Cyclobutanations, cycloadditions, benzene

Cycloaddition of 9-vinylcarbazoIe to tetracyanoethylene took place in benzene solution giving 103, but in methanol the diene 104 was obtained cyclobutane (103) itself was transformed into 104 in methanol. Other... 

Cycloaddition is, in general, an effective way of generating cyclobutane rings. The lactone (304) is converted in this way on irradiation into both head-to-tail (305) and head-to-head (306) dimers.248 Direct irradiation of 5,7-dimethoxycoumarin in acetonitrile or benzene similarly affords the syn head-to-tail dimer via the singlet excited state, whereas... 

Many examples of the intramolecular [2 + 2] photocydoaddition of alkenes to benzene derivatives have been reported. The acetophenone derivative 41 undergoes an efficient [2 + 2] photocydoaddition, leading to the cyclobutane derivative 42 (Scheme 5.9, reaction 18) [46, 47]. It was shown that, in this case, a nn triplet state is involved. The presence of a nitrile group in compound 43 induces a [2 + 2] cycloaddition at position 1,2 of the aromatic moiety, leading to intermediate VIII (reaction 19) [48]. Following tautomerization, the final product 44 is formed. 

Substituted cis-3,4-epoxyocta-7-en-l-ynes are converted into tricyclic 5,6-dihydropyran-2-ones 36 and 37 on treatment with Co2(CO)8 in benzene a Co-stabilised cyclic allene intermediate is proposed. The cyclopentanone-fused products 36 arise from tandem [5+1] -[2+2+1] cycloadditions, while the cyclobutane-fused derivatives 37 are produced through a tandem cyclocarbonylation and [2+2] cycloaddition <07JOC567>. 

Cyclobutene has been reported to undergo 1,3 cycloaddition to benzene under photolysis conditions.355 Although the endo stereochemistry of the cyclobutane ring in this adduct is assumed (see 371), pyrolysis of the hydrocarbon at ca 250 °C gave dihydrotriquinacene 372 as the major product. Using cis-3,4-dimethylcyclo-butene, 373 was obtained and similarly transformed to 374 without loss of stereochemistry by thermolysis.376 ... 

The [2 + 2] cycloaddition reaction is an excellent method for the synthesis of cyclophanes.691 For example, irradiation of the commercially available m-divinylbenzene (124) in dry benzene yields the cyclobutane 125, which then slowly photocyclizes to isomeric cyclophanes in a low chemical yield (<10% each) (Scheme 6.54).720... 

Many examples of cyclobutane formation involving cyano-substituted alkenes or enone components have been reported over the years. Some of these reactions arise as a result of SET to an electron-accepting sensitizer such as 1,4-dicyanonaphthalene. One such reaction among the many reported involves the photoreaction of the diene 244 in benzene solution. Under these conditions the (2 + 2)-cycloadduct 245 is formed in reasonable yields ca 70%). In acetonitrile, however, a different reaction occurs leading to the allylation of the sensitizer affording 246, which undergoes (2 + 2)-cycloaddition to yield the cyclobutane 247 ". Other intramolecular cycloadditions have also been reported, such as the formation of the cage compound 248 from irradiation of the pentaene 249 " ". ... 

Maleic anhydride and its methyl derivatives can dimerise to cyclobutane rings under direct irradiation or in the presence of benzophenone as sensitiser, in benzene ordioxane as solvents. The rate of dimer production is little affected by methyl substitution (a slight decrease is reported) in the sensitised reaction, but more strongly in the unsensitised process In analogous sensitised 1,2-cycloadditions of dimethylmaleic anhydride to several olefins, effects of substituents are also limited, as appears from the few examples below ". 

Another interesting photochemical reaction that occurs with the monolayers is dimerization. This is exempUfled by the photochemical behaviour of the SAM of 7-(10-thiodecoxy)coumarin (52) on polycrystaUine gold. Irradiation at 350 nm results in the (2 -f 2)-cycloaddition of the coumarin moieties. The photodimerization is a reversible process by irradiating at 254 nm. Better regioselectivity in the cycloaddition is obtained when the solid monolayer is irradiated rather than when it is in contact with benzene. The dimer formed is the yn-head-to-head dimer identified as 53 . Self-assembled monolayers of cis- and frani-4-cyano-4 -(10-thiodecoxy)stilbene (54) are also photochemically reactive. Irradiation of a thin film in benzene solution using A, > 350 nm results in the formation of a photostationary state with 80% of the cis-isomer present. Irradiation in the solid shows that cis.trans isomerism occurs but that trans.cis-isomerism fails. Prolonged irradiation brings about (2 - - 2)-cycloaddition of the stilbene units to afford cyclobutane adducts. Such dimerization is a well established process . The influence of irradiation at 254 nm or 350 nm of self-assembled monolayers of 10-thiodecyl 2-anthryl ether on polycrystaUine... 

Photochemical [2+ 2] cycloadditions remain the most widely used method for the formation of cyclobutanes. The dimerization of maleic anhydride in carbon tetrachloride solution gave the anti-dimer (43). The same structure is produced by irradiation of maleic anhydride in the solid phase. Irradiation of trans,trans-dibenzyli-dene acetone in benzene solution gave the head to head truxinic type of dimer (44). The photodimerization of 2-penten-4-oIide in acetonitrile solution led to the formation of three isomeric dimers. The structures of all have been established the major product was the head to tail dimer (45). 

The irradiation of benzene solutions of CA in the presence of the 1,1-diarylethenes 46 yields the [27H-27i]-cyclobutane adducts 47 and the substituted quinones 48, both of which undergo further photochemistry to give the products 49 and 50 of intramolecular arene-ethene cycloaddition and 6Jt-elec-trocychzation, respectively (Figure 87.10). It is proposed that products 48 arise from a single electron-transfer process from the donor ethene to the Tj chloranil, and that the formation of the cyclobutane adducts are formed from the 1,4-biradicals without the involvement of an electron-transfer process. This conclusion is supported by the observation that the ratio of 47 48 increases with an increase in both the oxidation potential of the diarylethenes and in the positive AG value for electron transfer between the addends furthermore, the formation of 48 is greatly favored by an increase in solvent polarity. 

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