Homogeneous, Heterogeneous and Nanocatalysis - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 15

condensation of aldehydes (or ketones) with primary amines. The required
carbonyl compound is generally prepared by the oxidation of the corres-
ponding alcohol. With a view to creating environmentally benign processes,
the development of catalytic systems for tandem reactions has attracted
considerable attention. From this standpoint, oxidative coupling of alcohols
and amines should be the most direct way to produce imines. The dicopper
complex [Cu 2 (bpnp)(m-OH)(HCOO) 3 ] (Scheme 1.12), stabilized by the bpnp
ligand, is an example of an e cient catalyst for the oxidative coupling of
alcohols with amines to yield the corresponding imines in the presence of
oxygen under solvent-free conditions. 56 Even though more detailed studies
are needed to study the effect of the metal centers, this method holds the
promise of general applicability, producing good yields of products with
various benzyl alcohols and amines, low catalyst loading and solvent-free
reaction conditions.
Yu and co-workers reported on the palladium-catalyzed aerobic oxidative
synthesis of imines from alcohols and amines under ambient conditions
(Scheme 1.13). 57 Ligand, TEMPO and base proved to be key factors for the
reaction and, from a detailed screening, the best results were obtained with
Pd(OAc) 2 , TEMPO and Et 3 N as the base. The imine synthesis occurs in one pot
from alcohols and amines via a low-loading palladium-catalyzed tandem
aerobic alcohol oxidation-dehydrative condensation reaction that can be
readily carried out in open air at room temperature. The same group also
reported findings in the area of air-promoted metal-catalyzed aerobic
N-alkylation. 58 Their method is rather general in catalyst and substrate scope
and uses many simpler, cheaper, readily available ligand-free metal catalysts
and a wide range of amines, amides and alcohols which behave with respect to
activities more effectively than those under conventional anaerobic con-
ditions. Although a few N-alkylation reactions have been carried out under
aerobic conditions and found to be more ecient than the anaerobic re-
actions, the role played by air in such reactions remains to be fully elucidated.

d n 4 r 4 n g | 1

.

N N

+ ArNH 3
catalyst 1 mol %

Ar

OH

N

HO

N N

Cu Cu

catalyst =

CsOHxH 2 O O 2 CH 3

H 3 CO 2 O O

Y

Y

H

Scheme 1.12 Imine formation from arylamines and benzyl alcohol catalyzed by the

dicopper complex [Cu 2 (bpnp)(m-OH)(HCOO) 3 ].

R 2 NH 2

cat. Pd/L
TEMPO, base, air

R 2

R 1

R 1 OH

O

R 1 N

base, Pd

Scheme 1.13 Proposed reaction path for the Pd-catalyzed aerobic oxidative prepar-
ation of imines from alcohols and amines.