Homogeneous, Heterogeneous and Nanocatalysis - Transition Metal Catalysis in Aerobic Alcohol Oxidation - page 16

1.2.3.5 Nitrile Synthesis
Nitriles are widely used in both chemical and biological applications as they
can be important building blocks in the pharmaceutical, fine chemical, dye
and agrochemical industries. However, despite the potential usefulness of
their available synthetic protocols, 59 the use of toxic solvents and expensive
reagents along with the production of large amounts of inorganic waste and
tedious work-up procedures limit their application from a green chemistry
perspective. The oxidative conversion of alcohols to nitriles is an attractive
alternative and the development of a highly ecient and environmentally
benign solvent-free protocol is still a challenging task. As an example,
Bhanage and co-workers exploited a copper catalyst for the aerobic oxidative
reaction of benzylic and allylic alcohols, using ammonium formate as the
nitrogen source, for the synthesis of nitrile compounds under an air at-
mosphere and solvent-free conditions (Scheme 1.14). A wide range of sub-
strates were well tolerated in the reaction, which produced water as a
by-product. 60 From a green perspective, the developed protocol uses an in-
expensive and readily available CuCl 2 6H 2 O catalyst; the system circum-
vents the use of oxidants such as 2,3-dichloro-5,6-dicyano-1,4-benzoquinone
(DDQ), I 2 ,H 5 IO 6 and 1,3-diiodo-5,5-dimethylhydantoin (DIH) and the use of
a handy and inexpensive nitrogen source such as ammonium formate.

d n 4 r 4 n g | 1

.

1.2.3.6 Asymmetric Synthesis
Transition metal catalysis, organocatalysis and biocatalysis have attracted
much attention in the field of organic synthesis because they can provide
highly complex molecules in addition to good chemo-, regio- and stereo-
selectivity. In recent years, dual catalytic systems, which combine different
catalytic species among transition metal complexes, organic catalysts and
biocatalysts, have been reported for multifunctionalization and multistep,
one-pot reactions. 61 Although a number of multicatalytic reactions using
both organocatalysts and transition metal catalysts have been implemented
and their powerful advantages described, one of the limitations of dual
metal-organocatalytic reactions is that the rigorous reaction conditions re-
quire an inert atmosphere and purified solvents to maintain the reactivity of
the transition metal catalyst. Jang and co-workers 62 combined an aerobic
copper-catalyzed oxidation with an organo-catalyzed Michael addition
reaction (iminium catalysis). By controlling the amount of TEMPO, allylic

CuCl 2 x2H 2 O

CN

OH HCOOHNH 3

air, K 2 CO 3

R

R

135 °C, 24 h
R = H, OMe, NO 2 , Cl, Br, OPh, CH 3 , CH(CH 3 ) 2 , Ph

Scheme 1.14 Oxidative synthesis of nitriles from alcohols and ammonium formate.