Organocopper complexes oxidation reactions

A less convenient route for the preparation of cyclopentadienylcopper-ligand complexes involves the use of Cu(I) oxide. Purification of the organocopper complexes is difficult because of the decomposition products formed by subsequent hydrolysis by H2O produced in the metallation reaction, e.g. ° ... 

In inorganic and coordination chemistry, the Cu(II) state is the most abundant one, and is regarded as more stable than the Cu(II) state under normal conditions [32]. Although numerous examples of Cu(I) coordination complexes are known, their chemistry is rather limited and they are readily oxidized to Cu(II) species [32]. Of the common oxidation states, compounds derived from copper(III) are rare, with only 30-40 reported examples [32]. Despite the small number of isolated Cu(III) compounds, however, organocopper] 111) species have been proposed as important intermediates in copper-mediated organic reactions (Chapts. 4 and 10). 

Copper(I) catalysis is very well established to promote intramolecular [2+2] photocycloaddition reactions of l,n-dienes (review [351]). The methodology recently enjoyed a number of applications [352-354], It is assumed that CuOTf, which is commonly applied as the catalyst, coordinates the diene and in this way mediates a preorganization. The Ghosh group recently reported a number of CuOTf-catalyzed photochemical [2+2] cycloaddition reactions, in which an organocopper radical complex was proposed as a cyclization intermediate (which should, however, have a formal Cu(II) oxidation state) (selected references [355-357]). A radical complex must, however, not be invoked, since the process may either proceed by a [2+2] photocycloaddition in the coordination sphere of copper without changing the oxidation state or according to a cycloisomerization/reductive elimination process. 

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