Palladium -peroxo complex

Exposure of the crystalline palladium(O) complex 10 to air at room temperature caused a color change from deep red to pale yellow. The formation of the palladium(II)peroxocarbonate complex 12 was confirmed by spectroscopic analysis as well as a solid-state structure. Control reactions revealed that the first intermediate is a palladium(II)peroxo complex 11. 

The complexes can be obtained from the reaction of a Rh1 complex with 02 (equation 5),37 from the reaction of a Com-superoxo complex with a reduced metal (equation 6),38 from the acid hydrolysis of a platinum-peroxo complex3 (equation 7) or from the reaction of potassium superoxide with rhodium (equation 8)40 or palladium complexes.41... 

The low trans selectivity and increased propensity for 1,4-m addition is thought to arise from a direct interaction with the metal-porphyrin peroxo complex similar to that suggested for the 2,5-dimethoxyphenyl derivatives shown in Scheme 8-13. It was demonstrated that the 2,5-dimethoxyphenyl derivatives worked in the aerobic oxidation, even without p-benzoquinone being present [64], Interestingly, in this case the 1,4-cis addition product predominates. It was proposed that the (jr-allyl)palladium complex is activated as shown in Scheme 8-13. 

It is known that the CO2 present in air degrades palladium catalysts in oxidative processes leading to inactive carbonate salts, and that water can hydrolyze the peroxo-palladium complex in the proposed catalytic cycle. These competitive processes seem not to affect the robustness of this catalyst system. 

Buchwald described an oxypalladation reaction, followed by a C-H functionalization. This entirely intramolecular reaction is initiated through a 5-exo Wacker-type cychzation of 84. The resulting a-alkyl-paUadium intermediate M provides subsequent C-H activation at the neighboring arene, which allows a paUadium(II) intermediate N bearing a-alkyl and o-aryl substituents, respectively. Reductive elimination provides the C-C bond installation of 85 with the concomitant release of a paUadium(O) catalyst state. Reoxidation imder aerobic conditions, most probably through a palladium(II) -peroxo complex and protonolysis with the acetic add hber-ated in the previous steps, regenerates the original paUadium(II) diacetate catalyst. 

Mixed NHC-PR3 palladium complexes [(NHC)Pd(PR3)] and [(NHC)Pd(ri -02) (PR3)] were described by Gazin and co-workers as efficient catalysts for the oxidation of alcohols with O2 in the presence of acetic acid." The initial catalytic studies were performed using palladium peroxo complexes [(NHC)Pd(T -02) (PR3)]. Interestingly, further studies demonstrated that [(NHC)Pd(PR3)] exhibited similar performance in the oxidation of alcohols. Addition of acetic acid was necessary for both catalysts. This fact suggested the formation of palladium acetates as key intermediate species. In order to confirm the involvement of Pd acetates, [(IPr)Pd(OAc)2(PCy3)] was synthesized and proved efficient for... 

In contrast to inactive iridium(TTI)-Peroxo complexes, Irm-hydroperoxo species have been shown to transfer oxygen to a coordinated alkene, for example in the slightly catalytic oxidation of cyclooctene to cyclooctanone by 02 + H2 mixtures in the presence of IrHCl2(CgH12) (equation 95). 68 Oxygen transfer presumably occurs as for palladium hydroperoxides in equations (89) and (90). 

The other main group elements which form peroxo complexes are d6 and d8 systems in group VIII including iridium, palladium and platinum. The ji-peroxo complexes do not generally catalyse the epoxidation of olefins with hydrogen peroxide,95,96 but it has been found that trifluoromethyl-substituted Pd(II) and Pt(II) hydroperoxides will perform such a transformation.97... 

A very recent example of carbene/palladium chemistry is reported by Stahl in which (NHC)2Pd add molecular oxygen to form a peroxo complex. Oxidative chemistry involving this type of complex as catalyst is expected to follow. This represents a fundamental departure of NHC ligands from phosphines due to their stability to oxidation Konnick MM, Guzei lA, Stahl SS (2004) J Am Chem Soc 126 10212... 

Finally, re-oxidation with has been studied, in part because of the interest in using oxygen as a terminal oxidant for oxidation processes in general. This le-oxidation can occur by formation of a palladium-peroxo complex, LjPd(Oj), followed by protonation with the acid to form peroxide and flie starting palladium(II) species containing carboxylate, aUcoxide, sulfonamidate, or imidate ligands. This sequence is shown generically in Equation 16.130. 

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