Unexpected binuclear O-O cleavage and radical C-H activation mechanism for Cu-catalyzed desaturation of lactone

• Published in: Dalton transactions : an international journal of inorganic chemistry 2021-03, Vol.5 (8), p.2997-34
• Main Authors: Hu, Zhiyun, Yang, Xinzheng
• Format: Article
• Language: English
• Subjects: Cleavage; Density functional theory; Desaturation; Free energy; Hydrogen bonds
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d0dt04311g

A density functional theory study of Cu-catalyzed desaturation of δ-valerolactone into α,β-unsaturated counterparts reveals an unexpected binuclear di- tert -butyl peroxide (DTBP) homolysis with spin-crossover and a radical α-C-H bond activation mechanism. The rate-determining step in the reaction catalyzed by Cu I OAc-CyPPh 2 is the homolysis of the O-O bond in DTBP with a total free energy barrier of 26.9 kcal mol −1 , which is consistent with the observed first-order dependences on LCu I -PR 3 and DTBP, as well as the pseudo-zeroth-order with lactone. The α- and β-H transfer steps have 0.3 and 14.8 kcal mol −1 lower barriers than the O-O cleavage process, respectively. Such different barriers well explain the observed weak kinetic isotopic effect (KIE) at α-H and no KIE at β-H. In addition, we found that the replacement of CyPPh 2 for pyridine in the Cu complexes leads to much higher barriers for O-O bond cleavage and C-H bond activations with the formation of more stable binuclear Cu complexes. Density functional theory calculations reveal a binuclear O-O cleavage and radical C-H activation mechanism with spin-crossovers for Cu-catalyzed desaturation of lactone.