Rate constants for 5-exo secondary alkyl radical cyclizations onto hydrazones and oxime ethers via intramolecular competition experiments

The kinetics for the 5-exo cyclizations of secondary radicals onto various imine accepters (aryl-, alkyl-, and benzoylhydrazones, oxime ethers, etc.) have been determined. The rate constants have been established by competiti ve "internal radical clock" type cyclizations on the basis of the known rat e constants for alkene and N,N-diphenylhydrazone systems. In refluxing benz ene (80 degrees C) the rate constants for these 5-exo cyclizations fall wit hin the range of 16 x 10(7) to 4 x 10(7) s(-1) depending on the electron-wi thdrawing capacity of the imine acceptor (C=NR). The fastest rate constants were observed for the N-benzyoylhydrazone acceptor (cis, 1.2 x 10(8) s(-1) ), a value that is very similar to rate constant determined previously for N,N-diphenylhydrazones (cis, 1.1 x 10(8) s(-1)), These rate constants are a pproximately 200 times faster than those for the corresponding 5-exo cycliz ation onto alkenes.