Catalytic oxidations of alcohols - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 124

was concluded that these reactions involve a dehydrogenation mechanism,
followed by oxidation of hydride by dioxygen. More recently, noble metal-
catalyzed oxidative dehydrogenations have been widely applied to the
selective oxidations of alcohols and vicinal diols 4 . Similarly, following the
pioneering work of Heyns and Paulsen 5 , the liquid phase aerobic oxidation
of carbohydrates over supported noble metal catalysts has been extensively
studied by groups in Delft 6 , Eindhoven 7 , Lyon 8 and Zürich 9 .
Noble metal salts, e.g. of Pd(II) or Pt(II) undergo reduction by primary
and secondary alcohols in homogeneous solution. Indeed, the ability of
alcohols to reduce Pd(II) was already described in 1828 by Berzelius who
showed that was reduced to palladium metal in an aqueous
ethanolic solution 10 . The reaction involves a elimination from an
alkoxymetal intermediate and is a commonly used method for the
preparation of noble metal hydrides (Reaction 1). In the presence of
dioxygen this leads to catalytic oxidative dehydrogenation of the alcohol,
e.g. with palladium salts 11-15 .

The oxidation of primary and secondary alcohols to the corresponding
carbonyl compounds plays a central role in organic synthesis 16 .
Traditionally, such transformations have been performed with stoichiometric
quantities of inorganic oxidants, notably chromium VI reagents 17 . However,
from both an economic and an environmental viewpoint, there is a growing
demand for atom efficient, catalytic methods that employ clean oxidants
such as and In this review we will focus on the use of
homogeneous metal catalysts to mediate the selective oxidation of alcohols
using or as the primary oxidant. Heterogeneous catalysts
have been extensively reviewed elsewhere 4 and will be covered only where

they are relevant to the discussion.

2. MECHANISMS

As noted above, the aerobic oxidation of alcohols catalyzed by low-
valent late transition metal ions, particularly those of Group VIII elements,
involves an oxidative dehydrogenation mechanism. In the catalytic cycle
(see Figure 1) a hydridometal species, formed by elimination from
an alkoxymetal intermediate, is reoxidized by dioxygen, presumably via
insertion of into the M-H bond with formation of Alternatively, an
alkoxymetal species can decompose to a proton and the reduced form of the
catalyst (see Figure 1), either directly or via the intermediacy of a