Taming the excited state reactivity of imines - from non-radiative decay to aza Paternò-Büchi reaction.

This review highlights the excited state characteristics of imines and processes that govern their photochemical and photophysical properties. This review examines the pathways for deactivation and types of photochemical reactions that originate from excited imines. This review also features recent strategies that are developed to circumvent the fundamental issues that have plagued the development of the aza Paternò-Büchi reaction.

Understanding Conformational Preferences of Atropisomeric Hydrazides and Its Influence on Excited State Transformations in Crystalline Media.

Hydrazides derivatives were evaluated to understand the role of N-N bond in dictating the outcome of photoreactions in the solid state.

Transposed Paternò-Büchi Reaction.

A complementary strategy of utilizing ππ* excited state of alkene instead of nπ* excited state of the carbonyl chromophore in a "transposed Paternò-Büchi" reaction is evaluated with atropisomeric enamides as the model system. Based on photophysical investigations, the nature of excited states and the reactive pathway was deciphered leading to atropselective reaction. This new concept of switching of excited-state configuration should pave the way to control the stereochemical course of photoreaction due to the orbital approaches required for photochemical reactivity.

Engaging electronic effects for atropselective [5+2]-photocycloaddition of maleimides.

Atropisomeric maleimides were synthesized and subjected to atropselective [5+2]-photocycloaddition under direct irradiation to yield azepinone products with high enantio- (ee >98%) and diastereoselectivity (dr >98%). While the ee was dictated by the axial chirality, the dr was influenced by the substituent on the maleimide ring. Interestingly, by tuning the electronics of the substituent, the dr of the product can be reversed.

Diamine Functionalized Cubic Mesoporous Silica for Ibuprofen Controlled Delivery.

A diamine functionalized cubic mesostructured KIT-6 (N-KIT-6) has been prepared by post-synthetic method using calcined mesoporous KIT-6 with a diamine source, i.e., N-'[3-(tri methoxysilyl)- propyl]'ethylenediamine. The KIT-6 mesoporous silica used for N-KIT-6 was synthesized under weak acidic hydrothermal method using bitemplates, viz., Pluronic P123 and 1-butanol. The synthesized mesoporous materials, KIT-6 and N-KIT-6, have been characterized by the relevant instrumental techniques such as SAXS, N2 sorption isotherm, FT-IR, SEM, TEM and TGA to prove the standard mesoporous materials with the identification of diamine groups. The characterized mesoporous materials, KIT-6 and N-KIT-6, have been extensively used in the potential application of controlled drug delivery, where ibuprofen (IBU) employed as a model drug. The rate of IBU adsorption and release was monitored by UV vis-spectrometer. On the basis of the experimental results of controlled drug delivery system, the results of IBU adsorption and releasing rate in N-KIT-6 are higher than those of KIT-6 because of the higher hydrophobic nature as well as rich basic sites on the surface of inner pore wall silica.

Dictating photoreactivity through restricted bond rotations: cross-photoaddition of atropisomeric acrylimide derivatives under UV/visible-light irradiation.

Nonbiaryl atropisomeric acrylimides underwent facile [2 + 2] photocycloaddition leading to cross-cyclobutane adducts with very high stereospecificity (enantiomeric excess (ee): 99% and diastereomeric excess (de): 99%). The photoreactions proceeded smoothly in isotropic media for both direct and triplet sensitized irradiations. The reactions were also found to be very efficient in the solid state where the same cross-cyclobutane adduct was observed. Photophysical studies enabled us to understand the excited-state photochemistry of acrylimides. The triplet energy was found to be ∼63 kcal/mol. The reactions proceeded predominantly via a singlet excited state upon direct irradiation with very poor intersystem crossing that was ascertained by quantification of the generated singlet oxygen. The reactions progressed smoothly with triplet sensitization with UV or visible-light irradiations. Laser flash photolysis experiments established the triplet transient of atropisomeric acrylimides with a triplet lifetime at room temperature of ∼40 ns.

Tailoring atropisomeric maleimides for stereospecific [2 + 2] photocycloaddition--photochemical and photophysical investigations leading to visible-light photocatalysis.

Atropisomeric maleimides were synthesized and employed for stereospecific [2 + 2] photocycloaddition. Efficient reaction was observed under direct irradiation, triplet-sensitized UV irradiation, and non-metal catalyzed visible-light irradiation, leading to two regioisomeric (exo/endo) photoproducts with complete chemoselectivity (exclusive [2 + 2] photoproduct). High enantioselectivity (ee > 98%) and diastereoselectivity (dr > 99:1%) were observed under the employed reaction conditions and were largely dependent on the substituent on the maleimide double bond but minimally affected by the substituents on the alkenyl tether. On the basis of detailed photophysical studies, the triplet energies of the maleimides were estimated. The triplet lifetimes appeared to be relatively short at room temperature as a result of fast [2 + 2] photocycloaddition. For the visible-light mediated reaction, triplet energy transfer occurred with a rate constant close to the diffusion-limited value. The mechanism was established by generation of singlet oxygen from the excited maleimides. The high selectivity in the photoproduct upon reaction from the triplet excited state was rationalized on the basis of conformational factors as well as the type of diradical intermediate that was preferred during the photoreaction.

Supramolecular photocatalysis: combining confinement and non-covalent interactions to control light initiated reactions.

Using non-bonding interactions to control photochemical reactions requires an understanding of not only thermodynamics and kinetics of ground state and excited state processes but also the intricate interactions that dictate the dynamics within the system of interest. This review is geared towards a conceptual understanding of how one can control the reactivity and selectivity in the excited state by employing confinement and non-covalent interactions. Photochemical reactivity of organic molecules within confined containers and organized assemblies as well as organic templates that interact through H-bonding and/or cation-carbonyl/cation-π interactions is reviewed with an eye towards understanding supramolecular effects and photocatalysis.

Enantiospecific photochemical 6π-ring closure of α-substituted atropisomeric acrylanilides--role of alkali metal ions.

Direct irradiation of atropisomeric α-substituted acrylanilides in the presence of alkali metal ions gave high ee values in the 3,4-dihydro-2-quinolin-2-one photoproduct, while in the absence of alkali metal ions, racemic photoproduct was observed. The heavy atom effect leading to enhanced triplet yields alters the reactive pathway leading to the observed enantioselectivity in the photoproduct.

Intramolecular Paternò-Büchi reaction of atropisomeric α-oxoamides in solution and in the solid-state.

Atropisomeric α-oxoamides were synthesized and employed for intramolecular Paternò-Büchi reaction leading to very high enantio- and diastereoselectivity in the bicyclic oxetane photoproduct. A reversal of product selectivity was observed in solution and in the solid-state.

Enantiospecific photochemical transformations under elevated pressure.

Enantiospecific axial-to-point chiral transfer in light-induced transformations was efficient under elevated pressure at high temperatures. Model photoreactions with atropisomeric compounds showed higher enantioselectivity in the photoproducts under elevated pressure. The ee values in the photoproducts were rationalized based on the increased stability of optically pure atropisomeric compounds at elevated pressure, even at high temperatures.

Light-induced stereospecific intramolecular [2+2]-cycloaddition of atropisomeric 3,4-dihydro-2-pyridones.

Atropisomeric 3,4-dihydro-2-pyridones undergo stereospecific [2+2]-photocycloaddition in solution with high stereoselectivity (ee > 98% and de > 96%) in the product. The chiral transfer during phototransformation was rationalized based on the stability/reactivity of the biradical.

Fun with photons: selective light induced reactions in solution and in water soluble nano-containers.

Two distinct strategies for controlling selectivity, in particular stereoselectivity in photochemical reactions are reviewed. In the first strategy, supramolecular approach using cucurbituril nano-containers in catalytic amounts is employed to control selectivity during photochemical transformations. In the second approach, a generalized methodology for carrying out light-induced transformations in solution at ambient conditions is detailed where axially chiral motifs are employed to enantiospecifically transfer the axial chirality in the reactant to point chirality in the photoproduct(s).

Supramolecular photocatalysis: insights into cucurbit[8]uril catalyzed photodimerization of 6-methylcoumarin.

Guest induced shape change of the cucurbit[8]uril cavity is likely rate limiting in the supramolecular photocatalytic cycle for CB8 mediated photodimerization of 6-methylcoumarin.

Enantiospecific 6π-photocyclization of atropisomeric α-substituted acrylanilides in the solid-state: role of crystalline confinement on enantiospecificity.

Direct irradiation of optically-pure axially-chiral α-substituted acrylanilides in the crystalline state leads to 3,4-dihydro-2-quinolin-2-one photoproduct with an enantiomeric ratio of 85 : 15 while a racemic mixture of photoproduct is observed in solution.

Reactive spin state dependent enantiospecific photocyclization of axially chiral α-substituted acrylanilides.

The enantiomeric ratio (e.r.) in the 3,4-dihydroquinolin-2-one photoproduct during 6π-photocyclization of α-substituted axially chiral ortho-tert-butyl-acrylanilides depends on the nature of the reactive spin state (singlet or triplet), where the singlet-spin state reactivity gives a racemic mixture and the triplet reactivity gives an e.r. value >95 : 5.

Monocrotophos toxicity and bioenergetics of muscle weakness in the rat.

Acute organophosphate pesticide poisoning is a common medical emergency with high fatality in agricultural communities of Asia. Organophosphate compounds inhibit acetylcholinesterase and prolonged neuromuscular weakness is a major cause of morbidity and mortality of poisoning. Organophosphate pesticide induced muscle weakness may not only arise from inhibition of acetylcholinesterase but also from non-cholinergic pathomechanisms, particularly mitochondrial dysfunction, affecting the production of sufficient ATP for muscle function. This study examined whether muscle weakness in rats subject to monocrotophos toxicity (0.8LD50) was caused by inhibition of ATP synthesis, by oxidative phosphorylation and glycolysis, in addition to inhibition of muscle acetylcholinesterase. Severe muscle weakness in rats following monocrotophos administration was associated with inhibition of muscle acetylcholinesterase (30-60%) but not with reduced ATP production. The rats rapidly recovered muscle strength with no treatment. The ability of rats to spontaneously reactivate dimethoxy phosphorylated acetylcholinesterase and efficiently detoxify organophosphates may prevent severe inhibition of muscle acetylcholinesterase following acute severe monocrotophos poisoning. This may protect rodents against the development of prolonged muscle weakness induced by organophosphates.

Photochemical type II reaction of atropchiral benzoylformamides to point chiral oxazolidin-4-ones. Axial chiral memory leading to enantiomeric resolution of photoproducts.

Atropisomeric benzoylformamides 1 undergo Type II reaction leading to cis-2 and trans-2 oxazolidin-4-one photoproducts. The N-C(Aryl) chiral axis is maintained during the course of the phototransformation in spite the reaction proceeding through a near planar intermediate(s). As the rotational barrier of the N-C(Aryl) chiral axis in the cis-2 and trans-2 photoproducts is lowered when compared with the reactant 1, the isolated optically pure trans-2 isomer is converted to the ent-cis-2 isomer without affecting the C-5 stereogenic center, resulting in resolution of the cis-2 enantiomers.

Decoding stereocontrol during the photooxygenation of oxazolidinone-functionalized enecarbamates.

Systematically designed oxazolidinone-derived enecarbamates reveal that solvent and temperature effects on the stereoselectivity during photooxygenation are likely due to the conformational flexibility of the chiral phenethyl side chain (entropy factors); the extent of enantiomeric excess in the photoproduct is dictated by the alkene geometry.