Abstract
[2+2] Photocycloaddition of olefins is a very useful reaction in synthetic organic chemistry to obtain cyclobutane-containing molecules, which are almost inaccessible by other methods. The reaction, when performed in the crystalline state, occurs more efficiently and selectively than in homogeneous solution due to tight and regular molecular arrangement in the crystal state. Despite numerous examples for the solid-state [2+2] photodimerization of monoenes, however, it is still a challenge to prepare not only dimers but also higher oligomers and polymers from conjugated polyenes, which have multiple reactive double bonds in a molecule. In our recent studies of the solid-state photoreactions of α,ω-diarylpolyenes, noncovalent intermolecular interactions in crystals were effectively utilized to prealign molecules in stacking arrangements, suitable for the [2+2] reaction. With appropriate ring-substituents, [2+2] photodimerization and photopolymerization of the polyenes took place, although the degree of polymerization was relatively low. This review will describe the details of these reactions.
Highlights
Abstract: [2+2] Photocycloaddition of olefins is a very useful reaction in synthetic organic chemistry to obtain cyclobutane-containing molecules, which are almost inaccessible by other methods
The reaction, when performed in the crystalline state, occurs more efficiently and selectively than in homogeneous solution due to the tight and regular molecular arrangement found in crystals
In the [2+2] photocycloaddition of highly conjugated molecules, the delocalized π-electron systems of olefin monomers are broken by the formation of aliphatic cyclobutane rings of dimeric or polymeric products
Summary
[2+2] Photocycloaddition is one of the best known reactions of olefins in solid-state organic photochemistry [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16]. A number of strategies have been employed to prealign molecules in stacking arrangements favorable for the [2+2] reaction These include intramolecular substitution to control intermolecular interactions between reactant olefins in crystals [11,26,27,28,29,30,31,32], inclusion within host structures [11,14,15,16], and cocrystallization with hydrogenbond templates [33,34,35]. In our recent studies on the solid-state photoreactions of ring-substituted α,ω-diphenylpolyenes (Figure 1), noncovalent intermolecular interactions in crystals were effectively utilized to prealign molecules in stacking arrangements suitable for the [2+2] reaction. Unsubstituted parents of this class of molecules were all photochemically inert due to unfavorable crystal packing for the reaction. Introduction of appropriate ring-substituents into the benzene rings of DPB and DPH led to π-stacked arrangements in crystals favorable for the [2+2] reaction
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