Photoaddition of alkene

A recent development in the synthesis of 2-benzazepine-l,5-diones is the photoaddition of alkenes to A-alkylphthalimides. A-Alkylphthalimides capable of undergoing a Norrish Type II process follow the route outlined in Scheme 35 (path a) to yield hydrobenz-azepinediones (264) by ring opening of the azacyclobutanol (262) (78ACR407). 

Photoaddition of alkenes to p-quinones has also been observed cyclooctene, for example, undergoes addition to p-benzoquinone306 [Eq. (78)] and anthraquinone 307 to form oxetanes. Furthermore,... 

Stereoselective intermolecular photoadditions of alkenes to enones have been elegantly utilized in the synthesis of naturally occurring compounds or compounds of special interest. Sato and collaborators100 have applied the photoaddition of dioxinone 208 to the chiral r/.v-diol 207 for a one-pot synthesis of the Corey lactone 210, which possesses considerable utility in the preparation of prostaglandin derivatives (Scheme 45). 

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-... 

The use of silica gel surfaces to modify photoprocesses continues to be of interest. The present report deals with photoaddition of alkenes to the steroidal enone (44).It is presumed that, in the adsorbed state on silica gel, the steroid will present the normally hindered side, i.e., the g-face is attacked by the alkene and the a-face is adsorbed on to the gel. The photoadditions were compared with those carried out in methanol. Thus with allene the two photoproducts (45) and (46) were obtained in 90 and 10% yields, respectively, at -78 °C in methanol, while on silica gel, both at -78 °C and at ambient temperature, the same two products were obtained in 46 and 53% yield. Similar results were obtained using ethene and cyclopentene. The photocycloaddition of ethyne and propyne to progesterone and testosterone in solution yields the cycloadducts (47) and (48), with a preference for the former mode of addition being observed. These results are in conflict with those of de Mayo and co-workers. ... 

Photoaddition of alkenes to the azathymine (227) affords the adducts (228) the structures of which were determined by -ray crystallography. Acetone-sensitized irradiation of the azauracil derivative (229) yields the cyclobutane (230). Irradiation of the uracil (231) in isotropic solvents is known to yield all four (2-1 2)-dimers in low yield especially at low concentrations. A study has shown that dimerization in smectic media affords a high yield (94X) of the trans-anti dimer (232). In frozen solutions the specificity is reversed and the CIS-anti dimer (233) is formed. The influence of a variety of media on this process was studied in detail. A theoretical treatment of the photochemical addition of alkenes to psoralens such as (234) has been published. ... 

Photoaddition of alkenes to phenanthrenes also occurs across the... 

The photochemical addition of ethene at 0°C in methylene chloride to the enedione (77) affords a high yield of the adduct (78). This was converted to the monochloro derivative (79) which also undergoes photoaddition of ethene to yield the Z> adduct (80). This on elimination of HCl yielded the quinol (81) which can be oxidised to the quinone (82). Cycloaddition of alkenes (cyclopentene, cyclohexene, and cycloheptene) has been carried out to the same enedione (77) to yield the adducts (83). lyoda et al. have also described a convenient synthesis of the bicyclo-octanediones(84) by a photochemical addition of alkenes to the enedione (77). The adducts (84) can be reduced by zinc in acetic acid to the desired products. Cycloaddition of ethyne to the same enedione followed by reduction affords the bicyclooctanes (85). The photoaddition of alkenes to the dibromo-enedione (86) is also effective and yields, after reduction, the adducts (87). 

A single electron transfer is again involved in formation of the ketone (14) together with the well-known intramolecular adduct (15) when carvone is irradiated in the presence of triethylamine (Givens et al.). In related work, Bischof and Mattay showed that the presence of triethylamine deflected the normal course of intramolecular photoaddition within the enones (16) to produce the spiro compounds (17) preferentially. Regio- and stereospecific adducts from cyclopentenones and the bicyclo[2.2.1]heptene (18) have been used by Salomon et al. as a route to spatane diterpenes. Photoadditions of alkenes to enones can give oxetans and/or cyclobutanes. Cruciani et al. have reported that the use of acetonitrile as solvent favours the formation of cyclobutanes. The oxetanes may be formed via a contact ion pair whereas the cyclobutanes may arise from an exciplex. 

A variety of heterocyclic compounds of the indole and benzofuran type can be made via photoaddition of alkenes to 2-amino- or 2-hydroxy-1,4-naphthoquinone. These products, described by H. Suginome probably derive from an intramolecular trapping reaction of the intermediary 1,4-biradicals or 1,4-zwitterions. 

Like the 1,1-dioxides, the parent benzo[b]thiopbenes will also undergo photoaddition of alkenes to give cyclobutane derivatives. In the sensitized addition of 1,2-dichloroethene, the four possible stereoisomers 16 are formed which one predominates depends on the stereochemistry of the starting alkene.163 Those isomers which can undergo trans elimination of HC1 will react with base to give the cyclobutene derivative (cf. 14). Bcnzo[fc]thiophene will add photochemically across the C=N bond of 2,5-diphenyl-1,3,4-oxadiazole in the presence of a sensitizer, to give the trans-fused adduct 17 [R = R2 = H (trans)].164 Sensitized addition to 2- or 3-methylbenzo-[b]thiophene gives the more usual cis-fused products 17 [R1 = H, R2 = Me (cis)] and 17 [R1 = Me, R2 = H (cis)] respectively. However, with 3-methyl-benzofb] thiophene in the presence of iodine, the orientation of addition is reversed and the cis adduct 18 is obtained. 3-Pyrrolidinobenzo[b]thiophene... 

An interesting photoaddition of alkenes to 6-cyano-1,3-di-methyluracil results in 1,4-transfer of the cyano group to give the product (167) (Scheme 41). 

Further examples of the photoaddition of alkenes to the diketonatoboron difluoride (17) have been published. The irradiations are carried out in 1,4-dioxane or acetonitrile as solvent and use 350 nm light. Irradiation times are relatively long (20 h) but result in the formation of the expected adducts. Thus addition of the alkenes (18) affords the 1,5-diketones (19). These arise by ring opening of the initially formed cyclobutane adducts [e.g. (20)]. Similar results are... 

Intramolecular photoaddition of alkene part with aromatic ring takes place in non-conjugated aryl olefins when two Jt systems in a molecule are in close proximity and are separated by four sigma bonds. For example, cir-6-phenyl-2-hexene 1 in solution undergoes intramolecular 1,3-cycloaddition to give two mcto-adducts 2 and 3 by the formation of exciplex [26]. 

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. - ... 

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