Styrenes 2+2 photodimerization

The subject of the first section of this chapter is the photodimerization and photocycloaddition reactions of olefins. In this category we include only those compounds in which the photoreactive olefin or polyene is not part of an aromatic system although it may bear an aromatic substituent, such as in styrene. 

The only reaction in which a chromene appears to react as a heterodiene and not like a styrene is the addition of acetylene dicarboxylate to lapachenol, followed by thermal decomposition of the adduct (Scheme 10).254 The ready photodimerization of a chromene has been reported recently. 255 (See also cannabicyclol, Section V,B.)... 

The L-B films offer some advantages over aqueous-hydrocarbon interfaces of micelles and the related assemblies discussed above in terms of the magnitude of their orienting ability and the ease of interpretation of selectivity in photoreactions conducted in them. Molecules in the films have very little freedom of motion (stiff reaction cavities), their interfaces are very well defined, and therefore the alignment of reactant molecules can be readily expressed in the products. Photodimerization of stilbazole derivatives 62, N-octadecyl-l-(4-pyridyl)-4-(phenyl)-l,3-butadiene, (63), surfactant styrene derivatives 64 and 65, and cinnamic acids have been carried out in L-B films [18, 196-200], In all cases, single isomeric head-head dimers are obtained. Geometric isomerization of olefins has not been observed in competition with photodimerization. Independent of the location of the chromophore (i.e.,... 

An interesting conversion of a bis(styrene) crown precursor was reported recently <2001MI35>. The [2+21-photodimerization of 16 was found to occur in solution to afford a cyclobutane-containing crown ether. The reaction product was formed only in 10% yield when irradiated at >280 nm in aqueous base solution. However, when 7-cyclodextrin was present, the hydrophobic styryl residues were apparently confined within its interior cavity and the yield rose to 39%. A similar reaction was performed to afford the diaza-18-crown-6 derived cryptand. In the latter case, cyclization failed in the absence of the cyclodextrin (Figure 18). 

The photocyclodimerization of JV-vinylcarbazole, which was reported by Ledwith and Shirota, can be accounted for by this mechanism [71-73]. A chain process is involved in this photoreaction, and the quantum yield exceeds unity (the maximum quantum yield is 66). The hole transfer from the cyclobutane radical cation to a neutral JV-vinylcarbazole is a key reaction for the chain process. Similar photodimerizations of electron-rich alkenes such as aryl vinyl ethers [74-76], indenes [29, 77, 78], styrenes [79-80] and enamines [71] have been reported by several groups. The DCA-sensitized photodimerization of phenyl vinyl ether gives cis- and trans-1,2-diphenoxycyclobutanes. This photoreaction also involves a chain process although the chain length is short [75]. 

Hayashi has investigated in some detail the ionic photopolymerization of styrene monomers. Free ion lifetimes measured by pulse electrical conductivity measurements were found to agree with those calculated from steady-state conductance measurements. Other studies of interest on radical addition polymerization include the photodimerization of polymers containing thymine bases, diene polymerization by terbium complexes, polymerization of vinyl acetate, and preparation of light-sensitive polyacrylates. 

Kricka and Ledwith146 have reviewed the photochemical synthesis of cyclobutanes by dimerization of mono-olefins. Routes which involve radical cations, species of recent interest, are discussed. Yamamoto et a/.147 have shown that p-methoxystyrene undergoes photodimerization in acetonitrile solution with jp-cyanobenzene as an electron acceptor. These conditions (Pyrex filter) yield the f/wis-head-to-head-dimer [(226a), 13%]. Dimerization does not occur in non-polar solvents. A study of the emission characteristics of the styrene-cyanobenzene system shows that the fluorescence of the styrene is quenched and a broad exciplex emission is observed at 440 nm. The likely path for dimerization involves electron transfer as shown in Scheme 22. p-7V7V-Dimethylaminostyrene... 

The photodimerizations of substituted styrenes were among the earliest photochemical reactions to be studied. These compounds give good yields of dimers... 

Incorporation of a triplet sensitizer benzophenone into the macrocycle 33c has little influence on the self-assembly nor the dimension of the resulting nanotube, but the reaction environment of the cavity is totally changed. Photodimerization of the above enones could not happen within 33c-tube. Instead, the nanotube was able to act as a triplet sensitizer for the cis-trans photoisomerization of trans- >-mQ hy -styrene and selective oxidation of 2-methyl-2-butene to a primary allylic alcohol (Scheme 3.14) [70, 109]. As well, an enlarged nanotube 33d-tube was shown to facilitate the selective photodimerization of coumarin to the awr/-head-to-head product [71]. 

The effects of fluorine substitution on crystal packing and topological photodimerization have also been investigated for styrylcoumarins. " Styrylcoumarin 3 yields the centrosymmetric (a-mode, anti-head-to-tail [HT]) photodimer 5 when irradiated in the solid state however, the inclusion of a fluorine substituent dramatically alters the packing mode and steers molecules 4a and 4b to form stereospecdfic photodimers 6a and 6b (P-mode, sy -head-to-head [HH]) by reaction at the styrenic double bonds. 

Pyridones carrying an N-alkenyl substituent such as 94 photodimerize (Figure 103.1) and photoi-somerize (Figure 103.5), but in water the alkene undergoes hydration to give 149. Performance of this reaction in the presence of perchloric acid leads to an isolable intermediate 148. When the alkene is a styrene, (E),(Z)-isomerization and stilbene-hke cyclization reactions are also observed. ... 

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