Copper catalyzed photocycloaddition, alkenes

Norbornene structures can be linked by long chain diesters. The copper catalyzed photocycloaddition of these alkenes gives rise to the cyclobutane shown in Sch. 26 [38]. The intramolecular reaction is stereoselective. 

Avasthi, K., Raychaudhuri, S. R., Salomon, R. G., Copper Catalysis of Olefin Photoreactions. 13. Synthesis of Bicyclic Vinylcyclobutanes via Copper catalyzed Intramolecular 2ji + 2ji Photocycloadditions of Conjugated Dienes to Alkenes, J. Org. Chem. 1984, 49, 4322 4324. 

The thermo- and photocycloaddition of alkenes will be discussed in Chapter 12, on pericyclic reactions. On the other hand, transition-metals have effectively catalyzed some synthetically useful cycloaddition reactions in water. For example, Lubineau and co-worker reported a [4 + 3] cycloaddition by reacting a,a-dibromo ketones with furan or cyclopen-tadiene mediated by iron or copper, or a-chloro ketones in the presence of triethylamine (Eq. 3.48).185... 

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. 

K. Avatschi, S. R. Raychaunduri, and R. G. Salomon, Copper(I) catalysis of olefin photoreactions. 13. Synthesis of bicyclic vinylcyclobutanes via copper(I)-catalyzed intramolecular 2.pi. + 2.pi. photocycloadditions of conjugated dienes to alkenes, J. Org. Chem. 49,4322-4324 (1984). 

Langer, K., Mattay, J., Copper(I) catalyzed Intra and Intermolecular Photocycloaddition Reactions of Alkenes. In Horspool, W. M., Song, P. S. (eds), CRC Handbook of Organic Photochemistry and Photobiology, CRC Press, Boca Raton, FL, 1995, pp. 84 104. 

Copper(I)-Catalyzed Inter- and Intramolecular [2 + 2]-Photocycloaddition Reactions of Alkenes... 

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. 

Copper(I)-catalyzed intermolecular photocycloaddition requires that one of the reacting alkenes must be highly strained and reactive. It was already stated that norbornene 3 undergoes intermolecular addition to form the dimer 4 (Scheme 2) in 38% yield with CuBr and in 88% yield with CuOTf as the catalyst. This is the first example of copper(I)-catalyzed intermolecular (2 -t 2]-photocycloaddition." Similarly, endo-dicyclopentadiene 5, on irradiation in the presence of CuOTf, undergoes (2 + 2]-addition at the more stained 8,9-double bond to form the dimer 6 (Scheme 3). Cyclopentene does not undergo dimerization with CuBr, while with CuOTf it forms the dimers 11 (30%) and 12 (2%) (Scheme 5). ... 

Tricydic ring systems can also be constructed when one of the alkene units of the 1,6-heptadiene is part of a ring. For example, the dienol 40 leads to the tricyclic products 41 (68%) and 42 (23%) (Scheme 14) The loss of stereospecificity in the products probably arises due to competing copper(l)-catalyzed geometric isomerization of the frans-aUyhc alcohol during photocycloaddition. 

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