Nonconjugated 2+2 -photocycloaddition

Intramolecular [2 + 2] photocycloadditions of alkenes is an important method of formation of compounds containing four-membered rings.184 Direct irradiation of simple nonconjugated dienes leads to cyclobutanes.185 Strain makes the reaction unfavorable for 1,4-dienes but when the alkene units are separated by at least two carbon atoms cycloaddition becomes possible. 

A large number of [2 + 2] photocycloadditions ofvinylarene compounds have been reported in the literature [37], and these are particular suitable for the synthesis of cyclophanes. Based on the fact that conjugation between the olefinic and aromatic 11-systems is rather low, these reactions can be compared to corresponding reactions with nonconjugated alkenes. 

Malik, C.K., Vaultier, M., and Ghosh, S. (2007) Copper(I)-catalyzed [2 + 2] photocycloaddition of nonconjugated alkenes in room-temperature ionic liquids. Synthesis, 1247-1250. 

The [2 + 2]-photocycloaddition of nonconjugated alkenes was described in Chapter 5. This strategy can be used for synthesizing macrocyclic rings by using a long linker between the two alkene moieties (Scheme 9.47). Thus, bicyclic cyclobutanes can be obtained starting from enol ethers and cinnamates by means of electron-transfer sensitizers, such as DCN or DCA [80], and triplet photosensitizers such as benzo-phenone (BP) [81]. 

It is now well established that Cu(l)-catalyzed [2 + 2]-photocycloaddition of alkenes requires formation of a complex in which one copper is coordinatively linked with two alkene units. Among the various copper salts (CuCl, CuBr, CuOTf) used, the triflate anion in CuOTf has exceptional weak coordinating ability compared to halide ions, which compete with alkene for coordination with copper. Thus, CuOTf exhibits a strong tendency to form 1 2 Cu-alkene complexes compared to CuCl or CuBr. This is reflected in improved yields of adducts obtained with CuOTf rather than CuCl or CuBr (vide infra). In addition, CuOTf is soluble in most organic solvents and is stable under the photochemical reaction conditions. Hence, CuOTf is the catalyst of choice for [2 + 2]-photocycloaddition between two nonconjugated alkenes. 

---