Regioselectivity of Radical Attacks on Substituted Olefins. Application of the SCD Model

The SCD model is used to derive regiochemical trends in radical addition to olefins. Regiochemistry is discussed by appeal to two fundamental properties of the radical and the olefin. The first factor is the relative spin density in the ³ππ^* state of the olefin. Thus, radical attack is directed toward the olefinic carbon which possesses the highest spin density. The second factor is the relative bond strengths of the radical to the olefinic carbons. This factor directs the regiochemistry toward the olefinic terminus which forms the strongest bond with the radical. When the two effects join up, regioselectivity will be large, e.g., for CH₂=CHX (X = NR₂, CR, Cl, CN, Ph). When the two effects oppose one another, regioselectivity will be smaller, and regioselectivity crossovers are expected, e.g., for CF₂=CHF. The “normal” regiochemistry is shown to coincide with the spin density rule which makes identical predictions to the HOMO rule.