Carbon-oxygen bond formation at organopalladium centers: The reactions of PdMeR(L2) (R = Me, 4-tolyl; L2 = tmeda, bpy) with diaroyl peroxides and the involvement of organopalladium(IV) species

Article abstract of DOI:10.1021/om030644q

The group 16 oxidants dibenzoyl- and bis(4-trifluoromethylbenzoyl)-peroxide react with dimethylpalladium(II) and methyl(4-tolyl)palladium(II) complexes of the bidentate nitrogen donor ligands 2,2′-bipyridine and N,N,N′,N′-tetramethylethylenediamine in discrete stepwise processes as the temperature is raised from -70°C. Carbon-oxygen bonds are formed during this reaction sequence but not from those Pd(IV) complexes detected spectroscopically. The initial reaction gives undetected "Pd^IV(O₂CAr)₂MeR(L₂)" (Ar = Ph, Ar_F; R = Me, Tol; L₂ = bpy, tmeda), which immediately undergo methyl aroate exchange with Pd^IIMeR(L₂) to give Pd^II(O₂CAr)R(L₂) and Pd^IV(O₂CAr)Me₂R(L₂), where all products except for Pd^IV(O₂CAr)Me₂-Tol(tmeda) were detected by ¹H NMR spectroscopy. On raising the temperature, the Pd^IVMe₃ complexes reductively eliminate Me-Me, and the Pd^IVMe₂Tol complexes eliminate Me-Me and Tol-Me. The resultant Pd(II) complexes Pd^II(O₂CAr)R(L₂) react with (ArCO₂)₂ at higher temperatures to form Pd^II(O₂CAr)₂(L₂) and R-O₂CAr (R = Me, Tol), except for Pd^II(O₂CAr)Tol(tmeda), which forms Pd^II(O₂CAr)₂(tmeda) and 4,4′-bitolyl. Each reaction step has been confirmed by the independent synthesis of intermediates Pd^II(O₂CAr)₂(L₂) and Pd^II(O₂CAr)R(L₂) (Ar = Ph, Ar_F; R = Me, Tol; L₂ = bpy, tmeda) and Pd^IV(O₂CR)Me₂R(L₂) (R = Ph, Ar_F; R = Me, Tol; L₂ = bpy) by metathesis reactions of halogeno complexes with Ag[O₂CAr], followed by temperature-dependent studies of both the decomposition of Pd(IV) complexes and reactions of Pd(II) complexes with (ArCO₂)₂. Attempts to prepare "Pd^IV(O₂CAr)₂ MeR(bpy)" in a similar manner (and thus in the absence of PdMeR(bpy) with which they undergo exchange reactions) were unsuccessful, but the complexes Pd^IVI₂MeR(bpy) (R = Me, Tol) that formed on reaction of diiodine with PdMeR(L₂) were detected and found to reductively eliminate iodomethane. X-ray structural studies are reported for the square-planar palladium(II) complexes Pd(O₂CPh)₂(bpy), Pd(O₂CAr)₂(tmeda) (Ar = Ph, Ar_F), and Pd(O₂CPh)(Tol) (bpy)?CH₂Cl₂.