Photodimerization micelles

Surfactants have also been of interest for their ability to support reactions in normally inhospitable environments. Reactions such as hydrolysis, aminolysis, solvolysis, and, in inorganic chemistry, of aquation of complex ions, may be retarded, accelerated, or differently sensitive to catalysts relative to the behavior in ordinary solutions (see Refs. 205 and 206 for reviews). The acid-base chemistry in micellar solutions has been investigated by Drummond and co-workers [207]. A useful model has been the pseudophase model [206-209] in which reactants are either in solution or solubilized in micelles and partition between the two as though two distinct phases were involved. In inverse micelles in nonpolar media, water is concentrated in the micellar core and reactions in the micelle may be greatly accelerated [206, 210]. The confining environment of a solubilized reactant may lead to stereochemical consequences as in photodimerization reactions in micelles [211] or vesicles [212] or in the generation of radical pairs [213]. 

Figure 4 shows typical SEC profiles of two PS-3-PI diblock copolymers before and after the photodimerization reaction. [46] The appearance of a peak after the UV irradiation at a short retention time with a doubled molar mass indicates the formation of PS-3-PI-C-PI-3-PS copolymer chains. In contrast, the irradiation of SI 42 in THF without the self-assembly only leads to a slight increase of the molar mass. It confirms that the self-assembly helps to concentrate and expose the reactive ends of precursor diblock copolymers on the periphery of the core-shell micelles, which greatly increases the coupling efficiency of the photodimmerization reaction between two coumarin end groups. 

In order to overcome this lack of selectivity, photodimerizations have been performed in micelles,16,17 in supercritical fluids,18 in inclusion compounds19 and in the solid state.20 Nevertheless, such reactions are difficult to run on a preparative scale, and better results can be obtained by careful choice of an appropriate solvent. Enantioselcctive gas chromatography combined with GC/MS analysis proved to be a very efficient tool for the direct assignment of constitution and configuration of the photocyclodimers formed.21 In this manner, /ram-1,2-di-vinylcyclobutane has been prepared by sensitized irradiation of buta-1,3-diene.22... 

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.,... 

Example of head-to-tail and head-to-head photodimerization of an enone in solution and at the interface of a micelle... 

The j5-chlorobenzenesulfonate salt of N-octadecyl-trans-4-stil-bazole (27) fails to dimerize in solution or when incorporated into CTAB micelles (66a,b). However, irradiation in the solid state or in monolayer assemblies yields the syn head-to-head dimer 23 accompanied by long-wavelength fluorescence attributed to the excimer. Evidently, aggregation of the hydrophobic hydrocarbon tails is more important than like-charge repulsion in determining the stereochemistry of photodimerization in the case of 27. Whitten and co-workers (66c) recently reported the observation of strong red-shifted fluorescence attributed to... 

The photodimerization of coumarin (60) is affected by the presence of micelles (Scheme XXI)52). Alterations in the nature of the micelle by employing anionic (SDS), cationic (HDTBr) and non-ionic (Triton X-100) micelles slightly affect the relative quantum efficiencies of the cycloaddition and supports the contention that... 

The useful ability of micelles to produce high local concentrations of bound organic reactants at low bulk concentrations has been used in photodimerization reactions see references [712, 713] for reviews. The increase in reactant concentration per unit volume of the micelle promotes the probability of encounters between two molecules resulting in up to 1000-fold rate enhancements. The photodimerization of acenaphthylene illustrates this effect nicely cf. Eq. (5-161) [720, 721],... 

Solution-phase photodimerization of the coumarins (112) affords the syn head-to-tail dimers (113), which were the only products formed when dimerizaton was similarly carried out in micelles. 

Quina, F H., and Wliitten, D. G. Photochemical Reactions in Organized Monolayer Assemblies. 4. Photodimerization, Photoisomerization, and Excimer Formation with Surfactant Olefins and Dienes in Monolayer Assemblies, Crystals, and Micelles. J. Am. Chem. Soc. 99, 877 (1977). 

Over the years there have been many reports that photoreactions, particularly dimerization processes, carried out in micelles, microemulsions, and cyclodextrin cavities frequently show a higher selectivity and occur at a greater rate than the corresponding process in simple solution. It is,therefore, of interest but no great surprise to note that the photodimerization of water-soluble anthracenes such as 2-anthracene sulphonate in... 

Quina and Whitten (5,6) studied the photodimerization, photoisomerization and excimer fluorescence of stilbazolium derivatives in homogeneous solution, CTAB micelles, monolayer assemblies, and the solid state. The photoproducts of /V-(1-Octadecyl)-4-stilbazolium salts (CisStzX, where X is the counterion) were found to be extremely dependent upon the molecular environment. In acetonitrile, CisStzX exhibits a weak structureless fluorescence with a maximum peak at 430nm. Irradiation of a nearly saturated solution results in trans- cis isomerization without evidence of bimolecular interactions occurring. Prolonged irradiation of the CisStzX solution results in cyclization to the corresponding azaphenathrene salt from c/s-CisStzX. The photochemical behavior of CisStzX in CTAB micelles was found to be quite similar to that observed in homogeneous solution. Trans- cis isomerization with eventual cyclization occurs upon irradiation. No dimerization was observed. 

A study of the photodimerization of the pyridones (245) in a micellar environment has been reported.The results show that there is an alignment of the molecules in the micelle. The pyrazinone derivative (246) is photochemically unreactive when it is irradiated in solution at room temperature. However when it is irradiated in the solid state at room temperature a [4 + 4]-dimer (247) is formed. 

The rates or efficiencies of bimolecular photoreactions such as excimer formation, photodimerization, and photoaddition can be strongly affected by using micellar solvents when the reactants associate with the micelles When low concentrations of reactants are solubilized in solutions of high concentrations of micelles, the reactants will be separated by association to different micelles and the reaction will be inhibited. Under opposite conditions high local concentrations will cause an increase of quantum yields compared to homogeneous solutions of equal analytical concentration. Thus in micellar solution the efficiency of bimolecular... 

Quantum yields of the photodimerization of some anthracene derivates A in ionic surfactant solutions are given in Table 3 as functions of the micelle concen-... 

Photodimerization of 2-pyridones has also been effected in other media. Nakamura reported the result of reactions in micelles and reverse micelles using pyridones bearing aliphatic chains on nitrogen. A high level of abnormal cis and head-to-head photoproducts were observed. ... 

In addition to the irreversible disruption triggered by NIR light, reversible photoisomerization reactions of some chromophores, such as the photodimerization of coumarin, have also been explored in order to design photocontrollable BCP micelles. The use of reversible photoisomerization or irreversible photocleavage reactions of various chromophores helps to design reversible or irreversible light-dissociable BCP micelles. 

Aqueous medium containing micelles have a great influence on the photodimerization of 3-n-butylcyclopentenone (106) and isophorone (109) (Scheme 5.35). Reversed orientations in aqueous micellar medium, with respect to organic solvent, were also observed in the photodimerization of 2-substituted naphthalenes, 2-pyridones ° (a [4 + 4] cycloaddition), and in the photocycloaddition reaction of 1-heptenylacetate with 3-n-butyl-cyclopenten-2-one (106) (Scheme 5.36). 

Nakamura, Y, Kato, T. and Morita, Y, A micellar alignment effect in the photodimerizations of n-co-carboxyalkyl-2-pyridones and their 4-alkyl derivatives in micelle or reversed micelle, Tefraftedron Lett., 1981, 22, 1025-1028. 

In its deprotonated form, cinnamic acid already presents some prerequisites of surfactants and is easily incorporated in reversed micelles, as demonstrated bySawaki and co-workers " " and others.Then-results support the view that increased local concentration and alignment can be achieved at micellar interfaces. Substantially higher conversion of cinnamate to its photodimers is observed upon irradiation of reversed micelles formed by laurylammonium or hexadecyltrimethylammonium cinnamate in carbon tetrachloride. The photoproducts are mainly the P- and 5-truxinates, consistent with the requirements imposed by the assembly for the formation of contact ion pairs. Unfortunately, these are also the main products observed by Reiser upon irradiation of ethyl cinnamate (albeit in different proportions). Interestingly, however, the observed dimer distribution varies as a function of added water that is proposed to He near the ion pair interface. Similar results were also obtained for indene-2-carboxylate and using sodium dodecyl sulfate microemulsions in water. " The photodimerization of coumarin in micelles has been investigated by Ramamurthy and co-workers. ... 

Takagi, K., Fukaya, H., Miyake, N., and Sawaki, Y, Organized photodimerization of cinnamic acid in cationic reversed micelle, Chem. Lett., 1053, 1988. 

Takagi, K., Itoh, M., Usami, H., Imae, T, and Sawaki, Y., Organized photodimerization of unsaturated carboxylates. Selectivity control by normal and reversed micelles, /. Chem. Soc., Perkin Trans. 2, 1003, 1994. 

Muthuramu, K. and Ramamurthy, V., Selectivity in chemical reactions in micellar media photodimerization of substituted coumarins in micelles, Indian J. Chem. Soc. Sect. B, 23B, 502, 1984. Ramnath, N. and Ramamurthy, V., Photochemical reactions in constrained systems changes in mode of solubilization due to long-chain hydrophobic groups,/. Org. Chem., 49, 2827, 1984. 

Mayer, H. and Sauer, J., Photodimerization of acenaphthylene derivatives in solutions and micelles. Tetrahedron Lett., 24, 4091,1983. 

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