RESUMEN
A curious and noticeable structural feature in Schiff bases from 2-aminoaldoses is the fact that imino tautomers arranged equatorially in the most stable ring conformation exhibit a counterintuitive reverse anomeric effect (RAE) in the mutarotational equilibrium, i.e., the most stable and abundant anomer is the equatorial one (ß). As shown by our very recent research, this effect arises from the total or partial inhibition of the exo-anomeric effect due to the presence of an intramolecular hydrogen bond between the anomeric hydroxyl and the iminic nitrogen in the axial anomer (α). When the Schiff base adopts either an enamine structure or the imino group is protonated, the exo-anomeric effect is restored, and the axial α-anomer becomes the most stable species. Although the intramolecular H-bonding should appropriately be interpreted as a genuine stereoelectronic effect, the magnitude of the RAE could be affected by other structural parameters. Herein and through a comprehensive analysis of benzylidene, cinnamylidene, naphthalene, phenanthrene, and anthracene aldehydes, we show the robustness of the RAE effect, which is similar in extent to simple aldehydes screened so far, irrespective of the size and/or hydrophobicity of the substituent at the nitrogen atom.
RESUMEN
We report counter-intuitive axial preferences in non-stereochemically biased, selectively fluorinated methoxycyclohexanes. These pseudo-anomeric effects are apparent when electronegative CF2 groups are placed at the C-2, C-4 and C-6 positions of the cyclohexane ring to render the C-3/5 axial hydrogen atoms electropositive. The electrostatic interaction between these axial hydrogen atoms and the -OMe oxygen is stabilising. The effect is explored using high-level ab initio and DFT calculations in the framework of NBO, QTAIM and NCI analysis across a range of derivatives, and experimentally (19 F{1 H}-NMR at -80 °C) for some illustrative examples. The effect is significant in energy terms for a weak interaction, and illustrates a new stereoelectronic aspect attributed to selective fluorine substitution in organic chemistry.
RESUMEN
Stereoelectronic interactions control reactivity of peroxycarbenium cations, the key intermediates in (per)oxidation chemistry. Computational analysis suggests that alcohol involvement as a third component in the carbonyl/peroxide reactions remained invisible due to the absence of sufficiently deep kinetic traps needed to prevent the escape of mixed alcohol/peroxide products to the more stable bisperoxides. Synthesis of ß-alkoxy-ß-peroxylactones, a new type of organic peroxides, was accomplished by interrupting a thermodynamically driven peroxidation cascade. The higher energy ß-alkoxy-ß-peroxylactones do not transform into the more stable bisperoxides due to the stereoelectronically imposed instability of a cyclic peroxycarbenium intermediate as a consequence of amplified inverse alpha-effect. The practical consequence of this fundamental finding is the first three-component cyclization/condensation of ß-ketoesters, H2 O2 , and alcohols that provides ß-alkoxy-ß-peroxylactones in 15-80 % yields.
RESUMEN
The instability of hydroxy peroxyesters, the elusive Criegee intermediates of the Baeyer-Villiger rearrangement, can be alleviated by selective deactivation of the stereoelectronic effects that promote the 1,2-alkyl shift. Stable cyclic Criegee intermediates constrained within a five-membered ring can be prepared by mild reduction of the respective hydroperoxy peroxyesters (ß-hydroperoxy-ß-peroxylactones) which were formed in high yields in reaction of ß-ketoesters with BF3 â Et2 O/H2 O2 .
RESUMEN
Deprotonation of biomass sugars is investigated by high-level electronic structure calculations, considering α- and ß-anomers as well as all O and C sites. Structural alterations are generally focused on the deprotonated sites. Proton migration or/and ring opening occurs in 15 of 42 deprotonation processes and more frequently for D-fructofuranose than D-glucopyranose conformers. Other regular patterns of structural changes are observed; e.g., deprotonation at anomeric sites often cause larger structural perturbations, strong H-bonds of O(i)H(i)â¢â¢â¢C(i) type are constructed by deprotonation at C sites. Then the enthalpy changes and Gibbs free energies for deprotonation at the various O/C sites are calculated by an economical and sufficiently testified composite method, which are consistent with previous results available. Deprotonation of anomeric groups is always preferred, while not the case for C sites. D-fructofuranose is more acidic than D-glucopyranose, whether for O or C sites. Although O sites are known to deprotonate, some C sites are found to be comparable, or even preferential. Whether for O or C sites, deprotonation is highly regioselective and this clearly indicates the potential utilization for selective conversion of biomass sugars.