Cinnamic acids, photochemistry

The examples of ex situ steady-state X-ray photodiffraction utihzed to follow the photodimerizations of olefin bonds in a single-crystal-to-single-crystal (or nearly so) manner are ubiquitous in the chemical literature. The interest of sohd-state chemists in this reaction dates back to the work of Cohen and Schmidt [30, 31], and it has become much of a guinea pig in organic solid-state photochemistry. In 1993, Enkelmann and collaborators published two seminal papers in the Journal of the American Chemical Society [32] and in Angewandte Chemie [33], where they presented a series of structures of a-tra s-cinnamic acid crystals reacted to various extents. These reports laid the way for a plethora of later studies on the olefin photodimerization reaction. The convenience of the high conversion and the simple mechanism, combined with the relatively small structural perturbation that it requires, has turned this reaction into a very useful tool to probe intermolecular... 

Cinnamates occupy an important place in the history of photochemistry. Schmidt and his co-workers [18] used the solid state photochemistry of cinnamic acid and its derivatives to develop the idea of topochemical control of photochemistry in the crystalline state. Minsk [19] developed poly(vinyl cinnamate) as the first polymer for photoimaging. The cinnamate chromophore is still commonly incorporated in photopolymers of all types, including LC polymers, to enable them to be photochemically cross-linked [20], and a number of reports of the photochemistry of such MCLC and SCLC polymers are summarized below. 

Figure 8 Photochemistry of cinnamic acids, infiuence of the isomeric form on photoreactivity. Source. From Ref. 10.

The experimental proof for anisotropic far-reaching molecular migrations within crystals by AFM analysis is particularly important with the former standard of topochemistry the a-cinnamic acid 1 photochemistry to give predominantly a-truxillic acid 2 (Scheme 2.1.1). 

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. 

Photochemical [2+2] cycloaddition of alkenes in the crystalline state is synthetically very useful because it usually produces only one stereoisomer predicted ftom the crystal structure. On the other hand, this stereospeciflcity of the reaction can be a disadvantage because of inaccessibility to other stereoisomers. In order to circumvent such a problem, we explored compelled orientational control of the photodimerization of particular compounds like ranj-cinnamic acids and anthracenecarboxylic acids [74-78]. During our study, photochemistry of fluoro- and chloro-substituted ranj-stilbene-4-carboxylic acids and their methyl esters and alkaline and alkaline earth salts in the crystalline phase was likewise studied in order to synthesize specific stereoisomers selectively (Scheme 41) [79]. Most of these stilbene compounds dimerized to give exclusively or mainly syn head-to-head cyclobutane dimers. Some were photochemicaUy inert. 

It has long been known that the photochemistry of crystalline compounds proceeds differently than upon irradiation in solution one of the best examples is that of the cinnamic acids and the research of Ciamician and Silber in 1902. Control of solid-state reactions was postidated by Cohen and Schmidt in 1964 to involve minimum molecular motion in the crystal. This approach has been termed the topochemical principle, perhaps otherwise known as least motion. Cohen and Schmidt also advanced the idea of a molecule reacting in a cavity created by surrounding neighbors. 

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). 

It stands out that the photochemistry is also dependent of the crystalline phases where the topochemical relations between nearest-neighbor molecules are not the same in the pure guest crystals and in the TOT clathrates. When the TOT/cw-stilbene clathrate was irradiated, photoconversion to trans-stilbene occurred smoothly with the additional formation of small amounts of phenanthrene and of an unidentified product. On the contrary the TOT/tru j-stilbene clathrate remained unchanged when irradiated for long periods of time. In the majority of cases, crystalline cw-cinnamic acid derivatives are entirely converted to tran -isomers on irradiation and the trans-to-cis isomerization is not observed However both TOT clathrates with the trans- and cis-isomers of methyl cinnamate photoisomerized under similar conditions. It is noteworthy that these clathrates and those of the stilbenes (Sect. 2.1.4) are isomorphous. 

Minsk et al. [117] may have been the first to synthesize a photocross-linkable polymer, namely poly (vinyl cinnamate). The photochemistry of this compound is similar to the photo-cyclization of cinnamic acid that is discussed bellow in this section. It is interesting that the reaction of cyclization of cinnamic acid can take place even in the solid crystalline stage. This illustrates that the reaction requires very little molecular motirai. Similar reactions occur in polymeric materials that are functionalized with cinnamate groups. The photocross-linking of poly(vinyl cinnamate) is illustrated below ... 

Lahav, M. and Schmidt, G. M. J. Topochemistry. Part XVIII. The solid-state photochemistry of some heterocyclic analogues of trans-cinnamic acid.. Chem. Soc. B 239-243, 1967. 

Cohen MD, Schmidt GMJ, Sonntag H (1964) Topochemistry. Part II. The photochemistry of trans-cinnamic acids. J Chem Soc 384 2000-2013... 

Yong, H. Photochemistry and proton transfer reaction chemistry of selected cinnamic acid derivatives in hydrogen bonded environments. Int. J. Mass Spectrom. Ion Proc. 1998, 175, 187-204. 

Schmidt and coworkers have extensively investigated the [2-1-2] cycloaddition reactions of different polymorphs that laid foundations for solid-state organic photochemistry as well as crystal engineering. The photochemical reactivities of different polymorphic forms of organic solids containing C=C bonds have been found to form distinct products. For example, cinnamic acid was found to crystallize in three polymorphic forms and in all three, the olefins are packed as linear stacks and they... 

Coates et al. used stacking interactions in the phenyl-perfluorophenyl system to direct co-crystal-lization of mixed stilbene and cinnamic acid co-crystals, thereby directing the formation of the mixed P-truxinic acid derivative in very high yield. Ito has recently reviewed the solid-state organic photochemistry of mixed crystals. " ... 

Vaida, M., Shimon, L. J. W., van Mil, J., Ernst-Cabrera, K., Addadi, L., Leiserowitz, L., and Lahav, M., Absolute asymmetric photochemistry using centrosymmetric single crystals the host/guest system ( )-cinnamamide/( )-cinnamic acid,/.Am. Chem. Soc., Ill, 1029-1034, 1989. 

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