Photodimerization reaction

In this type of process an excited molecule adds to a second — identical — molecule in its ground state, usually with formation of a ring. These dimerizations are thus most commonly intermolecular reactions, but obviously the two reactive moieties can also be linked together, e.g. by an alkyl chain. Such intramolecular photodimerization reactions have been studied in detail422). 

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Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

Photosensitized Reactions for Polymers. The economic and technical features for photocross-linking, photosolubilizafion, and photopolymerization reactions have been reviewed (55). The widely used poly(vinyl ciunamates) (PVCN) photocross-link by a photodimerization reaction. 

Photodimerization reactions of some other simple alkenes and dienes follow/39-30 36-182 Although not a dimerization reaction, photochemical ring closures to yield cyclobutane derivatives are analogous and are included in this section 31-35 ... 

The structural changes that accompanied the [2 + 2] photodimerization of the metastable a -polymorph of ort/zo-ethoxy-tranx-cinnamic acid have been studied [93]. In this study, the photochemical reaction was carried out at 293 K, and observed in situ by single-crystal X-ray diffraction. In the structure of the title compound, the three molecules in the asymmetric unit are arranged to form two potential reaction sites, but only one of these was found to be photoreactive. Since only two out of three molecules in the asymmetric unit take place in the photodimerization reaction, the crystal of the final product contains an ordered arrangement of the photodimer and the unreacted monomer. 

The first photochemical reactions to be correlated with PMO theory were the dimerizations of anthracene, tetracene, pentacene, and acenaphthylene. 36> More detailed energy surfaces for the photodimerization reactions of butadiene have also been calculated. 30> In the category of simplified calculations lie studies of the regiospecificity of Diels-Alder reactions 37>, and reactivity in oxetane-forming reactions. 38,39) jn these... 

The majority of photodimerization reactions that have been studied in the solid afford products in which chemically equivalent double bonds have combined, as, for example, in 62 and 64 above. This does not have... 

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). 

Figure 4 shows typical SEC profiles of two PS-3-PI diblock copolymers before and after the photodimerization reaction. [46] The appearance of a peak after the UV irradiation at a short retention time with a doubled molar mass indicates the formation of PS-3-PI-C-PI-3-PS copolymer chains. In contrast, the irradiation of SI 42 in THF without the self-assembly only leads to a slight increase of the molar mass. It confirms that the self-assembly helps to concentrate and expose the reactive ends of precursor diblock copolymers on the periphery of the core-shell micelles, which greatly increases the coupling efficiency of the photodimmerization reaction between two coumarin end groups. 

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... 

The synthetic methods developed for the formation of tricyclic compounds containing a central cyclobutane ring systems continue to be dominated by photochemical approaches <1996CHEC-II(7)841>. In particular, photodimerization reactions of maleimides (Equation 40) have attracted significant recent attention in the field of copolymeriz-able photoinitiators <1995JOC2353, 2002CEJ4199>. 

The photodimerization reaction of anthracene in supercritical CO2 was studied systematically at different CO2 densities. Unlike in normal liqnid solvents, the reaction in supercritical CO2 is significant even at anthracene concentrations as low as a few micromolar. At comparable anthracene concentrations, the photodimerization reaction is one order of magnitude more efficient in CO2 than in normal liqnid solvents. The results also show that the efficient photodimerization reaction of anthracene is hardly affected by the local density augmentation (or solute-solvent clustering) in supercritical CO2 (Bunker et al., 1997). 

Some derivatives have also been subjected to photodimerization reactions [69JCS(C)1729 82JHC1319]. 

An important problem concerns the nature of the first excited state actually involved in the photodimerization reaction (the lowest excited singlet or triplet states). The simple 77-HMO method does not distinguish between the excited singlet and triplet state. Its results concern the first excited state and cannot be correlated precisely with the different multiplicities. For such a correlation, the more refined approximations of the molecular orbital method, which eliminate spin degeneracy, must be used. 

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... 

Takaoka, K., Kawano, M., Ozeki, T., Fujita, M., Crystallographic observation of an olefin photodimerization reaction that takes place via thermal molecular tumbling within a self-assembled host. Chem. Commun. 2006, 1625-1627. 

Since photochemical reactions in inclusion compounds have been described in one chapter of the previous book [1], enantioselective (3-lactam formation reactions are summarized in this present chapter as a typical application of the inclusion technique for enantioselective photosynthesis. In addition, as a representative enantioselective single-crystal-to-single-crystal photoreaction, the photodimerization reactions of coumarin and thiocoumarin in their inclusion compound with a chiral host are also described. Furthermore, a host-catalyzed photodimerization reaction of chalcone and 2-pyridone in the solid state is also added to this chapter as a unique example of the application of inclusion techniques to selective photoreaction. 

The steric course of the photodimerization reactions of coumarin (37a) and thiocoumarin (37b) succeeded in being controlled almost perfectly by carrying out the reaction in inclusion complexes using various host compounds. Furthermore, enantioselective dimerization reactions of 37a and 37b were found to proceed through a single-crystal to single-crystal process. 

Figure 2 Host-catalyzed photodimerization reaction of 43 in the solid state.

Since the thymine photodimer was first isolated, its structure and the mechanism of the photodimerization reaction were studied extensively. Four different isomers of cyclobutane thymine dimer were proposed, but NMR spectroscopy was demonstrated to be insufficient to distinguish between them [46]. The structures of thymine photodimers are presented in Figure 13.3. A few years later the structure of thymine photodimers... 

Ab initio calculations have also been carried out <2003JFC(124)99> for the photodimerization reaction of trifluoro-thioacetyl fluoride 13 which affords approximately equal amounts of trans- and >-2,4-difluoro-2,4-bis(trifluoromethyl-1,3-dithietane) 14 (Scheme 1) <1965JOC1375>. The calculations indicate that both dimers are thermodynamically stable relative to the monomer, and of almost equal stability. The Ec and G2gs.is values obtained in the G3(MP2) calculations of the dimers relative to monomers gave 115 and 45 kj mol-1 for cis-14, while the corresponding values for trans-14 were 112 and 44 kj mol-1. For both isomers, the 1,3-dithietane ring was found to be planar <2003JFC(124)99>. 

Chiral crystals generated from non-chiral molecules have served as reactants for the performance of so-called absolute asymmetric synthesis. The chiral environments of such crystals exert asymmetric induction in photochemical, thermal and heterogeneous reactions [41]. Early reports on successful absolute asymmetric synthesis include the y-ray-induced isotactic polymerization of frans-frans-l,3-pentadiene in an all-frans perhydropheny-lene crystal by Farina et al. [42] and the gas-solid asymmetric bromination ofpjp -chmethyl chalcone, yielding the chiral dibromo compound, by Penzien and Schmidt [43]. These studies were followed by the 2n + 2n photodimerization reactions of non-chiral dienes, resulting in the formation of chiral cyclobutanes [44-48]. In recent years more than a dozen such syntheses have been reported. They include unimolecular di- r-methane rearrangements and the Nourish Type II photoreactions [49] of an achiral oxo- [50] and athio-amide [51] into optically active /Mactams, photo-isomerization of alkyl-cobalt complexes [52], asymmetric synthesis of two-component molecular crystals composed from achiral molecules [53] and, more recently, the conversion of non-chiral aldehydes into homochiral alcohols [54,55]. 

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