VI. Summary

Generating a carbon-centered radical represents the beginning point for most radical reac­tions of carbohydrates. The identity of the radical formed is determined by the rate constant for atom or group transfer from the carbo­hy­drate derivative to a radical that usually is centered on a tin or silicon atom. Compounds that contain iodine or bromine atoms are attractive starting materials for radical reactions because the rate constants for transfer of these atoms to tin- or silicon-centered radicals are quite large. Once a carbon-centered radical has formed, most reactions of importance include a radical trans­forming step (e.g., addition of a radical to a multiple bond or radical cyclization). Rate constants for radical trans­for­mation must be large enough that the desired reaction can take place before a competing process, often hydrogen-atom abstraction, intervenes. If a propagation step in a reaction is slower than a chain-terminating reac­tion, the reaction will undergo chain collapse.