Structure–activity relationship of alkanes and alkane derivatives for the abilities of C(sp3)H bonds toward their H-atom transfer reactions

Abstract

Hydrogen atom-donating ability of alkane is a research hotspot and has been extensively studied. In this article, the second-order rate constants of 20 hydrogen atom transfer (HAT) reactions between aliphatic, benzylic, and allylic alkanes and alkane derivatives with CumO^• in acetonitrile at 298 K were studied. The thermo-kinetic parameter ΔG^≠o(XH), bond dissociation free energy ΔG^o(XH), and kinetic intrinsic resistance energy ΔG^≠_XH/X were determined and used to evaluate the H-donating abilities of these substrates in thermodynamics, kinetics, and HAT reactions. Structure–activity relationships including the factors (electronic, stereoelectronic, and steric effects), introduction of CH₃, Ph, or Cl in alkanes, and introduction of N atom in cycloalkane were discussed carefully. The results show that the order of H-donating abilities is allylic alkanes > cycloalkanes > chain alkanes ≈ benzylic alkanes > haloalkanes. The introduction of CH₃, Ph, or Cl in alkanes and the introduction of N atom to the carbon ring reduce ΔG^o(XH) but increase ΔG^≠_XH/X, and ΔG^≠o(XH) is the synthesis result of these two parameters. The reliability of ΔG^≠o(XH) was verified, and the accuracy and reliability of the parameters were proved. Through the study of this paper, not only the ΔG^o(XH), ΔG^≠_XH/X, and ΔG^≠o(XH) of these alkanes and derivatives in HAT reaction can be quantitatively evaluated but also the structure–activity relationship of alkane is clearly researched.