Asymmetric Photodecarboxylations

1 Asymmetric Photodecarboxylations - Acridine and diphenylacetic acid are both achiral molecules, but despite this fact the two compounds self-assemble to form dextro- and /aevo-rotatory chiral mixed crystals from acetonitrile solution. Irradiation of either the dextro- or /aevo-rotating crystals causes a stereospecific decarboxylating condensation, which yields excess of (+)- or (—)-(68), respectively, with about 35% ee.  

An efficient method has been claimed for the enantiomeric enrichment of racemic tartaric acid by irradiation with highly intense, circularly polarized light from an excimer laser.The enrichment is presumed to occur by enantioselec-tive decarboxylation, and further degradation, of the tartaric acid molecules. All previous attempts to achieve such enantioselection with circularly polarized light have produced very low enantiomeric excesses, as predicted by theory. The highly intense laser beam increases the probability of two-photon absorption, and this could result in higher enantioselectivity than attained with lower power sources. It seems advisable, however, to await further confirmatory results before this approach to the resolution of racemic mixtures should be considered viable. 

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Component Molecular Crystal of Acridine and Diphenylacetic Acid and Its Absolute Asymmetric Photodecarboxylating Condensation, J. Am. Chem. Soc., 118, 12059-12065. (b) Koshima,H., Nakata, A.,... 

Despite acridine and diphenylacetic acid being achiral molecules, it was found that these two compounds formed a chiral cocrystal on spontaneous crystallization, and further irradiation of this caused photodecarboxylation and then enantio-selective radical coupling to give a chiral condensation product (Scheme 35) [31]. This kind of absolute asymmetric photodecarboxylation is not a topochemical reaction but is accompanied by gradual decomposition of the initial crystal structure as the reaction proceeds. This pathway is different from the four examples of [2 + 2] photocycloadditions described above. 

Scheme 35 Absolute asymmetric photodecarboxylative condensation, in the cocrystal of acridine and diphenylacetic acid.

Koshima, FI., Ding, K., Chisaka, Y., and Matsuura, X, Generation of chirality in a two-component molecular crystal of acridine and diphenylacetic acid and its absolute asymmetric photodecarboxylation condensation, /. Am. Chem. Soc., 118,12059, 1996. 

When two different achiral molecules form a chiral cocrystal by spontaneous chiral cocrystallization, the occurrence of absolute asymmetric intermolecular photoreaction can be expected. In fact, we have achieved enantio- and diastereo-selective photodecarboxylative condensation, as well as absolute asymmetric pho-todecarboxylative condensation. The development of intermolecular photoreao tions leads to an extension of the scope of solid-state chiral photochemistry. Reactivity in a cocrystal is controlled by the crystal packing arrangement, so the key point is the preparation of photoreactive cocrystals. 

A possible way to induce selectivity in the photodecarboxylation process could be through photosensitized reactions in the soHd state. In fact, when a two-component molecular crystal of phenanthridine and 3-indoleacetic acid is irradiated at low temperature (-70°C), 3-methyHndole is formed in high yield as the sole product by contrast, when the same reaction is carried out in acetonitrile solution, four products are obtained.Furthermore, irradiation of two-component molecular crystals of arylalkyl carboxylic acids with stoichiometric amounts of electron acceptor causes decarboxylative condensation between the two components with important selectivities. " Thus, irradiation of (S)-naproxen in a chiral crystal with 1,2,4,5-tetracyanobenzene produces a decarboxylated condensation product retaining the initial chirality." Photolysis of an enantiomorphous bimolecular crystal of acridine with the R or S enantiomer of 2-phenylpropionic acid causes stereoselective condensation to give three optically active products. An absolute asymmetric synthesis has also been achieved by the enantioselective decarboxylative condensation of a chiral molecular crystal formed from achiral diphenylacetic acid and acridine (Scheme 9). ... 

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