Percorrer por autor "Givens, Richard S."
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- Competing pathways for photoremovable protecting groups: the effects of solvent, oxygen and encapsulationPublication . Field, Thomas; Peterson, Julie; Ma, Chicheng; Jagadesan, Pradeepkumar; Da Silva, José Paulo; Rubina, Marina; Ramamurthy, V.; Givens, Richard S.Extending the applications of Photoremovable Protecting Groups (PPGs) to "cage" phenols has generally met with unusually complex PPG byproducts. In this study, we demonstrate that thep-hydroxyphenacyl (pHP) cage for both simple and complex phenolics, including tyrosine, dispenses free phenols. With the simpler unsubstituted phenols, the reaction is governed by their Bronsted Leaving Group ability. On the other hand, the byproducts of the cage vary with these phenols. For the more acidic phenols the cage byproduct follows the Favorskii rearrangement to formp-hydroxyphenylacetic acid whereas for the weaker phenols other reactions such as reduction and hydrolysis begin to emerge. When the photolysis is conducted in octa acid (OA) containers, non-Favorskii, unrearranged fragments of the cage and other byproducts arise.
- Melding caged compounds with supramolecular containers: photogeneration and miscreant behavior of the coumarylmethyl carbocationPublication . Kamatham, Nareshbabu; Da Silva, José Paulo; Givens, Richard S.; Ramamurthy, V.By merging well-established concepts of supramolecular chemistry, protecting group strategy, and photochemistry, we have solubilized in water hydrophobic organic molecules consisting of a photoactive protecting group and masked carboxylic acids, released the desired acid, and confined a reactive carbocation intermediate within a capsule. Confinement of the photogenerated carbo cation brought out the latent radical-like behavior. This observation is consistent with the recent theoretical prediction of the 7-(diethylamino)coumarinyl-4-methyl carbocation having a triplet diradical ground-state electronic contribution.
- Photorelease of incarcerated caged acids from hydrophobic coumaryl esters into aqueous solutionPublication . Kamatham, Nareshbabu; Mendes, Débora C.; Silva, José P. da; Givens, Richard S.; Ramamurthy, V.Photolysis of aqueous solutions of carboxylic acid esters of 7-(methoxycoumaryl)-4-methanol included within the capsule made up of two molecules of octaacid released the acids in water. The trigger 7-(methoxycoumaryl)-4-methyl chromophore remains within octaacid either as the alcohol or as an adduct with the host octaacid through a hydrogen abstraction process. The method established here offers a procedure to release hydrophobic acid molecules in water at will in a timely manner with light. In addition, the system offers an unanticipated opportunity to probe the mechanistic dichotomy of a diradicaloid intermediate expressing both radical and ionic behavior when generated by coumarylmethyl ester photolysis in a hydrophobic environment.
- Photorelease of incarcerated guests in aqueous solution with phenacyl esters as the triggerPublication . Jagadesan, Pradeepkumar; Silva, José P. da; Givens, Richard S.; Ramamurthy, V.We report the clean, efficient photorelease of a series of carboxylic acids embedded in octa acid (OA) host and protected by a p-hydroxyphenacyl cage. A key role is played by the cage by providing hydrophobicity for entry into the OA enclosure and yet readily removable as a photoactivated protecting group for release from the host. The rapid photo-Favorskii rearrangement of the departing chromophore does not react with the host OA but diminishes hydrophobicity of the OA contents, leading to their facile release into the solvent.
- Supramolecular photochemistry of encapsulated caged ortho-nitrobenzyl triggersPublication . Kamatham, Nareshbabu; Raj, A. Mohan; Givens, Richard S.; Da Silva, José Paulo; Ramamurthy, V.ortho-Nitrobenzyl (oNB) triggers have been extensively used to release various molecules of interest. However, the toxicity and reactivity of the spent chromophore, o-nitrosobenzaldehyde, remains an unaddressed difficulty. In this study we have applied the well-established supramolecular photochemical concepts to retain the spent trigger o-nitrosobenzaldehyde within the organic capsule after release of water-soluble acids and alcohols. The sequestering power of organic capsules for spent chromophores during photorelease from ortho-nitrobenzyl esters, ethers and alcohols is demonstrated with several examples.