The study of the kinetics of intramolecular radical cyclizations of acylsilanes via the intramolecular competition method

The kinetics of the radical cyclizations of acylsilanes is probed by the co nstruction of an intramolecular competition system. In this system, known r ate constants of olefin cyclizations are used as the internal clock. At-ben zene reflux temperature (80 degrees C), the cyclization rate constants for 5-exo cyclizations of acylsilanes with primary radicals are on the order of 10(6) s(-1). The corresponding rate constants for 5-exo cyclizations with secondary radicals and 6-exo cyclizations with primary radicals lie on the order of 10(5) s(-1). The 6-exo cyclizations of acylsilanes with secondary radicals are slower and fall in the range of 10(4) s(-1). Substituents on t he silicon of the acylsilanes affect the rates of cyclization. Large silyl groups reduce the rate, and this is more serious for the g-exo type of cycl izations. Electron-withdrawing groups increase the rates of cyclization.