Spin-Forbidden Reactions with Triplet Oxygen

2 Spin-Forbidden Reactions with Triplet Oxygen 

A limited number of organic substrates are able to react spontaneously with triplet oxygen in the absence of any catalyst or hght at mild conditions [17]. Seip and Braner have discovered that heU-anthrene (HEL) interacts with Pj in nonpolar solvents in the dark to give endoperoxide (HELPO) (Eq. 14.10) [23]. 

Solvent effects, in particular, a pronounced external heavy atom effect [24] (rate acceleration upon replacement of toluene for iodobenzene) found for the dark HEL oxidation suggested spin inversion (thermal intersystem crossing (ISC)) in the rate-liniiting step, leading to the formation of a polar zwitterionic intermediate ( Z). 

Kholdeeva and Rossi have found that oxidation of 2-methyl-1-naphthol (MNL) with molecular oxygen smoothly proceeds in nonpolar organic solvents under mild conditions (3 atm O, 20-80°C) in the absence of any catalyst, in dark to give 2-methyl-1,4-naphthoquinone (MNQ, menadione, or 

The driving force for the spin-forbidden reactions is supposed to be either a strong strain in the substrate molecule, which reduces significantly in the product (the case of HEL and HELPO) (Ref. 23 and references therein), or resonance effects [26, 27]. 

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Seip M, Brauer H-D. Endoperoxide formation of helianthrene with triplet molecular oxygen. A spin-forbidden reaction. J Am Chem Soc 1992 114 4486-4490. 

The first question that arises is how does triplet oxygen interact with reduced flavin in what is a spin-forbidden reaction We originally noted the similarity, both electronic and structural, between reduced flavin and tetrakis(dimethylamino)ethylene (33). In the presence of triplet oxygen, 33 reacts rapidly to give the corresponding urea. This... 

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