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

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posted on 2023-05-26, 21:41 authored by Denney, MC
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.


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Copyright 2003 the author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s). Appendix A is the following published article: Canty, A. J., Done, M. C., Skelton, B. W., White, A. H., 2001. Carbon-oxygen bond formation at organopalladium centres, Inorganic chemistry communications, 4(11), 648-650.

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