ADDITION OF ELECTROPHILES TO UNSYMMETRIC ALKENES - EFFECTS OF BETA-OXYGEN SUBSTITUENTS ON THE STEREO-CHEMISTRY AND REGIO-CHEMISTRY OF POSITIVE HALOGEN ADDITION

The stereo-and regio-chemistry of two step additions of positive halog en electrophiles to 5-methylene2-phenyl-1, 3-dioxane, 1-tert-butyl-4-m ethylenecyclohexane and 3-methoxy-2-(methoxymethyl)prop-1-ene have bee n determined. The direction of initial electrophile attack is in line with the frontier orbital and electrostatic considerations described b y us previously. The regiochemistry of addition is strongly affected b y hyperconjugative effects, acting between the intermediate epihaloniu m ion and the beta C-X bonds. Where the beta C-X bonds bear a fixed pe riplanar relation to the epihalonium ion and X is more electronegative than hydrogen, anti-Markownikoff addition is strongly promoted and be comes exclusive when two such beta C-X bonds are present. If the beta C-X bond is free to rotate away from periplanarity, then beta C-H bond s will adopt the geometry required for hyperconjugation and Markowniko ff regiochemistry will be favoured. The results are consistent with ab initio theoretical calculations and can be rationalised using a simpl e electrostatic model.