Alkenes oxidations, palladium® acetate

Dienes can be obtained by direct coupling of alkenes with palladium acetate. However, this reaction is seldom applied as a synthetic procedure, since the yields are low and side products due to oxidation of the double bond also contaminate the reaction. 

The importance of palladium acetate lies in its ability to catalyse a wide range of organic syntheses functionalizing C-H bonds in alkanes and in aromatics, and in oxidizing alkenes. It has been used industrially in the... 

Alkenes can also be oxidized with metallic acetates such as lead tetraacetate or thallium(III) acetate " to give bis-acetates of glycols. Oxidizing agents such as benzoquinone, Mn02, or 02, along with palladium acetate, have been used to convert conjugated dienes to l,4-diacetoxy-2-alkenes (1,4 addition). ... 

In contrast, the closely related palladium acetate-promoted intramolecular alkylation of alkenes by tri-methylsilyl enol ethers (Scheme 4)6,7 has been used to synthesize a large number of bridged carbocyclic systems (Table 1). In principle, this process should be capable of being made catalytic in palladium(II), since silyl enol ethers are stable to a range of oxidants used to carry the Pd° -> Pd11 redox chemistry required for catalysis. In practice, catalytically efficient conditions have not yet been developed, and the reaction is usually carried out using a full equivalent of palladium(II) acetate. This chemistry has been used in the synthesis of quadrone (equation 2).8 With the more electrophilic palladium(II) trifluoroace-tate, methyl enol ethers underwent this cyclization process (equation 3).9... 

Commercially available palladium compounds in the presence of various ligands have often been used as catalysts (Table 3-1). The first choice is often the air-stable and relatively inexpensive palladium acetate however, several of the other published variants can be preferable in certain applications. It is commonly assumed that the palladium(II) species is reduced in situ by the solvent, the alkene [11], the amine [12] or the added ligand (frequently a phosphane, which is oxidized to a phosphane oxide) [13]. In some cases, highly dispersed elemental palladium on charcoal can be applied. In the case of alkenyl or aryl bromides, phosphanes are necessary to avoid precipitation of palladium black (c.f., however. Section 3.2.4.), whereas iodides have been reported to be less reactive in the presence of phosphanes. Triflates have been found to be more reactive in the presence of chloride ions, as the chloride ligand is more easily removed from palladium than the tiiflate ion [14], However, this also has become questionable, because successful coupling reactions of alkenyl triflates have been performed in the absence of chloride ions [15]. 

The selectivity of palladium and gold for alkene oxidation to aldehydes 28,29,170) was attributed initially to adsorption strength. However, electrooxidation in the presence of palladium ions indicates possible homogeneous alkene insertion, similar to the Wacker process 304). Homogeneous reaction is also involved in redox oxidations of hydrocarbons. In this case, the nature of the metal ions is expected to control selectivity. Indeed, toluene yields 20% benzaldehyde in electrolytes containing Ce salts, while oxidation proceeds to benzoic acid with Cr redox catalysts 311). In addition, the concentration of redox catalysts appears to affect yields in nonelectrochemical oxidation of ethylene large amounts of palladium chloride promote butene formation at the expense of acetaldehyde 312). Finally, the role of the electrolyte and solvent should not be ignored. For instance, electrooxidation of ethylene on carbon, in aqueous solution of acetic acid yields acetaldehyde 313) in the... 

Allylic acetoxylation with palladium(II) salts is well known however, no selective and catalytic conditions have been described for the transformation of an unsubstituted olefin. In the present system use 1s made of the ability of palladium acetate to give allylic functionalization (most probably via a palladium-ir-allyl complex) and to be easily regenerated by a co-oxidant (the combination of benzoquinone-manganese dioxide). In contrast to copper(II) chloride (CuClj) as a reoxidant,8 our catalyst combination is completely regioselective for allcyclic alkenes with aliphatic substrates, evidently, both allylic positions become substituted. As yet, no allylic oxidation reagent is able to distinguish between the two allylic positions in linear olefins this disadvantage is overcome when the allylic acetates are to... 

Oxidative Amination of Alkenes. Palladium acetate-catalyzed aerobic oxidation of cyclopentene and cyclooctene using TcesNH2 affords the corresponding allylic amine products (eq 8). Small amounts of isomeric alkene products are also obtained in this reaction. 

With hydrogen peroxide or, better, t-butylhydroperoxide as the oxidant, the combination of palladium acetate and hydroquinone forms an efficient catalyst system, which does not require other cocatalysts. Using t-butylhydroperoxide as the oxidant, it is possible to react a series of different carboxylates with alkenes in methylene chloride solution. For instance, (5)-0-acetylmandelic acid was reacted with cyclohexene to give the addition product with a modest chiral induction (de ca. 20%). Also, intramolecular cyclization could be achieved for example, 2-cyclopenten-l-acetic acid gave mainly the allylic lactone in good yield, accompanied by a small amount of the double bond isomer (Scheme 9). 

Earlier studies have also shown that a catalyst system consisting of palladium(II) and copper salts plus oxygen for the reoxidation did not work well,t in contrast to the result with the Wacker oxidation. However, if quinone or hydroquinone was added to a mixture of palladium acetate and copper acetate, oxygen could be used as an efficient oxidant for conversion of alkenes into allylic acetates. Thus, cyclohexene gave better than 85% cyclohexenyl acetate (Scheme 10). The combination of oxygen and cobalt or manganese acetate also works, but less well.t ... 

This cyclization has been used in an efficient synthesis of chokol A 11.219, an anti-fungal compound (Scheme 11.74). The acetal 11.216, in which the tartrate moiety functions as an economical chiral auxiliary, cyclized using palladium acetate to give an 8.5 1 mixture of stereoisomers. Aeidic hydrolysis removed the acetal to give a ketone 11.217, which could be taken through to the natural product 11.219 by selective reduction of the electron-poor alkene, stereoselective addition of a methyl group, allylic oxidation and desilylation. 

Carbonyl Compounds by Oxidation of Alcohols and Aldehydes. Salts of palladium, in particular PdCl2 in the presence of a base, catalyze the CCI4 oxidation of alcohols to aldehydes and ketones. Allylic alcohols carrying a terminal double bond are transformed to 4,4,4-trichloro ketones at 110 °C, but yield halo-hydrins at 40 °C. These can be transformed to the corresponding trichloro ketones under catalysis of palladium acetate (eq 56). The latter transformation could be useful for the formation of ketones from internal alkenes provided the halohydrin formation is regioselective. 

Palladium-catalyzed Direct Arylation of Indoles and Thiophenes. Five-membered ring heterocycles possessing only one heteroatom and no Af-oxide function can also be arylated using palladium(II) complexes, a phosphine ligand, and an inorganic base. In one example, a tandem palladium-catalyzed Heck coupling reaction and direct intramolecular C2 arylation reaction on a )V-(2-chlorobenzyl)-5-bromoindole was reported (eq 30). The procedure, which is catalyzed by palladium acetate, uses tn-tert-butylphosphonium tetrafluoroborate as a ligand and ferf-butyl acrylate as the alkene for the Heck reaction (eq 30). ... 

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