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CaY, activated under different conditions, was characterized with 1H, 31P, and 1H/27Al double resonance MAS NMR. The 1H MAS NMR spectra of CaY, calcined in an oven at 500 °C, shows resonances from H2O (bound to Ca2+ and the zeolite framework), CaOH+, aluminum hydroxides, silanols, and Brønsted acid sites. No evidence for Lewis acidity is observed on adsorption of trimethylphosphine, and an estimate of ≈16 Brønsted acid sites per unit cell is obtained for this sample. CaY activated in an oven at higher temperatures contains less water, but all the other species are still present. In contrast, CaY activated by slow ramping of the temperature under vacuum to 500 or 600 °C shows a much lower concentration of Brønsted acid sites (<1/unit cell). Again, no evidence for Lewis acidity was observed. These NMR results have been utilized to understand the very different product distributions that are observed for reactions of 1,1- and 1,2-diarylethylenes in zeolite CaY activated in an oven (in air) and under vacuum. Samples with high concentrations of Brønsted acid sites react stoichiometrically with these sites, yielding diarylalkanes. At low concentrations, the Brønsted acid sites can act catalytically resulting in isomerization reactions.

Geometric isomerizations of alkenes and cyclopropanes have played a key role in the development of mechanistic organic photochemistry. 1 Although novel strategies toward obtaining cis geometric isomers have been developed for alkene photoisomerization 2 no method is currently available for selective geometric isomerization of trans-diarylcyclopropanes. We now present a strategy based on differential cation-π interaction that has enabled us to selectively convert trans-diphenylcyclopropane (DPC) to the corresponding cis isomer.The ability of cations to interact with π-systems, especially phenyl rings, has been implicated in a number of important processes. 3 The enhanced stability resulting from binding of a potassium ion to more than one benzene ring has been quantified in the gas phase, 4 while crystal structures are available in which sodium ions form sandwich structures. 5 Hence the "bowl"-shaped cis-DPC with optimally poised phenyl rings may be expected to bind to a cation more strongly than the trans isomer ( Figure 1). Ab initio calculations on alkali cation complexes with cis-DPC (vide infra) reveal that the binding energies are comparable to those of sandwich structures with free benzene ligands, and also are cation dependent. These insights have allowed us to selectively convert trans-DPC to the cis isomer. Since Y zeolite contains a large number of cations that are free to interact with the guest molecules, we have employed it as the medium for the above selective trans to cis photoisomerization. 6 As reported in the literature, triplet sensitization of DPC in acetonitrile solution gave a photostationary mixture consisting of both cis and trans isomers (45% cis and 55% trans; p-methoxyacetophenone as sensitizer in acetonitrile). 1b,7 For zeolite irradiations, both triplet sensitizer (xanthone, p-methoxyacetophenone, and benzophenone) and DPC were included within alkali cation (Li + , Na + , K + , Rb + , and Cs + ) exchanged Y zeolites. 8 All photostationary state measurements were made from both transand cis-DPC. The loading levels of the sensitizer and DPC were maintained at one molecule per 10 supercages. The above samples were dried on a vacuum line (10 -4 Torr at 65°C for 12 h), transferred to dry hexane (10 mL) and following bubbling with nitrogen irradiated as hexane slurries. The products were extracted with methylene chloride-diethyl ether and analyzed by GC. The geometric isomers accounted for over 90% of the extracted materials (GC, calibration method). The photoreaction was clean and the photostationary state was reached within 48 h. The photostationary state composition was cation dependent (percent cis at the photostationary state: Li + Y, 91%; Na + Y, 92%; K + Y, 88%; Rb + Y, 85% and Cs + Y, 65%; the error limit was (4% based on at least six independent runs).A slightly modified conventional potential energy surface for the photoreaction helps one understand the unexpected selective isomerization observed in this study ( Figure 1). The photostationary state composition is generally cont...

The nature of the lowest triplet excited state of acetophenones included in zeolites has been inferred through steady-state and time-resolved emission spectra. Acetophenone shows cation-dependent state switching. Within NaLiY and NaY zeolites, the emitting state is identified to have ππ* character, whereas in NaRbY and NaCsY, two emissions characteristic of nπ* and ππ* were observed. In contrast, 4′-methoxyacetophenone does not show cation-dependent state switching; in all alkali cation-exchanged zeolites, the lowest triplet is identified to have ππ* character. The results are attributed to a specific cation-acetophenone interaction. Static, MAS, and CP-MAS spectra of 13 C-enriched acetophenone included in MY zeolites confirm the presence of such an interaction. The data reveals that the extent of interaction, as reflected by the molecular mobility, depends on the cation. Small cations such as Li + and Na + interact strongly whereas large cations such as Rb + and Cs + interact weakly with acetophenone. Consistent with these trends, small cations are found to switch the lowest triplet to ππ* character, whereas the large cations leave the nπ* and ππ* triplet states of acetophenone close to each other. Computational studies provide strong support for these interpretations. B3LYP/6-31G* calculations were carried out on acetophenone and 4′-methoxyacetophenone as well as their Li + and Na + complexes. Geometries with cations bound to the carbonyl, phenyl, and methoxy groups were examined. The most-stable structures involve a cation-carbonyl interaction, which stabilizes the n orbital and, in turn, destabilizes the nπ* triplet state. Excited-state energetics were quantified using TDDFT/6-31+G* calculations. Consistent with experimental observations, acetophenone and 4′-methoxyacetophenone are predicted to have nπ* and ππ* as their lowest triplet states, respectively. Complexation with Li + or Na + is predicted to lead to a ππ* triplet as the lowest excited state for both compounds. The present study, combining steady-state and time-resolved emission spectra, solid state NMR, and computations, demonstrates the occurrence of cation-dependent state switching in acetophenones and offers an internally consistent explanation of the effect in terms of specific cation-carbonyl interaction.The generality and simplicity of the orbital-based nomenclature for electronic transitions introduced by Kasha 1 allowed photochemists to classify and predict photoreactions on the basis of the orbital nature of the excited state. 2 Since the pioneering studies by Hammond on the photoreduction of benzophenone, it has become clear that in carbonyl compounds the nπ* state is more reactive than the ππ* state. 3 This has led photochemists to seek conditions under which a given carbonyl compound would possess nπ* as the lowest excited state. Since the early 1960s, it has been known that aryl-alkyl ketones and diaryl ketones may possess closely placed nπ* and ππ* triplet states. 4 The relative energies of the two states, each with its own distinc...

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Diarylethenes spontaneously form the corresponding radical cations and carbocations upon inclusion within activated Ca Y zeolite; oxygen plays an important role in the generation of the radical cations.

Through a systematic study of several diphenylcyclopropane derivatives, we have inferred that the cations present within a zeolite control the excited-state chemistry of these systems. In the parent 1,2-diphenylcylopropane, the cation binds to the two phenyl rings in a sandwich-type arrangement, and such a mode of binding prevents cis-to-trans isomerization. Once an ester or amide group is introduced into the system (derivatives of 2beta,3beta-diphenylcyclopropane-1alpha-carboxylic acid), the cation binds to the carbonyl group present in these chromophores and such a binding has no influence on the cis-trans isomerization process. Cation-reactant structures computed at density functional theory level have been very valuable in rationalizing the observed photochemical behavior of diphenylcyclopropane derivatives included in zeolites. While the parent system, 1,2-diphenylcylopropane, has been extensively investigated in the context of chiral induction in solution, owing to its failure to isomerize from cis to trans, the same could not be investigated in zeolites. However, esters of 2beta,3beta-diphenylcyclopropane-1alpha-carboxylic acid could be studied within zeolites in the context of chiral induction. Chiral induction as high 20% ee and 55% de has been obtained with selected systems. These numbers, although low, are much higher than what has been obtained in solution with the same system or with the parent system by other investigators (maximum approximately 10% ee).

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