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Asymmetric photochemistry I

02 July 2008 - Definitions

In asymmetric photochemistry (also called photochirogenesis) light is used to synthesise asymmetric molecules. Circularly polarized light is chiral and processes like circular dichroism exploit the difference in absorption of UV radiation between two optical isomers. All the more reason chemists have been exploring asymmetric photochemistry for over 80 years.

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 humulene DOI 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.

Several attempts were launched at true photochemical asymmetric synthesis starting from racemic substrates between 1933 and 1945 but these attempts suffered from low optical yields and irreproducibility. Karagunis in 1930 added chlorine to an unsymmetrically substituted triphenylmethyl radical DOI, Davis in 1935 brominated and chlorinated trinitrostilbene ( DOI) ( DOI) and in 1945 reported the bishydroxylation of diethyl fumarate with hydrogen peroxide ( DOI).

The first verified photochirosynthesis was obtained with photochemical formation of an helicene by Kagan in 1971 ( DOI DOI DOI DOI). This reaction is a trans-cis isomerisation followed by rearrangement reaction and oxidation. Calvin in 1972 was scooped (DOI DOI DOI) but extensively investigated helicene formation mechanism.

Asymmetric photochemistry III

Homochirality - 9 July 2008

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.

In 1996 Inoue et al. ( DOI) demonstrated photoderacemization in cyclooctene. The trans isomer can be resolved as a pair of enantiomers with very high optical rotation.

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.