Biradicals carbon atom reactivity

Reactivity in the elimination reaction is probably largely determined by the nature of the substituents (in addition to hydrogen) attached to the y-carbon atom. If the reaction is stepwise, one would expect faster reaction with either 2-hexanone or methoxyacetone than with 2-pentanone, because the biradical intermediates would be more stable in the former two cases. Such a conclusion follows from the similarity shown by various ketone triplets in intermolecular reactions333 to the known selectivity of attack by /-butoxy radicals in both inter-353 and intramolecular reactions.354... 

Consider, for example, the radical anion from styrene, PhCH=CH2. Dimerization of styrene to a biradical would remove the two terminal carbon atoms from conjugation. The resulting loss in n energy per styrene unit would then be Nt being the corresponding reactivity number, namely... 

The problem of the synthesis of highly substituted olefins from ketones according to this principle was solved by D.H.R. Barton. The ketones are first connected to azines by hydrazine and secondly treated with hydrogen sulfide to yield 1,3,4-thiadiazolidines. In this heterocycle the substituents of the prospective olefin are too far from each other to produce problems. Mild oxidation of the hydrazine nitrogens produces d -l,3,4-thiadiazolines. The decisive step of carbon-carbon bond formation is achieved in a thermal reaction a nitrogen molecule is cleaved off and the biradical formed recombines immediately since its two reactive centers are hold together by the sulfur atom. The thiirane (episulfide) can be finally desulfurized by phosphines or phosphites, and the desired olefin is formed. With very large substituents the 1,3,4-thiadiazolidines do not form with hydrazine. In such cases, however, direct thiadiazoline formation from thiones and diazo compounds is often possible, or a thermal reaction between alkylideneazinophosphoranes and thiones may be successful (D.H.R. Barton, 1972, 1974, 1975). 

Irradiation of ketone (182) with a mercury lamp generates biradicals which are formed via n-TT electron transition. The reactive oxygen center of the biradical abstracts a hydrogen atom via 1,6-H shift, and the formed carbon-centered biradical couples intramolecularly to produce 5-membered compound (183) (eq. 3.70) Eq. 3.71 shows the same type of photochemical cyclization through the generation of a biradical, 1,9-H shift, and finally intramolecular coupling of the formed carbon-centered biradical [192-199]. 

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