O2-promoted allylic acetoxylation of alkenes: Assessment of “push” versus “pull” mechanisms and comparison between O2 and benzoquinone

• Published in: Polyhedron Vol. 84; pp. 96 - 102
• Main Authors: Diao, Tianning, Stahl, Shannon S.
• Format: Journal Article
• Language: English
• Published: Elsevier Ltd 14.12.2014
• Subjects: Aerobic; Allylic; C–H activation; Oxidation; Palladium; C–H activation; Palladium; Oxidation; Aerobic; Allylic
• ISSN: 0277-5387
• DOI: 10.1016/j.poly.2014.06.038

Mechanistic studies of PdII-mediated allylic acetoxylation suggest that O2 and benzoquinone promote C–O bond formation by different pathways: BQ participates directly in C–O bond formation via an oxidatively induced reductive elimination pathway, while O2 traps Pd0 following reversible C–O bond formation from an allyl-palladium(II) species. [Display omitted] Palladium-catalyzed acetoxylation of allylic C–H bonds has been the subject of extensive study. These reactions proceed via allyl-palladium(II) intermediates that react with acetate to afford the allyl acetate product. Benzoquinone and molecular oxygen are two common oxidants for these reactions. Benzoquinone has been shown to promote allyl acetate formation from well-defined π-allyl palladium(II) complexes. Here, we assess the ability of O2 to promote similar reactions with a series of “unligated” π-allyl palladium(II) complexes (i.e., in the absence of ancillary phosphorus, nitrogen or related donor ligands). Stoichiometric and catalytic allyl acetate formation is observed under aerobic conditions with several different alkenes. Mechanistic studies are most consistent with a “pull” mechanism in which O2 traps the Pd0 intermediate following reversible C–O bond formation from an allyl-palladium(II) species. A “push” mechanism, involving oxidatively induced C–O bond formation, does not appear to participate. These results and conclusions are compared with benzoquinone-promoted allylic acetoxylation, in which a “push” mechanism seems to be operative.