Dimerisations 4 + 4 -photochemical

Example Ester (59) was needed for a photochemical synthesis of chrysanthemate ester (60), a component of the pyrethrin insecticides. The a,B disconnection (59a) gives synthon (61) and aldehyde (62). This 8,y-unsaturated compound could be made by dehydration of (63) as the double bond can appear in only the required position. On page T 149 we discussed the synthesis of (62) by the aldol dimerisation of (64), An alternative strategy is to work at the ester oxidation level (65) which means synthon (66) is needed to combine with (64). 

This is a general situation for thermal, concerted additions those involving Ane + 2ne systems proceed readily, e.g. the Diels-Alder reaction, whereas those involving 2ne + 2ne systems, e.g. the cyclo-dimerisation of alkenes, do not. We might, however, expect that photochemical cyclodimerisation of alkenes would be symmetry... 

The theory again accords the observation that dimerisation is almost invariably a photochemical process. Such conclusions can also be reached qualitatively without detailed calculations, but for precise estimations of the energies involved in the more complex cases, elaborate calculations are necessary. 

Scheme 1.1. Some characteristic reactions of radicals. A and B are any two types of reactive radical. A = B reaction [1]+ is a dimerisation. The reverse [ 1 ] is bond homolysis and may be induced thermally or photochemically. Reaction [2] represents addition to an alkene derivative. Reaction [3], hydrogen atom transfer, is one of the most important displacement reactions of radicals.

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). 

DNA absorbs UV light with a wavelength of <260 nm, leading to photochemical reactions, for instance, the dimerisation of pyrimidine (Eq. 9.2). 

The photocycloaddition of chloroprene to methyl 2,4-dioxopentanoate (4) has been reported. Only two de Mayo style products were obtained from this process and these were identified as the adducts (5) and (6) arising from the two paths of addition of the enol (7) to the diene. Precise kinetic data has been obtained for the photochemical dimerisation of the cyclopentanone derivative (8). ... 

The photochemical dimerisation of (193) results in the formation of some major products, one of which has been identified as the adduct (194). The photochemical dimerisation of single crystals of ( )-2,6-di-Bu -4-[2-(4-methyl-phenyl)-ethenyl]pyrylium salts has been studied." ... 

For most of the compounds examined the excited state has an enhanced dipole moment compared with the ground state. This reflects the greater basicity of the double bond of the excited vinylarene and rationalises the fast and efficient protonation reaction which is observed. 9-Vinylanthracene was found to be an exception in that the excited state dipole moment was determined to be zero. This correlates with the fact that the dominant photochemical reaction of vinylanthracene is dimerisation to give (297) rather than hydration. 

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. ... 

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