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.     

cis-[Rh2(mu-O2CCH3)2(CH3CN)6]2+ as a photoactivated cisplatin analog

The complex cis-[Rh2(mu-O2CCH3)2(CH3CN)6]2+ (1) exchanges the two axial CH3CN ligands for solvent molecules in water to yield cis-[Rh2(mu-O2CCH3)2(CH3CN)4(H2O)2]2+ (2). Photolysis of 2 in H2O results in the photoaquation of two equatorial acetonitrile ligands to yield [Rh2(mu-O2CCH3)2(CH3CN)2(H2O)4]2+ (3), which is able to covalently bind to free 2,2'-bipyridine (bpy) and 9-ethylguanine in solution, as well as double-stranded DNA (lambdairr >/= 455 nm). Complex 2 exhibits 20-fold lower cytotoxicity towards human skin cells than hematoporphyrin in the dark, and its toxicity increases by a factor of 34 when irradiated with visible light (400-700 nm, 30 min). This increase in cytotoxicity by 2 upon irradiation is approximately 7 times greater than that measured for hematoporphyrin. These properties make 2 a promising photo-cisplatin analog and a potential agent for photodynamic therapy. To our knowledge, 2 is the first metal-metal bonded complex to bind to DNA upon irradiation with visible light.     

Oximetry of oxygen supersaturated solutions using nitroxides as EPR probe

The broadening of the three hyperfine EPR nitroxide lines in oxygen supersaturated solutions was examined. The solutions were supersaturated with oxygen either by thermal decomposition of 1,4-peroxy-1,4-dimethylnaphthalene or by pressurizing molecular oxygen above the sample solution. The linear relationship between the Lorentzian component of the line width and the O(2) concentration was proven to hold even when the hyperfine splitting is unresolved. In this region of line broadening, the requirement for spectral simulation is discussed.     

Photoproduction of remarkably stable benzylic radicals in cyclodextrin inclusion complexes

Photolysis of cyclodextrin inclusion complexes of diastereomers of ,′-dimethyldibenzyl ketone in the solid state resulted in stable benzylic radicals at room temperature. These radicals were characterized by their electron spin resonance (ESR) signals, emission and excitation spectra.     

Porphyrins as photosensitizers to enhance night vision

Relative bleaching rates of bovine rhodopsin (rod outer segments) in the presence and absence of seven porphyrins and methylene blue were measured under exposure to lambdamax = 675 nm light, using UV-vis spectroscopy. Rate enhancements on the order of up to three times compared to the bleaching of rhodopsin alone where observed. Fluorescence measurements and other data suggests that the porphyrins act as photosensitizers and excite the visual pigment via electron or triplet state energy transfer. These mechanisms suggest that rhodopsin possesses a pocket, proximal to the Schiff base so that porphyrins act as photosensitizers.     

A Tr Esr study of the quenching of photoexcited dioxouranium (VI) salts by stable nitroxyl free radicals

TR ESR spectroscopy was applied to the study of the quenching of excited dioxouranium (VI) (uranyl) nitrate and sulfate by stable nitroxyl radicals of the 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) family. Photoexcitation of uranyl in solutions of alcohols of moderate viscosity (η = 3-10 cP) in the presence of TEMPO leads to CIDEP signals of TEMPO due to a radical triplet pair mechanism (RTPM). Polarized nitroxyls were also observed in solutions of polyelectrolyte sodium poly(styrenesulfonate), NaPSS, in the presence of the nitroxyl with a positively charged trimethylammonium group. Photolysis of uranyl salts in solutions of alcohols leads to the generation of free radicals of alcohols. No CIDEP of these radicals was observed, distinguishing U₂ ^2+* from its organic analog, the triplet benzophenone. The probable reason for the lack of polarization in uranyl photoreduction reactions is the difficult access of free radicals to the U atom of the solvated radical UO₂₊ (V); this atom bears the unpaired electron. The role of polyelectrolytes in the enhancement of the quenching of excited states is discussed. Results are in agreement with the statement that photoexcited uranyl has a triplet multiplicity.     

REMARKABLE INHIBITION OF OXYGEN QUENCHING OF PHOSPHORESCENCE BY COMPLEXATION WITH CYCLODEXTRINS

The phosphorescence of the 4-bromo-l-naphthoyl group is readily quenched by molecular oxygen in homogeneous solvents. However, when this lumophore is complexed with γ-cyclodextrin in aqueous solution at room temperature, its phosphorescence is observed even under 1 atm of oxygen! Phosphorescence decay data indicated that two types of probe/cyclodcxtrin complexes are formed with lifetimes of 600 u,s and 3.5 ms. Oxygen completely quenches the fast decay, but only partially quenches the slow decay.     

Supramolecular effects on the dynamics of radicals in MFI zeolites: a direct EPR investigation

Photolysis of the supramolecular complexes (dibenzyl ketones@ZSM-5) produced supramolecular complexes of benzyl radicals@ZSM-5, which were directly detected by CW-EPR spectroscopy, and provided information on the dynamics of the radicals. The lifetimes of the radicals increased as the group X attached to the carbon atom at the radical center increases from X = H (t(1/2) ca. 2 min) to X = (CH(2))(4)CH(3) (t(1/2) > 200 min). In addition, line broadening of the EPR signal was observed as the group X increases. Experiments involving cation-exchanged zeolites (MZSM-5; M = Li, Na, K, Rb, Cs) showed a strong dependence of the radical lifetime on the size of the cation (t(1/2) ca. 10 min for Li and t(1/2) > 200 min for Cs). The results are discussed in terms of supramolecular steric effects on the radical-radical reactions in the zeolite supercages.     

THE EFFECT OF ALIPHATIC ALCOHOLS ON FLUORESCENCE QUENCHING AND FLUORESCENCE POLARIZATION OF LUMINESCENCE PROBES IN HEXADECYLTRIMETHYLAMMONIUM BROMIDE MICELLES

Abstract— The fluorescence quenching of the indole chromophore by NO⁻₂ and the fluorescence depolarization of several luminescence probes in aqueous solutions containing hexadecyltrimethylammonium bromide (HDTBr) were measured as a function of added C₂–C₄ aliphatic alcohol concentration. The fluorescence decay profiles of pyrene in the micellar solutions were also measured to estimate the aggregation number of the micelles. The addition of n -butyl alcohol significantly reduces the fluorescence quenching rate and the aggregation number and increases the extent of fluorescence depolarization in HDTBr micellar systems. The addition of ethyl alcohol shows a similar but smaller effect.