Photodimerization selectivity

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

The interlayer region of LDHs can provide a novel environment for photochemical reactions of guest molecules [201-204]. For example, Takagi et al. reported that the controlled photodimerization of a variety of unsaturated carboxylate species intercalated in the interlayer galleries of Mg/Al LDH could occur between the layers [203]. Syn head-to-head cyclodimers were selectively formed in the irradiation of intercalated cinnamate ions, whereas two isomers of syn head-to-head and syn head-to-taU cyclodimers were formed for the case of phenylethenylbenzoates. The product selectivity was shown to be controlled by the Mg/Al ratio of the host LDH, and hence the packing density of the anions in the interlayer region [204]. 

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... 

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.,... 

The purpose of this section is to demonstrate the reaction rate constant and selectivity of a bimolecular reaction may be influenced by solute-solute clustering as the pressure is lowered towards the critical point. The solvent sensitive photodimerization of 2-cyclohexen-l-one is an ideal candidate as both the rate of dimerization and the product selectivity may be monitored. 

Experimental. All photodimerizations were carried out in a stainless steel fixed volume cell (1.75 cm ID with a 1.0 cm path length) with sapphire windows under the irradiation of a Hanovia medium pressure mercury lamp filtered through water and Pyrex for a 13.5 hour exposure. The cell and lamp assembly have been described previously (31). For selected runs a custom built 0.9 mL variable-volume pump was connected to the cell and the pressure was varied to determine the exact location of the phase boundary, based on light scattering measured in a Cary 2290 UV-Vis spectrophotometer (Varian Inst.). The spectrophotometer was also used to measure the concentrations of the monomeric cyclohexenone before and after reaction. 

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. 

In this system, however, intermolecular dimerization may take place competitively with intramolecular dimerization. To rule out this possibility, compound 5, in which two anthracenes are linked by two polyether chains, was synthesized.171 It was found that intramolecular photodimerization proceeds rapidly in the presence of Na+ as the template metal cation. Compound 6 was also synthesized.181 Although this compound has not been applied in a photoswitch system, it displays a remarkable fluorescence change upon binding with RbC104 or H3N+(CH2)7NHj.[81 Yama-shita et al.[9] also synthesized 7, in which intermolecular photodimerization of anthracene is completely suppressed. The photochemically produced cyclic form 8 displayed excellent Na+ selectivity. 

Figure 8.17 Pairwise-selective [2 + 2] photodimerization of acenaphthylene (18) and 5-ethoxy-l,4-naphthoquinone (20) within cage 2.

In an approach to a dihydrooritidine analogue, the intermolecular [2 + 2]-photo-cycloaddition of 2, 3 -0-isopropylideneuridine to chiral and achiral acrylates was found to be unsatisfactory both with respect to regio- and diastereoselectivity. The intramolecular approach was more successful, and uridine 143 produced selectively the single diastereomerically pure product 144. Due to concurrent photodimerization and polymerization reactions, however, the yield was only moderate (Scheme 6.50) [136]. 

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-... 

Photodimers have been reported as the principal products from irradiation of numerous arene substrates. While there is little selectivity to be considered in most of these reactions, it has been reported that almost all photolyses of 9-substituted anthracenes give head-to-head dimers as depicted in U10). ° An intramolecular version of these photodimerizations has also been reported. Irradiation of di(a-2-naphthylmethyl) ether gave a mixture of the endo cyclomer (111) and the exo cyclomer (112). ° The photodimerization of 2-pyridones has been the subject of extensive investigation for a number of years." "" In most instances the head-to-tail or trans-anti isomer was the major or exclusive product in these reactions. Irradiation of A -methyl-2-pyridone for 15 h provided the trans-anti dimer (113) in 51% yield. This material was accompanied by much smaller quantities of other dimeric species. Vari-... 

Intermolecular Additions.- Open Chain Systems. The involvement of a singlet exciplex is proposed in the selective photoaddition of 2,3-dimethylbut-2-ene to the /-auf-nitrile (38) yielding the (2- -2)-adducts (39). The photodimerization of (40) affords the cyclobutane derivative (41). ... 

Over the years there have been many reports that photoreactions, particularly dimerization processes, carried out in micelles, microemulsions, and cyclodextrin cavities frequently show a higher selectivity and occur at a greater rate than the corresponding process in simple solution. It is,therefore, of interest but no great surprise to note that the photodimerization of water-soluble anthracenes such as 2-anthracene sulphonate in... 

In studies where cholesteric phase order appears to influence the course of bimolecular reactions, solute diffusion and collisional orientations appear to be affected by solvent anisotropy. An example is the stereoselective photodimerization of 1,3-dlmethylthymine(24). While all four possible cis-fused cyclobutane photodimers are produced in isotropic solutions and disordered glasses, the cis-syn dimer is formed almost exclusively in liquid-crystalline media. Furthermore, selectivity of reaction products in the mesophases is greatly decreased upon addition of an isotropic diluent which disturbs local solvent order (e.g., dioxane or DMSO). Since all of the... 

Similarly, the symmetry of the mixed crystal system composed of (E)-cin-namamide (host) and (E)-2-thienylacrylamide (guest, 8% occluded) is lower than that of the host crystal (P2j/c). The selective occlusion of the guest arises from repulsive sulfur- n interactions. The -l-b end and the -b end of the mixed crystal are enantiomorphic (space group PI) and underwent topochemical [2-1-2] photodimerization in 40-69% enantiomeric excess (Scheme 8) [29]. 

Photochemical [2+2] cycloaddition of alkenes in the crystalline state is synthetically very useful because it usually produces only one stereoisomer predicted ftom the crystal structure. On the other hand, this stereospeciflcity of the reaction can be a disadvantage because of inaccessibility to other stereoisomers. In order to circumvent such a problem, we explored compelled orientational control of the photodimerization of particular compounds like ranj-cinnamic acids and anthracenecarboxylic acids [74-78]. During our study, photochemistry of fluoro- and chloro-substituted ranj-stilbene-4-carboxylic acids and their methyl esters and alkaline and alkaline earth salts in the crystalline phase was likewise studied in order to synthesize specific stereoisomers selectively (Scheme 41) [79]. Most of these stilbene compounds dimerized to give exclusively or mainly syn head-to-head cyclobutane dimers. Some were photochemicaUy inert. 

Cycloaddition of thioparabanates to a variety of electron-donor and electron-acceptor alkenes, giving the appropriate thietans in high yield, has been reported.200 With ethoxyethylene, for example, the adduct (270) is obtained in 88% yield. The use of light with > 400 nm is essential as it provides selective excitation to the state on which the success of the addition depends. The formation of thietans by the addition of thiophosgene201 and aromatic thionecarbonates202 to alkenes has also been described. An unusual photodimerization of 1-hydroxy-pyridine-2-thione (271) has been reported 208 the dimer (272) is further converted into 1,6-diazathianthrene (273) on heating at 100 °C. 

Similarly to 2-AS, 1-AS photodimerizes in 1 2 y-CD complexes efficiently but nonspecifically. However, it does not react in the presence of p-CD because of the absence of 2 2 complexes (Table 21). Inclusion of 1- and 9-anthracene carboxylate gives complexes of very little stability [132]. Ueno and co-workers appended the reactants to the macrocycle to create a CD template, which proved to be much more useful for high selectivity in the production of photodimers (see Section IV.D). 

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