Alkenes photoaddition reactions with benzene

The possibility of the intermediacy of the triplet state of benzene itself has been discussed by Atkins et al. [108], Photoaddition of alkenes to arenes is often accompanied by the formation of dimers of the alkene, a reaction sensitized by triplet benzene. With methyl acrylate and methyl vinyl ketone, however, it was found that the ratio of ortho cycloadducts to alkene dimers increased with the concentration of benzene. Because the yield of T, benzene increases with benzene concentration, these results might indicate that ortho photocycloaddition of aery-... 

Aromatic compoimds also undergo photocycloaddition reactions with alkenes, leading to 1,2-, 1,3-, and (less often) 1,4-adducts, as shown for the reaction of benzene with ethene in equation 12.70. Olefins with strongly electron-withdrawing or electron-donating substituents tend to give 1,2-photoaddition products, while olefins with alkyl substituents tend to give mostly 1,3-photoaddition. 

In an attempt to trap fulvene 2 by the Diels-Alder reaction with maleic anhydride, another new photoreaction was discovered/ A cycloadduct was obtained which was suggested to be formed by direct addition to the excited state of benzene. Since then, photoadditions of alkenes to benzene and substituted benzenes have been studied extensively and become a powerful tool for synthetic approaches to complex polycycHc compounds. - ... 

Only the head-to-tail adducts were obtained in the [2+2] photoaddition of 4-hydroxy-l-phenyl[l,8]naphthyridin-2(l//)-one with various alkenes in methanol (Scheme 3). The photolysis of the hypoiodites generated by the in situ reaction of the cycloadducts with excess mercury(ll) oxide-iodine reagent in benzene induced a regioselective scission of the non-ring junction bond of the alkoxyl radical to give substituted 3,9-dihydro-9-phenylyfuro[2,3- ][l,8]naphthyridin-4(2//)-one and/or 3,5-dihydro-5-phenylfuro[3,2-f][l,8]naphthyridin-4-(2//)-ones <1996T6125>. 

A kinetic scheme was proposed [122] with the fluorescent exciplex as precursor of the photoproducts (ortho as well as meta adducts). Quantum yields of adduct formation, exciplex emission, and benzene fluorescence were measured as a function of alkene concentration. The kinetic data fit the proposed reaction scheme. The authors have also attempted to prove the intermediacy of the exciplex in the photoaddition by adding a quencher to the system benzene + 2,2-dimethyl-... 

Thietanes are produced efficiently from [2 + 2] photoaddition of thiones to alkenes . The photochemical reaction of thiosuccinimides (e.g., 6) with 1,1-diphenylethene in benzene affords spirothietanes 7 in high yield (Scheme 13) <2003CC2218>. Sulfenes (RCH=S02) add to electron-rich alkenes to give thietane 1,1-dioxides . 

Many examples exist of the reaction of anthracenes with alkenes and dienes to give products resulting from addition across the 9,10-positions of the anthracene ring. Yang has used this reaction to generate the (4-I-41 adducts (129) which are then converted to the novel para, para linked species (130). These comjpounds are chemiluminescent on thermolysis and yield benzene and, in the case of R=H, singlet excited anthroic acid. The (4+21 photoaddition of... 

In contrast to the preferred meta mode of intramolecular photoaddition of 5-phenyl-l-pentenes, where the alkene and benzene groups are separated by three atoms, irradiation of the styrene (64) yields a single stereoisomer of the ortho adduct (65). In (64), not only are the reacting units separated by 4 atoms, but also it is the styrene rather than the benzene which is excited. Comparable photoreactivity is seen for phenanthrene-styrene systems such as (66) which yield 2+2 adducts (67) along with products derived from competing Paterno-Btichi reaction of the ester carbonyl with the alkene side chain. The photochemical cycloaddition also proceeds in an intermolecular fashion between the ester of 9-phenanthrene carboxylic acid and para-methoxy-0-methylstyrene. The mechanism of this reaction is shown to involve addition of the styrene to the singlet excited state of the phenanthrene derivative. °... 

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