Solution, cinnamic acid derivatives

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. ... 

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 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-... 

In TFA solution, electron-rich examples of cinnamic acid derivatives and phenols give high yields of 4-aryl-3,4-dihydrocoumarins at room temperature with good regioselectivity <05JOC2881,05T9291>. 

NMR spectroscopy has been used to study the species formed in solution by interaction of cinnamic acid derivatives with asymmetric hydrogenation catalysts. Such studies are necessarily limited to those species which accumulate in adequate concentration and have sufficiently long lifetimes for observation by NMR. In catalytic reactions as rapid as those described here, such complexes appear likely to be outside rather than in the operating catalytic cycle. ... 

Various rhodium catalysts, such as [(l,5-hexadiene)RhCl]2, RhCl3, or Rh black, in methanol solution under H2 reduce and esterify unsaturated carboxylic acids. Even the phenyl ring of cinnamic acid derivatives is reduced at ambient conditions as shown in equation (3). The substrate must contain a C=C double... 

For the separation of solutes with high boiling points Kelker et al. [135] synthesized cinnamic acid derivatives which may be used up to about 360°C. These liquid crystals are well suited for the separation of fatty acids. 

TABLE 19.1 Cyclophanes Prepared from Stilbene, Vinyl Ether, and Cinnamic Acid Derivatives in Solution and Solid-State Reactions... 

Reactions. Heating an aqueous solution of malonic acid above 70°C results in its decomposition to acetic acid and carbon dioxide. Malonic acid is a useful tool for synthesizing a-unsaturated carboxyUc acids because of its abiUty to undergo decarboxylation and condensation with aldehydes or ketones at the methylene group. Cinnamic acids are formed from the reaction of malonic acid and benzaldehyde derivatives (1). If aUphatic aldehydes are used acryhc acids result (2). Similarly this facile decarboxylation combined with the condensation with an activated double bond yields a-substituted acetic acid derivatives. For example, 4-thiazohdine acetic acids (2) are readily prepared from 2,5-dihydro-l,3-thiazoles (3). A further feature of malonic acid is that it does not form an anhydride when heated with phosphorous pentoxide [1314-56-3] but rather carbon suboxide [504-64-3] [0=C=C=0], a toxic gas that reacts with water to reform malonic acid. 

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. 

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... 

Crystalline traras-cinnamic acid forms cyclobutane derivatives upon irradiation of the crystal when C=C double bonds in neighboring molecules interact with each other to give cyclobutane derivatives, as illustrated in Figure 18.1. In solution or in the molten state, the only reaction observed is isomerization from trans- to cfs-cinnamic acid, but in the crystalline state the stereospecific reaction to form cyclobutane derivatives may occur. 

Intramolecular Additions - The truxinic acid derivative (23) is formed on irradiation of (24) in methanol using a Pyrex filter. The reaction is quantitative and has a quantum yield of 0.55. Similar reactivity is observed when crystals of (24) are irradiated. The use of a benzene ring as a constraint has provided a method for the formation of a single stereoisomer in high yield this is seen best of all with the yield this is cinnamate ester (25). Irradiation of (25) for 21 h in methylene chloride solution affords an 88% yield of (26). A less efficient reaction... 

This work focused on frans-cinnamic acid and some of its ring substituted derivatives. In homogeneous solution, the distribution of the photoproducts is dependent on the steric and electronic effects of the reagents. Irradiation of a melt or solution of frans-cinnamic acid does not cause dimerization, only isomerization (9,10). Dimers of frans-cinnamic acid can form in the solid state with retention of crystal symmetry because the monomers are held rigidly within the lattice in a uniform and repeating manner forcing an association between monomers. 

Cyanoanthracene has a 3-type crystal structure but produces a head-to-tail photodimer. Unlike cinnamic acids and their derivatives, the stereochemistry of the photoproduct dimer from anthracenes cannot often be predicted on the basis of their crystal packing [78]. 9-Methoxyanthracene has a photostable y-type crystal structure. 9-CNA host crystals doped with 9-MeOA form a solid solution, which upon irradiation yield a heterodimer (3> = 0.12). A reaction mechanism via an exciplex is proposed. The exciplex fluorescence was observed [32] ... 

---