Spin-Trapping Evidence for the Formation of Alkyl, Alkoxyl, and Alkylperoxyl Radicals in the Reactions of Dialkylzincs with Oxygen

• Published in: Chemistry : a European journal Vol. 17; no. 5; pp. 1586 - 1595
• Main Authors: Maury, Julien, Feray, Laurence, Bazin, Samantha, Clément, Jean-Louis, Marque, Sylvain R. A., Siri, Didier, Bertrand, Michèle P.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 01.02.2011; WILEY‐VCH Verlag; Wiley Subscription Services, Inc
• Subjects: Bonding; Bonding strength; Chemistry; dialkylzinc; Enthalpy; EPR spectroscopy; Formations; Mathematical analysis; Oxygen; radical reactions; Radicals; Spectroscopy; spin traps; Trapping; Vices
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201002616

The reactions of dialkylzincs (Me2Zn, Et2Zn, and nBu2Zn) 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 R2Zn and vice versa). The results have demonstrated that Et2Zn and, to a lesser extent, nBu2Zn are much better traps for oxygen‐centered species than Me2Zn. 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 Et2Zn and nBu2Zn than for Me2Zn. 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. All trapped! The reactions of dialkylzincs with oxygen (see scheme) have been investigated with EPR spectroscopy by using spin‐trapping techniques. The use of 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. A detailed mechanism is discussed in the light of theoretical calculations of the CZn, OZn, and OO bond dissociation enthalpies of the proposed reaction intermediates.