Copper complexes cycloisomerization

Inspired by the success of intramolecular addition and tautomerization of aldehydes with a pendant alkyne through cooperative catalysis of a secondary amine and an An complex, in 2008, Yang et al. reported a cascade reaction with the combination of a copper complex and an achiral secondary amine catalyst for the synthesis of attractive carbocycles [48]. This chemistry merged a pyrrolidine-promoted Michael addition via iminium ion intermediates and a Cu-catalyzed cycloisomerization protocol (Scheme 9.54). Various ketones and alkyne-tethered active methylene compounds could be converted into densely functionalized cyclopentene derivatives. Although the asymmetric version was not given, the chemistry described here was amenable for the implementation of asymmetric synthesis of such functionalized molecules by a combination of chiral amines and suitable Au complexes. 

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. 

The roles of NHC complexes of copper, silver, and gold in conjugate additions, allylic alkylations, reduction, boration, hydrofunctionalizations, hydrations, cross-couplings, and enyne cycloisomerization have been discussed. ... 

The cycloisomerization reaction was also catalyzed by copper(I) complexes. The examples were well reviewed by Fehr [267]. Cycloisomerization of 1,5-enyne compounds 411 was explored using transition metal-catalyzed conditions (Scheme 1.191) [268]. Among the metal complexes examined, Cu(CH3CN)4BF4 provided the best results for the formation of tricyclic cyclopropanes 412. 1,6-Enyne 413 also underwent a similar cycloisomerization reaction to give polycyclic cyclopropanes 414 (Scheme 1.192) [269]. 

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