Study on the Factors Affecting the H-Donating Abilities of α-CH Bonds in Cyclohexane Derivatives

The stable chair conformation of cyclohexane and derivatives are often used to study the selectivity and activity in hydrogen atom transfer reactions (HATs) because of their special structures. In this article, the kinetic studies of 21 HATs between mono- and di-substituted cyclohexane derivatives with CumO^• in CH3CN at 298 K were researched. The bond dissociation free energies ΔG°(XH), the intrinsic resistance energies ΔG^≠XH/X and thermo-kinetic parameters ΔG^≠°(XH) were obtained to evaluate the H-donating abilities of these substrates in thermodynamics, kinetics and HAT reactions. The factors that govern H-donating ability of the substrate were discussed in detail, including electronic effect (induction and conjugation effects), steric, polar, and stereoelectronic effects. Through this study, not only the ΔG°(XH), ΔG^≠ _XH/X and ΔG^≠°(XH) of mono- and di-substituted cyclohexane derivatives in HATs but also the influence of introducing groups tosubstrates on the reaction center and reactivity, can be quantitatively evaluated. This paper made the HAT reactivity of the axial and equatorial C(sp³)H bonds in the chair conformation of cycloalkanes concrete. The results of this study will enable researchers to better understand the various effects of substituents on cyclohexane ring, and lay a theoretical foundation for efficient design and synthesis of more H-donors.