Photodimerizations cinnamic acids

Figure 10.1 Photodimerization of anthracene (a) and trans-cinnamic acid (b).

Intermolecular photocycloadditions of alkenes can be carried out by photosensitization with mercury or directly with short-wavelength light.179 Relatively little preparative use has been made of this reaction for simple alkenes. Dienes can be photosensitized using benzophenone, butane-2,3-dione, and acetophenone.180 The photodimerization of derivatives of cinnamic acid was among the earliest photochemical reactions to be studied.181 Good yields of dimers are obtained when irradiation is carried out in the crystalline state. In solution, cis-trans isomerization is the dominant reaction. 

Chloranil, photocycloaddition with butadiene, 474 Chlorophylls, 552 Chu, N. Y. C 439 Ciamician, G., 1, 2,459 Cinnamic acid, solid state photodimerization, 476... 

The structural changes that accompanied the [2 + 2] photodimerization of the metastable a -polymorph of ort/zo-ethoxy-tranx-cinnamic acid have been studied [93]. In this study, the photochemical reaction was carried out at 293 K, and observed in situ by single-crystal X-ray diffraction. In the structure of the title compound, the three molecules in the asymmetric unit are arranged to form two potential reaction sites, but only one of these was found to be photoreactive. Since only two out of three molecules in the asymmetric unit take place in the photodimerization reaction, the crystal of the final product contains an ordered arrangement of the photodimer and the unreacted monomer. 

Many derivatives of quinones, cinnamic acids, and mucconic acids photodimerize in solid phases to give results 16> that in many cases are not in agreement with the general PMO rule of head-to-head reaction. However, it is clear that those reactions are controlled by topochemical effects, i.e. the geometry and proximity of the reactants in the solid phase. 135> Consequently, PMO theory will not be useful for calculating reactions of that type. 

An asymmetric photosynthesis may be performed inside a crystal of -cinnamide grown in the presence of E-cinnamic acid and considered in terms of the analysis presented before on the reduction of crystal symmetry (Section IV-J). We envisage the reaction as follows The amide molecules are interlinked by NH O hydrogen bonds along the b axis to form a ribbon motif. Ribbons that are related to one another across a center of inversion are enantiomeric and are labeled / and d (or / and d ) (Figure 39). Molecules of -cinnamic acid will be occluded into the d ribbon preferentially from the +b side of the crystal and into the / ribbon from the — b side. It is well documented that E-cinnamide photodimerizes in the solid state to yield the centrosymmetric dimer tnixillamide. Such a reaction takes place between close-packed amide molecules of two enantiomeric ribbons, d and lord and / (95). It has also been established that solid solutions yield the mixed dimers (Ila) and (lib) (Figure 39) (96). Therefore, we expect preferential formation of the chiral dimer 11a at the + b end of the crystal and of the enantiomeric dimer lib at the —b end of the crystal. Preliminary experimental results are in accordance with this model (97). 

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. 

Feldman and Campbell, on the other hand, used hydrogen-bonding interactions to enforce a particular stereo- and regiochemical outcome of the sohd-state photocycloaddition of a naphthoic acid-derived cinnamic acid [45]. In a conceptually similar approach, Scheffer demonstrated that diamines can form double salts with a variety of trans-cinnamic acid derivatives. The locking in place of the double bonds steers the sohd-state [2+2] photodimerization [46]. 

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... 

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

In the absence of defects, the reactivity of organic solids is mainly determined by molecular packing. Reactions in which the crystal structure holds sway over intrinsic molecular reactivity are said to be topochemically controlled (Thomas, 1974). A classic example of a topochemically controlled organic reaction in the solid state is the photodimerization of rrans-cinnamic acids studied by Schmidt et al. (see Ginsburg,... 

A great number of olefinic compounds are known to photodimerize in the crystalline state (1,2). Formation of a-truxillic and / -truxinic acids from two types of cinnamic acid crystals was interpreted by Bernstein and Quimby in 1943 to be a crystal lattice controlled reaction (5). In 1964 their hypothesis on cinnamic acid crystals was visualized by Schmidt and co-workers, who correlated the crystal structure of several olefin derivatives with photoreactivity and configuration of the products (4). In these olefinic crystals the potentially reactive double bonds are oriented in parallel to each other and are separated by approximately 4 A, favorable for [2+2] cycloaddition with minimal atomic and molecular motion. In general, the environment of olefinic double bonds in these crystals conforms to one of three principal types (a) the -type crystal, in which the double bonds of neighboring molecules make contact at a distance of -3.7 A across a center of symmetry to give a centrosymmetric dimer (1-dimer) (b) the / -type crystal, characterized by a lattice having one axial length of... 

Photodimerization behavior of 4-formyl-, 3,4-dichloro-, and several other cinnamic acid derivatives is greatly influenced by other molecules outside of the crystal (9,10). For example, 4-formylcinnamic acid 1 crystallizes in two modifications, photoreactive and photostable forms. The photoreactive crystals of 1 (mp 249 °C), on photoirradiation at room temperature in the presence of even a trace of moisture, dimerize to crystalline dimer 2 containing one molecule of water. The continuous change of the x-ray diffraction pattern during the photodimerization indicates a typical crystal-to-crystal transformation process. On the other hand, the same crystal 1 photodimerizes into amorphous dimer 2 in the absence of water. The same cyclobutane derivative is produced in very high yield in both reactions. However, highly crystalline dimer 2 is obtained only by the photodimerization of 1 in the presence of water and is not regenerated by any attempted recrystallization procedures from various aqueous solutions of 2. 

In addition to the influence on the dimer morphology, the presence of water molecules strikingly affect apparent photoreaction rate and temperature dependence of the rate (12). Since the topochemical reaction deteriorates pronouncedly at reaction temperatures close to the melting point of the starting crystal, maximal reaction rate is necessarily observed at a specific temperature for individual crystals, for example, at ca. 20"C for a -form crystal of cinnamic acid (mp 132°C) (13). In an aqueous dispersant the apparent maximal rate of photodimerization of 1 is observed about 15°C while the temperature for maximal rate in a non-aqueous dispersant is about 35 °C. The... 

The L-B films offer some advantages over aqueous-hydrocarbon interfaces of micelles and the related assemblies discussed above in terms of the magnitude of their orienting ability and the ease of interpretation of selectivity in photoreactions conducted in them. Molecules in the films have very little freedom of motion (stiff reaction cavities), their interfaces are very well defined, and therefore the alignment of reactant molecules can be readily expressed in the products. Photodimerization of stilbazole derivatives 62, N-octadecyl-l-(4-pyridyl)-4-(phenyl)-l,3-butadiene, (63), surfactant styrene derivatives 64 and 65, and cinnamic acids have been carried out in L-B films [18, 196-200], In all cases, single isomeric head-head dimers are obtained. Geometric isomerization of olefins has not been observed in competition with photodimerization. Independent of the location of the chromophore (i.e.,... 

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). 

Keywords cinnamic acid derivative, [2+2]photodimerization, a-truxillic acid, cyclobutane... 

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... 

Steric Compression Inhibition of [2+2] Photocycloaddition. Following the pioneering work of Schmidt and co-workers on the solid state photodimerization reactions of the cinnamic acids... 

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. 

Table 1 Examples of Photodimerization of Halocoumarins and Cinnamic Acids in the Solid State...

---