Addition of electrophiles to unsymmetric alkenes. Effects of β oxygen substituents on the stereo- and regio-chemistry of positive halogen addition

Abstract

The stereo- and regio-chemistry of two step additions of positive halogen electrophiles to 5-methylene-2-phenyl-1,3-dioxane, 1-tert-butyl-4-methylenecyclohexane and 3-methoxy-2-(methoxymethyl)prop-1-ene have been determined. The direction of initial electrophile attack is in line with the frontier orbital and electrostatic considerations described by us previously. The regiochemistry of addition is strongly affected by hyperconjugative effects, acting between the intermediate epihalonium ion and the β C–X bonds. Where the β C–X bonds bear a fixed periplanar relation to the epihalonium ion and X is more electronegative than hydrogen, anti-Markownikoff addition is strongly promoted and becomes exclusive when two such β C–X bonds are present. If the β C–X bond is free to rotate away from periplanarity, then β C–H bonds will adopt the geometry required for hyperconjugation and Markownikoff regiochemistry will be favoured. The results are consistent with ab initio theoretical calculations and can be rationalised using a simple electrostatic model.