Overestimated Halogen\nAtom Transfer Reactivity of\nα‑Aminoalkyl Radicals

Halogen\natom transfer (XAT) is a versatile method for\ngenerating\ncarbon radicals. Recent interest has focused on α-aminoalkyl\nradicals as potential XAT reagents, previously reported to exhibit\nreactivity comparable to tin radicals. Utilizing an advanced time-resolved\nEPR technique, the XAT reactions between α-aminoalkyl radicals\nand organic halides were examined, allowing direct observation of\nthe process through EPR spectroscopy and analysis of radical kinetics.\nSecond-order rate constants for these reactions were determined, with\nsome validated using transient absorption spectroscopy. The key finding\nis that the reactivity of α-aminoalkyl radicals in XAT reactions\nis 10<sup>3</sup> to 10<sup>5</sup> times lower than that of tin and\nsilicon radicals and only slightly higher than alkyl radicals. This\nchallenges the belief that α-aminoalkyl radicals are as reactive\nas tin radicals. The study on the solvent effect indicates that the\nXAT reaction of α-aminoalkyl radicals does not involve a highly\npolarized transition state, suggesting that the kinetic polar effect\nin this XAT process is not as significant as previously believed.\nThe present study provides a reliable XAT reactivity scale for α-aminoalkyl\nradicals, which is crucial for designing XAT reactions and understanding\ntheir mechanisms.