Importance of Polar Effects in Halogen-Atom Transfer from Alkyl Iodides to α‑Aminoalkyl Radicals. A Kinetic and Computational Evaluation of the Role of Structural and Medium Effects

α-Aminoalkyl radicals have emerged as practical
halogen-atom
transfer (XAT) agents, enabling the formation of carbon radicals from
unactivated alkyl and aryl halides under mild conditions. A recent
study correctly revised some of the XAT rate constants for these radicals
downward, prompting broad mechanistic conclusions that contradict
the key importance of polar effects and challenge the XAT reactivity
for α-aminobenzyl radicals. Herein we report a comprehensive
kinetic and computational analysis of XAT reactions of α-aminoalkyl
and α-aminobenzyl radicals with alkyl iodides under both thermal
and photochemical conditions. Laser flash photolysis experiments reveal
a clear structure–reactivity relationship shaped by polar and
steric effects, which is well supported by the computational results.
These findings reaffirm the key importance of the nucleophilic character
of α-aminoalkyl radicals in providing significant transition
state stabilization to XAT processes. This confirms the prominent
role played by polar effects in these reactions and the possibility
for reactivity tuning using α-aminobenzyl radicals.