Photoaddition, cyclohexene

The stereospecificity of the photoaddition of maleic anhydride to cyclohexene can be conveniently rationalized as follows ... 

Reports of [ 2 + 2] cycloaddition of nitrogen containing heterocycles to alkenes are so numerous that attention can be drawn here to only a few. Recent examples include the acetone-sensitized photoaddition of 4-oxazolin-2-one (248) to ethylene to give the cis-adduct 249,203 the photocycloadditions of N-substituted imidazoles to acrylonitrile204 and of N-methyl-4-hydroxy-quinol-2-one to cyclohexene,205 and the photoaddition of pentafluoro-pyridine to ethylene to give the 1 1- and 1 2-adducts 250 and 251,... 

Full details of the [3 + 2] photoaddition of nitrobenzene (296) to cyclohexene (297) to give the 1,3,2-dioxazole (298), stable at -70°C, have now been published.244 An analogous intramolecular cycloaddition has been proposed to account for the novel photocyclization of the nitrouracil 299 to the triazole 300 and the pathway is outlined in Scheme 9.245 A 2-azaallyt radical has been shown to be an intermediate in the photoaddition of benzo-... 

A further mechanism for photoaddition that applies to cyclohexenes or cydoheptenes begins with formation of the highly reactive trans isomer of the cycloalkene. In this way 1-methylcydoheptene gives an ether on irradiation in methanol (2.53), and I-methylcydohexene an acetate with acetic acid (2.54). In both cases a... 

The overall course of reaction depends on the relative rate constants for the various secondary radical processes. Aliphatic ketones are often photoreduced to secondary alcohols (4.121, but although there are interesting features in the stereochemistry of the reduction, the method is not a worthwhile alternative to thermal reduction using hydride reagents, except in cases where the substrate is sensitive to basic conditions. Photoaddition of methanol is promoted in the presence of titaniurnfiv) chloride, both for acyclic and cyclic (4.33) ketones the titanium involvement probably starts in the early steps of the reaction, but the detailed mechanism is not known. Addition may also be a major pathway when cyclohexene is used as hydrogen source (4.341 unlike many other simple alkenes, cydohexene does not readily give oxetanes by photocycloaddition (see p. 126). 

The photoaddition of nitrobenzene to cyclohexene at — 70° results in the formation of the first reported 1,3,2-dioxazole [Eq. (93)].342 A novel dimerization is observed in the photolysis of 2-methoxy-6-phenyltropone (318) to the furan derivative (319) in low yield.343 This is regarded as the first 1,8-dipolar photoaddition to be observed. 

Addition to six-membered oxygen heterocycles is also common. The photocycloaddition of 5,7-dimethoxycoumarin to tetramethylethylene has been described,269 and 4-hydroxycoumarin (326) undergoes facile addition to cyclohexene on direct irradiation to give the cyclobutane (327)270 analogous additions to a variety of other alkenes have been reported, and the cycloaddition of 4-methoxycoumarin to 2-methylpropene has been employed in a synthesis of l,2-dihydrocyclobuta[c]coumarin.271 Photoaddition of the 1,2-bisenol lactone (328) to tran.s-stilbene yields propellane (329),272 and [ 2 + 2] cycloaddition is observed along with other competing photoreactions on irradiation of chromone in the presence of alkenes.273... 

The cycloaddition of a variety of sulfur heterocycles to alkenes has also been reported. Dibromomaleic anhydride undergoes [ 2 + 2] cycloaddition to thiophen295 and to benzo[b]thiophen.296 The photoaddition of benzo-[(>]thiophen 1,1-dioxides to alkenes also affords cyclobutane derivatives.297 Of particular interest is the addition of 3-acetoxybenzo[6]thiophen (350) to cyclohexene to give the adduct (351), which on treatment with base, undergoes a retro-aldol ring opening to benzo[6]thiepinone (352).298 A mixture of stereoisomeric cyclobutane derivatives were obtained on irradiation of 2,6-diphenyl-4//-thiopyran-4-one 1,1-dioxide in cyclohexene.299 The [ 2 + 2] cycloaddition of sulfolen to maleic anhydride and to maleimide has been reported.300... 

The first examples of exclusive oxetane formation upon olefin photoaddition to the cyclohexen-l,4-dione 4-oxoisophorone 45, leading to two novel 2-substituted l-oxaspiro[3,5]non-5-en-7-ones 46 and 47, respectively, was reported by Catalani and coworkers (Sch. 12) [42]. They demonstrated that the chemoselectivity of the olefin-enone photochemistry... 

Table 12. Solvent effect on the product yields in various photoaddition of A, A -dimethylallylamine (252) to 2-cyclohexen-l-one (251). ...

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). 

Numerous examples of [ 2 -I- 2] photocycloaddition of five-membered oxygen-containing heterocycles to alkenes have been reported, many of which have useful synthetic applications. The regiochemistry and stereochemistry of each adduct has not always been fully established. The lactone (311) undergoes photoaddition to cyclopentene to afford the cis,anti,cis adduct (312), whereas three stereoisomeric adducts were obtained with cyclohexene. Analogous additions of substituted lactones to alkenes have been employed in the syntheses of ( )-dihydrofomannosin acetate and cyclobutane analog of chrysanthemumic acid. The photoaddition of... 

The photoaddition product of 3-acetoxybenzo[b]thiophene 1,1-dioxide with cyclohexene undergoes ring expansion in the presence of base, to give the benzothiepine derivative 15.162... 

The photoaddition of diketene to the anhydrides (366) yields the [2 + 2] adducts (367) and (368). A study of the photocycloaddition of cyclohexene to maleic anhydride sensitized by an insoluble benzoylated polystyrene has shown that the technique is nearly as efficient as the use of free benzophenone. The cycloadducts (369) and (370) are obtained from the acetone-sensitized addition of 3,3-dimethylbut-l-yne to the anhydride (371). The product (372) is also found in the reaction mixture. This material is light sensitive and is converted by... 

The photoaddition of A -ethylphthalimide (413) to cyclohexene in methanol affords the two adducts (414) and (415). The incorporation of methanol could result from trapping a radical cation. Such a mechanism has been put forward for the formation of the products (416) and (417) from the phthal-imide derivatives (418). The cyclized products (419, 420) are formed on the irradiation of the phthalimide derivatives (421a—c) in methanol. In the case of... 

Non-stereospecific photochemical [2+2]-cycloadditions occur in the dimerization of phenyl cyclohexene 23 in the presence of a sensitizer to produce 24 and 25 [17], and in reactions of Z/E-2 butene with cyclohexenone 26 to give 27 and 28 [18] through the formation of intermediate diradicals. The photoaddition of cyclohexene to an enolised form of 1,3 diketone 29 gives 30 in a concerted process via the formation of an unstable cycloadduct [18]. 

---