Unveiling the Delicate Balance of Polarity and Strain Effect in Determining the Site-Selective Intramolecular Hydrogen Atom Transfers

Abstracting hydrogen atoms from different non-activated
C(sp3)–H bonds to generate various carbon radicals
provides
an efficient way to further broaden the application of intramolecular
hydrogen atom transfer (HAT) reactions. This study thoroughly explores
the roles of kinetic factors (polarity and strain effects) and thermodynamic
factors (bond dissociation energy) in affecting the reactivity of
intramolecular HAT reactions, contributing to the rational design
of suitable substrates. Furthermore, the site-selectivity, as represented
by the barrier height difference (ΔΔG1, n–1, n′≠, where 1,n and 1,n′ denotes
1,n-HAT and 1,n′-HAT reaction)
correlates reasonably well with local electrophilicity index difference
(ΔΔω1,n–1,n′loc), demonstrating the vital role of polarity in achieving site-selective
intramolecular HAT reactions. Additionally, strain effects are considered
alongside polar effects to explain the observed site-selective products,
offering fundamental kinetic insight into balancing these effects
in determining the proper hydrogen atom engaged in the reaction.