The main methods (For reviews: Feringa et al. 1998 DOI Inoue 1992 DOI) are:
* photodestruction, the conversion of one enantiomer leading to the enrichment of the other enantiomer. Also called asymmetric photolysis.
* Photoresolution, a deracemization process of photochemically interconvertable enantiomers.
* Asymmetric photosynthesis, an enantioselective photochemical formation of an optically active compound from a prochiral substrate. Also called absolute asymmetric synthesis.
* Asymmetric photosensitization with an optically active compound
Asymmetric photochemistry II
05 July 2008 - History
Le Bel in 1874 and van 't Hoff in 1894 were the first to propose the possibility of asymmetric synthesis with left- or right circularly polarized light. Cotton of the Cotton effect was the first one to try (photolysis of an alkaline solution of copper tartrate) in 1896 but failed. Kuhn in 1929 was the first to succesfully conduct a photodestruction experiment with ethyl-alpha-bromopropionate and others followed like Mitchell 1930 with the dinitrogentrioxide adduct of humuleneDOI and with chloro-nitroso-diphenylbutane in 1944DOI. In both experiments left or right handed CPL yielded an maximum optical rotation of opposite sign at 50% conversion.
One possible explanation for the phenomenon of homochirality is that chiral molecules were brought to prebiotic Earth via meterorites such as the Murchison meteorite. These chiral molecules could very well have been formed via a photodecomposition process en route to Earth with CPL originating from synchrotron radiation from a neutron star although it must be stressed that even then there is no preference for handedness.
In 1977 Bonner et al ( DOI) prepared optically active leucine (a biologically relevant compound) in around 2% enantiomeric excess from asymmetric photodecomposition of racemic leucine.
It is found that the (R)-form is isomerized to achiral cis-cyclooctene by l-CPL leaving the (S)-form in solution and vice versa. In this experiment synchrotron UV light was used to mimic interstellar conditions.
Asymmetric photochemistry IV
10 july 2008 - Asymmetric photosensitization
Hammond et al. in 1965 introduced asymmetric reactions involving a chiral photosensitizer (DOI). Racemic trans-diphenylcyclopropane was subjected to non-polarized UV radiation in presence of a chiral amide converting it to the achiral (meso) cis isomer up to 40%. The optical rotation of the remaining trans isomer was found to increase from 0 to 28° (7% optical yield). This effect is attributed to the formation of an excimer complex allowed transfer of chirality from the sensitizer to the reaction product.
Optical yields were improved by Inoue et al. in 1989(DOI) and Schuster et al. in 1990 ( DOI)
Inoue photoisomerized cis-cyclooctene to the trans isomer (chiral) with 12% optical purity with a photosensitizer based on chiral borneol. Schuster's system is based on a Diels-Alder reaction between beta-methylstyrene and cyclohexadiene and a complex Binaphthalene sensitizer.