Charge transfer and electron localization as the origin of the anomeric effect in the O─C─O segment of dimethoxymethane and spiroketals

An Interacting Quantum Atoms (IQA) and Relative Energy Gradient (REG) Analysis of the Anomeric Effect

The explanation of the anomeric effect in terms of underlying quantum properties is still controversial almost 70 years after its introduction. Here, we use a method called Relative Energy Gradient (REG), which is able to compute chemical insight with a view to explaining the anomeric effect. REG operates on atomic energy contributions generated by the quantum topological energy decomposition Interacting Quantum Atoms (IQA). Based on the case studies of dimethoxymethane and 2-fluorotetrahydropyran, we show that the anomeric effect is electrostatic in nature rather than governed by hyperconjugation.

Effect of the nO → π*C═O Interaction on the Conformational Preference of 1,3-Diketones: A Case Study of Riolozatrione Derivatives

The cyclopropane ring-opening reaction of riolozatrione, a natural product obtained from Jatropha dioica, afforded a 2,2-disubstituted 1,3-cyclohexandione displaying an alkyl methyl ether group at position 5. The conformational analysis of this product showed a high preference for the trans-diaxial conformation in both solution and solid state. Such conformation was possible from the noncovalent intramolecular n_X → π*_C═O interactions (X = an element having an unshared electron pair), allowing the determination of the interaction energies. Since the n_X → π*_C═O interactions can be regarded as additive, the energy values ranged from 4.52 to 6.51 kcal mol^-1 for each carbonyl group with a strong dependency on the interatomic distances. The rigorous analysis of the electron density in the topological theory of atoms in molecules framework clearly shows that the origin of O-C═O interactions are through the n_O → π*_C═O electron transfer mechanism. Such interactions are slightly weaker than a canonical hydrogen bond but seemingly stronger than a van der Waals interaction. This interaction must be considered as a stereoelectronic effect due the electronic transfer between the interacting groups, which are limited by their relative stereochemistry and can be represented by a bond-no bond interaction, causing the pyramidalization of the carbonyl, which is the charge acceptor group.

The contribution of non-classical CHaxOC hydrogen bonds to the anomeric effect in fluoro and oxa-methoxycyclohexanes

In this theory study we demonstrate the dominance of non-classical 1,3-diaxial CH_axOC hydrogen bonds (NCHBs) dictating a 'pseudo' anomeric effect in selectively fluorinated methoxycyclohexanes and also influencing the axial preference in the classical anomeric exhibitor 2-methoxytetrahydropyran, a phenomenon which is most often described as a consequence of hyperconjugation. Analogues of methoxycyclohexane where ring CH₂'s are replaced by CF₂ can switch to an axial preference and theory methods (NBO, QTAIM, NCI) indicate the dominance of 1,3-CH_axOMe interactions over hyperconjugation. For 2-methoxytetrahydropyran, it is revealed that the global contribution to the anomeric effect is from electrostatic interactions including NCHBs, not hyperconjugation, although hyperconjugation (n_O→σ*_CO or n_O→σ*_CC) remains the main contributor to the exo-anomeric phenomenon. When two and three ether oxygens are introduced into the ring, then both the NCHB interactions and hyperconjugative contributions become weaker, not stronger as might have been anticipated, and the equatorial anomers progressively dominate.