Oxidation reactions Palladium trifluoroacetate

Oxidation of alkenes and dienes involving an allene substituent as a formal nucleophile is a conceptually new reaction. Allcnc-substitutcd 1,3-cyclohexadienes 34 undergo a pal-ladium(II)-catalyzed oxidation to give bicydic compounds 35 or 36 in good yields (Scheme 17.14) [14]. When y-alkenylallenes, e.g. 37, 41 and 43, were treated with 1 mol% palladium trifluoroacetate, a similar oxidative carbocyclization took place [15]. In both reactions the new stereocenters are formed with high stereoselectivity. 

Palladium was also investigated for oxidative carbonylation using O2 as oxidant, but now CUCI2 was added to re-oxidize the palladium metal. These reactions were carried out in aqueous trifluoroacetic acid, and led to the formation of a mixture of methanol and its triflate ester. ... 

While such a process had initially been observed as an undesired side-reaction in transformations where copper salts were employed as re-oxidants [13], Chemler demonstrated that various aminohalogenation reactions proceed in THF or acetonitrile in the presence of potassium carbonate as base [14]. These reactions employ palladium trifluoroacetate or palladium dibromide as catalyst source and require a moderate excess of the copper oxidant (3-4 equiv) giving moderate to excellent yields. However, they usually suffer from rather low selectivity, either in the initial aminopalladation or via subsequent rearrangement pathways to provide mixtures of pyrrolidines and piperazines (Scheme 4.2, Eq. (4.3)). A stoichiometric control reaction in the presence of palladium bromide led only to the Wacker cydization together with an alkene isomerization product, suggesting that the presence of copper(II) salts is crucial for the overall process. The exact role of the copper(II) salts has not yet been darified and palladium intermediates of different oxidation states may be involved in the final stage of carbon-halogen bond formation. 

The oxaspirocyclization was applied to the synthesis of theaspirone and vitispirane (equations 26 and 27)59. Under slightly modified reaction conditions where water is employed as the major solvent, palladium-catalyzed 1,4-oxidation of 64 afforded 65. Alcohol 65 was oxidized to theaspirone, which was obtained as a 1 1 isomeric mixture of cis and trans isomers. When the analogous reaction was performed at a lower pH by the use of trifluoroacetic acid, vitispirane was formed in high yield, again as a 1 1 isomeric mixture of stereoisomers. 

Other bidentate N-heterocyclic carbenes were used to form stable chelate complexes. A fine example is the use of palladium NHC complex 24 in the catalytic conversion of methane to methanol (Fig. 10) [111]. In this case the stability of the complexes is a requirement, since the reaction takes place in an acidic medium (trifluoroacetic acid) at elevated temperatures (80 °C) mediated by strong oxidizing agents (potassium peroxodisulfate). 

Palladium catalysts are best known for oxidizing alkenes to ketones or vinyl derivatives. However, formation of a, unsaturated carbonyl compounds by UV irradiation of oxygenated solutions of alkoies in the presence of catalytic amounts of palladium s ts has been observed by Muzait - This reaction is believed to proceed through a ir-allylpalladium trifluoroacetate complex, e.g. (77). 

Palladium(II) catalysts, such as PdCl4 in trifluoroacetic acid solvent, have been used for the conversion of methane to methanol (7i). The system uses hydrogen peroxide as oxidant the overall reaction involving the addition of one mole of water is shown in Eq. 26. The function of the... 

Phenyl trifluoroacetate (149) undergoes oxidative addition to generate the acyl-palladium 150 [64], Based on this reaction, trifluoromethyl ketones such as 151 can be prepared by Suzuki coupling with arylboronic acids. Similarly, anisyl heptafluoropropyl ketone (154) was prepared by the Suzuki coupling of phenyl heptafluorobutyrate (152) with anisylboronic acid 153 [65]. 

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