The role of organopalladium (IV) species in carbon-oxygen and carbon-selenium bond forming reactions

The role of palladium in carbon-oxygen and carbon-selenium bond forming processes has been investigated with particular emphasis on the postulated intermediacy of organopalladium(IV) species. Reactions of the diorganopalladium(II) complexes PdMeR(L\\(_2\\)) [R = Me, 4-tolyl (Tol); L\\(_2\\) = 2,2'-bipyridirle (bpy), N,N,N',N'-tetramethylethylenediamine (tmeda)] with diaroyl peroxides (ArCO\\(_2\\))\\(_2\\) [Ar = Ph, 4-CF\\(_3\\)C\\(_6\\)H\\(_4\\) (Ar\\(_F\\))] were investigated using variable temperature \\(^1\\)H NMR spectroscopy, revealing a complex series of reactions. The complexes Pd\\(^{II}\\)MeR(L\\(_2\\)) are predicted to undergo initial oxidative addition of (ArCO\\(_2\\))\\(_2\\) to give an unobserved Pd(IV) intermediate, Pd\\(^{IV}\\)(O\\(_2\\)CAr)\\(_2\\)MeR(L\\(_2\\))', which immediately takes part in an exchange reaction with the reagents Pd\\(^{II}\\)MeR(L\\(_2\\)) to form the observed intermediates Pd\\(^{II}\\)(O\\(_2\\)CAr)R(L\\(_2\\)) and Pd\\(^{IV}\\)(O\\(_2\\)2CAr)Me\\(_2\\)R(L\\(_2\\)) (except in the case of R = Tol and L\\(_2\\) = tmeda, where no Pd(IV) intermediate was observed). Decomposition of Pd\\(^{IV}\\)(O\\(_2\\)CAr)Me\\(_2\\)R(L\\(_2\\)) then occurred via reductive elimination of carbon-carbon bonds with absence of carbon-oxygen coupling. Carbon-oxygen coupling to form R-O\\(_2\\)CAr occurred from the reaction of monoorganopalladium(II) species Pd\\(^{II}\\)(O\\(_2\\)CAr)R(L\\(_2\\)) with (ArCO\\(_2\\))\\(_2\\), with concurrent formation of Pd\\(^{II}\\)(O\\(_2\\)CAr)\\(_2\\)(L\\(_2\\)). The exception was the reaction of Pd\\(^{II}\\)(O\\(_2\\)CAr)Tol(tmeda) with (ArCO\\(_2\\))\\(_2\\), which proceeded in a 2:1 ratio of reactants to form 4,4'-bitolyl and Pd\\(^{II}\\)(O\\(_2\\)CAr)\\(_2\\)(tmeda). Observed PdII and Pd(IV) intermediates and products of the reactions were also prepared by independent syntheses. Attempts to prepare Pd\\(^{IV}\\)(O\\(_2\\)CAr)\\(_2\\)MeR(bpy) were unsuccessful. The use of palladium and platinum complexes incorporating an intramolecularly coordinating `NCN-pincer' ligand, [C\\(_6\\)H\\(_3\\)(CH\\(_2\\)NMe\\(_2\\))\\(_2\\)-2,6]\\(^-\\), as a potential means of preparing stable models for Pd(IV) intermediates has been investigated. No reaction was observed between Pd\\(^{II}\\)(O\\(_2\\)CPh)(NCN) and (PhCO\\(_2\\))\\(_2\\), while the reaction with the platinum analogue resulted in formation and isolation of the stable complex mer-Pt\\(^{IV}\\)(O\\(_2\\)CPh)\\(_3\\)(NCN). The reactions of M\\(^{II}\\) (O\\(_2\\)CPh)(NCN) (M = Pd, Pt) with iodomethane resulted in formation of M\\(^{II}\\) I(NCN) and Me-O\\(_2\\)CPh in a 1:1 ratio. Low temperature \\(^1\\)H NMR spectroscopic studies revealed the intermediacy of a Pt(IV) species, cis-Pt\\(^{IV}\\)(O\\(_2\\)CPh)2Me(NCN), but Pd(IV) intermediates were not observed. The complex cis-Pt\\(^{IV}\\)(O\\(_2\\)CPh)\\(_2\\)Me(NCN) has been isolated from the reaction of Pt\\(^{II}\\)(O\\(_2\\)CPh)(NCN) with excess Ag[O\\(_2\\)CPh] and iodomethane, and was found to decompose via first-order kinetics to form Pt\\(^{II}\\)(O\\(_2\\)CPh)(NCN) and Me-O\\(_2\\)CPh. The reaction of Pt\\(^{II}\\)Tol(NCN) with (PhCO\\(_2\\))\\(_2\\) led to the formation of cis-Pt\\(^{IV}\\)(O\\(_2\\)CPh)\\(_2\\)Tol(NCN). This species has been isolated from the reaction of PtI\\(_2\\)Tol(NCN) with Ag[O\\(_2\\)CPh] and found to be resistant to decomposition. Studies of the analogous palladium chemistry were not feasible due to the instability of Pd\\(^{II}\\)Tol(NCN). Carbon-selenium bond-forming reactions at palladium were studied as potential models for carbon-oxygen coupling. The complexes Pd\\(^{II}\\)MeAr(bpy) [Ar = Tol, 4-anisyl (Anis)] were found to react with bis(4-chlorophenyl) diselenide [(CIC\\(_6\\)H\\(_4\\)Se)\\(_2\\)] to form trans-Pd\\(^{IV}\\)(SeC\\(_6\\)H\\(_4\\)Cl)\\(_2\\)MeAr(bpy), in a rare example of reversible oxidative addition. The Pd(IV) intermediate decomposed by both carbon-carbon and methyl-selenium coupling, with the absence of aryl-selenium coupling. Reactions of Pd\\(^{II}\\)MeR(tmeda) (R = Me, Tol) with (CIC\\(_6\\)H\\(_4\\)Se)\\(_2\\) led to the formation of products arising from both carbon-carbon and carbon-selenium coupling. Palladium(IV) species are postulated as intermediates but have not been observed. Reactions of Pd\\(^{II}\\)MeR(dmpe) [R = Me, Tol; dmpe = 1,2-bis(dimethylphosphino)ethane] with (CIC\\(_6\\)H\\(_4\\)Se)\\(_2\\) resulted in carbon=selenium coupling only. The reaction of Pd\\(^{II}\\)MeTol(dmpe) with one equivalent of (CIC\\(_6\\)H\\(_4\\)Se)\\(_2\\) gave Me-SeC\\(_6\\)H\\(_4\\)Cl and Pd\\(^{II}\\)(SeC\\(_6\\)H\\(_4\\)Cl)Tol(dmpe) as the major products. However, addition of a second equivalent of (CIC\\(_6\\)H\\(_4\\)Se)\\(_2\\) resulted in aryl-selenium coupling, from the reaction of Pd\\(^{II}\\)(SeC\\(_6\\)H\\(_4\\)Cl)Tol(dmpe) with (CIC\\(_6\\)H\\(_4\\)Se)\\(_2\\). X-ray structural studies are reported for Pd(O\\(_2\\)CPh\\(_2\\)(bpy), Pd(O\\(_2\\)CAr)\\(_2\\)(tmeda) (Ar = Ph, Ar\\(_F\\)), Pd(O\\(_2\\)CPh)Tol(bpy), M(O\\(_2\\)CPh)(NCN) (M = Pd, Pt), Pt(O\\(_2\\)CPh)\\(_3\\)(NCN), Pt(O\\(_2\\)CPh)\\(_2\\)R(NCN) (R = Tol, Me) and Pd(SeC\\(_6\\)H\\(_4\\)Cl)Me(dmpe). Observations of the reactivity of M(II) and M(IV) (M = Pd; Pt) complexes have led to a better understanding of the possible role of Pd(IV) in carbon-oxygen bond forming processes in reactions catalysed by palladium complexes.