Palladium(II) tert-Butyl Peroxide Carboxylates. New Reagents for the Selective Oxidation of Terminal Olefins to Methyl Ketones. On the Role of Peroxymetalation in Selective Oxidative Processes

Article abstract of DOI:10.1021/ja00523a023

A new family of palladium(II) tert-butyl peroxidic complexes of general formula 4 was synthesized and characterized by physicochemical methods and X-ray crystallography.The crystal structure of 4 was determined.The molecule crystallizes with four formula units in the monoclinic space group C2/c with a = 17.867 (7) Angstroem, b = 14.371 (4) Angstroem, c = 21.126 (3) Angstroem, and β = 112.63 (2) deg.The final indices based on the least-squares refinements with 2154 observations having I > 3?(I) are R₁ = 0.055 and R₂ = 0.077.The tetrametric molecule has crystallographic C₂ symmetry and approximately D_2d symmetry.The four palladium atoms are coplanar and are located approximately at the corners of a square.Four trichloroacetato bridging anions are alternatively above and below this square.The terminal oxygen atoms of the four tert-butyl peroxidic anions are located in the plane of the four palladium atoms and bridge these atoms two by two.The average Pd(*)Pd separation is 2.91 Angstroem.The mean value of the peroxidic O-O bond distance is 1.49 Angstroem.These complexes are highly efficient reagents for the selective stoichiometric oxidation of terminal olefins to methyl ketones at ambient temperature, and catalysts for the ketonization of terminal olefins by tert-butyl hydroperoxide.The removal of one oxygen atom from the tert-butyl peroxidic group in 4 (PPT) by the terminal olefin first affords the palladium tert-butoxy complex and then the ?-allylic complex by rapid substitution of the tert-butoxide group for the olefin.Several ?-allylic complexes of general formula 2 were synthesized in this way from the reaction of PPT with both terminal and internal acyclic olefins.The consideration of the general features of the ketonization of terminal olefins by PPT has led us to suggest a mechanism involving a five-membered pseudocyclic peroxymetalation of the coordinated olefin.This mechanism strongly resembles that proposed for the epoxidation of olefins by molybdenum peroxo complexes and ketonization of terminal olefins by rhodium peroxo complexes involving a cyclic five-membered peroxymetalation of the olefin.This mechanism may afford a plausible explanation for the molybdenum-catalyzed epoxidation of olefins by alkyl hydroperoxides.