Overestimated Halogen Atom Transfer Reactivity of α-Aminoalkyl Radicals

Halogen atom transfer (XAT) is a versatile method for generating carbon radicals. Recent interest has focused on alpha-aminoalkyl radicals as potential XAT reagents, previously reported to exhibit reactivity comparable to tin radicals. Utilizing an advanced time-resolved EPR technique, the XAT reactions between alpha-aminoalkyl radicals and organic halides were examined, allowing direct observation of the process through EPR spectroscopy and analysis of radical kinetics. Second-order rate constants for these reactions were determined, with some validated using transient absorption spectroscopy. The key finding is that the reactivity of alpha-aminoalkyl radicals in XAT reactions is 10(3) to 10(5) times lower than that of tin and silicon radicals and only slightly higher than alkyl radicals. This challenges the belief that alpha-aminoalkyl radicals are as reactive as tin radicals. The study on the solvent effect indicates that the XAT reaction of alpha-aminoalkyl radicals does not involve a highly polarized transition state, suggesting that the kinetic polar effect in this XAT process is not as significant as previously believed. The present study provides a reliable XAT reactivity scale for alpha-aminoalkyl radicals, which is crucial for designing XAT reactions and understanding their mechanisms.