Reaction cavity photodimerization

A. Reaction Cavities with Active Walls as Templates Photodimerization... 

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.,... 

According to mechanism (III) even if the concentration of A in one of the two sites (7) is much smaller than the amount in site X, appreciable photoreaction from AY may occur through AxA Y energy transfer as outlined in Sch. 21 or if the quantum yield of reaction from A Y is much larger than that from A x. An example in which situation (III) operates is the photodimerization of 9-cyanoanthracene in the crystalline state presented above (Sch. 23) [138,139]. On a statistical basis, many more molecules within the crystalline bulk phase are expected to be excited than those at defect sites. However, the reaction cavities capable of supporting reaction are specific to defect sites. Efficient photodimerization is believed to occur from exciton migration from the inert bulk sites to the defect sites. 

General Considerations. In the well-known photodimerization of anthracenes in liquid solution 9, lO /lO, 9 -dimers (head-to-tail [4 + 4]) are formed in most cases. However, there have also been instances where head-to-head-photodimers (9,9 /10,10 ) are produced [19], and these were overseen previously. The solid phase photoreactions of anthracenes charged the topochemical postulate [7] for decades with hitherto unsolvable difficulties. All examples that contradict this assumption were eliminated without hesitation from the scope of topochemistry and termed to be crystal defect reactions, because the topochemically allowed processes were taken as support for topochemistry without further proof. The later provision, that the dimerizations occur within reaction cavities in the bulk of the crystal [20], did not help in this respect. A summary of the various arguments is given in Ref. 8. From examples 7 to 8 only b and perhaps c formally meet... 

The analysis of the regioselective reactivity of olefins in identical topochemical environments by three computational methods concludes that both steric factors (cavity and potential energy) and electronic factors (perturbation energy from orbital interactions) play important cooperative roles in determining which C—C double bond in a molecule reacts first in [2-1-2] photodimerization. The steric factor is considered to be effective in the movement of olefins at an early stage of the reaction, whereas the electronic factors are effective in the adduction of olefins at a later stage of the reaction. 

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). 

A SCSC [2+2] photodimerziation involving 7-hydroxy-4-methylcoumarin has been shown to occur in the cavity of (3-cyclodextrin, as reported by Stezowski and co-workers (Fig. 2.3.6) [53]. The SCSC reaction was facilitated by the inclusion of both reactants within a single cyclodextrin cavity. In that way, the photodimerization produced very small changes in the overall shape of the inclusion complex. In addition, water molecules of crystallization were shown to afford an environment favorable for the SCSC transformation by providing a way to relieve the strain induced by the reaction. 

Bimolecular Reactions j8-CD is known to hinder the [2-h2] photodimerization of stil-benes through complexation within its cavity. " In contrast, y-CD can simultaneously include two stilbenes in its larger cavity in both solution and solid phase to significantly facihtate the [2-i-2] photodimerization reaction. 

The initial quantum yields for the dimerization at the lower limit are 0.5, 0.3 and 0.2 without and with 6-cyclodextrin, and in the cavity of the cyclodextrin at the solid state, respectively, at 312 nm. This indicates that these photodimerizations all belong to one and the same type of reaction and they proceed more efficiently than that in solution The quantum efficiencies for formation of cis- and trans-head-to-head dimers were determined as 4. 4 X lO in acetonitrile and 4.4-35 X 10 in various solvents such as acetonitrile, dioxane and carbon tetrachloride, respectively [17]. The reason for the difference between the quantum yields in the solid and the solutions [15,17] could not be ex-... 

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]. 

An aqueous solution of 2-anthracenesulfonate 8 was irradiated in the presence of y-cyclodextrin. The cyclodextrin accelerated the photodimerization to dimers 9,10, and 11 by almost one order of magnitude (Scheme 5). Even at dilute concentrations, a sandwich complex of two anthracene molecules in the host cavity (8.5 A) is formed which accelerates the cycloaddition reaction. In the presence of y-cyclodextrin, the relative yields of the three isomers did not significantly change, suggesting that the guest molecules are nonspedficaUy trapped in the cavity. 

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