Dimerization, cinnamic acid derivatives

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]. 

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. 

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-... 

Cinnamic acid derivative 36 crystallizes in the chiral space group P2t and gives the optically pure dimer 37 upon irradiation in the solid state [22], Chiral crystals of 38 gave, upon irradiation, the optically active dimer 39 of 90% ee, whereas the corresponding methyl ester gave a highly crystalline linear polymer through a typical [2 + 2] topochemical photopolymerization [23],... 

Based on some interesting reactions in certain inorganic crystalline compounds, Kohlschutter [9,10] proposed that the nature and properties of the products obtained take place on the surface or within the solid state. Indeed, he coined the term topochemistry for such reactions in the solid state. However, systematic investigations of photoinduced reactions in crystals began from 1964 onward by Schmidt and Cohen [11], Their studies on the 2tt + 2tt photoreaction of cinnamic acid derivatives in the crystalline state and correlation with the molecular organization in these crystals led to what are now known as Topochemical Principles. The most important conclusions reached by them are as follows (1) The necessary conditions for the reactions to take place are that the reactive double bonds are parallel to one another and the center-to-center distance be within 4.1 A (2) there is one-to-one correspondence between the stereochemistry of the photoproduct and the symmetry relationship between the reactants. The centrosymmet-ric relationship (called the a-form) leads to centrosymmetric cyclobutane (anti-HT), whereas the mirror symmetric arrangements (called the (5-form) produce mirror symmetric dimer (yy -HH). 

The alcohols formed from some cinnamic acid derivatives, namely /7-coumaryl alcohol, coniferyl alcohol (LI), and sinapyl alcohol (L2), commonly known as monolignols, undergo dimerization reactions that yield lignans such as (-l-)-pinoresinol (L3), (-l-)-sesamin (L4), (-)-matairesinol (L5), and podophyllotoxin (L6) (Fig. 13). Several thousand lignans are found to occur in nature. Lignins, the structural components of plant cell walls, are polymers of monolignols and/or lignans. 

The principles of such topochemical reactions have been established by G. M. J. Schmidt for the dimerization of cinnamic acid derivatives which represents the other classical organic solid-state reaction... 

Photodimerization of a,p-unsaturated ketones, esters, and lactones bearing aryl groups has been investigated to give cyclobutane derivatives. Irradiation of benzoxepinone (267) afforded quantitatively diaster-eoisomeric head-to-head dimers (268) and (269). The minor dimer (268) containing trans-ving fusion underwent spontaneous isomerization to the major cis-transoid-cis diastereoisomer (269). ° Irradiation of 2, 4 -dihydroxychalcone (270) and its related compounds mainly afforded two head-to-tail cyclobutanes (271, 272), which had never been found in nature previously (Scheme 59). ° Photocyclodimerization of cinnamic acid derivatives (275) as natural products was also reported. ... 

As mentioned above, an example where lattice control over the course of an organic solid state reaction is explicit is provided by the solid state photodimerization of trans-cinnamic acid and many of its derivatives.When irradiated in the melt or in solution, cinnamic acid derivatives do not dimerize — the presumed singlet photoexcited state being too short-lived for reaction in such mobile phases. The only consequence of irradiation is trans cis isomerization. Irradiation of crystalline solids, however, was found to result in one of three distinct events — and that which occurs found to depend upon the solvent of crystallization— see Scheme 6.1. The contribution of Schmidt, Cohen, and co-workers at the Weizmarm Institute in Israel was first to appreciate the importance of polymorphism and then to establish a direct correlation between the molecular packing within a particular polymorphic phase and the nature of the photodimer which results. ... 

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. 

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. 

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... 

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... 

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.,... 

Schmidt et al. found that a distinct product phase appeared at less than 10% conversion in some cases (indicating heterogeneous reaction), while in others none was evident at greater than 70% conversion [63]. Certain derivatives, (3-truxinic acids in particular, were formed in a modification different from that into which they could be recrystallized. Schmidt interpreted these results in terms of a phase separation of the forming product which occurs when it reaches a limiting solubility in the lattice of the reactant. He further pointed out that recrystallization may be necessary to improve yields in the dimerization of 0-cinnamic acids in certain instances where monomer molecules may be stranded in nonreactive sites such as M2.. M.. M2 (Section IV.B.5.). 5-Bromo-2-hydroxy-cinnamic acid is an example of a case where there is no evidence for phase separation and reaction is found to be slow and proceed in low yield [63,108]. 

This lactone derives from the shikimic acid-cinnamic acid pathway, via oxidative dimerization of coniferyl alcohol. Whereas the presence of yatein itself was not detected in the plants producing steganes, presteganes A and B were isolated in Steganotaenia araliaceae and constitute therefore viable biogenetic precursors [82,83]. 

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