Thiocoumarin photodimerization

Thiocoumarin photodimerizes more efficiently than does coumarin in solution and yields the head-to-head product. The photocycloaddition with 2,3-dimethylbut-2-ene affords a 4 1 mixture of the as- and trans-fused adducts, but terminal alkenes give only the m-fused cyclobutane in which the unsubstituted C atom has added to C-3 of the thiocoumarin (Equation 68) <1991JPH(57)231 >. Thiocoumarin reacts ca. 5 times faster under irradiation with tetrachloroethene than does isothiocoumarin (Scheme 70). Both thiocoumarins afford the appropriate m-fused adduct (Equation 69) <1997HCA1865>. 

Single crystal-to-single crystal enantioselective photodimerizations of cou-marin (169a) and thiocoumarin (169b) were found to proceed efficiently in... 

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. 

Tanaka K, Toda F, Mochizuki E, Yasui N, Kai Y, Miyahara I, Hirotsu K (1999) Enantiose-lective single-crystal-to-single-crystal photodimerization of coumarin and thiocoumarin in inclusion compounds with chiral host compounds. Angew Chem Int Ed Engl 38 3523-3525... 

Keywords thiocoumarin, inclusion crystal, [2+2]photodimerization, cyclobutane... 

The enantioselective photodimerization of thiocoumarin (102) to optically pure (+)-a ft -head-to-head dimer (103) in the 1 1 inclusion complex of 102 with 12b was also found to proceed in a single-crystal to single-crystal manner. For example, when a mixture of thiocoumarin (102) and optically active host compound 12b in butyl ether was kept at room temperature for 12 hr, a 1 1 inclusion complex of 103 with 12b was obtained as colorless needles. Photoirradiation of a 1 1 inclusion complex in the solid state (400-W high-pressure Hg lamp, Pyrex filter, room temperature, 2 hr) gave a 2 1 complex of 12b with 103,... 

The single-crystal to single-crystal nature and the steric course of the photodimerization of thiocoumarin (102) to (+)-a t/-head-to-head dimer (103) in the inclusion complex were investigated by x-ray crystallographic analysis and x-ray powder diffraction spectroscopy [54],... 

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. 

Single-Crystal to Single-Crystal Photodimerization of Coumarin and Thiocoumarin... 

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. 

Irradiation of 1 1 host-guest crystals of coumarin 85a with (/ ,/ )-(— )-6a derived from tartaric acid gave the (— )-antvdtedd-to-head dimer 86a of 96% ee [88]. Enantiospecific photodimerization of thiocoumarin 85b gave optically pure (+ )-anti-head-to-head dimer 86b when the 1 1 complex with R,R)- — )-6b was used. X-ray structure analysis revealed that the distance between the two ethylenic double bonds was short enough (3.59 and 3.42 A for 85a and 3.73 and 3.41 A for 85b) for addition to occur and topochemically [89]. Further, both reactions were found to proceed via a single crystal-to-single crystal transformation. 

Tanaka, K., Mochizuki, E., Yasui, N., Kai, Y, Miyahara, I., Hirotsu, K., and Toda, E, Single-crystal-to-single-crystal enantioselective [2 -I- 2] photodimerization of coumarin, thiocoumarin and cyclohex-2-enone in the inclusion complexes with chiral host compounds. Tetrahedron, 56, 6853, 2000. 

The [2-1-2]-cycloaddition reactions in inclusion crystals, or host-guest crystals, have been thoroughly reviewed. Coumarin 61a, thiocoumarin 61b, and cyclohex-2-enone 62 efficiently undergo single-crystal-to-single-crystal enantioselective photodimerization in inclusion complexes with chiral host compounds (R,R)-(-)-fra s 63a, (R,R)- -)-trans 63b, and (-)-64, respectively. The products are (-)-anri-HH 65a (100% EE), +)-anti-HH 65b (100% EE), and -)-syn-trans 66 (48% EE) dimers. An example for enantioselective intramolecular [2+2]-photocycloadditions in inclusion crystals is the reaction of guest 67 in chiral host 68. Irradiation of the powdered inclusion crystals in water suspension affords the optically active photocyclization product 69 with 100% EE in 90% yield. 

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