Dimerization, cinnamic acid

As pointed out by Schmidt in the context of his study of cinnamic acid dimerizations, the very early stages of any solid-state reaction which is not initiated at a defect site can be described by a dilute mixed crystal of the product in the crystal phase of the reactant (Scheme 6) [63]. Mixed crystals or solid solutions are... 

D Auria and Racioppi have reported that the arylacrylonitriles (11) undergo facile (2+2)-cycloaddition when subjected to benzophenone-sensitized irradiation in acetonitrile solution. The products obtained from this treatment and the yields obtained are shown under the appropriate structures in Scheme 3. Again a mixture of addition types is encountered in line with results obtained from the cycloaddition reactions with the cinnamic acids. Dimerization of the nitrile (12) was also studied and yielded the two adducts (13) and (14). Irradiation of 3-(estran-16-yl)acrylates and 2-(estran-16-yl)vinyl ketones brings about the formation of dimers and also isomerization of the unsaturated side chains. ... 

Aloisi, G.G., Mazzucato, U., Bartocci, G., Cavicchio, G., Maravigna, R, and Montaudo, G., Luminescence and photolytic cycloreversion of cyclobutane derivatives cinnamic acid dimers and their diamides, Z. Phys. Chem., 138, 207, 1983. 

Rennert, J. and Grossman, D., Photochemical scission of cinnamic acid dimers, a-truxillic and 5-truxinic acids, /. Photochem., 3, 163,1974. 

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. 

Other a,0-unsaturated carbonyl and acid derivatives, such as dimethyl-fumarate, 2,5-dimethylquinone, and Pmy-cinnamic acid, are known to dimerize in the crystalline state and will be discussed in Section 10.4. 

Topochemical control of solid state dimerizations is well illustrated by the example of the frows-cinnamic acids.(112) The a form of ftmv-cinnamic acid is known to have a molecular separation of 3.6 A between double bonds and the molecules are arranged in a head-to-tail fashion. -Cinnamic acid has approximately the same intermolecular distance in the crystal but the molecules are arranged in a parallel head-to-head manner. a-Truxillic (101) and /3-truxinic (102) acids are the products expected and observed ... 

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). 

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.

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. 

The sensitivity to temperature is reminiscent of that seen in Ron s study (188) of o-methoxy-m-cinnamic acid, 129. When crystals of this substance are exposed to light at low temperatures (< — 80°C), there occurs a gradual isomerization of the molecules to the trans form, 130, with formation of a little of the (2 + 2) photodimer, 131, of the latter. If this irradiated material is warmed in the dark to + 60°C, recooled, and reirradiated for a short while, there is a jump in the amount of dimer. This behavior is interpreted as follows ... 

Polyesters of cinnamic acid derivatives used as photorresists, whose main photochemical feature is [2tt -I- 2-77] dimerization, are reported to experiment PFR along with trans-cis isomerization as secondary reactions [234-240]. 

It is well established that cinnamic acid and some substituted cinnamic acids (including frans-p-coumaric acid but not ferulic acid) can be dimerized in vitro by sunlight to truxillic and truxinic acids and their derivatives... 

Neither the anilide of cinnamic acid94 nor the diphenyl substituted acroyl anilide 138 95 yields any product of rearrangement or cyclodimerization. Upon irradiation of 138 in benzene solution in a Pyrex reactor, only the isomeric /9-lactams 139 (2.3%) and 140 (37%), in addition to dihydrocarbostyril 141 (5%), were isolated. The latter is the major product upon irradiation of alkyl substituted acroyl anilides.96 On the other hand, the closely related phenyl cinnamate rearranges regularly to the ortho- and para-positions97 and does not dimerize as the other alkyl esters of cinnamic acid.98... 

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.,... 

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-... 

Thallium(lII) trifluoroacetate is a versatile oxidant for organic compounds [55, 56, 57] It reacts with alkenes at room temperature to form oxiranes, ketones, and 1,2 diols [55] Usually these oxidations are accompanied by cyclizations and rearrangements The reaction of thallium(III) trifluoroacetate with substituted cinnamic acids results in instantaneous oxidative dimerization leading to bislactone lignans, which belong to a naturally occurring family of compounds [56] (equation 26)... 

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. 

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