Some Basic Chemistry of Molecular Oxygen

Oxygen possesses two characteristic properties, its paramagnetic nature and its electrophilic character (5). As a paramagnetic diradical, it tends to react readily with other paramagnetic species. It combines with organic compounds in their triplet state, with certain transition metal complexes, and even with itself (72). It also reacts with strained compounds which might have free radical character (75, J4) and readily with free radicals in the familiar process of chain autoxidation (75, 76). 

On the other hand, as it is an electrophile, it can form charge-transfer complexes and can also undergo reduction by accepting one, two, three or four electrons which are conveniently accommodated in its half-filled 7t and empty a molecular orbitals (5). 

It has been pointed out that the spin-forbidden nature of singlet-triplet reactions can be viewed as a problem of time-scale (5). The time needed for spin inversion is usually longer than the life-time of a collision 

Recent studies 19) have shown that reaction of triplet molecular oxygen with carbanions in the gas-phase proceeds only if the electron affinity of the corresponding organic radical is less than 20 kcal. In such cases, spin inversion apparently occurs by the device of an electron jumping from the carbanion to the oxygen molecule to generate the radical pair which subsequently collapses to the singlet peroxy anion (eq. 3). 

Having now at this preliminary stage introduced the idea that activation of oxygen by the metal co-factor may be unimportant and that the substrate could be activated instead, we will first examine in some detail the precedents for oxygen-activation and later we will outline, where possible, from the evidence, mechanistic schemes for substrate activation . 

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