Inverted regioselectivities in the reactions of chlorine atoms with heteroarylmethanes

Article abstract of DOI:10.1021/ja00041a015

The relative reactivities of the two methyl groups of 2,6-dimethylquinoline (2,6-DMQ) and 2,7-dimethylquinoline (2,7-DMQ) toward several radicals were assessed by intramolecular competitions. The regioselectivities observed for t-BuO* and Br* (k₆/k₂, k₇/k₂ > 1) are consistent with electrophilic characteristics of these radicals and estimates of relative stability of the resulting radicals. In contrast, Cl* displays inverted selectivities (k₆/k₂, k₇/k₂< 1 suggesting a directive effect of the DMQ nitrogen. Complexation of Cl* by the N lone pair of dimethylquinolines, followed by a novel intramolecular hydrogen abstraction through a 5-center cyclic transition state, explains the regioselectivities observed in these reactions. Although bromine atoms also complex with heteroaromatics, these Br* complexes have a low reactivity toward benzylic hydrogens. Evidence from intermolecular competition experiments, laser flash photolysis studies, semiempirical MO calculations, and dimethylbutane quenching experiments is presented in support of the proposed intramolecular hydrogen abstraction process.