Photodimerization chalcones

Keywords chalcone, [2+2]photodimerization, molten-state, cyclobutane... 

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. 

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]. 

The photodimerization of the chalcones (90) as a complex with the hexa-diynediol (91) in the crystalline state yields a single product identified as (92). Caldwell and Singh have measured the triplet lifetimes of several chalcones. 

Under rare circumstances it may happen that the molecular shape of two monomers and the dimer, and their crystal parameters, are so nearly identical that a topotactic phototransformation occurs single crystal into single crystal. This is realized in intramolecular cases like [2,2]-(9,10)-an-thracenophane 17 and its [4 + 4]-photoisomer 18 [31]. However, it was also possible to find some rare intermolecular cases of topotactic single crystal into single crystal photodimerizations with chalcones like 2-ben-zyl-5-benzylidene-cyclopentanone 19 [32]. Since there is virtually no mol-... 

The compound has functional groups that support dimerization type crosslinking and cationic polymerization upon UV exposure (A, = 300—360 nm).. Photodimerization of the chalcone-epoxy compound was confirmed by UV-visible and IR absorbance changes of the C=C double bond of the chalcone unit. Additions of small amounts of onium salts will also photoinitiate cationic polymerization of the epoxy groups present in the above chalcone-epoxy compound by exposine to UV. This ultra-violet light cured chalcone-epoxy compound was reported to possess excellent thermal stability and compares well with conventional UV-cured Bisphenol A type epoxy resins. (see Chapter 3)... 

The photodimerization of chalcone /7a is not easy both in solution and in the solid state. For example, irradiation of 11a in solution gives a mixture of 11a and its cis isomeror a polymer . Irradiation of 11a in the solid state results in a complex mixture of all possible stereoisomeric photodimers in low yield . X-Ray crystal structural studies of two dimorphs oi 11a show that the distances between the double bond centers are 5.2 A in one dimorph and greater than 4.8 A in another dimorph although the molecules are packed in parallel manner in both cases Those distances are longer than the limit of an intermolecular reaction (4.2 A) and therefore the two dimorphs are photoinactive. 

Figure 4 General reaction scheme for the [2-1-2] photodimerization reactions of chalcone and dibenzylidene acetone. In each case, the dimerization reaction can potentially give rise to four possible dimers syn head-to-head (product 1), anti head-to-head (product 2), syn head-to-tail (product 4) and anti head-to-tail (product 3).

Figure 5 The products obtained in the [2+2] photodimerization reaction of trawi-chalcone in different phases (host A is defined in Figure 1).

Alternatively, cinnamate groups may be present in the backbone, just as chalcone groups may be pendant. The photodimerization process is illustrated for poly(vinyl cinnamate) (14) in Scheme 6. Photoexcited (14) is shown to dimerize with ground state (14) to yield the crosslinked polymer. The cinnamate dimer is shown in the head-to-tail configuration. 

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