples of aerobic oxidation are described in two
subsections. The first concerns the ligand design and the second focuses on
the use of fine-tuned homogeneous catalytic systems for the one-pot syn-
thesis of high-value products such as heterocyclics or nitrogen-containing
compounds which are suitable either as building blocks or for fine-chemical
synthesis.

d n 4 r 4 n g | 1

1.2.2 Tailored Catalysts for the Selective Oxidation
of Alcohols: Ligand Design
Many different ligands - such as pyridine, N-heterocyclic carbene chelated
N-O-, N-N-, tridentate, pincer and tetradentate ligands - are used for the
preparation of copper, palladium and iron complexes which are active in the
aerobic oxidation of alcohols.

.

1.2.2.1 N-O-Ligands
N-O-dentate ligands for copper-catalyzed alcohol oxidation under air or
oxygen conditions were first investigated by Punniyamurthy and co-workers,
who used a salen analog N-O-ligand to achieve the ecient oxidation of
primary alcohols to the corresponding aldehydes under oxygen conditions. 19
In this field, Ding and co-workers recently reported highly ecient
performances - in particular for secondary alcohols - of the commercially
available and inexpensive N-O-bidentate ligand L -proline under mild con-
ditions (Scheme 1.3). 20 When using 5 mol% of Cu( I ) as the metal precursor,
in the presence of 5 mol% of TEMPO as the co-catalyst and air as the oxidant,
a wide range of primary and secondary benzylic alcohols are transformed
smoothly into corresponding aldehydes and ketones with high yields and
selectivities in DMF at room temperature.

5 mol% CuI, 5 mol% L-proline
5 mol% TEMPO, 1.0 equiv.

OH

O

t -BuOK

R 1 R 2 DMF, air, 25 °C

R 1 R 2

Scheme 1.3 Reaction conditions for the aerobic alcohol oxidation of secondary
alcohols to ketones.