Photodimerization of Olefins and Polyenes

The photodimerization of simple isolated olefinic bonds is rarely observed because of the absorption of these compounds in the high-energy or vacuum-ultraviolet region. One case reported is that of the photo-dimerization of 2-butene. Irradiation of liquid c r-2-butene with light from a cadmium (A = 229, 227, 214 nm) or zinc (A = 214 nm) lamp was reported to lead to dimers (1) and (2)  

Similarly, irradiation of liquid /ranr-2-butene yielded dimers (1) and (3)  

It can be seen that these products result from stereospecific dimerization of the corresponding 2-butenes. Irradiation of mixtures of cis- and /roiu-2-butene resulted in a fourth isomer (4) in addition to the above products  

The stereospecificity of these reactions is surprising in light of the large energies absorbpd by these molecules. Indeed, the major photochemical product of these photolyses was the alternate olefin isomer (1-butene was also observed). These results indicate that free rotation about the photo-excited double bond does not occur in those molecules that dimerize. This suggests the participation of ground state complexes or excimers in the photodimerization. This view is supported by the observations that dilution of m-2-butene with neopentane (1 1) decreased the yield of dimers and a 1 4 dilution almost completely suppressed dimerization. 

A photosensitized dimerization of an isolated olefin, norbomene, has been reported by Scharf and Korte. Irradiation in acetone or in the presence of acetophenone ( , = 74 kcal/mole) produced dimers (5) and (6) as major products. However, benzophenone ( = 69 kcal/mole) failed to sensitize the reaction to (5) and (6), but in ether solution led to the quantitative formation of benzpinacol and in benzene to the oxetane (7) in 80% yield. Sensitizers of intermediate energy, such as xanthone Et = 72 kcal/mole), demonstrated a competition between energy transfer to form triplet norbomene and cycloaddition to form the oxetane  

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