Photodimerization cycloalkenes

As mentioned above, some arylalkenes, such as stilbene, form complexes with Ag+. Such complexes are also formed between Ag+, as added silver perchlorate, and simpler alkenes. Typical of this is the interaction with 1-methylenecyclohexane when a crystalline complex is formed. Irradiation of this complex in the polycrystalline state or in solution in methanol affords isomerization to 1-methylcyclohexene by a 1,3-hydrogen migration path. Further irradiation brings about the stereospecific formation of the exo,trans,exo-dimer (24) of 1-methylcyclohexene. Less specific photodimerization is also reported for the irradiation of the /S-pinene (25)/silver perchlorate complex69 The mechanism was thought to involve a silver/cyclohexenyl radical similar in type to that observed in the y-radiolysis of silver/cycloalkene complexes70,71. 

Copper(i) triflate, CuSOsF, is an efficient catalyst for the photodimerization of norbornene. The proposed mechanism of this reaction (Scheme 3), which is supported by quantum-yield studies, involves photoexcitation of a 2 1 alkene-CuSOsF complex followed by unimolecular collapse to products. An alternative mechanism involving photoexcitation of a 1 1 copper(i)-alkene complex and a subsequent termolecular interaction with two ground-state alkenes to give the dimer, a mechanism proposed for the photodimerization of norbomene-catalysed by copper(i) halides, is not consistent with the present studies. Photodimerizations catalysed by CuCl have only been observed with strained cycloalkenes. However, CuSOgF is an efficient catalyst for the photodimerization of simple alkenes e.g. cyclopentene. 

---