catalysts lead to prohibited undesired hydrogenation of the alkene group and accelerative H 2 activation, respectively.
3 Metal NP-Catalyzed Selective Oxidations Using O 2
The selective oxidation of organic compounds is a fundamental challenge in synthetic chemistry. Although the economic and environmental advantages of molecular oxygen as a chemical oxidant are apparent, few reports on the selective
introduction of oxygen functions into organic compounds exist because oxygenation
often involves radical reactions that are difficult to control. The oxidation reactivity
of organic compounds greatly varies depending on their structures. For example, in
the oxidation of ethylene using silver catalyst, ethylene oxide is produced using
molecular oxygen through a silver-superoxo intermediate. In contrast, when the
substrate is changed from ethylene to propylene, propylene is not converted to
propylene oxide but completely oxidized to CO 2 and H 2 O. This is attributed to the
low bond dissociation energy of the allylic C–H bond. Preferential dissociation of
the allylic C–H bond in organic compounds generally induces unselective reactions
or combustion. Therefore, only limited compounds with tertiary or benzylic C–H
bonds can undergo selective oxygenation reactions.
The Wacker oxidation of alkenes is an important oxygenation reaction. The
Wacker oxidation is conducted under an oxygen atmosphere, but the carbonyl
oxygen introduced into the product is derived from water, not molecular oxygen.
Molecular oxygen reoxidizes the metal and is not added into the substrate. An
oxygen nucleophile from water attacks the positively charged C¼C double bond
through coordination to the metal center to form methyl ketones. Alternatively, the
resulting metal–hydrogen species generated from the substrate through dehydrogenation is oxidized by molecular oxygen, completing the catalytic cycle. In this
reaction, high selectivity for methyl ketone products can be achieved because the
radical character of oxygen is not involved.
Some oxidation reactions that use molecular oxygen as an oxidant give low
selectivity for oxygenated compounds, while oxidations involving dehydrogenation
give high selectivity. In this section, we describe metal NP-catalyzed selective
oxidation reactions involving dehydrogenation, in which molecular oxygen acts as
an oxidant but is not incorporated into the substrate.
3.1 Aerobic Oxidation of Alcohols Using Pd NPs
There have been numerous reports on the catalytic oxidation of alcohols under an
oxygen atmosphere [55]. In particular, Pd had been used as a metal center both in
homogeneous and heterogeneous systems [56]. PdHAP, in which a mononuclear Pd
species is adsorbed to hydroxyapatite (HAP), was reduced to a Pd NP (~5 nm)
62
K. Jitsukawa and T. Mitsudome
3 Metal NP-Catalyzed Selective Oxidations Using O 2
The selective oxidation of organic compounds is a fundamental challenge in synthetic chemistry. Although the economic and environmental advantages of molecular oxygen as a chemical oxidant are apparent, few reports on the selective
introduction of oxygen functions into organic compounds exist because oxygenation
often involves radical reactions that are difficult to control. The oxidation reactivity
of organic compounds greatly varies depending on their structures. For example, in
the oxidation of ethylene using silver catalyst, ethylene oxide is produced using
molecular oxygen through a silver-superoxo intermediate. In contrast, when the
substrate is changed from ethylene to propylene, propylene is not converted to
propylene oxide but completely oxidized to CO 2 and H 2 O. This is attributed to the
low bond dissociation energy of the allylic C–H bond. Preferential dissociation of
the allylic C–H bond in organic compounds generally induces unselective reactions
or combustion. Therefore, only limited compounds with tertiary or benzylic C–H
bonds can undergo selective oxygenation reactions.
The Wacker oxidation of alkenes is an important oxygenation reaction. The
Wacker oxidation is conducted under an oxygen atmosphere, but the carbonyl
oxygen introduced into the product is derived from water, not molecular oxygen.
Molecular oxygen reoxidizes the metal and is not added into the substrate. An
oxygen nucleophile from water attacks the positively charged C¼C double bond
through coordination to the metal center to form methyl ketones. Alternatively, the
resulting metal–hydrogen species generated from the substrate through dehydrogenation is oxidized by molecular oxygen, completing the catalytic cycle. In this
reaction, high selectivity for methyl ketone products can be achieved because the
radical character of oxygen is not involved.
Some oxidation reactions that use molecular oxygen as an oxidant give low
selectivity for oxygenated compounds, while oxidations involving dehydrogenation
give high selectivity. In this section, we describe metal NP-catalyzed selective
oxidation reactions involving dehydrogenation, in which molecular oxygen acts as
an oxidant but is not incorporated into the substrate.
3.1 Aerobic Oxidation of Alcohols Using Pd NPs
There have been numerous reports on the catalytic oxidation of alcohols under an
oxygen atmosphere [55]. In particular, Pd had been used as a metal center both in
homogeneous and heterogeneous systems [56]. PdHAP, in which a mononuclear Pd
species is adsorbed to hydroxyapatite (HAP), was reduced to a Pd NP (~5 nm)
62
K. Jitsukawa and T. Mitsudome