Photodimerization of cinnamic acids

Photodimerization of cinnamic acids and its derivatives generally proceeds with high efficiency in the crystal (176), but very inefficiently in fluid phases (177). This low efficiency in the latter phases is apparently due to the rapid deactivation of excited monomers in such phases. However, in systems in which pairs of molecules are constrained so that potentially reactive double bonds are close to one another, the reaction may proceed in reasonable yield even in fluid and disordered states. The major practical application has been for production of photoresists, that is, insoluble photoformed polymers used for image-transfer systems (printed circuits, lithography, etc.) (178). Another application, of more interest here, is the use that has been made of mono- and dicinnamates for asymmetric synthesis (179), in studies of molecular association (180), and in the mapping of the geometry of complex molecules in fluid phases (181). In all of these it is tacitly assumed that there is quasi-topochemical control in other words, that the stereochemistry of the cyclobutane dimer is related to the prereaction geometry of the monomers in the same way as for the solid-state processes. 

Enkehnann V, Wegner G (1993) Single-crystal-to-single-crystal photodimerization of cinnamic acid. J Am Chem Soc 115 10390-10391... 

The photodimerization of cinnamic acid and similar molecules is observed in crystals, but reactions of the same type occur in some polymers as well. Polymers such as polystyrene are made of long, saturated hydrocarbon chains with pendant groups in close contact dangling from the chain these chromophores can then interact in bimolecular photoaddition reactions. Polyvinyl car bazole and its derivatives are important examples of polymers which lead to such bimolecular interactions (e.g. exciplex formation). 

It is well established, in a qualitative sense, that chemical reactions occurring in crystals are subject to restrictive forces, not found in solution, which limit the allowable range of atomic and molecular motions along the reaction coordinate. This often leads to differences, either in the product structures or the product ratios, in going from solution to the solid state. This was first demonstrated in a systematic way by Cohen and Schmidt in 1964 in their studies on the solid state photodimerization of cinnamic acid and its derivatives (1 ). This work led to the formulation of the famous topochemical principle which states, in... 

Tomotaka and coworkers [202] studied the photodimerization of cinnamic acids incorporated in vesicles (Fig. 39). They mixed equimolarly the cinnamic acid with alkyldimethylamine N-oxide (C DAO) to produce the ion pair 72 (Fig. 39). These ion pairs form stable vesicular aggregates in water. Whereas photoirradiation of the cinnamic acids in methanol resulted in only the cis-trans isomerization to form 71, in these vesicle medium, three dimers, 68-70, were obtained. 

In a systematic study of the 2ir + 2tt photodimerization of cinnamic acids it was shown that reactions in crystals occur with molecularity, selectivity, and efficiencies that are quite different from those observed in solution. Cinnamic acids and their derivatives were shown to crystallize in three distinct packing arrangements known as a-, (3-, and 7-forms. The packing arrangement of a given crystal form determines whether or not a 2tt + 2tt photodimerization may occur in the solid state and which of the possible products will form (Scheme 1). Al-... 

Kaupp, G. Photodimerization of cinnamic acid in the solid state new insights on application of atomic force microscopy. Angew. Chem., Inti. Ed. Engl. 31, 592-595 (1992). 

The strategy for crystal engineering has been mostly directed to photodimerizations of cinnamic acids and related compounds. In crystalline photochemistry, however, their photoreactivities are usually classified only into two categories, i.e., photoreactive or photostable. Evidently, this is not enough since the reactivity is by nature a continuous property. Quantum yield measurements for solid-state photoreactions are highly desirable from this viewpoint. 

Despite a large number of reports on the solid-state bimolecular photoreactions (photodimerization) of cinnamic acids [2], bimolecular photoreactions between different organic compounds in the solid state had been little studied when the present author undertook investigations about the bimolecular photoreaction of solid mixtures [87]. I began this study because with no special equipment needed... 

A comparison of the photosensitivity of poly(vinyl cinnamate) with the photodimerization of cinnamic acid in poly(vinyl alcohol) coatings was carried out by Nakamura and Kikuchi 44>. Cinnamic acid in polyvinyl alcohol) coatings was found to undergo photoisomerization as well as photodimerization. The ultraviolet absorption spectra used to follow the course of the reaction showed one isosbestic point together with other nonisosbestic curve intersections. Such complex curve changes indicated that photoisomerization and at least one other reaction, photodimerization, was occurring. 

Among the numerous examples of organic solid state photopolymerizations (e.g., 9-12) only two systems have been thoroughly studied with regard to the relationship between crystal and molecular structure. The first is the so called four-center type photopolymerization (Reaction 1) which was investigated by Hasegawa and co-workers (13) this is a straightforward translation of the work of G. M. J. Schmidt (3) and his students on the photodimerization of cinnamic acids in the crystalline state into polymer chemistry. 

Scheme 6.2 Photodimerization of cinnamic acid to produce a-truxillic acid (II) and (3-truxinic acid (III).

Photocyclization is a particularly valuable route to the formation of cyclic compounds. There is a wide variety of photocyclization reactions reported in the literature of organic photochemistry, but relatively few of these have been carried out in solid polymers. The earliest reports concern the photodimerization of cinnamic acid derivatives, leading to crosslinking in solid polymers. These polymers have important applications as commercial photoresists. The chemistry has been reviewed by Delzenne (46) and Williams (47). 

Photodimerization of cinnamic acid and its esters is controlled by the crystal lattice,... 

Studies of electronic structures in photoisomerization and photodimerization of cinnamic acid showed that phosphorescence of cinnamic groups occurs at about 20,000 cm Also, it was demonstrated when photosensitizers are present, the critical distance between donor, sensitizer, and acceptor molecules (cinnamic acid) is about 10 angstroms.Although all the details of incipient photocrosslinking of poly(vinyl cinnamate) have still not been fully worked out, most accept that all three mechanisms take place. These are dimerizations to truxillic and truxinic acid type structures and polymerizations through the double bonds. The excited states of the molecules can be produced by direct irradiation and also through intersystem crossing from an appropriate photosensitizer. ... 

The photodimerization of cinnamic acid has long been believed to occur onfy in the crystalline state of appropriate geometry in which the distance between nearest-neighbor double bonds is smalla- than 4 A Thus it was considered, by the md of... 

The photodimerizations of cinnamic acids were among the earliest photochemical reactions to be studied.These compounds give good yields of dimers... 

The first systematic study concerning the relationship between the chemical reactivity of an organic solid and its crystal structure is generally associated with the work of Schmidt and his co-workers on the photodimerization of cinnamic acid and its derivatives. The main conclusions were summarized by Schmidt in 1971 and phrases such as topochemistry and crystal engineering were set to enter the lexicon of the organic solid state chemist. ... 

Nakanishi, E, Nakanishi, H., Tsuchiya, M., and Hasegawa, M., Water participation in the crystal-line-state photodimerization of cinnamic acid derivatives. A new type of organic photoreaction, Bull. Chem. Soc. Jpn., 49, 3096,1976. 

Takagi, K., Fukaya, H., Miyake, N., and Sawaki, Y, Organized photodimerization of cinnamic acid in cationic reversed micelle, Chem. Lett., 1053, 1988. 

Nakamura, Y., Regioselective photodimerization of cinnamic acid in a micellar solution, /. Chem. Soc., Chem. Commun., 477, 1988. 

Intensive studies concerning topochemical reactions have been reported on the photodimerization of cinnamic acid and its derivatives. Schmidt and co-workers proposed a geometrical criterion for the photodimerization in the crystalline state that the reacting double bonds should be situated within about 4.2 A of each other and aligned parallel. In the case of the photoactive single crystal of 1-n-octylthymine obtained from acetonitrile solution (Form III), however, the distance between the reacting double bonds of the thymine bases was 4.47 A. It is difficult to apply Schmidt s rule to the photodimerization of the 1-n-octylthymine crystals. Therefore, it is necessary to determine the feature within the crystal of 1-n-octylthymine for the photodimerization reaction in the crystaUine state. 

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