Catalytic oxidations of alcohols - Advances in Catalytic Activation of Dioxygen by Metal Complexes - page 129

Primary alcohols give the corresponding aldehydes in high selectivity,
e.g. 1-octanol affords 1-octanal in >99% selectivity. Over-oxidation to the
corresponding carboxylic acid, normally a rather facile process, is
completely suppressed in the presence of a catalytic amount of TEMPO. For
example, attempted oxidation of octanal under the reaction conditions, in the
presence of 3m% TEMPO, gave no reaction in one week. In contrast, in the
absence of TEMPO octanal was completely converted to octanoic acid
within 1 h under the same conditions. These results are consistent with over-
oxidation of aldehydes occurring via a free radical autoxidation mechanism.
TEMPO suppresses this reaction by efficiently scavenging free radical
intermediates resulting in the termination of free radical chains, i.e. it acts as
an antioxidant.
Allylic alcohols were selectively converted to the corresponding
unsaturated aldehydes in high yields. No formation of the isomeric saturated
ketones via intramolecular hydrogen transfer, which is known to be
promoted by ruthenium phosphine complexes 30 , was observed.
Although, in separate experiments, secondary alcohols are oxidized
faster than primary ones, in competition experiments the Ru/TEMPO system
displayed a preference for primary over secondary alcohols. This can be
explained by assuming that initial complex formation between the alcohol
and the ruthenium precedes rate-limiting hydrogen transfer and determines
substrate specificity, i.e. complex formation with a primary alcohol is
favoured over a secondary one.
An oxidative hydrogenation mechanism, analogous to that proposed by
Bäckvall for the Ru/quinone system (see earlier), can be envisaged for the
Ru/TEMPO system (see Figure 4).
The intermediate hydridoruthenium species is most probably
as was observed in hydrogen transfer
reactions 31 . The observation that exhibits the same activity as
in the Ru/TEMPO catalyzed aerobic oxidation of 2-octanol is
consistent with this notion.