Crystal structure cinnamic acids

Solid matrices affect the efficiency of photodimerization as illustrated in Fig. 25 A decrea% in quantum yield for photodimerization in a solid film of poly(vinyl cinnamate) was observed, along with a virtual standstill of reaction below 50% conversion of the cinnamate group. Ibis is in contrast to a constant quantum yield for the reaction in the crystal of cinnamic acid (A). Since all chromophores in the system have the same structure, the lack of reactivity in some of the diromophores must be due to their environment. Thus, the reactive site distribution profiles in amorphous solids were represented by using a histogram technique Egerton et al. considered that the reactive site distribution is static (which means that each site does not change from... 

To date, the crystal structures of more than 200 mesogenic compounds are known. In this review, we wish to present a general overview of the crystal structures of mesogenic compounds up to the end of 1997. Unfortunately, it is not possible to consider the crystal structure determinations of carbohydrate liquid crystals [13, 14], metallomesogens [15-18], phasmid and biforked mesogens [19-22], perfluorinated mesogenic compounds [23-27], benzoic acids [6, 28-31], cinnamic acids [7, 32, 33], dicarboxylic acids [34, 35], cinnamate compounds [8, 36-40], and discotic liquid crystals [41-43] due to the lack of space. 

A single-to-single crystal phase transition was found to take place at 333 K in a new polymorph of ort/zo-ethoxy-trans-cinnamic acid [77]. In this study, the structures of the title compound obtained at two temperatures above the transition point were determined in addition to the structures of the stabilized forms existing at lower temperatures. It was found that the phase transition involved a cooperative conformational transformation coupled with a shift in layers of the constituent molecules. 

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. 

Figure 39. Four ribbons of cinnamide (phenyl = ) molecules. Ribbons / and l, d and d are related by translation. Ribbons d and /, d and l make plane-to-plane contacts of 4 A across centers of symmetry. Ribbon / is above d, and ribbon d is below l. Cinnamic acid molecules (filled circles) have been introduced into the structure in the allowed sites, assuming the crystal grows from the center in the two opposite directions +b and —b. The dimers obtained at the two opposite sides are enantiomeric.

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... 

Fig. 6 Results from Rietveld refinement of the disordered crystal structure of the P polymorph of p-formyl-tranj--cinnamic acid. The disorder concerns two orientations of the formyl group as shown in (a). The crystal structure in (b) shows only the disorder component of higher occupancy. The results from Rietveld refinement shown at the bottom are for (c) an ordered model comprising only the major orientation of the formyl group, and (d) the final disordered model (right side). Apart from the description of the order/disorder of the formyl group, all other aspects of these refinement calculations were the same. A slight improvement in the quality of the Rietveld fit for the disordered model is evident...

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... 

For instance, head-head photodimers are predicted from the crystal structures of 9-cyanoanthracene and 9-anthraldehyde, but the head-tail isomer is produced. Craig and Sarti-Fantoni and later others found that photoreactions of 9-cyanoanthracene and 9-anthraldehyde take place at defect sites [96,215], Systematic photochemical and crystallographic studies by Schmidt and co-workers uncovered many cases of substituted anthracenes which behave in an unexpected fashion (Scheme 40) [216,217]. Examples shown in Scheme 40 clearly illustrate that, unlike cinnamic acid derivatives, the stereochemistry of the product dimer from anthracenes cannot be predicted on the basis of crystal packing. An example from the laboratories of Venkatesan is noteworthy in this context [218], Irradiation of crystals of 7-... 

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... 

Even more important point to note is that often times one cannot even extend the chemistry of the same molecule in the same medium under different conditions. The best examples are found in the crystalline state where molecules tend to crystallize as different polymorphs. It has been very well established that the different polymorphic forms of a compound show significant differences in photochemical behavior. A classic example that shows polymorphic form dependent reactivity is cinnamic acid. Substituted cinnamic acids crystallize in three polymorphic forms namely a, (3, and y and provide photodimers characteristic of the crystal packing (Sch. 1). One of the interesting crystal structure dependent cyclizations is that of tetrabenzoyl-ethylene [143]. Photolysis of only one of the two dimorphic modifications gives rise to a product while the other is inert to UY radiation (Sch. 25). 

In addition to the identification of crystal moditications, SSNMR has been used to monitor reactivity and phase changes in different polymorphic forms. For instance, Harris and Thomas (1991) followed the photochemical conversion of formyl-fran -cinnamic acid with SSNMR (see also Section 6.4). Variable temperature techniques have been used to study the interconversion of four polymorphic modifications of sulphanilamide (/ -amino-benzenesulphonamide), including interpretation of at least some of the molecular motions during the course of the transformation (Frydman et al. 1990). A similar combination was augmented with colourimetric techniques to study the coexistence of two phases in the course of a phase transition (Schmidt et al. 1999). Of course, differences between unsolvated and solvated or hydrated crystal moditications may also be readily characterized by the SSNMR technique, as was done with the anhydrous and monohydrate of oxyphenbutazone (Stoltz et al. 1991). Due to the availability of the crystal structures for both modifications the SSNMR results could be interpreted directly in terms of the different atomic environments, especially for the differences in hydrogen bonding in the presence... 

The crystal structures of the cinnamic acid salts with (+)-l-phenyleth-ylamine salt with and (+)-l-(4-isopropylphenyl)ethylamine were determined using single-crystal X-ray analysis, where it was foimd that two amine components and two acid components formed a helical column through hydrogen bonding in each crystal [47]. On the basis of structural... 

Cinnamoyl- 6-CD (6-CiO-/3-CD) was sparingly soluble in water, although most 6-substituted 6-CDs are soluble. However, 6-CiO-/3-CD was solubilized in water on the addition of adamantane carboxylic acid or p-iodoaniline which could be included in a 6-CD cavity. These results suggest that 6-CiO-/l-CD formed supramolecular polymers in the solid state, as shown in the proposed structure in Fig. 17. The X-ray powder pattern of 6-CiO-/l-CD was similar to that of the complex between p-CD and ethyl cinnamate, in which /3-CDs formed a layer structure. The crystal structure of 6-aminocinnamoyl-/3-CD (6-aminoCiO-/l-CD) is shown in Fig. 12 and we discussed the relationship between crystal packing and the substituent group in Sect. 2.8. 

Certain crystal structures, however, that appear to fit the criteria just listed for a or /3 cinnamic acids do not, in fact, produce photodimers. For example, 3,4-dihydroxy-traras-cinnamic acid is photostable.It is suggested that this is because symmetry-related molecules are held together by strong hydrogen bonding which does not permit the molecular flexibility that requires the 4.0 A interaction to be reduced to approximately 1.5 A. It appears that there needs to be sufficient space in the crystal reaction cavity for the reaction to take place, and sufficient flexibility in the overall crystal packing for the required atomic spatial reorganization to occur. 

The authors attributed the difrerence in binding to increased steric bulk and lower ir-acidity of ketones as compared to aldehydes. No crystal structures of FpValdehyde complexes are available in order to determine whether Fp has a similar amphichelic binding property but (Hi3P)(CO)2Fe binds cinnamal-dehyde in an fashion. Additionally, it is worth noting that both of the Senium complexes shown here are chiral and it has been shown that in the enantiomerically pure form, they undergo nucleophilic additions to the carbonyls with hi enantioselectivities. " Finally, the significance of the phenylace-taldehyde crystal structure should not escape attention. This is the first crystal structure of a nonchelated. 

The alternative intramolecular cyclizations are also catalyzed by STS and CHS which, however, use 4-coumaroyl-CoA as starter molecule, leading to the formation of stilbenes and flavonoids, respectively (1,2). CoA esters of cinnamic acids are accepted by neither BPS nor BIS (Table 1). Recently, the crystal structure of STS from Pinus sylvestris has been resolved and the mutagenic... 

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