Understanding the origin of the enantioselectivity and the mechanism of the asymmetric reduction of ketimine generated from acetophenone with oxazaborolidine catalyst

The Role of the Catalyst on the Reactivity and Mechanism in the Diels–Alder Cycloaddition Step of the Povarov Reaction for the Synthesis of a Biological Active Quinoline Derivative: Experimental and Theoretical Investigations

An experimental and theoretical study of the reactivity and mechanism of the non-catalyzed and catalyzed Povarov reaction for the preparation of a 4-ethoxy-2,3,4,4a-tetrahydro-2-phenylquinoline as a biological active quinoline derivative has been performed. The optimization of experimental conditions indicate that the use of a catalyst, namely Lewis acid with an electron-releasing group, creates the best experimental conditions for this kind of reaction. The chemical structure was characterized by the usual spectroscopic methods. The prepared quinoline derivative has been also tested in vitro for antibacterial activity, which displays moderate inhibitory activity against both Escherichia coli and Staphylococcus aureus. The antioxidant activity was investigated in vitro by evaluating their reaction with 1,1-diphenyl-2-picrylhydrazyl DPPH radical, which reveals high reactivity. The computational study was performed on the Diels–Alder step of the Povarov reaction using a B3LYP/6-31G(d,p) level of theory. The conceptual DFT reactivity indices explain well the reactivity and the meta regioselectivity experimentally observed. Both catalysts enhance the reactivity of the imine, favoring the formation of the meta regioisomers with a low activation energy, and they change the mechanism to highly synchronous for the Lewis acid and to stepwise for the Brønsted acid. The reaction of imine with allyl alcohol does not give any product, which requires high activation energy.

London Dispersion Interactions Rather than Steric Hindrance Determine the Enantioselectivity of the Corey-Bakshi-Shibata Reduction

The well-known Corey-Bakshi-Shibata (CBS) reduction is a powerful method for the asymmetric synthesis of alcohols from prochiral ketones, often featuring high yields and excellent selectivities. While steric repulsion has been regarded as the key director of the observed high enantioselectivity for many years, we show that London dispersion (LD) interactions are at least as important for enantiodiscrimination. We exemplify this through a combination of detailed computational and experimental studies for a series of modified CBS catalysts equipped with dispersion energy donors (DEDs) in the catalysts and the substrates. Our results demonstrate that attractive LD interactions between the catalyst and the substrate, rather than steric repulsion, determine the selectivity. As a key outcome of our study, we were able to improve the catalyst design for some challenging CBS reductions.

A MEDT computational study of the mechanism, reactivity and selectivity of non-polar [3+2] cycloaddition between quinazoline-3-oxide and methyl 3-methoxyacrylate

The Molecular Electron Density Theory (MEDT) was used for the study of the mechanism and the selectivity of the [3+2] cycloaddition reaction between quinazoline-3-oxide and methyl 3-methoxyacrylate, using the B3LYP/6-31G(d,p) DFT method. In gas phase, this [3+2] cycloaddition reaction is characterized by a completely ortho regioselectivity and a moderate exo stereoselectivity. Dichloroethane solvent did not modify the selectivities obtained in gas phase but increase the activation energies and decrease the exothermic character. Analysis of thermodynamic characters indicates that by the inclusion of the experimental conditions, the reaction becomes endergonic and thereby under thermodynamic control favouring the formation of the most stable product as observed experimentally, explaining the exo stereoselectivity. The analysis of the global electron density transfer (GEDT) at the transition states and bond order (BO) show that this reaction takes place via a very slightly synchronous and non-polar one-step mechanism. Conceptual DFT reactivity indices analysis accounts for the electrophilic character of the reagents, explaining the high obtained free activation energies, while local Parr functions analysis allows us to explain the ortho regioselectivity observed experimentally. ELF topological analysis of the most favoured reactive pathways indicates that mechanism of this 32CA reaction is one stage, one step, synchronous and non-concerted. The stability of the favourable cycloadduct is attributed to the presence of different non-conventional hydrogen bonds interactions as indicated by NCI and QTAIM analyses. Graphical Abstract.