On the 6-endo selectivity in 4-penten-1-oxyl radical cyclizations.

Regioselectivities in cyclizations of 4-substituted 4-penten-1-oxyl radicals have been investigated in a combined experimental and computational study (density functional theory). The progressive increase of the 6-endo-trig selectivity along the series of 4-substituents H < CH(3) < C(CH(3))(3) < C(6)H(5) has been interpreted to originate from a balance between strain and FMO interactions. Torsional strain, which is associated with geometrical changes upon an approach of the reacting entities, is relevant for the 6-endo-trig but not for the 5-exo-trig reactions, as seen, for instance, in selective tetrahydrofuran formation from the 4-penten-1-oxyl radical and its 4-methyl derivative. The preference for tetrahydropyran formation in cyclizations of the 4-tert-butyl and the 4-phenyl-4-penten-1-oxyl radical has been attributed to FMO interactions between the terminal carbon atom of the pi bond and the O-radical center thus favoring the 6-endo-trig reaction on the basis of lower transition state energies.