Steric effects Wagner-Meerwein rearrangement

Reaction with unsaturated bicyclic hydrocarbons. From a study of the chlorination of unsaturated bicyclic compounds, Masson and Thuillier1 conclude that the reaction follows, a radical addition mechanism when initiated thermally or photo-chemically. The stereochemistry of the addition is markedly influenced by steric effects. No Wagner-Meerwein rearrangements are observed under these conditions. An ionic mechanism is involved without initiation or in the presence of trifluoroacetic acid in this case the usual carbonium ion rearrangements are observed. 

A major opponent of this nonclassical ion intermediate was Brown,who published dissenting views throughout the latter half of the twentieth century. He insisted on the existence of a rapid equilibrium between the two classical carbocation forms facilitated via Wagner-Meerwein rearrangement. Exo- and endo-T tQ ratios were attributed to steric effects, as strain caused e (3to-isomers to exhibit more hindrance to ionization. Finally, Brown criticized the bridged intermediate model for not providing sufficient electrons for all bonds. ... 

---