Factors Controlling the Addition of Carbon Centered Radicals to Alkenes and Alkynes

Abstract

More than a decade ago Tedder [1] and Giese [2] reviewed the title topic on the basis of the few rate constants and quantum chemical calculations then available. Their main conclusions were: (a) The regioselectivity of addition is determined by steric substituent effects (b) Substituents at the attacked carbon atom (α-substituents) influence the rates by steric and polar effects whereas those at the non attacked carbon (β-substituents) exert polar but only minor steric effects. (c) Unsubstituted or electron donor substituted radicals as methyl, cyclohexyl and tert-butyl behave nucleophilically and acceptor substituted ones as dicyanomethyl and trifluoromethyl behave electrophilically. (d) Except for extreme cases of radical stabilization by phenyl groups, the variation of the reaction enthalpy with radical or substrate substitution has little influence. (e) In accord with an early transition state the polar effects are due to stabilizing SOMO-LUMO and SOMO-HOMO interactions lowering the reaction barrier. These views were nicely supported by Minisci’s extensive studies on radical additions to heteroaromatic compounds [3] and form the present credo [4].