Coumarin, photodimerization

The photodimerization of a, (3-unsaturated carboriy1 compounds occurs from either an ( , 7c ) oi a (u, 7r ) state which may be a singlet or a triplet. For example, coumarins photodimerize to give two main products as shown below ... 

Coumarins, natural from plants, general review 78FOR(35)199. Coumarins, photodimerization in solid phase 86UK1282. 

Hoffman, R., WeUs, R, and Morrison, H., Organic photochemistry. XII. Further studies on the mechanism of coumarin photodimerization observation of an unusual heavy atom effect, /. Org. Chem., 36, 102, 1971. 

Dimerization. A coumarin dimer is formed by prolonged exposure of coumarin to sunlight or uv radiation. Photodimerization is also catalyzed by boron trifluoride (30). 

The photodimerization of coumarin has been rather extensively investi-gated.<90 92) Irradiation of coumarin in solution yields a mixture of four stereoisomeric dimers (78)—(81) with dimers (78) and (79) strongly predominating ... 

Another illustrative example concerns the photodimerization of coumarin. The direct irradiation in ethanol, Eq. 49,... 

Photodimerization of coumarin and its derivatives have been widely investigated and used.[37-45] It is known that under a UV irradiation with light wavelength longer than 300 nm, coumarin and its derivatives can undergo [2+2] dimerization to form four types of dimmers depending on the reaction conditions. Importantly, such formed photodimers can be... 

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. 

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... 

Another example of a carbonyl compound whose excited singlet reacts before crossing over to a triplet is provided by coumarin. Direct irradiation in ethanol gives very low yields of a cw-fused dimer.432 However, inclusion of benzophenone results in only trans-fused dimer, even if all the light is initially absorbed by the coumarin.433 Apparently, excited singlet coumarin transfers its singlet excitation to benzophenone, which then crosses over to its triplet and transfers its triplet excitation to coumarin. Triplet coumarin then dimerizes fairly efficiently ( = 0.25) to a tranj-fused system. Morrison has demonstrated sizeable solvent effects on this photodimerization.434 Yields of the cis-dimer are increased by polar solvents and probably arise from excited singlet excimers.119... 

The photodimerization of coumarin has been studied in several solvents and their nature has an effect on this rather complex reaction. In a polar medium such as methanol, the only product is the cis head-to-head isomer (244) but in acetonitrile, this is accompanied by the trans head-to-head dimer (245), which becomes the main product of the reaction in non-polar solvents like benzene or dioxan. Small amounts of the head-to-tail isomers (246) are also formed in non-polar solvents (66JA5415). In some solvents, the presence of benzophenone as sensitizer is essential and it also alters the relative proportion of dimeric isomers (64JA3103, 66CB625). Photolysis of osthole (247) in the presence of acetophenone gives a mixture of two quite different dimers (248) and (249) (80MI22302), representing addition at the exocyclic and endocyclic double bond respectively. 

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

Enantioselective photodimerization of coumarin (97) was accomplished in the inclusion complex (100) with 12a. For example, irradiation of the 1 1 inclusion compound of 97 with 12a gave the anti-head-to-head dimer (—)-98 of 96% ee in 96% yield [52], This photochemical reaction of 100 to the complex (101) of 98 was also found to proceed by a single-crystal to single-crystal manner. However, 1 2 inclusion compound of 11 and 97 gave the. vw-head-to-head dimer (99) upon irradiation in 75% yield [53],... 

The efficient enantioselective dimerization reaction of 97 in the inclusion complex (100) with 12a suggests that two coumarin molecules are arranged in chiral positions in 100 and their [2 + 2] photodimerization occurs by keeping the chirality in 100. The chiral arrangement of 97 was detected by CD spectral measurement of the inclusion complex in Nujol mulls. The 1 1 complex of 97 with 12a and 13a showed CD spectra with a mirror-imaged relation (Fig. 17). After photoirradiation, the CD absorptions of the complex (100) at 225, 275, 300, and 330 nm disappeared, and the new CD absorption due to the inclusion complex (101) of 98 at 240 nm appeared. The photodimerization of 97 was also followed by measurement of infrared (IR) spectra as Nujol mulls. Upon photoirradiation, the nCO absorption of 97 in 100 at 1700 cm 1 decreased gradually and finally disappeared after 4 hr, and new vCO absorption due to 98 in 101 appeared at 1740 cm-1. 

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. 

The [2+2] photodimerization of coumarin has been extensively studied and previously reviewed in this series <1984CHEC>. An enantioselective variant of this transformation has recently been described <20050L1501>. Irradiation of coumarin or 6-methylcoumarin in the presence of an optically active host compound provided anti-head-to-head dimers with up to 98% ee (Equation 15). 

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>. 

The direct irradiation of the parent coumarin in the presence of alkenes results only in an inefficient photodimerization and [2 + 2]-photocycloaddition. Lewis acid coordination appears to increase the singlet state lifetime, and leads to improved yields in the stereospecific [2 + 2]-photocycloaddition [95]. Alternatively, triplet sensitization can be employed to facilitate a [2 + 2]-photocycloaddition. Yields of intramolecular [2 + 2]-photocycloadditions remain, however, even with electron-rich alkenes in the medium range at best. The preference for HT addition and for formation of the exo-product is in line with mechanistic considerations discussed earlier for other triplet [2 + 2]-photocycloadditions [96, 97]. Substituted coumarins were found to react more efficiently than the parent compound, even under conditions of direct irradiation. 3-Substituted coumarins, for example, 3-methoxy-carbonylcoumarin [98], are most useful and have been exploited extensively. The reaction of 3-ethoxycarbonylcoumarin (100) with 3-methyl-l-butene yielded cleanly the cyclobutane 101 (Scheme 6.36) with a pronounced preference for the exo-product (d.r. = 91/9). Product 101 underwent a ring-opening/ring-closure sequence upon treatment with dimethylsulfoxonium methylide to generate a tetrahydrodibenzofur-an, which was further converted into the natural product ( )-linderol A (102) [99]. 

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