Catalytic oxidations of alcohols - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 144

5. COPPER-CATALYZED OXIDATIONS WITH

Copper would seem to be an appropriate choice of metal for the catalytic
oxidation of alcohols with dioxygen since it comprises the catalytic centre in
a variety of enzymes, e.g. galactose oxidase, which catalyze this conversion
in vivo 69,70 . However, despite extensive efforts 71 synthetically useful copper-
based systems have generally not been forthcoming.
Semmelhack reported 72 that the combination of CuCl and 4-hydroxy
TEMPO catalyzes the aerobic oxidation of alcohols. However, the scope was
limited to active benzylic and allylic alcohols and activities were low

(10mol% of catalyst was needed for smooth reaction).
More recently, Marko and coworkers 73 , 74 reported that a combination
of CuCl (5m%), phenanthroline (5m%) and di-tert-butylazodicarboxylate,
DBAD (5m%), in the presence of 2 equivalents of catalyzes the
aerobic oxidation of allylic and benzylic alcohols (Reaction 18). Primary
aliphatic alcohols, e.g. 1-decanol, could be oxidized but required 10m%
catalyst for smooth conversion.

The nature of the copper counterion was critical, with chloride, acetate
and triflate proving to be the most effective. Polar solvents such as
acetonitrile inhibit the reaction whereas smooth oxidation takes place in
apolar solvents such as toluene. An advantage of the system is that it
tolerates a variety of functional groups (see Table 9 for examples). Serious
drawbacks of the system are the low activity, the need for two equivalents of
(relative to substrate) and the expensive DBAD as a cocatalyst.
According to a later report 75 the amount of can be reduced to 0.25
equivalents by changing the solvent to fluorobenzene.