A radical and an electron transfer process are compared in their regioselectivities towards a molecule with two different C-I bonds: Effect of stericcongestion

Steric compression in 1,4-diiodo-2,6-dimethylbenzene (2a) makes the C-I bon d flanked by methyls substantially weaker (a buttressing effect) than the u nhindered C-I bond. Calculations also confirm the weaker bonding interactio n 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(R N)1 reaction with the enolate ion of a ketone - which requires the conversi on 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 intermediat e resulting from cleavage of the weaker C-I bond of 2a(.-) shows a modest b ut detectable discrimination between reduction or substitution, this once a gain being due to the steric congestion.