Cavitand octa acid forms a nonpolar capsuleplex dependent on the molecular size and hydrophobicity of the guest.

We have been exploring the use of a deep cavity cavitand known by the trivial name 'octa acid' as a photochemical reaction cavity for manipulating photochemical and photophysical properties of organic molecules. In the current study, we have monitored the micropolarity of the interior of the cavitand by recording the fluorescence of five different organic probes. They all indicate that the interior of octa acid capsuleplex (2:1, H/G complex) is nonpolar and does not contain water molecules in spite of the complex being present in water. The nature of the octa acid-probe complex in each case has been characterized by 1H NMR data to be a 2:1 capsuleplex. Photophysical and 1H NMR experiments were employed to probe the factors that control the structure of the complex, 2:2, 2:1, and 1:1. The data we have on hand suggest that the structure of the host/guest complex depends on the size and hydrophobicity of the guest molecule.

Chiral photochemistry within a confined space: diastereoselective photorearrangements of a tropolone and a cyclohexadienone included in a synthetic cavitand.

The value of a supramolecular assembly to enforce a closer interaction between a chiral auxiliary and a reaction center has been established using photoreactions of tropolone and cyclohexadienone derivatives. Two probe molecules utilized to establish the concept undergo 4 e- electrocyclization and oxa-di-pi-methane rearrangement from excited singlet and triplet state, respectively. The chiral auxiliaries investigated here has no/little effect in acetonitrile solution during phototransformations of the probe molecules to yield products with new chiral centers. On the other hand the same ones are able to enforce diastereoselectivities to the extent of approximately 30% when the reactions occur within the restricted space of a capsule made up of a synthetic cavitand commonly known as octa acid. Extensive NMR studies have been utilized to characterize the guest-host supramolecular structures. The results presented here should be of value in the overall understanding of chiral induction in photochemical reactions.

Controlling photochemical geometric isomerization of a stilbene and dimerization of a styrene using a confined reaction cavity in water.

Utility of a water-soluble deep cavity cavitand, octa acid, as a reaction medium is illustrated by carrying out photochemical reactions of a stilbene and a styrene included within the octa acid in water. Geometric isomerization of trans-4,4'-dimethyl stilbene is restricted while dimerization of 4-methyl styrene is facilitated within the octa acid cavity. The excited-state chemistry of both systems is different in this medium from that in organic solvents. The change in chemistry is attributed to the supramolecular effects provided by the host cavity.

Crystal engineering principles applied to solution photochemistry: controlling the photodimerization of stilbazolium salts within gamma-cyclodextrin and cucurbit[8]uril in water.

Borrowing concepts from crystal engineering techniques we have been able to steer the photodimerization of stilbazolium salts included in gamma-cyclodextrin towards a desired dimer.

Value of zeolites in asymmetric induction during photocyclization of pyridones, cyclohexadienones and naphthalenones.

Two strategies, namely chiral inductor and chiral auxiliary approaches, have been examined within zeolites with the aim of achieving asymmetric induction during the photocyclization of cyclohexadienone, naphthalenone and pyridone derivatives. Within zeolites, enantioselectivity as high as 55% and diastereoselectivity as high as 88% have been obtained. The observed stereoselectivities are significant given the fact that these reactions gave very little stereoselectivities in isotropic solution media. The results obtained on the photocyclization of dienones, naphthalenones and N-alkyl pyridones within zeolites compliment our earlier investigations on the photocyclization of tropolone derivatives, the geometric isomerization of 1,2-diphenylcyclopropanes and 2,3-diphenyl-1-benzoyl cyclopropanes, and the Norrish type II reaction of alpha-oxoamides, phenyl adamantyl ketones, phenyl norbornyl ketones and phenyl cyclohexyl ketones. With the help of these examples, we have established the importance of zeolite and its charge compensating cations in effecting asymmetric induction in photochemical reactions.

Preorientation of olefins toward a single photodimer: cucurbituril-mediated photodimerization of protonated azastilbenes in water.

With the view to establishing the generality of cucurbit[8]uril as a template, the photodimerization of hydrochloride salts of eight azastilbenes has been investigated in an aqueous medium. Whereas in solution upon excitation all of these olefins yield products of geometric isomerization, cyclization, and hydration, in the presence of cucurbit[8]uril the predominant product is that of dimerization. Such a change in product distribution is attributed to the localization of the olefins by the host cucurbit[8]uril. Most importantly, instead of a mixture of dimers, predominantly a single dimer was obtained in each case. The nature of the dimer that was formed could be rationalized on the basis of the principles of "best fit" and "minimization of electrostatic repulsion". The superior ability of cucurbit[8]uril compared to micelles to act as a templating agent is attributed to its ability to provide a reaction cavity that is tight and time-independent.

Photodimerization of acenaphthylene within a nanocapsule: excited state lifetime dependent dimer selectivity.

Direct excitation of acenaphthylene molecules included in a syn fashion within the octa acid nanocapsule dimerizes quantitatively to a syn dimer, and upon triplet sensitization, yields both syn and anti dimers probably by reacting within and outside the capsule.

Photo-Fries reaction in water made selective with a capsule.

The water soluble capsule formed by a deep cavity cavitand with eight carboxylic acid groups controls product distribution during photo-Fries rearrangement of naphthyl esters in water by restricting the mobility of primary singlet radical pair.

Templating photodimerization of stilbazoles with water-soluble calixarenes.

Water soluble six and eight membered calixarenes template the dimerization of trans-stilbazoles. In the absence of calixarenes at the concentrations employed stilbazoles mainly isomerize to the corresponding cis isomers. Calixarenes help to localize the olefins and orient them in a specific geometry to yield anti-head-tail dimers. Electrostatic interaction between the sulfonate anion and the pyridinium ion of the olefin and hydrophobic interaction between the olefin and the host cavity are believed to be responsible for the observed selectivity. (1)H NMR spectra provide evidence for complexation but do not suggest the exact structure of the host-guest complex.

Regioselective photodimerization of cinnamic acids in water: Templation with cucurbiturils.

Cinnamic acids upon irradiation in solution undergo geometric isomerization while dimerizing to different dimers in the crystalline state. Controlling the nature of the dimer formed upon irradiation remains a challenging task. We have aligned a variety of cinnamic acid molecules in a head-head fashion employing cucurbit[8]uril, a weakly water soluble host as a template. The water solubility of cucurbit[8]uril is enhanced by inclusion of water soluble cinnamic acids and positions the olefins in an arrangement that favors the formation of syn head-head cyclobutanes in near quantitative yields. This methodology works in both solid state as well as in aqueous solution. Irradiation of cinnamic acid complexes with gamma-cyclodextrin has been carried out as a comparison. We find that while cucurbit[8]uril functions well both in solid state and aqueous solution, cyclodextrin works best as solid complexes only. Consistent with the postulated requirement of large cavities for templating olefins to dimerization, irradiation of complexes of cinnamic acid with cucurbit[7]uril resulted in only the corresponding cis isomers.

Asymmetric induction during electron transfer mediated photoreduction of carbonyl compounds: role of zeolites.

Photochemistry of 17 aryl alkyl ketones included within cation exchanged zeolites has been examined. In solution five of the 17 ketones undergo intramolecular hydrogen abstraction reaction even in the presence of a chiral amine and the rest are photoreduced to the corresponding alcohol. Within zeolites all 17 ketones yielded in presence of a chiral amine, the corresponding alcohol as the major product. When a chiral amine was used as the coadsorbent within alkali ion exchanged zeolites, enantiomerically enriched alcohol was formed in all cases. The best chiral induction was obtained with phenyl cyclohexyl ketone (enantiomeric excess: 68%). 1H-13C Cross Polarization Magic Angle Spinning (CP-MAS) experiments, with a model ketone (perdeuterated acetophenone) and chiral amine (pseudoephedrine) included within MY zeolites, suggested that the cation brings the reactant and the chiral amine closer. The role of the cation in such a process is also revealed by the computation results. The results presented here highlight the importance of a supramolecular structure in forcing a closer interaction between a reactant and a chiral inductor that could be used to achieve asymmetric induction in photoproducts.

Photoproduct selectivity in reactions involving singlet and triplet excited states within bile salt micelles.

Generally, photochemical reactions tend to give more than one product. For such reactions to be useful one should be able to control them to yield a single product. Of the many approaches used in this context, the use of reaction media with features different from those of isotropic solutions has been very effective. We provide results of our studies on four reactions within bile salt micelles (cholic acid and deoxycholic acid). These four reactions involve homolytic cleavage of a C-C or C-O bond to yield either a singlet or triplet radical pair. The bile salt micelles control the rotational and translational mobilities of the radical pair, resulting in photoproduct selectivity. The dynamic nature of the bile salt micelles results in differential effects on the singlet and triplet radical pairs.

Control of chirality by cations in confined spaces: Photooxidation of enecarbamates inside zeolite supercages.

On photooxygenation of the optically active Z/E enecarbamates 1 (X = i-Pr) and 2 (X = Me) equipped with the oxazolidinone chiral auxiliary in methylene-blue (MB)-incorporated, alkali-metal (M = Li, Na, K, Cs, Rb), exchanged Y-type zeolites (MY-MB), oxidative cleavage of the alkenyl functionality releases the enantiomerically enriched methyldesoxybenzoin (MDB) product. The extent (%ee) and/or the sense (R or S) of the stereoselectivity in the formation of the MDB product depends on the choice of the alkyl substiuent (i-Pr or Me) at the C-4 position of the oxazolidinone chiral auxiliary, the Z/E configuration of the alkene functionality in the enecarbamates, and the type of alkali metal in the zeolite. Most significantly-the highlight of this study-is the reversed sense (R or S) in the stereoselection when the photooxygenation is run in CDCl3 solution versus inside the MY-MB zeolite. As a mechanistic rationale for this novel stereochemical behavior, we propose the combined action of spatial confinement and metal-ion coordination (assessed by density-functional calculations) of the substrate within the zeolite supercage, both of which greatly reduce the freedom of the substrate and entropically manipulate the stereochemical outcome.

Alkali ion exchanged Nafion as a confining medium for photochemical reactions.

Methanol-swollen Nafion beads were used as microreactors to control the photochemical reaction pathways. Product selectivity in three unimolecular reactions, namely, the photo-Fries rearrangement of naphthyl esters, Norrish Type I reaction of 1-phenyl-3-p-tolyl-propan-2-one and Norrish Type I and Type II reactions of benzoin alkyl ethers were examined. The influence of cations over the photodimerization of acenaphthylene and cross-photodimerization between acenaphthylene and N-benzyl maleimide included within Nafion were also examined. The photochemical behaviors of the above substrates were significantly altered within Nafion compared with their solution photochemistry. Of particular interest, the product distributions were found to depend on the counter cations of Nafion.

Role of cations and confinement in asymmetric photochemistry: enantio- and diastereo-selective photocyclization of tropolone derivatives within zeolites.

Asymmetric induction in photochemical reactions has been explored using the photochemistry of tropolones as a model. Three approaches have been examined: chiral inductor, chiral auxiliary and [chiral inductor + chiral auxiliary]. All three methods gave excellent asymmetric induction in zeolite and very little or zero induction in solution. Results presented on tropolones clearly illustrate the remarkable influence that a confined space studded with cations can have on asymmetric induction. Tropolone derivatives, upon irradiation undergo 4pi-electron electrocyclization to yield a bicyclic product and a rearranged product. Enantiomeric excess up to 68% has been achieved in the cyclized product. In systems where a chiral inductor has been covalently linked, diastereomeric excess as high as 88% has been achieved within a zeolite while the same system in solution gave 10%.

A latent photoreaction predominates within water-soluble calixarenes: photochemistry of benzoin alkyl ethers.

Benzoin alkyl ethers encapsulated in a cisoid conformation within water-soluble p-sulfonatocalix[8]arenes preferentially yield the Norrish Type II reaction product deoxybenzoin.

Dendrimers as photochemical reaction media. Photochemical behavior of unimolecular and bimolecular reactions in water-soluble dendrimers.

Synthesis and studies of poly(alkyl aryl ether) dendrimers, possessing carboxylic acid functionalities at their peripheries, are reported. 5-Bromopentyloxy methylisophthalate was utilized as the monomer to O-alkylate the phenolic hydroxyl groups of poly(alkyl aryl ether) dendrimers. Dendrimers of first, second, and third generations, possessing 6, 12, and 24 carboxylic acids, respectively, were thus prepared. These dendrimers were soluble in alkaline aqueous solutions, and the ensuing microenvironmental properties of the aqueous solutions were assessed by pyrene solubilization studies. Upon establishing the presence of nonpolar microenvironments within the dendritic structures, solubilizations of few organic substrates were conducted and their photochemical behaviors were assessed. Specifically, the photolysis of 1-phenyl-3-p-tolyl-propan-2-one and benzoin ethyl ether and photodimerization of acenaphthylene were conducted. These studies revealed that the product distribution and the "cage effect" were more distinct and efficient for the third generation dendrimer, than for the first and second generation dendrimers. The photochemical studies of carboxylic acid functionalized dendrimers were compared to that of hydroxyl group terminated poly(alkyl aryl ether) dendrimers.

A hydrophobic nanocapsule controls the photophysics of aromatic molecules by suppressing their favored solution pathways.

A combination of hydrophobic forces and guest templation drive the assembly of cavitands into molecular capsules. Remarkably, anthracene that dimerizes with unit efficiency in solution does not dimerize within the capsule despite forming 2:2 complex. The capsule allows an unprecedented examination of the anthracene excimer.

Templating photodimerization of trans-cinnamic acids with cucurbit[8]uril and gamma-cyclodextrin.

Cucurbit[8]uril and gamma-cyclodextrin are able to align two olefin molecules in a head-head fashion within their large cavities. Excitation of such templated olefins results in syn head-head cyclobutanes in nearly quantitative yields. The methodology revealed here works with trans-cinnamic acids that do not dimerize either in solution or in the solid state and with the ones that yield only anti head-tail dimer in the solid state. [reaction: see text]