Solid state photodimerization

By examining any correlation between excimer formation (as evidenced by characteristic excimer fluorescence) and dimerization quantum yield, one could perhaps determine whether dimerization is dependent upon prior excimer formation. Excimer fluorescence from anthracene solutions at room temperature is negligible although it has been observed in the solid state at low temperature.<75) Unfortunately, the data for substituted anthracenes allow no firm conclusions to be drawn. Some derivatives dimerize but do not exhibit excimer fluorescence. Others both dimerize and show excimer fluorescence. Still others show excimer fluorescence but do not dimerize and finally, some neither dimerize nor show excimer fluorescence. Hopefully, further work will determine what role excimer formation plays in this photodimerization. 

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

Cinnamylideneacetic acid, photodimerization in solid state, 477 Cinnamylidenemalonic acid, photodimerization in solid state, 477... 

Forster, Th 211, 278, 282, 285 Forster resonance energy transfer, 282 Forster singlet energy transfer, 378 Franck-Condon factors, 23 Franck-Condon principle, 5 Franck-Condon transition, 5 French, C. S., 555 Friedman, G., 353 Fritzsche, J., 37 Frosch, R. P 252, 267, 269 Fumaronitrile, photodimerization in solid state, 478... 

Irradiation of cycloocta-2,4-dien-l-one (55) in pentane gives a racemic photodimer, anti-tricyclofSAO.O Jhexadeca- , 11 -diene-3,16-dione (60) in 10% yield along with polymeric materials 34). Efficient and enantioselective photodimerization of 58 was achieved by irradiation of the 2 1 inclusion complex 59 formed between 2 a and 5813). When a solution of 2a and an equimolar amount of 58 in ether-hexane (1 1) was kept at room temperature for 12 h, 59 was obtained as colorless needles of mp 105 to 108 °C. Irradiation of 59 in the solid state for 48 h gave (—)-60 of 78 % ee in 55 % yield. 

Some quinolizinium derivatives such as MPB-07 60 have importance as chloride channel activators. This compound has been shown to be photolabile in aqueous solution when exposed to daylight, being transformed into the phenolic derivative 62 with the deprotonated form 61 as an intermediate, as shown in Scheme 1 <2002JPS324>. A highly regioselective solid-state photodimerization of naphthoquinolizinium salts has also been described <2002EJO2624>. 

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). 

Most products of solid-state (2 + 2) photodimerization are found to have skeletal configuration 66, while a few have configuration 67. This distribution clearly reflects the tendency of the monomers to crystallize in the particular corresponding structures. Reports (153) of solid-state photodimerizations that yield configuration 68 suggest that the range of product stereochemistries available via photodimerization is indeed large. 

Some examples from the large number of known solid-state photodimerizations may be used to illustrate the extraordinary selectivity that is routinely observed. 

To the organic chemist, the most striking feature of solid-state reactions is the stereochemical purity of die product obtained in most cases. This feature allows conversion by conventional methods of the solid-state product to other materials of desired stereochemistries. We illustrate this by some examples of reactions starting from the cyclobutanes obtained by solid-state (2 + 2) photodimerization. 

An obvious extension of the studies on photodimerization of crystalline olefins is to solid-state vinyl polymerization (with light, if absorbed, or y-irradiation), with the aim of achieving stereoregular polymers. In fact, an immense effort has been made in this direction, but with singular lack of success. The explanation is that, for various reasons, the lattice in the vicinity of the chain front becomes progressively more damaged as polymerization proceeds, so that after relatively few steps there is loss of stereochemical control. 

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 culmination of the studies on asymmetric photodimerization reactions in the solid state was the successful elaboration of chemical systems that are achiral but crystallize in chiral structures, and that yield, on irradiation, dimers, trimers, and higher oligomers in quantitative enantiomeric yield (175,258). 

Toda et al. reported that the topotactic and enantioselective photodimerization of coumarin and thiocoumarin takes place in single crystals without significant molecular rearrangements [49]. Molecular motion needs to be called upon to explain the photochemically activated cycloaddition reaction of 2-benzyl-5-benzylidenecyclopentanone. The dimer molecules, once formed, move smoothly in the reactant crystal to form the product crystal [50]. Harris et al. investigated the reactivity of 10-hydroxy-10,9-boroxophenanthrene in the solid state and the mechanism of the solid-state reaction was characterized by both X-ray diffraction and thermal analysis [51]. It was demonstrated that the solution chemistry of 10-hydroxy-10,9-boroxophenanthrene is different from that in the solid state, where it undergoes dimerization and dehydration to form a monohydride derivative. 

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 synthesis of cis-1,4 polymers was also tried by e use of monomers with an s-cis conformation. The solid-state photopolymerization of pyridone derivatives, which is a six-membered cyclic diene amide and is a tautomer of 2-hydroxypyridine, was attempted [100]. Pyridones make hydrogen-bonded cocrystals with a carboxylic acid in the crystalline state. Because the cyclic structure fixes its s-cis conformation, if the polymerization proceeds, a cis-2,5 polymer would be obtained. Actually, however, the photopolymerization did not occur, contrary to our expectation, but [4-1-4] photodimerization proceeded when the carbon-to-carbon distance for the dimerization was small (less than 4 A) [101]. A closer stacking distance of the 2-pyridone moieties might be required for the topochemical polymerization of cychc diene monomers. 

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... 

Schmidt GMJ (1971) Photodimerization in the solid state. Pme Appl Chem 27 647-678... 

Kaftory M, Shteiman V, Lavy T, Scheffer JR, Yang J, Enkelmann V (2005) Discrintination in the solid state photodimerization of l-methyl-5,6-diphenylpyrazin-2-one. Eur J Org Chem 847-853... 

Deng-Ke C, Thekku VS, Mark B, Gilad G, Jason BB, Menahem K (2010) Kinetics of solid state photodimerization of l,4-dimethyl-2-pyridinone in its molecular compound. J Phys Chem A 114 7377-7381... 

Solid-state photodimerization of quinones is another example of such a phenomenon, as shown by the dimerization of 2,6 dimethyl-benzoquinone18 (7,8). There is little evidence regarding the mechanism... 

Marubayashi et al. <1997J(P2)1309> have also shown that solid-state dimerization is possible and propose that there is a buffer zone in the crystal structure of 1,4-dihydropyridines that governs the solid-state photodimerization process. This is exemplified by the fact that dimethyl l,4-dihydro-2,6-dimethyl-4-(3-nitrophenyl)pyridine-3,5-dicar-boxylate 83 cannot undergo solid-state photodimerization (Equation 21), whereas the structurally related (4/ 3, l / 3 )-methyl-l-phenyl-2-piperidinoethyl-l,4-dihydro-2,6-dimethyl-4-(2-thienyl)pyridine-3,5-dicarboxylate 84 affords a single product 5 (Scheme 2). Interestingly, when the photodimerization conditions are applied to the corresponding solution-phase reaction, the sole product is that of aromatization giving product 85. 

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,... 

In order to overcome this lack of selectivity, photodimerizations have been performed in micelles,16,17 in supercritical fluids,18 in inclusion compounds19 and in the solid state.20 Nevertheless, such reactions are difficult to run on a preparative scale, and better results can be obtained by careful choice of an appropriate solvent. Enantioselcctive gas chromatography combined with GC/MS analysis proved to be a very efficient tool for the direct assignment of constitution and configuration of the photocyclodimers formed.21 In this manner, /ram-1,2-di-vinylcyclobutane has been prepared by sensitized irradiation of buta-1,3-diene.22... 

Ultraviolet irradiation of the methiodide or hydrochloride of 2-styryl-pyridine (XCIII) in the solid state results in transformation of the trans-isomers to the corresponding dimers (XCVa, b) on the other hand irradiation in benzene solution gives both isomerization and dimerization.309 Dimer (XCVa) was produced in low yield on irradiation of XCIII in the powdered form in the presence of air.308 This is in contrast to the reported stability of XCIII toward photodimerization.109 Similar dimerizations have been reported in the case of 2,4-dichloro-3-cyano-6-styrylpyridine,164 2-styrylquinaldine (XCV) and frans-4,4 -diguanyl-stilbene bis(2-hydroxyethane sulfonate) (stilbamidine) (XCIV).82,83... 

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