Anchimeric Assistance in Hydrogen-Atom Transfer to Bromine

The free-radical benzylic brominations of series of phenylalanine derivatives and O-phenylalkyl benzoates and N-phenylalkylamides with N-bromosuccinimide exhibit anchimeric assistance by neighbouring ester and amido groups. Rate enhancement occurs through electron donation to the electropositive carbon centre that develops in the transition state of the hydrogen-atom transfer to bromine. The extent of the effect depends on the electron demand at the benzylic position and the electron-donating ability of the neighbouring group.