Quantitative evaluation of H-donating abilities of C(sp³)–H bonds of nitrogen-containing heterocycles in hydrogen atom transfer reaction

Abstract

Nitrogen-containing heterocycles are an important class of antioxidants, and their reactivity and selectivity in hydrogen atom reactions have attracted significant interest from chemists.

Thermodynamic H-Abstraction Abilities of Nitrogen Centered Radical Cations as Potential Hydrogen Atom Transfer Catalysts in Y–H Bond Functionalization

“…According to the thermodynamic results of H-affinity for X •+ , nitrogen centered radical cations are definitely potential H-abstractors in Y–H bond functionalization. For a Y–H bond, the bond dissociation free energy (BDFE) of the Y–H bond, BDFE(Y–H), is a very important thermodynamic parameter to evaluate the bond strength and breaking difficulty. − Based on the H-affinities of X •+ and the BDFE(Y–H) values, the thermodynamic driving forces of HAT from the Y–H bond to X •+ (X •+ + Y–H → XH + + Y • ) could be calculated by subsequent eq Δ G H A T false( X • / Y H false) = Δ G H A false( X • + false) + B D E F ( Y H ) false) …”

“…Herein, the BDFE of various representative Y–H bonds, − consisting of C sp 3 –H, C sp 2 –H, C sp –H, N–H, and O–H bonds, in AN are gathered and displayed in Scheme , along with the H-affinities of X •+ (X 1 •+ –X 4 •+ ). As shown in Scheme , it can be discovered that BDFE­(Y–H) ranges from 74.6 kcal/mol for C sp 3 –H bond in benzylalcohol (α-HO-C sp 3 –H) to 128.4 kcal/mol for C sp –H bond in CHCH among the 24 Y–H bonds, covering a very large thermodynamic scale by 53.8 kcal/mol.…”