A Radical and An Electron Transfer Process are Compared in their Regioselectivities Towards a Molecule with Two Different C-I Bonds: Effect of Steric Congestion

Steric compression in 1,4-diiodo-2,6-dimethylbenzene (2a) makes the C-I bond flanked by methyls substantially weaker (a buttressing effect) than the unhindered C-I bond. Calculations also confirm the weaker bonding interaction of the hindered C-I bond of 2a. This causes a remarkable regioselectivity toward the weaker bond in dehalogenation by stannyl radicals. Conversely, a much lower regioselectivity is found for a process -a photostimulated S(RN)1 reaction with the enolate ion of a ketone - which requires the conversion of 2a into a radical anion. A calculation of the BDE of the C-I bond for aa ArI.- system is offered. Finally, the hindered aryl radical intermediate resulting from cleavage of the weaker C-I bond of 2a(.-) shows a modest but detectable discrimination between reduction or substitution, this once again being due to the steric congestion.