Photo dimerisation

Photolysis of 1-phenylcyclopentene (106) and 1,4-dicyanobenzene in acetonitrile gives [2+4] cycloadducts based upon the tetralin skeleton, and a 2 1 adduct derived from the substrates. However, although 1-phenylcyclohexene (107) leads to l-cyano-2-phenyl-cyclohexane, no 1 1 products are formed. Radical cations are seen which have been shown by the PM3 method to be fully planar (106 ), and which will dimerise by reaction with (106) in contrast, the optimised structure of (107 ) has a chair-like structure. Styryl dyes containing both a crown ether group and a heteroaromatic residue with a sulfoalkyl N-substituent in acetonitrile solution and in the presence of Mg ions will undergo photo-dimerisation. Only one cyclobutane dimer is produced and this is interpreted... 

High resolution NMR has been used to investigate the photo-dimerisation of 9-methylanthracene. In the solid state, only the trans dimer is formed whereas in benzene solution both the trans and cis dimers are produced. Studies show that the maximum domain size of the minor component is about 0.3 pm, and that the reaction occurs at crystal defects in the monomer. Photodimerisation of the 9-substituted anthracenes An-CHzN- MejBr- (166), AnCH2C02 Na-" (167), AnCH20H (168), AnCOMe... 

Numerous systems have been examined for photochromic activity and many of these involve the isomerisation of aromatic ring systems a monograph has been published which surveys many of these rearrangements. A short review, written in Chinese, on the photo-dimerisation of aromatic compounds and the photoaddition of enones and alkynes to aromatic rings has been reported. ... 

Addition reactions.- A wide variety of intermolecular and intramolecular [ 2 + jj2] photocycloaddition reactions have again been described in nitrogen-containing systems. [ 2 + 2] Photo-dimerisation has been reported for (E)-1,2-bis(1, 3 -benzoxazol-2 -... 

Of a comparatively small range of photochemical reactions described for quinolines and isoquinolines, perhaps the most intriguing are some hetero-ring rearrangements of quaternary derivatives, which can be illustrated by the ring expansions of their A-oxides to 3,1-benzoxazepines. As with 2-pyridones, 2-quinolones undergo 2h-2 photo-dimerisation involving the C-3-C-4 double bond. ... 

The basic principles that govern this kind of reactions were established from pioneering work of Schmidt and co-workers on the [2 -I- 2] photo-dimerisation of cinnamic acids in the solid state. This compound... 

Other elegant examples on the use of homosynthons have been reported by Wheeler and co-workers, using flexible chiral sulphonamide cinnamic acids, which form dimers via self-complementary hydrogen bonds and via formation of carbojgrlic acids homosynthons. These compounds are reminiscent of the /-shaped dicarbojgrlic acid reported by Feldman and Campbell, which form similar dimers via the same homosynthons (Scheme 7). In such assemblies, the double bonds are oriented in a parallel fashion at suitable distances for the photo-dimerisation upon UV irradiation. 

Formally an ozonide, this photo-peroxide explodes at 18°C [1], and its dimerisation, catalysed by carbon disulfide, was exothermic and sometimes explosive at ambient temperature [2]. 

In the absence of nucleophilic species, the radical cations generated in the photo-induced electron-transfer reactions may undergo other reactions. Thus, 1-phenoxypropene undergoes cis-trans isomerisation on irradiation in the presence of electron acceptors such as dicyanobenzene (Majima et al., 1979). Some alkenes undergo dimerisation giving cyclobutanes on irradiation in the... 

The construction and properties of monolayers has been well documented by Kuhn (1979) and the photochemical reactions which occur in such systems reviewed (Whitten et al., 1977). Molecules in monolayers are usually ordered and in the case of rru/i -azastilbenes irradiation of the ordered array produces excimer emission and dimers (Whitten, 1979 Quina et al, 1976 Quina and Whitten, 1977). This contrasts with what is found when the fra/jj-isomers of such compounds are incorporated into micelles. In such systems the predominant reaction is cis-trans isomerisation excimer emission is lacking. It is suggested that the lack of isomerisation in the fatty acid monolayers is due to the tight packing and consequent high viscosity of such systems. Styrene also dimerises in a fatty acid monolayer. Interestingly, the products formed on photo-oxidation of protoporphyrins are dependent upon whether the reaction is carried out in a monolayer or a micelle (Whitten et al., 1978). Zinc octa-ethylporphyrin exhibits excimer emission in monolayers (Zachariasse and Whitten, 1973). Porphyrins are photoreduced by amines in monolayers (Mercer-Smith and Whitten, 1979). Electron-transfer reactions have been carried out with monolayers of stearic acid containing chlorophyll and electron acceptors such as quinones (Janzen et al., 1979 Janzen and Bolton, 1979). 

The photo-reduction of perfluorobenzophenone in isopropanol [243] provides an interesting contrast with the classical reduction of benzophenone to benzopinacol under the same conditions (Figure 9.98). This difference between the two systems has been attributed to the greater resonance stabihsation of the radical 9.98A in comparison with 9.98B and to electron-pair repulsions inhibiting the dimerisation of 9.98A. 

KAU 14b] Kaur G., Johnston P., Saito K., Photo-reversible dimerisation reactions and their applications in polymeric systems . Polymer Chemistry, vol. 5, pp. 2171-2186, 2014. 

Al-Ekabi, H., and P. De Mayo The Cds Photo-induced Dimerisation of N-Vinyl Carbazoles. Tetrahedron 42, 6277 (1986). 

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