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

A density functional theory study of Cu-catalyzed desaturation of delta-valerolactone into alpha,beta-unsaturated counterparts reveals an unexpected binuclear di-tert-butyl peroxide (DTBP) homolysis with spin-crossover and a radical alpha-C-H bond activation mechanism. The rate-determining step in the reaction catalyzed by (CuOAc)-O-I-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 alpha- and beta-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 alpha-H and no KIE at beta-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.

Automated summary

The study using density functional theory revealed unexpected binuclear di-tert-butyl peroxide (DTBP) homolysis with spin-crossover and a radical alpha-C-H bond activation mechanism in Cu-catalyzed desaturation of delta-valerolactone into alpha,beta-unsaturated counterparts. The rate-determining step was found to be the homolysis of the O-O bond in DTBP, while the alpha- and beta-H transfer steps had lower barriers than the O-O cleavage process. The substitution of CyPPh2 for pyridine in Cu complexes resulted in higher barriers for O-O bond cleavage and C-H bond activations with the formation of more stable binuclear Cu complexes.