A latent photoreaction enhanced upon cyclodextrin encapsulation: Photochemistry of α-alkyl dibenzyl ketones in water

α-Alkyl dibenzyl ketones that are capable of undergoing both Norrish Type I and Type II reactions generally yield products of Norrish Type I reaction in organic solvents. These water insoluble ketones form water-soluble host–guest complexes with β and γ-cyclodextrins. Irradiation of the above β-cyclodextrin complexes in water leads to products of Norrish Type II reaction. This change in behavior between the guest alone in organic solvent and as host–guest complexes in water is consistent with the influence of other ordered media such as micelles and reactants such as benzoin alkyl ethers and alkyl deoxy benzoins.     

An unexpected Paternò-Büchi reaction in the crystalline state

[reaction: see text]. The solution and solid-state photochemistry of a series of aryl 1-phenylcyclopentyl ketones (1) was investigated. While typical Norrish type I products were formed in solution, irradiation of crystals of 1 afforded the novel oxetanes 3 and 4 regiospecifically. The formation of the oxetanes is believed to occur through Norrish type I cleavage and hydrogen abstraction, producing an alkene and an aldehyde, followed by a Paternò-Büchi reaction within the crystal lattice cage.     

Controlling photochemistry with distinct hydrophobic nanoenvironments

A combination of hydrophobic forces and guest templation drive the assembly of cavitands into molecular capsules. Encapsulated guests such as dibenzyl ketones reside in an essentially dry environment, and upon irradiation, undergo rearrangement processes that are templated by the shape of the 1 nm x 2 nm cavity.     

Interfacial regions governing internal cavities of dendrimers. Studies of poly(alkyl aryl ether) dendrimers constituted with linkers of varying alkyl chain length

This report deals with a study of the properties of internal cavities of dendritic macromolecules that are capable of encapsulating and mediating photoreactions of guest molecules. The internal cavity structures of dendrimers are determined by the interfacial regions between the aqueous exterior and hydrocarbon like interior constituted by the linkers that connect symmetrically sited branch points constituting the dendrimer and head groups that cap the dendrimers. Phloroglucinol-based poly(alkyl aryl ether) dendrimers constituted with a homologous series of alkyl linkers were undertaken for the current study. Twelve dendrimers within first, second, and third generations, having ethyl, n-propyl, n-butyl, and n-pentyl groups as the linkers and hydroxyl groups at peripheries in each generation, were synthesized. Encapsulation of pyrene and coumarins by aqueous basic solutions of dendrimers were monitored by UV-vis and fluorescence spectroscopies, which showed that a lower generation dendrimer with an optimal alkyl linker presented better encapsulation abilities than a higher generation dendrimer. Norrish type I photoreaction of dibenzyl ketone was carried out within the above series of dendrimers to probe their abilities to hold guests and reactive intermediate radical pairs within themselves. The extent of cage effect from the series of third generation dendrimers was observed to be higher with dendrimers having an n-pentyl group as the linker.     

ITC and NMR analysis of the encapsulation of fatty acids within a water-soluble cavitand and its dimeric capsule

We report here NMR and Isothermal Titration Calorimetry studies of the binding of ionisable guests (carboxylate acids) to a deep-cavity cavitand. These studies reveal that the shortest guests favoured 1:1 complex formation, but the longer the alkyl chain the more the 2:1 host-guest capsule is favoured. For intermediate-sized guests, the equilibrium between these two states is controlled by pH; at low values the capsule containing the carboxylic acid guest is favoured, whereas as the pH is raised deprotonation of the guest favours the 1:1 complex. Interestingly, for one host–guest pair the energy required to decap the 2:1 capsular complex and form the 1:1 complex is sufficient to shift the pK_a of the guest by ~3–4 orders of magnitude (4.1–5.4 kcal mol^?1). The two largest guests examined form stable 2:1 capsules, with in both cases the guest adopting a relatively high energy J-shaped motif. Furthermore, these 2:1 complexes are sufficiently stable that at high pH guest deprotonation occurs without decapping of the capsule.     

H-abstraction prevails over α-cleavage in the solution and solid state photochemistry of cis-2,6-di(1-cyclohexenyl)cyclohexanone

The photochemistry of cis-2,6-di(1-cyclohexenyl)cyclohexanone was studied in solution and in crystals to determine its photoreactivity and chemoselectivity. Although 1,3-acyl shifts and the oxadi-π-methane rearrangement products are possible for the β,γ-unsaturated chromophore, an efficient intramolecular abstraction of an allylic γ-hydrogen and small amounts of α-cleavage and decarbonylation were observed. The mechanism of the Norrish type-II reaction and the selectivity of product formation were analyzed in terms of structural information obtained by X-ray diffraction analysis.     

Predicting conformations and orientations of guests within a water soluble host: a molecular docking approach

In this study we have examined conformations and orientations of guests within a water-soluble host known by the trivial name Octa Acid (OA). Docking program Vina, which was originally developed for screening drug-like molecules, has been used to identify binding modes and affinities of select guest molecules with OA. Hydrophobic guests were encapsulated into the nonpolar cavity of OA capsule owing to solvophobic interactions. Amphiphilic guests were bound by keeping the nonpolar part within the cavity of OA, while pointing the polar anionic group out of the cavity. All these results obtained from the docking study were in accord with experimental findings. The post-complexation attributes of the guests were regulated by available free space and the specific interactions between guest–OA pair, which led to unusual conformations and orientations. This study showed that scoring function available with Vina, which was derived from protein–ligand data set, could successfully predict post-complexed structural features of guests within OA, thus opening opportunities to modulate physical and chemical behavior of guest molecules.     

Unsurpassed cage effect for the photolysis of dibenzyl ketones in water-soluble dendrimers

Amphiphilic water-soluble poly(alkyl aryl ether) dendrimers Gn (n = 1-3) with charge-neutral tetraethylene glycol monomethyl ethers at their periphery were synthesized as microreactors to control the photochemical reactions of dibenzyl ketone derivatives in aqueous solutions. Photophysical studies demonstrated that Gn can encapsulate organic molecules and provide a hydrophobic microenvironment. The product distribution of photolysis of dibenzyl ketone derivatives can be successfully controlled by encapsulating the substrates within dendrimers, and an unsurpassed cage effect of 1.00 is reached in high generation dendrimers, revealing that a thick and compact "shell" was formed at the periphery of the dendrimers. The cage effect is also significantly influenced by the substituent at the para-position of the guest molecules. The higher generation dendrimers exhibit a better confined microenvironment and the aggregates possess more compact cavities to "lock" the guests than the corresponding unimolecular dendrimers. After photolysis, the separation of products can be easily achieved by extracting from the dendrimer solutions and the dendrimers are simply recovered and reused.     

Adiabatic and nonadiabatic bond cleavages in Norrish type I reaction

One of the fundamental photoreactions for ketones is Norrish type I reaction, which has been extensively studied both experimentally and theoretically. Its α bond-cleavage mechanisms are usually explained in an adiabatic picture based on the involved excited-state potential energy surfaces, but scarcely investigated in terms of a nonadiabatic picture. In this work, the S(1) α bond-cleavage reactions of CH(3)OC(O)Cl have been investigated by using the CASSCF and MRCI-SD calculations, and the ab initio based time-dependent quantum wavepacket simulation. The numerical results indicate that the photoinduced dissociation dynamics of CH(3)OC(O)Cl could exhibit strong nonadiabatic bond-fission characteristics for the S(1) α C-Cl bond cleavage, while the dynamics of the S(1) α C-O bond cleavage is mainly of adiabatic characteristics. This nonadiabatic mechanism for Norrish type I reaction of CH(3)OC(O)Cl is uncovered for the first time. The quantum wavepacket dynamics, based on the reduced-dimensional coupled potential energy surfaces, to some extent illustrates the significance of the nonadiabatic effect in the transition-state region on the dynamics of Norrish type I reaction.     

Asymmetric environments in encapsulation complexes

Symmetrical, self-assembled capsules capable of surrounding two guests offer a new approach to enantioselection through coencapsulation: when one guest is chiral, the space remaining is also chiral. This notion is explored within a cylindrical capsule. The dimensions of the capsule select appropriately sized combinations of guests, the shape of the capsule prevents tumbling of rigid molecules, and the chemical surface of the capsule orients polar functions within. Chiral carboxylic acids such as mandelic acid and alpha-Br-butyric acid are identified as promising compounds for this purpose, but diastereoselection is modest (<25% de).     

Solid‐State Asymmetric Photochemical Studies Using the Ionic Chiral Auxiliary Approach

The ionic chiral auxiliary approach to asymmetric photochemical synthesis discovered by the Scheffer group has been successfully applied to many reactions in the solid state. Enantiomeric excesses of up to 99 % were obtained using this method. After a brief introduction of absolute asymmetric synthesis, chirally modified zeolites and host–guest assemblies in asymmetric photochemistry, the bulk of this review will summarize and discuss the application of the solid state ionic chiral auxiliary technique to the Norrish type II reaction, the di‐π‐methane photorearrangement, and to a novel retro‐Claisen photorearrangement.     

Chiral Encapsulation by Directional Interactions

The complexation of chiral guests in the cavity of dimeric self‐assembled chiral capsule 1 ₂ was studied by using NMR spectroscopy and X‐ray crystallography. Capsule 1 ₂ has walls composed of amino acid backbones forming numerous directional binding sites that are arranged in a chiral manner. The polar character of the interior dictates the encapsulation preferences towards hydrophilic guests and the ability of the capsule to extract guests from water into an organic phase. Chiral discrimination towards hydroxy acids was evaluated by using association constants and competition experiments, and moderate de values were observed (up to 59 %). Complexes with one or two guest molecules in the cavity were formed. For 1:1 complexes, solvent molecules are coencapsulated; this influences guest dynamics and makes the chiral recognition solvent dependent. Reversal of the preferences can be induced by coencapsulation of a nonchiral solvent in the chiral internal environment. For complexes with two guests, filling of the capsule’s internal space can be very effective and packing coefficients of up to 70 % can be reached. The X‐ray crystal structure of complex 1 ₂?((S) ‐6 )₂ with well‐resolved guest molecules reveals a recognition motif that is based on an extensive system of hydrogen bonds. The optimal arrangement of interactions with the alternating positively and negatively charged groups of the capsule’s walls is fulfilled by the guest carboxylic groups acting simultaneously as hydrogen‐bond donors and acceptors. An additional guest molecule interacting externally with the capsule reveals a possible entrance mechanism involving a polar gate. In solution, the structural features and dynamic behavior of the D₄‐symmetric homochiral capsule were analyzed by variable‐temperature NMR spectroscopy and the results were compared with those for the S₈‐symmetric heterochiral capsule.     

Guest encapsulation and self-assembly of a cavitand-based coordination capsule

The synthesis and spectroscopic characterization of a cavitand-based coordination capsule 14 BF4 of nanometer dimensions is described. Encapsulation studies of large aromatic guests as well as aliphatic guests were performed by using 1H NMR spectroscopy in [D1]chloroform. In addition to the computational analysis of the shape and geometry of the capsule, an experimental approach to estimate the interior size of the cavity is discussed. The cavity provides a highly rigid binding space in which molecules with lengths of approximately 14 A can be selectively accommodated. The rigid cavity distinguished slight structural differences in the flexible alkyl-chain guests as well as the rigid aromatic guests. The detailed thermodynamic studies revealed that not only CH-pi interactions between the methyl groups on the guest termini and the aromatic cavity walls, but also desolvation of the inner cavity play a key role in the guest encapsulation. The cavity preferentially selected the hydrogen-bonded heterodimers of a mixture of two or three carboxylic acids 18-20. The chiral capsule encapsulated a chiral guest to show diastereoselection.     

Beta-phenyl quenching of triplet excited ketones: how critical is the geometry for deactivation?

The phenomenon of beta-phenyl quenching has been examined by laser-flash photolysis in a series of alpha- and/or beta-substituted ketones 4-8 with similar excited-state characteristics. It is found that alpha-substitution markedly increases the triplet lifetimes in contrast to beta-substitution. The force field calculations for the various staggered conformers of ketones 4-6 and 8-syn show that the lowest-energy conformation in all these ketones has the carbonyl group and the beta-phenyl ring gauche to each other. Despite this geometrical requirement, the longer lifetimes observed are interpreted as being due to the influence of the alpha-substituent on the rotational freedom of the planar benzoyl moiety as a whole. The experimental results are suggestive of the attainment of what appears to be a critical geometry for quenching. This scenario may be likened to Norrish type II reactions, where the alpha-substituent has long been known to suppress the elimination pathway and promote Yang cyclization. In addition, we have shown that the diastereomers of alpha,beta-disubstituted ketones exhibit distinct lifetimes.     

Photo-oxidation of polymers—Part V: A new chain scission mechanism in polyolefins

The photo-oxidation of an ethylene-propylene copolymer has been studied at 310 and 365 nm. The results indicate that the Norrish type II reaction occurs but that another chain scission mechanism is also operative. This involves the reaction of free radicals produced by the photo-decomposition of hydroperoxide groups with neighbouring ketones.     

Hydrogen-bonded encapsulation complexes in protic solvents

We describe here the behavior of the hydrogen-bonded capsule 1.1 and its complexes in protic solvents. The kinetics and thermodynamics of the encapsulation process were determined through conventional (1)H NMR methods. The enthalpies and entropies of encapsulation are both positive, indicating a process that liberates solvent molecules. The rates of dissociation-association of the capsule were comparable to the rates for the in-out exchange of large guests, which suggests that guest exchange occurs by complete dissociation of the capsule in protic solvents. The stability of the hydrogen-bonded capsule 1.1 toward protic solvents depends strongly on the guests, with the best guest being dimethylstilbene 8. The results establish guidelines for the properties of capsules that could be accessed in water.     

Restricted guest tumbling in phosphorylated self-assembled capsules

ABii diphosphonatocavitands self-assemble in chloroform solution to form dimeric molecular capsules. The molecular capsules can incarcerate an N-methylpyridinium or N-methylpicolinium guest. We have demonstrated that the supramolecular assembly acts as a molecular rotor as a result of the restricted motion of the guest inside the molecular cavity. In the solid state, X-ray diffraction analysis of the free host showed that two cavitands interact through strong hydrogen bonds to give the supramolecular self-assembled capsule. The solid-state structure of the N-methylpicolinium complex is comparable to that of the free host and indicates that the guest is not a prerequisite for the formation of the capsule. DOSY NMR studies provided a definitive argument for the formation of the free and complexed supramolecular capsule in CDCl(3) solution. In solution, the tumbling of the N-methylpyridinium and N-methylpicolinium guests about the equatorial axes of the host can be frozen and differs by the respective energy barriers, with the larger picolinium substrate having a larger value (ΔG(++) = 69.7 kJ mol(-1)) than the shorter pyridinium guest (ΔG(++) = 44.8 kJ mol(-1)). This behavior corresponds to the restricted rotation of a rotator in a supramolecular rotor.     

Successive stepwise evolution of host layer‐stacking framework upon the intercalation of mobile vapor guests within side‐chain layers

For the ordered phases of hairy‐rod semiconductive poly(2,5‐bis(3‐tetradecylthiophene‐2‐yl)thieno[3,2‐b]thiophene) (PBTTT) sandwiched in between crystalline platelets of hexamethylbenzene, the successive stepwise evolution of layer‐stacking framework upon guest intercalation has been studied in this research. The direct consequence of the guest intercalation into side‐chain layers is evaluated to cause the lateral shift of thiophene backbones along π–π stacking, resulting in stepwise shift of ultraviolet absorption wavelength. The thermal motions of vapor guests within disordering side‐chain layers subsequently cause progressive expansion of host stacking framework. With the increase in side‐chain length, thicker layers of disordering side chains in liquid crystals (LCs) accommodate additional vapor guests and larger amplitudes of thermal motions of guests, hence promoting the level of reversible d‐spacing change. The mixing between mobile vapor guests and aliphatic side chains is clarified as the mechanism of guest intercalation, which rationalizes successive guest intercalation during heating and the contribution of disordering side‐chain layers. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1448–1456     

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.     

An NMR study of the rates of single-molecule exchange in a cylindrical host capsule

Self-assembled cylindrical capsule 1 is reversibly formed from dimerization of two tetraimide resorcinarenes. Studies of guest exchange involving host capsule 1 reveal a mechanistic continuum for exchange that depends on the structure of the guest. Kinetic and dynamic NMR measurements demonstrate the direct displacement of one guest by another. Surprisingly, in the case of benzene exchange in the pairwise encapsulation of benzene and p-xylene, the incoming benzene occupies the same half of the capsule as the outgoing benzene. As the size of the guests increases, solvent-bridged intermediates determine the rates; empty volumes on the molecular scale need not be invoked.