Spin-trapping evidence for the formation of alkyl, alkoxyl, and alkylperoxyl radicals in the reactions of dialkylzincs with oxygen.
The reactions of dialkylzincs (Me(2)Zn, Et(2)Zn, and nBu(2)Zn) with oxygen have been investigated by EPR spectroscopy using spin-trapping techniques. The use of 5-diethoxyphosphoryl-5-methyl-1-pyrrroline N-oxide (DEPMPO) as a spin-trap has allowed the involvement of alkyl, alkylperoxyl, and alkoxyl radicals in this process to be probed for the first time. The relative ratio of the corresponding spin-adducts depends strongly on the nature of the R group, which controls the C-Zn bond dissociation enthalpy, and on the experimental conditions (excess of spin-trap compared with R(2)Zn and vice versa). The results have demonstrated that Et(2)Zn and, to a lesser extent, nBu(2)Zn are much better traps for oxygen-centered species than Me(2)Zn. When the dialkylzincs were used in excess with respect to the spin-trap, the concentration of the oxygen-centered radical adducts of DEPMPO was much lower for Et(2)Zn and nBu(2)Zn than for Me(2)Zn. A detailed reaction mechanism is discussed and C-Zn, O-Zn, and O-O bond dissociation enthalpies for the proposed reaction intermediates were calculated at the UB3LYP/6-311++G(3df,3pd)//UB3LYP/6-31G(d,p) level of theory to support the rationale.
Laboratoire Chimie Provence, UMR 6264, Faculté des Sciences et Techniques de Saint-Jérôme, Avenue Normandie-Niemen, Aix-Marseille Université, 13397 Marseille Cedex 20, France.
Julien
J
Maury
Laurence
L
Feray
Samantha
S
Bazin
Jean-Louis
JL
Clément
Sylvain R A
SR
Marque
Didier
D
Siri
Michèle P
MP
Bertrand