Photodecarboxylative condensation

Photoirradiation of chiral salt crystals formed from base 48 and acid 49 in the solid state gave the optically active product 50 of about 35% ee in 37% yield via photodecarboxylative condensation reaction [27],... 

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.,... 

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. 

Acridine 94 and (R)-( — )-2-phenylpropionic acid (R)-95 formed the chiral 3 2 cocrystal 94 (/ )-95. Irradiation of these cocrystals gave enantiomeric products 96 and 98 as well as the diastereomeric product 97 through photodecarboxylative condensation, with all positive [a]j> values (Scheme 22) [94]. Similarly, the opposite handed cocrystal afforded 96-98 with negative [a] d values. In contrast, photolysis of a 1 1 solution of 94 and (/ )-95 in acetonitrile resulted injhe formation of racemic 96 and biacridane 99. 

Scheme 21 Chirality memory for the photodecarboxylative condensation in the CT crystal.

Scheme 22 Enantio- and diastereoselective photodecarboxylative condensation in the cocrystals.

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

On the subject of spontaneous generation of chirality, it is of interest to know that spontaneous formation of chiral aggregates from nonchiral monomers is known to occur, e.g. the assembly of tetra-alkyl benzimidocyanins 3 as monitored by CD (circular dichroism). Formation of chiral crystals from achiral monomers is also reported, e.g. by photodimerization in the solid state. " In a recent example, chiral crystals of acridine 4 and diphenylacetic acid 5 give excess of the (.S)-product 6 upon a photodecarboxylating condensation reaction. Symmetry breaking is also known to occur for supramolecular complexes of achiral components e.g. glu-tarimide 7 and the diaminopyridine 8, and, as will be discussed below, in monolayers at the air-water interface. ... 

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. 

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. 

Irradiation of the M-crystals caused solid-state photodecarboxylation, and then enantioselective condensation occurred, to give the optically active condensation product (S)-( —)-151 as the main product with [a] = —30 in 35% ee and in 37% chemical yield (Scheme 35). Conversely, irradiation of the P-crystals resulted in formation of the opposite handed condensation product (/ )-(+ )-151 with [a]r = +30 in 33% ee and in 38% chemical yield. For a comparison, solution phase photolysis of acridine 150 and diphenylacetic acid DPA in acetonitrile did not produce chiral product 151 but rather gave the achiral condensation product 153 in 74% as the major product at complete conversion of DPA. 

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. ... 

Photolysis of solutions of C6o(OH)ig at low solute concentration leads to [C6o(OH)i8] by electron transfer from Me2C(OH) radicals or from hydrated electrons, and this has enabled the reduction potential of the C6o(OH)ig/ [C6o(OH)ig] couple to be estimated. The kinetics of the photoreduction of hexanal using RhCl(PMe3)2CO as catalyst have been measured and the feasibility of a photocatalytic synthesis of hexanol from pentane, CO, and H2 in the presence of rhodium complexes has been demonstrated. Irradiation of a chiral bimolecular crystal of acridine and R-(-)- or S-(+)-2-phenylpropionic acid induces photodecarboxylation followed by stereoselective condensation to give a mixture of three optically active products, and the 3-0-S-methyl dithiocarbo-nate derivatives of oleanolic and ursolic methyl esters have been used as models for the photodeoxygenation of alcohols. ... 

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). ... 

Irradiation of a chiral bimolecular crystals formed from acridine with diphenylacetic acid and R/S-2-phenylpropionic acid (2-PPA) results in enantioselective photodecarboxylation, which is followed by stereoselective condensation between the CHMePh radical and the hydroacridine radical species (Scheme 4, enantioselective photodecarboxylation in a chiral bimolecular crystal). The radical coupling occurs in the crystal lattice to give the optically active products II to IV. On the other hand, photolysis in solution phase results in the formation of the optically inactive II and biacridine IV. 

Solid-state irradiation of two component molecular crystals of thienylacetic acids with aza aromatic compounds (acridine and phenanthridine) ° results in photodecarboxylation and gives decarboxylated and condensation products. Two-component molecular crystals of the above azo aromatic compounds with 3-indolepropionic acid and 1-naphthylacetic acid, upon solid-state irradiation, give radical intermediates via electron transfer and ultimately afford decarboxylated compounds in near quantitative yield. Irradiation of crystalline charge-transfer complexes of 3-indoleacetic acid and 2-naphthylacetic acid with 1,2,4,5-tetracyanobenzene gives methylnaphthalene (decarboxylation) and naphthyl(2,4,5-tricy-ano)methane (dehydrocyanating condensation) in the solid state. 

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