Asymmetric induction with β cyclodextrin: cis-trans photoisomerization of diphenyl-cyclopropane and its derivatives

Cyclodextrins due to their inherently chiral nature can bring about moderate enantio and diastereoselectivity during the photoreactions of included guest molecules. Photoisomerization of diphenylcyclopropane and its derivatives have been used as a useful probe for this purpose. Interestingly, the optical isomers that are favoured by β-CD during the photoisomerization of diphenylcyclopropane and its derivatives are the same one that are selectively included when a racemic mixture is equlibrated with β-CD

Surface photochemistry of the herbicide napropamide. The role of the media and environmental factors in directing the fates of intermediates

The photochemical behaviour of the herbicide napropamide is studied on cellulose and silica surfaces, using steady-state and laser-flash diffuse ectance techniques. The results are used to probe how the reaction sites of the host matrices influence the photo-reactive pathways. Napropamide undergoes reaction when irradiated with UV (lamps) or visible (sunlight) radiation on both solid supports. The nature of the intermediates and final products depend strongly on the presence or absence of molecular oxygen. The triplet state of napropamide adsorbed on cellulose is detected by both time-resolved luminescence and transient absorption spectroscopies. The triplet sate was not observed on silica, but transients which include the participation of molecular oxygen are detected during. ash photolysis studies. The keto intermediates of the photo-Claisen rearrangement products are observed on both solids. Substituted 1-naphthols from photo-Claisen reactions and 1-naphthol are among the main reaction products. 1,4-Naphthoquinone is a major photoproduct in the presence of molecular oxygen and is expected to be prevalent when napropamide undergoes photodegradation in the environment (i. e., after being applied to plants and fields)

Cyclodextrin mediated enantio and diastereoselective geometric photoisomerization of diphenylcyclopropane and its derivatives

cis-Diphenylcyclopropanes upon direct excitation and triplet sensitization undergo geometric isomerization to the corresponding trans isomers. In solution the trans isomers are formed as a 1:1 enantiomeric or diasteromeric mixture. Upon inclusion within β-cyclodextrin the same molecules give the trans isomers enriched in one optical isomer. Enantiomeric excess and diasteromeric excess induced by the optically active host β-cyclodextrin although small is mechanistically significant. Graphi

Persistent carbocations from 4,4'-dimethylaminodiphenylethylenes within CaY zeolite: intrazeolite-water controls the structure of the carbocation

Inclusion of 4,4'-dimethylaminodiphenylethylene and related olefins within activated CaY results in the formation of persistent monomer carbocations. The structure of the cation, consequently the color of the zeolite, is controlled by water molecules present within zeolites. Under 'dry' conditions the zeolite is light yellow and under 'wet' conditions it is blue colored. Blue color results from the quininoidal structure adopted by 1,1-bis-(4-dimethylaminophenyl)ethyl cation through participation of the lone pair electrons present on the amino substituent at the para position

Asymmetric induction with cyclodextrins: photocyclization of tropolone alkyl ethers

Employing the photobehavior of tropolone alkyl ethers as the probe, we have established that cyclodextrins could as a serve as a medium to bring about enantioselective photoreactions. The enantioselectivity observed in this study is moderate at best. The enantioselectivity observed in the solid state in comparison to aqueous solution suggests that a rigid environment may be essential to achieve chiral induction during photoreactions of organic molecules. In this context the much higher enantioselectivity observed for the same systems within zeolites is noteworthy

Cyclodextrin-mediated regioselective photo-Fries reaction of 1-naphthyl phenyl acylates

1-Naphthyl phenyl acylates upon irradiation in solution yield eight products via β-cleavage process. However, excitation of these molecules as included in γ-cyclodextrin results in a single product (>95%). This medium dependent product selectivity is attributed to conformational and translational restrictions enforced on the reactant as well as intermediates by the cyclodextrin cavity