Reoxidants copper® acetate

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 ... 

A very recent addition to the already powerful spectrum of microwave Heck chemistry has been the development of a general procedure for carrying out oxidative Heck couplings, that is, the palladium)11)-catalyzed carbon-carbon coupling of arylboronic acids with alkenes using copper(II) acetate as a reoxidant [25], In a 2003 publication (Scheme 6.6), Larhed and coworkers utilized lithium acetate as a base and the polar and aprotic N,N-dimethylformamide as solvent. The coupling... 

Scheme 6.6 Oxidative Heck coupling of boronic acids and alkenes using copper(ll) acetate as a reoxidant.

When acetaldehyde is oxidized in the presence of copper (II), the noncatalytic reaction between acetaldehyde and peracetic acid may be the main route to acetic acid. Since this reaction is slow, one would expect the presence of a significant concentration of peroxide in the reactor product, and we have confirmed this experimentally. Acetic acid can also be produced by oxidizing acetyl radicals by copper (II) the copper(I) formed could be easily reoxidized by oxygen. The by-products when using copper (II) acetates must be produced mainly by degradation of peracetoxy radicals by Reaction 14 and 16 since peracetic acid decomposition is negligible and the reaction of acetaldehyde with peracetic acid produces essentially only acetic acid. 

In fact, the role of copper and oxygen in the Wacker Process is certainly more complicated than indicated in equations (151) and (152) and in Scheme 10, and could be similar to that previously discussed for the rhodium/copper-catalyzed ketonization of terminal alkenes. Hosokawa and coworkers have recently studied the Wacker-type asymmetric intramolecular oxidative cyclization of irons-2-(2-butenyl)phenol (132) by 02 in the presence of (+)-(3,2,10-i -pinene)palladium(II) acetate (133) and Cu(OAc)2 (equation 156).413 It has been shown that the chiral pinanyl ligand is retained by palladium throughout the reaction, and therefore it is suggested that the active catalyst consists of copper and palladium linked by an acetate bridge. The role of copper would be to act as an oxygen carrier capable of rapidly reoxidizing palladium hydride into a hydroperoxide species (equation 157).413 Such a process is also likely to occur in the palladium-catalyzed acetoxylation of alkenes (see Section 61.3.4.3). 

Reduction of cupric acetate An unreduced solution of cupric acetate in quinoline is a dark green color the fully reduced solution is a clear ruby red. When quinone has been reduced, as described above, further treatment with hydrogen produces metallic copper. In contrast, when cupric ion has all been reduced to cuprous ion, no further reduction occurs within convenient experimental times, and the solution remains clear. The reduced cupric solution can be reoxidized rapidly by oxygen at room temperature. 

Both reactions were at the origin of the boom in palladium chemistry, scientifically as well as industrially. To render the previously cited reactions catalytic, the reduced form of Pd is reoxidized with the Cu2+/Cu+ redox couple, with the reduced form of copper finally being reoxidized with dioxygen. This chemistry is performed industrially with the chloride salts of Pd2+ and Cu2+ with ethylene as a feed, either in an aqueous or acetic acid medium. Products are acetaldehyde and vinyl acetate, respectively. 

A reaction mechanism for the above reactions was proposed which consists of initial formation of the copper precursor complexes of Fig. 3 (without coordinated phenolate), coordination of phenolate, electron transfer from phenolate to Cu2+ and subsequent reduction to Cu1+ with formation of a phenoxy radical, and reoxidation of Cu1+ to Cu2+ with oxygen. Various copper(II) catalysts having different stereochemistries (octahedral or tetrahedral coordination) due to coordination of amines like pyridine (Py) or acetate (OAc) groups in different ligand sites were observed by NMR and electron paramagnetic resonance techniques. 

Few examples of carboaminations of normal double and triple bonds are known. Similarly to allylic alcohols, tosylated allylic amines with (cthoxy)ethene form 2-ethoxy-4-vinylpyrrolidines 2. The reaction, mediated by stoichiometric amounts of palladium(II) acetate, can be changed to catalytic if copper(II) acetate is added as reoxidant of palladium(O). Moderate stereoselectivity (d.r. 64 36) is reported, however, without further structural assignment. 

The reaction is an equilibrium that can be shifted to the right by introduction of a base sodium acetate is one of the reagents of choice with branched olefins.4 For unbranched alkenes, dimethyl-formamide is preferred.5 Inorganic bases have also been used6 but are less satisfactory. Addition of copper(I) chloride7 improves the yields, since any Pd (II) that is reduced by the olefin (which in turn is converted into an aldehyde or ketone) is reoxidized. 

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... 

The irradiation of an alkane solution in acetonitrile with visible light in the presence of catalytic amounts of quinone and copper(II) acetate gives rise to the formation of almost pure alkyl hydroperoxide which is decomposed only very slowly under these conditions to produce the ketone and alcohol [64c,d]. It has been proposed [64d] that the first step of the reaction is a hydrogen atom abstraction from the alkane, RH, by a photoexcited quinone species to generate the alkyl radical R and semiquinone. The former is rapidly transformed into ROO and then alkyl hydroperoxide, while the latter is reoxidized by Cu(II) into the initial quinone (Scheme IX. 10). 

Purified preparations of cytochrome oxidase in the oxidized state display an absorption band (maximum 830 nm) in the near-infrared region which disappears on reduction and reappears on reoxidation at rates commensurate with those of the band at 605 nm (Wharton and Tzagoloflf, 1964). This chromophore can be gradually and irreversibly destroyed by treating the oxidase with the copper-specific chelator bathocuproinesulfonate in the presence of acetate buffer below pH 5 or by dialyzing... 

The acetoxylation of ethylene to form the vinyl acetate monomer (VAM) can be catalyzed by homogeneous catalysts comprised of PdCl/CuCl salts and carried out in glacial acetic acidl . The reaction is catalyzed by the Pd + species which are reduced by the adsorption of ethylene and subsequently reoxidized by oxygen and copper chloride. The speculated mechanism is as follows... 

In the presence of water, Wacker cheudstiy tends to predominate, whereby ethylene reacts with water rather than acetic acid and forms acetaldehyde as the primary product. Heniy and Pandeyl l showed that the addition of alkali metal acetates can help to shift the product spectrum in order to form vinyl acetate in higher yields. The reaction is thought to involve the nucleophillic attack of ethylene by acetate to form a C2H4-OAC-Pd complex which subsequently undergoes /3-C-H activation to form VAM. Acetic acid or HCl can desorb from the complex to form Pd ° which is reoxidized back to copper chloride to regenerate Pd2+. 

Allylpalladium halides are formed very readily, for example from alkenes and palladium salts. The reaction is assisted by a base such as sodium acetate, which removes hydrogen chloride. Yields are improved if copper(II) acetate is added to reoxidize any palladium metal which may be formed in side reactions (cf. p. 380). Allylpalladium reagents have found considerable application in organic synthesis, both in stoichiometric and catalytic reactions (p. 261). 

Since, as shown in equation (34), palladium metal is precipitated as a byproduct of the reaction, it is necessary to reoxidize it back to the Pd " " state. This is accomplished with a palladium-copper couple, as depicted in equations (35) and (36), which is driven by oxygen. The reaction is carried out by contacting a mixture of ethylene and oxygen with a mixture of acetic acid, lithium acetate, and the palladium-copper couple at temperatures of 80 to 150 °C. The vapor-phase process is carried out under pressure at high temperatures (120 to 150 °C) using a fixed-bed palladium catalyst [218]. The oxidative acylation of ethylene can also be used for the preparation of the higher vinyl esters, although it is not currently used for that purpose, due to the low demand for those materials. 

Optimum conditions for the reduction of saturated ketones by the complex reducing agent formed from sodium hydride, sodium t-amylate, and Ni" acetate (NiCRA) have been delineated.Reoxidation of the secondary alcohol products is dramatically postponed by the addition of alkali- or alkaline-earth-metal salts, and catalytic ketone reductions are achieved with NiCRA-MgBr2 mixtures. Full details of the reducing properties of various complex metal hydrides (12) of copper, formed by reaction of UAIH4 with appropriate lithium methylcuprates [equation (1)], have been published for example enones are reduced pre-... 

---