In recent years, organic synthesis has improved to the points of being environmentally-friendly. Many excellent methodologies have been devised for green chemistry and environmentally-friendly manufacturing. Oxidation reactions employing atmospheric oxygen or molecular oxygen as oxidant is one of the methodologies of green chemistry. The earth is filled with air, and about 20% of air is oxygen. If oxygen could be utilized as the oxidant, this would provide a method that is not only economically prudent, but also environmentally clean. However, the oxidation capability of atmospheric oxygen and molecular oxygen is not powerful enough when used alone. Therefore, there is much active R&D underway to make improvements in the oxidation capability with the presence of transition metal complex catalysts and radical producing catalysts. For example, Markó and co-workers have reported that tetrapropylammonium perruthenate (TPAP) can be used for the oxidation of alcohols to aldehydes and ketones using molecular oxygen as the oxidant.¹⁾ Similarly, Uemura and co-workers have demonstrated that palladium(II) diacetate is also a catalyst for the same type of oxidations.²⁾ Katsuki and co-workers reported that the oxidation method of alcohols using (nitrosyl)Ru-Salen complex as a catalyst.³⁾ This catalyst is activated by photo-irradiation and can be used to oxidize primary alcohols to aldehydes selectively. Fukuzumi and co-workers reported that the oxygenation using 9-aromatic substituted acridinium derivatives as effective electron-transfer photocatalysts.⁴⁾ Ishii and his group have reported a catalytic carbon radical formation method using N-hydroxyphthalimide (NHPI).⁵⁾ In this method, the hydrogen atom of the hydroxyimino group in NHPI is pulled off by molecular oxygen, thereby producing a phthalimide N-oxyl (PINO) radical. The PINO radical then pulls hydrogen atoms from carbon-hydrogen bonds such as alkanes and alcohols to furnish the corresponding carbon radicals. The resulting carbon radicals readily react with different types of molecules, thereby producing oxygen-containing compounds such as carboxylic acids under an oxygen atmosphere.
Typical transition metal complex catalysts and carbon radical producing catalysts for oxidation using molecular oxygen are as follows.

Transition Metal Complexes

Radical Producing Catalysts

Redox Catalysts

Ligands

Transition Metal Complexes

C0373	M0042	V0016
T0746	C1944	A1424
T1559	I0079	D1997

Radical Producing Catalysts

M1775	D3428	D3429
B2897	M1774	M2072
H1036	H0395

Redox Catalysts

T1560

H0186

Ligands

P0221	P0879	B1733
P0160	A0869	M1132
P1297	T1438	P0052
B0989	D1678	T1359
T0434	T0519