Light allowed conversions Photochemically

These observations are relevant to the general problem of photochemical light-energy conversion. Thus, retardation of the back electron-transfer reaction is a major prerequisite in any artificial photosynthetic system (9). In the specific Ir(II) /DMB (sol-gel) system the absence of an effective back reaction allows (at acidic pH) the strong reductant Ir(II) to react with water yielding molecular H2 according to (7) ... 

In the skin of animals, 7-dehydrocholesterol is converted to vitamin D, by the reaction sequence that follows. The first step in this process, the conversion of 7-dchy-drocholesterol to pre-cholecalciferol, requires light. This is an electrocyclic reaction and must occur by a conrotatory motion to avoid the formation of a highly strained trans double bond in one of the rings. Conrotation involving three pairs of electrons must occur photochemically to be allowed. 

Further examples using rapidly equilibrating screwed/helical/spiral molecules that happen to crystallize in chiral space groups have been provided by Sakamoto. [11c] These are photochemical formations of chiral oxetanes, / -lactames, oxazolidine-2,4-diones, aziridines, oxazolines, phthalides, and -thio-lactames. [11c] In most cases the ee values had to be improved by going to -78 °C or to low conversion . Whereas the latter technique is frequently preferred it is often dubious for the normal selective irradiations (only the educt absorbing the light), if the crystal reacts from its surface down into the bulk, multilayer by multilayer as these become transparent at very advanced local conversions. [14, 15] Thus, no basis exists for assumptions that the optical yield may increase to quantitative values at very low conversions, while the error limits will not allow to secure such claims. Nevertheless, there is the risk that... 

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