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 CuIOAc-CyPPh2 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 LCuI-PR3 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 CyPPh2 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.