Topotactic photodimerization

Toda et al. reported that the topotactic and enantioselective photodimerization of coumarin and thiocoumarin takes place in single crystals without significant molecular rearrangements [49]. Molecular motion needs to be called upon to explain the photochemically activated cycloaddition reaction of 2-benzyl-5-benzylidenecyclopentanone. The dimer molecules, once formed, move smoothly in the reactant crystal to form the product crystal [50]. Harris et al. investigated the reactivity of 10-hydroxy-10,9-boroxophenanthrene in the solid state and the mechanism of the solid-state reaction was characterized by both X-ray diffraction and thermal analysis [51]. It was demonstrated that the solution chemistry of 10-hydroxy-10,9-boroxophenanthrene is different from that in the solid state, where it undergoes dimerization and dehydration to form a monohydride derivative. 

It may be suspected that the genuinely topotactic (as secured by the molecular precision of the AFM [18]) photodimerization of 2-benzyl-5-benzyli-denecyclopentanone [118] might be a good candidate for a quantitative preparative photo dimerization to give the head-to-tail anti-[2+2] dimer. Early quantitative solid-state [2-1-2] photodimerizations (most of the published mechanistic interpretations of which can no longer be accepted) are listed in [110]. These deal with the anti dimerization of acenaphthylene-1,2-dicarboxylic anhydride, the head-to-head syn dimerization of acenaphthylene-1-carboxylic acid, the syn dimerization of 5,6-dichloroacenaphthylene, and the thermally reversible head-to-tail anti dimerization of seven ( )-2,6-di-f-butyl-4-(2-aryl-ethenyl)pyrylium-trifluoromethanesulfonates. All of these reactions proceed fully specific. On the other hand, quantitative photoconversions of a 1 1 mixed crystal of ethyl and propyl a-cyano-4-[2-(4-pyridyl)ethenyl]cinnamates gives mixtures of diesters with one (A>410 nm) or two cyclobutane rings (no cutoff filter). 

Topotactic forces affect the ordinary reaction in some other ways. There is the case of a small molecule partidpating in crystalline-state photodimerization (see Sect, m.a.), e.g. small molecules such as water or n-hexane in the atmosphere are involved in the reaction29,30. These small molecules, sometimes even the moisture in the air, not only greatly affect the rate and conversion of the reaction but are also included in the crystal of the photoproduct as a crystal molecule at the end of the reaction. In such dimer crystals, a small molecule may be useful for filling up the cave in the crystal, thus stabilizing the... 

Theocharis, C. R., and Jones, W. Topotactic and topochemical photodimerization of benzylidenecyclopentanones. In Organic Solid State Chemistry. (Ed., Desiraju, G. R.) Ch. 2., pp. 47-68. Elsevier Amsterdam, Oxford, New York, Tokyo (1987). 

Under rare circumstances it may happen that the molecular shape of two monomers and the dimer, and their crystal parameters, are so nearly identical that a topotactic phototransformation occurs single crystal into single crystal. This is realized in intramolecular cases like [2,2]-(9,10)-an-thracenophane 17 and its [4 + 4]-photoisomer 18 [31]. However, it was also possible to find some rare intermolecular cases of topotactic single crystal into single crystal photodimerizations with chalcones like 2-ben-zyl-5-benzylidene-cyclopentanone 19 [32]. Since there is virtually no mol-... 

Topotactic processes are described more comprehensively in Mechanical Preparation of Crystalline Materials. An Oxymoron and Templated [2 + 2] Photodimerizations in the Solid State, Supramolecular Materials Chemistry. Since its original definition in 1959, the term topotaxy has evolved " to mean a solid-state chemical reaction (not necessarily reversible) that completely transforms a single crystal such that the product resembles that of the initial material both in terms of its three-dimensional structure and the orientation thereof. Topotactic is synonymous with topochemical... 

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