Homogeneous, Heterogeneous and Nanocatalysis - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 8

catalyst 0.01 mol%

OH

O

NaOA c, PEG 40 0

O 2 ,120°C

R 1 R 2

R 1 R 2

d n 4 r 4 n g | 1

+ Br -

N

N N

N

nBu

O HO

or

Pd Cl N Pd Br

catalysts =

O O

nBu

NN

N

N

Scheme 1.5 Palladium NCN and CNC pincer complexes effective in the aerobic
oxidation of secondary benzyl alcohols.

1.2.2.4 Water-Soluble Copper Complexes
Following the pioneering work of the groups of Sheldon, 25 Reedijk 26 and
Repo 27 on the use of Cu-based, water-tolerant oxidation catalysts, Pombeiro
and co-workers recently developed several copper complexes for the TEMPO-
mediated aerobic oxidation of alcohols in water. 28 Catalytic processes in
aqueous media may require constrained active particles in the aqueous
phase and the design of homogeneous water-soluble catalysts usually in-
volves the use of a suitable ligand with a hydrophilic function. 29 In most
cases, such a ligand can be obtained by attaching a water-solubilizing group,
e.g. a sulfonate or carboxylate group. Another major challenge is the re-
versible tuning of the acid-base properties of the catalyst, which can en-
hance the activity/selectivity towards a certain product and/or reversibly
change the hydrophilicity. In addition, many processes require a certain pH
interval. Therefore, a complex is needed that, in addition to its main catalytic
function, can maintain the pH within a certain range (i.e. have buffer
properties). Hence the synthesis of water-soluble complexes with several pH-
tunable sites for protonation-deprotonation is of interest for new green
catalytic processes. In this field, the coordination chemistry of copper has
been explored with N-ethyldiethanolamines for the formation of dicopper( II )
alkoxo-bridged complexes using a self-assembly method. 28 Azo derivatives of
b-diketones (ADBs) - in particular o-hydroxy-substituted ADBs 30 and their
congeners - contain sites for possible pH regulation properties, especially
sulfo and amino groups such as 3-(5-chloro-2-hydroxy-3-sulfophenyl-
hydrazo)pentane-2,4-dione. 31 All of the above-mentioned systems have shown
to be significantly active in the aerobic TEMPO-mediated oxidation of benzyl
alcohols to aldehydes. A more recent study investigated the template for-
mation of alkoxy-1,3,5-triazapentadien(e/ato)copper( II ) complexes: 32 the lat-
ter are active in the oxidative conversion of primary and secondary alcohols
to the corresponding carbonyl, making possible molar yields of aldehydes of

.