Kaleidoscopic topochemical behaviour of diolefin crystals

At present, based on the topochemical rule, the configuration of photoproducts, as well as photoreactivity, can be precisely predicted from the crystal structure of the starting olefin compounds, with certain exceptions. 

On the other hand, the crystallization process of diolefin compounds often plays a significant role in determining their topochemical behaviour, by changing their crystal structure or by forming solvent inclusion complexes. Furthermore, topochemical photoreactions of crystals with )8-type packing are accompanied by thermal processes under moderate control by the reacting crystal lattice (see p. 140). These factors seriously complicate the whole reaction scheme. 

2 OPr-)8-dimer (as-prepared). On photoirradiation, 2 OPr-j8-dimer (recrystallized) does not afford 2 OPr-cyclo at all, but rather an oligomer. 

All these types of behaviour are reasonably interpreted by crystallographic analysis of these compounds, based on topochemical consideration. 

In this section, various types of topochemical behaviour such as the even-numbered degree of polymerization mechanism, topochemical induction into the syndiotactic structure, stereo- and enantio-selective reactions, and the formation of highly strained cyclophanes are described. 

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