Mechanisms copper regeneration

Scheme 9 Proposed mechanism for regeneration of copper complexes in ARGET and ICAR ATRP...

The reaction mechanism is not rigorously known, but is likely to involve the following steps." " First the arenediazonium ion species 1 is reduced by a reaction with copper-(l) salt 2 to give an aryl radical species 4. In a second step the aryl radical abstracts a halogen atom from the CuXa compound 5, which is thus reduced to the copper-1 salt 2. Since the copper-(l) species is regenerated in the second step, it serves as a catalyst in the overall process. 

The proposed reaction mechanism involves intermolecular nucleophilic addition of the amido ligand to the olefin to produce a zwitterionic intermediate, followed by proton transfer to form a new copper amido complex. Reaction with additional amine (presnmably via coordination to Cn) yields the hydroamination prodnct and regenerates the original copper catalyst (Scheme 2.15). In addition to the NHC complexes 94 and 95, copper amido complexes with the chelating diphosphine l,2-bis-(di-tert-bntylphosphino)-ethane also catalyse the reaction [81, 82]. 

A copper-centered mechanism for the Cu-TEMPO-catalyzed aerobic oxidation of alcohols was proposed by Sheldon and co-workers, wherein the active catalytic Cu" species is generated by oxidation of a Cu species with TEMPO, in the presence of alcohol, with formation of TEMPOH (Scheme 3) [146]. The resulting Cu" species is then capable of oxidizing the alcoholate to the aldehyde or ketone species. Regeneration of the TEMPO radical species was achieved by rapid oxidation of TEMPOH with O2. 

Copper(I) salts catalyze the allylic oxidation of alkenes in the presence of peresters, such as tert-BuOjCOPh, to afford the corresponding allylic benzoate esters. In the case of terminal alkenes, internal allylic esters are formed in preference over the terminal isomers. The mechanism is believed to involve addition of an allylic radical to copper(II) benzoate. " Rearrangement of the copper(III) intermediate then produces the product and regenerates the copper(I) catalyst. 

The mechanism of the Sonogashira cross-coupling follows the expected oxidative addition-reductive elimination pathway. However, the structure of the catalytically active species and the precise role of the Cul catalyst is unknown. The reaction commences with the generation of a coordinatively unsaturated Pd species from a Pd " complex by reduction with the alkyne substrate or with an added phosphine ligand. The Pd " then undergoes oxidative addition with the aryl or vinyl halide followed by transmetallation by the copper(l)-acetylide. Reductive elimination affords the coupled product and the regeneration of the catalyst completes the catalytic cycle. 

The reaction mechanism proceeds via a transmetallation giving an aryl-copper-nitrogen species, followed by a reductive elimination, but the difference from the aryl hahde reaction is that this generates a species (Cu(0)) that cannot enter into a catalytic cycle. However, methods that are catalytic in copper have been developed, using an oxidant to regenerate Cu(ll), although here there is the possibility for a variation in... 

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