Unusual Organic Biradicals

A large group of paramagnetic organic molecules containing even numbers of electrons are known. Compounds for which a conjugated, biradical structure can be formulated actually exist predominantly as quinoidal singlets.78 

Meta placement of the radical sites or substituents which impose steric interference to conjugation result in stabilization of the biradical forms.79 

Since 2,4,6-trisubstituted phenols are readily oxidized to fairly stable phenoxy radicals, this system has been used to construct various biradical species. The bisphenol 6 below loses three hydrogens to form the very stable crystalline radical galvinoxyl.86 

Substitution of the two methylene hydrogens in 6 with methyl groups and oxidation yields a compound consisting primarily of the 

However, the more sterically constrained bisphenol 8 is oxidized to a product that apparently is an equilibrium mixture of a fully bonded singlet 9 and a biradical triplet species 10.88 

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The authors proposed the following mechanism to explain the unusual organic transformations. The /z-peroxo-/i-oxo complex, 15, reacts in a biradical fashion with the protons on a methylenic carbon to generate a carbene and two hydroxy radicals that recombine to give a gem-diol. This diol will in turn collapse to the ketone product. The diketonization of acetylenes was explained in a similar fashion with the formation of a four-member ring intermediate (Scheme 2) (36, 37). 

As a final suggestion for future research, cyclobutanones have also provided the organic photochemist with the opportunity of investigating the existence of unusual and reactive intermediates oxacarbenes, trimethylene biradicals, trimethylenemethane biradicals, acyl alkyl biradicals, and ketenes. Evidence for the intervention of oxacarbenes in the ring-expansion reaction is quite compelling however, their unusual behavior relative to "typical" carbenes (e.g., failure to form cyclopropane adducts with some olefinic substrates) makes them prime subjects for further study and characterization. Unlike oxacarbenes, the existence of acyl alkyl biradicals (e.g., [30]) is tenuous at best. Ideally,... 

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