I Catalyzed Cycloadditions of Olefins

Among the simple inorganic catalysts employed in photochemistry of organic substrates Cu(I) salts are the most important and most extensively investigated. Cu(II) is not effective in reactions of unfunctionalized olefins, since it does not form complexes with C=C bonds and in cases Cu(II) salts have been added as catalysts these are reduced in situ to Cu(I). 

A variety of photoreactions of olefins such as rearrangements of diolefins and of strained systems, 2+2 cycloadditions as well as 2+4 cycloadditions are effectively catalyzed by Cu(I) salts. Cu(I) halides (CuCl, CuBr), which are highly insoluble in water in the presence of an olefin show limited solubility in organic solvents, hydrocarbons and in particular ether. Subsequently to the use of halides the trifluoromethanesulfonate (triflate, OTf) of Cu(I) was introduced by Kochi et al. [5], which was found to be a superior reagent, due to its higher solubility and to the higher ionic character of the Cu-OTf-bond that facilitates olefin bonding. The solubility is due to complex formation, either of 1 1 or 1 2 Cu olefin-stoichiometry. In many cases complexes cannot be isolated but are formed in pre-equilibria in solution. The 1,5-cyclooctadiene (COD) complex 1, isolable as a compound and reasonably stable in solution, has been studied with respect to photoconversion of the complex itself as well as its role as a catalyst. 

A molecular orbital treatment [7] on the HFS (Hartree-Fock-Slater) level of the Cu-ethylene Cu(C2H4) and CuX(C2H4) model complexes with X = 

Either concerted electrocyclic reactions of the complexed olefin(s) or a sequence involving the formation of Cu-bound alkyl radicals produced through 7t/a-rearrangement, ring closure to an intermediate cupracycle followed by reductive elimination has been considered. In particular in the framework of the ionic Cu model, approximated closest by CuOTf, ethylene-7i mixes into Cu-3d to an appreciable amount. Excitation then has more resemblance to an intraligand transition and thus parallels the photochemistry of the pure olefins. 

Apart from specific excitation of a Cu complex one major role of the Cu catalyst in these reactions is undoubtedly to keep the two reacting olefinic double bonds in close proximity by complexing them to a single metal centre (see below). At the same time it is clear that in a Cu-bis(olefin)-complex excitation will lead to activation of both olefinic ligands whereby the difficulty of any excited state reaction of two unrelated double bonds, i.e. the simultaneous excitation of both partners, or the formation of exciplexes will be removed. More consideration of mechanistic details will be given with the examples. 

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