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Insertion of Molecular Oxygen into a Palladium−Hydride Bond:  Computational Evidence for Two Nearly Isoenergetic Pathways

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journal contribution
posted on 11.04.2007, 00:00 by Brian V. Popp, Shannon S. Stahl
The reaction of a palladiumII−hydride species with molecular oxygen to form palladiumII−hydroperoxide has been proposed as a key step in Pd-catalyzed aerobic oxidation reactions. We recently reported one of the first experimental precedents for such a step (Angew. Chem., Int. Ed. 2006, 45, 2904−2907). DFT calculations have been used to probe the mechanism for this reaction, which consists of formal insertion of O2 into the palladium−hydride bond of trans-(NHC)2Pd(H)OAc (NHC = N-heterocyclic carbene). Four different pathways were considered:  (1) hydrogen atom abstraction (HAA) of the Pd−H bond by molecular oxygen, (2) reductive elimination of HX followed by oxygenation of Pd0 and protonolysis of the (η2-peroxo)−PdII species, (3) oxygenation of palladiumII−hydride with subsequent reductive elimination of the O−H bond from an η2-peroxo−PdIV center, and (4) formation of a cis-superoxide adduct of the palladium−hydride species followed by O−H bond formation via hydrogen atom migration. The calculations reveal that pathways 1 and 2 are preferred energetically, and both pathways exhibit very similar kinetic barriers. This result suggests that more than one pathway is possible for catalyst reoxidation in Pd-catalyzed aerobic oxidation reactions.