Cinnamic acid, solid state photodimerization

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

General Considerations. Cinnamic acid solid state photodimerizations are the prototype for the topochemical postulate and reasoning [7], The a-modification of frans-cinnamic acid a-4 gives a-truxillic acid 5 (head-to-tail), while the /i-modification (/J-4) gives /i-truxinic acid 6 predominantly... 

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... 

For conceptually similar approaches, see (a) Ito, Y. Borecka, B. Trotter, J. Scheffer, J. R. Control of Solid-State Photodimerization of fraws-Cinnamic Acid by Double Salt Formation with Diamines. Tetrahedron Lett. 1995, 36, 6083-6086 (b) Amirsakis, D. G. Garcia-Garibay, M. A. Rowan, S. J. Stoddart, J. F. White, A. J. P. Williams, D. J. Host-Guest Chemistry Aids and Abets a Stereospecific Photodimerization in the... 

The photosensitivity of poly(vinyl cinnamate) derives from the reactivity of the einnamoyl group. Cinnamic acid is a naffirally occurring compound (found in oil of cinnamon, from which it derives its name) and is readily synthesized. Its photosensitivity has long been known, though little use was made of this property until Mink and co-workers, while reviewing the literature in search of a potential cross-linking reaction, found it to be the only solid state photodimerization that was known at the... 

Ito. Y. Borecka. B. Trotter. J. Scheffer. J.R. Control of solid-state photodimerization of trans-cinnamic acid by double salt formation with diamines. Tetrahedron Lett. 1995. 36, 6083-6086. 

Unusual modes of dimerization of cinnamic acids have been found in the solid state photodimerization of the octadecyl esters of these acids. Bolt and co-workers have crystallized this ester of cinnamic acid in three different forms, one of which cyclized to an a-truxilic acid type of dimer, one of which was photo-inert, and one of which gave the 6-truxinic acid dimer. Two crystal forms of the ester of trans-p-chloro-cinnamic acid were isolated, one of which cyclized to an a-truxillic acid, the other being photo-inert. The long-chain ester group presumably modifies the crystal structure, and use of orienting groups of this type may allow the formation of other dimers not usually obtained in solid state dimerizations. 

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. ... 

Ito, Y., Borecka, B., Trotter, J., and Scheffer, J. R., Control of solid-state photodimerization of trans-cinnamic acid by double salt formation with diamines. Tetrahedron Lett., 36,6083-6086,1995. Ito, Y., Borecka, B., Olovsson, G., Trotter, J., and Scheffer, J. R., Control of the solid-state photodimerization of some derivatives and analogs of trans-cinnamic acid by ethylenediamine. Tetrahedron Lett., 36, 6087-6090,1995. 

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. 

It may be suspected that the genuinely topotactic (as secured by the molecular precision of the AFM [18]) photodimerization of 2-benzyl-5-benzyli-denecyclopentanone [118] might be a good candidate for a quantitative preparative photo dimerization to give the head-to-tail anti-[2+2] dimer. Early quantitative solid-state [2-1-2] photodimerizations (most of the published mechanistic interpretations of which can no longer be accepted) are listed in [110]. These deal with the anti dimerization of acenaphthylene-1,2-dicarboxylic anhydride, the head-to-head syn dimerization of acenaphthylene-1-carboxylic acid, the syn dimerization of 5,6-dichloroacenaphthylene, and the thermally reversible head-to-tail anti dimerization of seven ( )-2,6-di-f-butyl-4-(2-aryl-ethenyl)pyrylium-trifluoromethanesulfonates. All of these reactions proceed fully specific. On the other hand, quantitative photoconversions of a 1 1 mixed crystal of ethyl and propyl a-cyano-4-[2-(4-pyridyl)ethenyl]cinnamates gives mixtures of diesters with one (A>410 nm) or two cyclobutane rings (no cutoff filter). 

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... 

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,... 

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

In a systematic study of the 2ir + 2tt photodimerization of cinnamic acids it was shown that reactions in crystals occur with molecularity, selectivity, and efficiencies that are quite different from those observed in solution. Cinnamic acids and their derivatives were shown to crystallize in three distinct packing arrangements known as a-, (3-, and 7-forms. The packing arrangement of a given crystal form determines whether or not a 2tt + 2tt photodimerization may occur in the solid state and which of the possible products will form (Scheme 1). Al-... 

Ito Y, Matsuura T. A simple method to estimate the approximate solid state quantum yield for photodimerization of trans-cinnamic acid. J Photochem Photobiol 1989 50 141-145. Ito Y, Matsuura T, Fukuyama K. Efficiency for solid-state photocyclization of 2,4,6-triisopropylbenzophenones. Tetrahedron Lett 1988 29 3087-3090. 

In contrast to solutions, solid drugs have a fixed conformation resulting in topochemical reactions. The majority of photoreactions in the solid state, described in the literature, deal with lattice-controlled examples and photodimerizations. A precondition for these reactions is the parallel position of the double bond of two adjacent molecules in the crystal lattice as shown by the example of the trimorphic, frans-cinnamic acid. Irradiation of the a- and the 5-modifications causes the formation of a-truxillic acid and (i-truxinic acid, respectively, whereas the y-modification is photostable due to the distance of the double bonds fixed by the lattice (Fig. 8) (10). 

Kaupp, G. Photodimerization of cinnamic acid in the solid state new insights on application of atomic force microscopy. Angew. Chem., Inti. Ed. Engl. 31, 592-595 (1992). 

The molecular assemblies described above have inspired us, in recent years, to develop finite assemblies in the solid state that exhibit chemical reactivity. Specifically, we,69 and others,70 have been utilizing principles of molecular recognition and self-assembly to develop a method to direct the formation of covalent bonds in organic solids. The method builds on the work of Schmidt on the reactivity of cinnamic acids in the organic solid state.45 Specifically, Schmidt has described topochemical postulates that dictate geometry criteria for a [2 + 2] photodimerization to occur in a solid. The postulates state that two carbon-carbon double (C=C) bonds should be aligned in parallel and separated by a distance <4.2 A to react. 

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. 

Despite a large number of reports on the solid-state bimolecular photoreactions (photodimerization) of cinnamic acids [2], bimolecular photoreactions between different organic compounds in the solid state had been little studied when the present author undertook investigations about the bimolecular photoreaction of solid mixtures [87]. I began this study because with no special equipment needed... 

Among the numerous examples of organic solid state photopolymerizations (e.g., 9-12) only two systems have been thoroughly studied with regard to the relationship between crystal and molecular structure. The first is the so called four-center type photopolymerization (Reaction 1) which was investigated by Hasegawa and co-workers (13) this is a straightforward translation of the work of G. M. J. Schmidt (3) and his students on the photodimerization of cinnamic acids in the crystalline state into polymer chemistry. 

The photochemical (2 + 2)-dimerization between raw -cinnamates contained within a photocrosslinkable dendrimer has been reported. The cycloaddition is accompanied by trans-cis isomerism. The (3-form of raw -2,4-dichlorocinnamic acid dimerizes on irradiation to give the (3-truxinic acid derivative in a first-order kinetic process. Others have studied the influence of substituents on the aryl ring of cinnamic acid, with a view to control crystal engineering. Ito et alJ have reported the photodimerization in the solid state of cinnamic acids (1) as their... 

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