Atoms in molecules interpretation of the anomeric effect in the OCO unit

A True Reverse Anomeric Effect Does Exist After All: A Hydrogen Bonding Stereocontrolling Effect in 2-Iminoaldoses

The reverse anomeric effect is usually associated with the equatorial preference of nitrogen substituents at the anomeric center. Once postulated as another anomeric effect with explanations ranging from electrostatic interactions to delocalization effects, it is now firmly considered to be essentially steric in nature. Through an extensive research on aryl imines from 2-amino-2-deoxyaldoses, spanning nearly two decades, we realized that such substances often show an anomalous anomeric behavior that cannot easily be rationalized on the basis of purely steric grounds. The apparent preference, or stabilization, of the β-anomer takes place to an extent that not only neutralizes but also overcomes the normal anomeric effect. Calculations indicate that there is no stereoelectronic effect opposing the anomeric effect, resulting from the repulsion between electron lone pairs on the imine nitrogen and the endocyclic oxygen. Such data and compelling structural evidence unravel why the exoanomeric effect is largely inhibited. We are now confident, as witnessed by 2-iminoaldoses, that elimination of the exo-anomeric effect in the α-anomer is due to the formation of an intramolecular hydrogen bond between the anomeric hydroxyl and the iminic nitrogen, thereby accounting for a true electronic effect. In addition, discrete solvation may help justify the observed preference for the β-anomer.

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.

Latent Nucleophilic Carbenes

Using DFT and ab initio calculations, we demonstrate that noncyclic formamidines can undergo thermal rearrangement into their isomeric aminocarbenes under rather mild conditions. We synthesized the silylformamidine, for which the lowest activation energy in this process was predicted. Experimental studies proved it to serve as a very reactive nucleophilic carbene. The reactions with acetylenes, benzenes, and trifluoromethane proceeded via insertion into sp, sp², and sp³ CH bonds. The carbene also reacted with the functional groups, such as CHO, COR, and CN at double or triple bonds, displaying high mobility of the trimethylsilyl group. The obtained silylformamidine can be considered as a latent nucleophilic carbene. It can be prepared in bulk quantities, stored, and used when the need arises. Calculation results predict similar behavior for some other silylated formamidines and related compounds.

Anomeric effect, hyperconjugation and electrostatics: lessons from complexity in a classic stereoelectronic phenomenon

Understanding the interplay of multiple components (steric, electrostatic, stereoelectronic, dispersive, etc.) that define the overall energy, structure, and reactivity of organic molecules can be a daunting task. The task becomes even more difficult when multiple approaches based on different physical premises disagree in their analysis of a multicomponent molecular system. Herein, we will use a classic conformational "oddity", the anomeric effect, to discuss the value of identifying the key contributors to reactivity that can guide chemical predictions. After providing the background related to the relevant types of hyperconjugation and a brief historic outline of the origins of the anomeric effect, we outline variations of its patterns and provide illustrative examples for the role of the anomeric effect in structure, stability, and spectroscopic properties. We show that the complete hyperconjugative model remains superior in explaining the interplay between structure and reactivity. We will use recent controversies regarding the origin of the anomeric effect to start a deeper discussion relevant to any electronic effect. Why are such questions inherently controversial? How to describe a complex quantum system using a model that is "as simple as possible, but no simpler"? What is a fair test for such a model? Perhaps, instead of asking "who is right and who is wrong?" one should ask "why do we disagree?". Stereoelectronic thinking can reconcile quantum complexity with chemical intuition and build the conceptual bridge between structure and reactivity. Even when many factors contribute to the observed structural and conformational trends, electron delocalization is a dominating force when the electronic demand is high (i.e., bonds are breaking as molecules distort from their equilibrium geometries). In these situations, the role of orbital interactions increases to the extent where they can define reactivity. For example, negative hyperconjugation can unleash the "underutilized" stereoelectronic power of unshared electrons (i.e., the lone pairs) to stabilize a developing positive charge at an anomeric carbon. This analysis paves the way for the broader discussion of the omnipresent importance of negative hyperconjugation in oxygen-containing functional groups. From that point of view, the stereoelectronic component of the anomeric effect plays a unique role in guiding reaction design.

Stereoelectronic power of oxygen in control of chemical reactivity: the anomeric effect is not alone

Although carbon is the central element of organic chemistry, oxygen is the central element of stereoelectronic control in organic chemistry. Generally, a molecule with a C-O bond has both a strong donor (a lone pair) and a strong acceptor (e.g., a σ*_C-O orbital), a combination that provides opportunities to influence chemical transformations at both ends of the electron demand spectrum. Oxygen is a stereoelectronic chameleon that adapts to the varying situations in radical, cationic, anionic, and metal-mediated transformations. Arguably, the most historically important stereoelectronic effect is the anomeric effect (AE), i.e., the axial preference of acceptor groups at the anomeric position of sugars. Although AE is generally attributed to hyperconjugative interactions of σ-acceptors with a lone pair at oxygen (negative hyperconjugation), recent literature reports suggested alternative explanations. In this context, it is timely to evaluate the fundamental connections between the AE and a broad variety of O-functional groups. Such connections illustrate the general role of hyperconjugation with oxygen lone pairs in reactivity. Lessons from the AE can be used as the conceptual framework for organizing disjointed observations into a logical body of knowledge. In contrast, neglect of hyperconjugation can be deeply misleading as it removes the stereoelectronic cornerstone on which, as we show in this review, the chemistry of organic oxygen functionalities is largely based. As negative hyperconjugation releases the "underutilized" stereoelectronic power of unshared electrons (the lone pairs) for the stabilization of a developing positive charge, the role of orbital interactions increases when the electronic demand is high and molecules distort from their equilibrium geometries. From this perspective, hyperconjugative anomeric interactions play a unique role in guiding reaction design. In this manuscript, we discuss the reactivity of organic O-functionalities, outline variations in the possible hyperconjugative patterns, and showcase the vast implications of AE for the structure and reactivity. On our journey through a variety of O-containing organic functional groups, from textbook to exotic, we will illustrate how this knowledge can predict chemical reactivity and unlock new useful synthetic transformations.

Graphical Transition Moment Decomposition and Conceptual Density Functional Theory Approaches to Study the Fundamental and Lower-Level Overtone Absorption Intensities of Some OH Stretching Vibrations

The investigation of electron density migrations caused by molecular structure changes is of central importance in various fields of chemistry. To address this topic in general and to study absorption intensities of vibrations, we analyze sensitive dipole moment functions (DMFs) of a molecule by combining the linear response function of conceptual DFT and bond dipoles separated by the quantum theory of atoms in molecules with a graphical transition moment decomposition scheme. The fundamental intensities of OH stretching vibrations depend strongly on the substituents but only weakly on the molecular conformations. Interestingly, in some alcohols, completely opposite trends have been observed for the lower-level overtone intensities: a weak substituent dependence but a stronger conformation dependence. It is well known that the formation of a hydrogen-bonded complex increases the OH stretching fundamental intensity, but less well known is the decrease in their overtone intensities. To investigate these characteristics comprehensively, we calculated their intensities (Δv = 1, 2, and 3) for conformers of ethanol and trifluoroethanol (TFE) and hydrogen-bonded phenol (PhOH) systems via the DFT method in the local mode model for the OH stretching coordinate ΔR. Their first and second derivatives of the electron density with respect to ΔR were calculated and interpreted using their bond moments. For ethanol and TFE, the OH, CC, and CH bond moments were found to make an important contribution to the molecular DMF derivatives parallel to the OH bond. The OH bond contributes only to the first derivative of DMF, and its conformational dependence is determined by the magnitude of the charge polarization of each structure. The electron density derivatives in the CC bond region were largely maintained during the internal rotation; thus, their conformation-dependent contributions were expressed by a geometrical factor of the CC bond direction. The CH bond at the antiperiplanar position of the OH bond was found to make a remarkably large contribution to the second derivative of DMF in the gauche conformer. The importance of electron density migration on substituents was also identified in the hydrogen-bonded phenol, in which the π-electron density change on the aromatic ring was clearly shown. This migration creates the DMF derivatives both perpendicular and parallel to the OH bond and strongly affects the absorption intensities. In all the cases, some bond moments on the substituents contribute to the first and second DMF derivatives in a structure-dependent manner, thus explaining their stereoelectronic effects.

Probing the anomeric effect and mechanism of isomerization of oxazinane rings by DFT methods

Mechanistic studies of the thermal amine-promoted isomerization of oxazinane rings by DFT methods showed that the isomerization proceeds through abstraction of the C-3 hydrogen atom by the amine nitrogen atom followed by its re-recruitment from C-3 that helps the oxazinane ring to avoid breaking, leading to the same or an isomeric conformer. Calculations also provided evidence that steric effects are responsible for the breaking of the O-N bond in the transition state of the thermal amine-promoted transformations of oxazinane rings, leading to the transformation of the 6-membered ring to a 5-membered ring. Extensive computational studies of the origin of the anomeric effect in the di-substituted oxazinane rings, bearing the EtO substituent at C-6 and CO2Et at C-3, and a series of analogous tetrahydro-2H-pyran ring conformers, revealed that the conformational preferences in both series of compounds are tuned by the balance of non-covalent (weak vDW, dipole-dipole, electrostatic forces, hydrogen bonding) steric effects and hyperconjugative interactions.

On the anomeric preference of the isothiocyanato group

Anomeric effect of the isothiocyanato group has been quantified for the first time in xylopyranose triacetates; both anomers being synthesized.

Catalytic application of sulfamic acid-functionalized magnetic Fe3O4 nanoparticles (SA-MNPs) for protection of aromatic carbonyl compounds and alcohols: experimental and theoretical studies

Protection techniques of functional groups within the structure of organic compounds are important synthetic methods against unwanted attacks from various reagents during synthetic sequences. Acetal and thioacetal groups are well known as protective functional groups in organic reactions. In this study, acetalization of carbonyl compounds with diols and dithiols and methoxymethylation of alcohols with formaldehyde dimethyl acetal (FDMA) have been carried out using sulfamic acid-functionalized magnetic Fe3O4 nanoparticles (SA-MNPs) as a heterogeneous solid acid catalyst. Products were characterized by FT-IR and NMR spectroscopies. The structural and electronic properties of some products were computed by quantum mechanical (QM) methods. Depending on the stereochemistry and electronic properties that were obtained by computational results, we have suggested that hyperconjugation plays a key role in the structural properties of 2-phenyl-1,3-dioxolane derivatives, and also the electron transfer between π-electrons of the aromatic ring with the 3d orbital of S-atoms influences the 2-phenyl-1,3-dithiane derivatives' structure.

Exploring the Origin of the Axial-Conformation Preferences in the 3-Halopiperidinium Cations: the Importance of the Coulombic Potential Energies

Although there are some published conclusions in the literature concerning the origin of the axial-conformation preference in 3-fluoropiperidinium cations (charge-dipole orientation effect), the origin of the axial-conformation preferences in the 3-halopiperidinium cations [halogen = F (1), Cl (2), Br (3)] has remained an open question. To explore the origin of the axial-conformation preferences in compounds 1-3, we assessed the roles and contributions of the hyperconjugative interactions, the Coulombic electrostatic interactions, the electrostatic model associated with dipole-dipole interactions, and the steric effects associated with the Pauli exchange-type repulsions on the conformational properties of compounds 1-3 utilizing the G3MP2, LC-ωPBE, and B3LYP methods and natural bond orbital (NBO) interpretations. Natural Coulombic potential energies are in favor of the axial conformations of compounds 1-3, and justify their corresponding total energy differences. The through-space hyperconjugative interactions between the donor lone pairs of halogen atoms (LP3X) and the acceptor antibonding orbitals of H-N bonds [σ*(H-N) ⊕], LP3X → σ*(H-N) ⊕, increase from compound 1 to compound 3. The inspection of the dipole moments of the parallel C-X and H-N bonds in the axial conformations of compounds 1-3 revealed that the variations of their corresponding four-center dipole-dipole interactions correlate well with their corresponding conformational behaviors. The steric effects associated with the Pauli exchange-type repulsions are strongly in favor of the equatorial conformations of compounds 1-3. Accordingly, the charge-dipole orienting effect associated with the four-center dipole-dipole interactions is a dominant factor in the conformational behaviors of compounds 1-3.

Fluorine-Induced Pseudo-Anomeric Effects in Methoxycyclohexanes through Electrostatic 1,3-Diaxial Interactions

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.

Stereochemistry of Transition Metal Complexes Controlled by the Metallo-Anomeric Effect

The use of stereoelectronic interactions to control reactivity and selectivity has a long history in chemistry. The anomeric effect, one of the fundamental concepts in organic chemistry, describes the preferences of a substituent at the anomeric carbon in glycosides to adopt axial configuration when the anomeric group is an electronegative element such as oxygen or a halogen. The origin of the anomeric effect has been the subject of intense debate. Explanations capitalizing on either the delocalization of the endocyclic oxygen lone pair into the antibonding σ*_(C-X) orbital or the minimization of the dipole-dipole interactions are currently the two leading theoretical models. Although the majority of experimental and theoretical studies have focused on the elements from groups 6 and 7, little is known about conformational preferences of tetrahydropyran rings substituted with a transition metal at the anomeric carbon and the role of these interactions in stereoselective synthesis. Here, we report studies on conformational and configurational preferences of organometallic complexes stabilized by vicinal heteroatoms. We provide computational evidence that late transition metals adopt the axial position in heterocycles or synclinal geometry in acyclic systems. Furthermore, the anomeric preferences of late transition metals correlate with the oxidation state of the metal and can be explained by hyperconjugative interactions between endocyclic heteroatom and the σ* acceptor orbitals of the C-M bond. In a broader context, this discovery provides insight into the role of previously unanticipated stereoelectronic effects that can be harnessed in the design of stereoselective reactions, including chemical glycosylation and enantioselective catalysis.

On the Unusual Synclinal Conformations of Hexafluorobutadiene and Structurally Similar Molecules

An explanation is presented for the unusual conformations of some molecules that contain the C═C-C═C core, namely, butadienes, biphenyls, and styrenes. Small substituents often induce a synclinal conformation, which brings the substituents into close proximity, and sometimes, there is no anticlinal minimum at all. This would not be predicted from steric repulsion arguments nor would it be expected that atoms that are nonbonded in a Lewis structure would approach closer than the sum of their van der Waals radii. Atomic energies calculated according to the quantum theory of atoms in molecules (QTAIM) do not show a consistent pattern for these structurally similar molecules, nor are intersubstituent bond paths consistently found, nor favorable diatomic interaction energies calculated using the interacting quantum atoms (IQA) scheme. Instead, the synclinal conformations are found to be driven by the attraction energy of the electron distribution of the carbon atoms and the nuclei of the molecule.

Intramolecular Photocycloaddition of 2(5 H)-Furanones to Temporarily Tethered Terminal Alkenes as a Stereoselective Source of Enantiomerically Pure Polyfunctionalyzed Cyclobutanes

Allyloxymethyloxymethyl and 4-pentenoyloxymethyl substituents have been used as tethering groups to study the intramolecular [2 + 2] photocycloaddition of chiral 5-substituted 2(5 H)-furanones. The photoreactions proceed in good yield and provide the expected regio- and diastereoselective tricyclic compounds with complementary regioselectivity, which depends on whether the vinyl chain is attached to the furanone by an acetal or an ester linkage. Computational simulations agree with experimental observations and indicate that the origin of the different observed regioselectivity in the intramolecular photochemical reaction of lactones 5 and 6 arises from the relative stability of the initial conformers. The synthetic potential of the enantiomerically pure photoadducts is illustrated by preparing an all- cis 1,2,3-trisubstituted cyclobutane bearing fully orthogonally protected hydroxyl groups.

New Insights into the Origin of the cis-Configuration Preferences in 1,2-Dihaloethenes: The Importance of the Bonding Orbital Deviations

The origin of the preferences for the cis-configurations in 1,2-difluoroethene (1), 1,2-dichloroethene (2), and 1,2-dibromoethene (3) were explored by means of the G3MP2, LC-ωPBE and CCSD(T) methods with the 6–311+G** basis set on all atoms, and natural bond orbital interpretation. On the basis of the results obtained, the cis-configurations preferences decrease in going from compound 1 to compound 3. Effectively, the deletions of the hyperconjugative interactions from the Fock matrices of the cis- and trans-configurations of compound 1 lead to the increase of the trans-conformation stability (by ~6.11 kcal mol−1) compared with its corresponding cis-conformation. However, the Pauli exchange-type repulsion difference between the cis- and trans-configurations of compound 1 is in favour of the trans-configuration (by ~6.25 kcal mol−1), revealing that the stabilization energies associated with the hyperconjugative interactions do not compensate the destabilizations associated with the exchange component and dipole-dipole interactions. Importantly, the C=C bond paths in the cis-configuration of compound 1 are bent in essentially the same direction (towards the C–F bonds), leading to an increased overlap and a stronger C–C bond, whereas the C–C bond paths in the trans-configuration are bent in opposite directions. Accordingly, the co-operative stabilizations associated with the bending of the C=C bond paths (towards the C–F bonds) and total hyperconjugative generalized anomeric effect overcome the destabilizations associated with the exchange component and dipole–dipole interactions, leading to the preference of the cis-configuration in compound 1. The deletions of all the donor–acceptor electronic interactions from the Fock matrices of the cis- and trans-configurations of compounds 2 and 3 lead to the increase of the trans-conformation stabilities compared with their corresponding cis-conformations, revealing the determining impacts of the hyperconjugative interactions on the configurational preferences in compounds 2 and 3.

Exploring the Origin of the Generalized Anomeric Effects in the Acyclic Nonplanar Systems

Contrary to the published conclusions in the literature concerning the origin of the generalized anomeric relationships in open-chain nonplanar systems, its origin has remained an open question. In order to explore the origin of the generalized anomeric relationships in open-chain nonplanar systems, we assessed the roles and contributions of the effective factors on the conformational properties of methyl propargyl ether (1), methyl propargyl sulfide (2), and methyl propargyl selenide (3) by means of the G3MP2, CCSD(T), MP2, LC-ωPBE, and B3LYP methods and natural bond orbital (NBO) interpretations. We examined the contributions of the hyperconjugative interactions on the conformational preferences of compounds 1-3 by the deletions of the orbitals overlapping from the Fock matrices of the gauche- and anti-conformations. The trend observed for energy changes in the Fock matrices justify the variations of the gauche-conformations preferences going from compound 1 to compound 3, revealing that the hyperconjugative interactions are solely responsible for the generalized anomeric relationships in compounds 1-3. Accordingly, the conclusions published in the literature concerning the origin of the generalized anomeric effect in the acyclic nonplanar compounds should be revised by these findings. The Pauli exchange type repulsions (PETR) are in favors of the gauche-conformations and the variations of the PETR differences between the gauche- and anti-conformations of compounds 1-3 correlate well with their gauche-conformations preferences, revealing that the generalized anomeric relationships in compounds 1-3 have also the Pauli exchange-type repulsions origin. The resemblance between the preorthogonal natural bond orbitals (that are involved in the hyperconjugative interactions) and their corresponding molecular orbitals have been investigated.

Assessing the effective factors affecting the conformational preferences and the early and late transition states of the unimolecular retro-ene decomposition reactions of ethyl cyanate, ethyl thiocyanate and ethyl selenocyanate

The variations of Δ[(HCGAE(X₃–C₄weakening) – HCGAE(X₃–C₄strengthening)] parameters correlate well with the variations of the retro-ene decomposition reactions barrier heights going from compound1to compound3.

The Importance of the Pauli Exchange-Type Repulsions and Hyperconjugative Interactions on the Conformational Properties of Halocarbonyl Isocyanates and Halocarbonyl Azides

To gain further insight into the origin of the anomeric relationships in planar open-chain (acyclic) compounds, we examined the effects of the hyperconjugative generalized anomeric effect (HC-GAE), Pauli exchange-type repulsion (PETR), the electrostatic model associated with the dipole–dipole interactions (EM-DDI), and the attractive electrostatic interactions (AEI) between the natural atomic charges (NACs) on the conformational properties of halocarbonyl isocyanates [halogen = F (1), Cl (2), Br (3)] and halocarbonyl azides [halogen = F (4), Cl (5), Br (6)] by means of G3MP2, CCSD, MP2, and B3LYP methods with the 6–311+G** basis set on all atoms and natural bond orbital interpretation. Importantly, the deletions of the through bond LPN3→σ*C2–X6 hyperconjugative interactions from the Fock matrices of the cis- and trans-conformations lead to the increase of the cis-conformations’ stability compared with their corresponding trans-conformations going from compound 1 to 3 and from compound 4 to 6, revealing the determining effects on the conformational preferences in compounds 1–3 and 4–6. Essentially, the effects of the through space (LPN3→σ*C4–O5 and LPNα→π*Nβ=Nω, respectively) hyperconjugative interactions on the conformational preferences in the isocyanate (1–3) and azide compounds (4–6) are negligible. The EM-DDI fails to account for the conformational preferences in compounds 2, 3, 5, and 6. Therefore, the generalized anomeric relationships in compounds 1–3 and 4–6 result from the cooperative effects of the HC-GAE and PETR. The variations of the AEIs revealed their opposite effects on the trend observed for the conformational preferences in compounds 1–3 and 4–6. Contrary to the usual assumption, the much larger barrier heights of the rotation around the C2–N3 bonds in the azide compounds (4–6) compared with those in the isocyanate compounds (1–3) result from the exchange components and have no hyperconjugative origin.

Exploring the origin of the anomeric relationships in 2-cyanooxane, 2-cyanothiane, 2-cyanoselenane and their corresponding isocyano isomers. Correlations between hyper-conjugative anomeric effect, hardness and electrostatic interactions

The impacts of the HCAE, hardness, PETR and electrostatic interactions on the anomeric relationships in 2-cyanooxane, -thiane, -selenane and their iso-cyano isomers have been explored.

Excluding hyperconjugation from the Z conformational preference and investigating its origin: formic acid and beyond

A scheme indicating that the preference for the Z conformer in proteins is chemically equivalent to that of amides. Other compounds, such as carboxylic acids, also exhibit the same conformational trend.

The origin of the anomeric effect: probing the impacts of stereoelectronic interactions

To gain further insight on the origin of the anomeric effect, the correlations between SE, EM, PETR, bond-orders, donor and acceptor orbital energies and occupancies, structural parameters and configurational behavior of dihalo-1,4-oxathianes were investigated.

Conformational behaviors of trans-2,3- and trans-2,5-dihalo-1,4-diselenanes. A complete basis set, hybrid-density functional theory study and natural bond orbital interpretations

Complete basis set CBS-4, hybrid-density functional theory (hybrid-DFT: B3LYP/6-311+G**) based methods and natural bond orbital (NBO) interpretations have been used to examine the contributions of the hyperconjugative, electrostatic, and steric effects on the conformational behaviors of trans-2,3-dihalo-1,4-diselenane [halo = F (1), Cl (2), Br (3)] and trans-2,5-dihalo-1,4-diselenane [halo = F (4), Cl (5), Br (6)]. Both levels of theory showed that the axial conformation stability, compared to its corresponding equatorial conformation, decreases from compounds 1 → 3 and 4 → 6. Based on the results obtained from the NBO analysis, there are significant anomeric effects for compounds 1-6. The anomeric effect associated with the electron delocalization is in favor of the axial conformation and increases from compounds 1 → 3 and 4 → 6. On the other hand, dipole moment differences between the axial and equatorial conformations [Δ(μ(eq)-μ(ax)] decrease from compounds 1 → 3. Although Δ(μ(eq)-μ(ax)) parameter decreases from compound 1 to compound 3, the dipole moment values of the axial conformations are smaller than those of their corresponding equatorial conformations. Therefore, the anomeric effect associated with the electron delocalizations (for halogen-C-Se segments) and the electrostatic model associated with the dipole-dipole interactions fail to account for the increase of the equatorial conformations stability on going from compound 1 to compound 3. Since there is no dipole moment for the axial and equatorial conformations of compounds 4-6, consequently, the conformational preferences in compounds 1-6 is in general dictated by the steric hindrance factor associated with the 1,3-syn-axial repulsions. Importantly, the CBS-4 results show that the entropy difference (∆S) between the equatorial axial conformations increases from compounds 1 → 3 and 4 → 6. This fact can be explained by the anomeric effect associated with the electron delocalization which affects the C₂-Se bond orders and increase the rigidity of the corresponding rings. The Gibbs free energy difference values between the axial and equatorial conformations (i.e. ΔG(ax-ax) and ΔG(eq-eq)) of compounds 1 and 4, 2 and 5 and also 3 and 6 have been calculated. The correlations between the anomeric effect, electrostatic model, ΔG(eq-ax), ΔG(ax-ax), ΔG(eq-eq), bond orders, dipole-dipole interactions, structural parameters and conformational behaviors of compounds 1-6 have been investigated.

How solvent influences the anomeric effect: roles of hyperconjugative versus steric interactions on the conformational preference

The block-localized wave function (BLW) method, which can derive optimal electron-localized state with intramolecular electron delocalization completely deactivated, has been combined with the polarizable continuum model (PCM) to probe the variation of the anomeric effect in solution. Currently both the hyperconjugation and electrostatic models have been called to interpret the anomeric effect in carbohydrate molecules. Here we employed the BLW-PCM scheme to analyze the energy differences between α and β anomers of substituted tetrahydropyran C5OH9Y (Y = F, Cl, OH, NH2, and CH3) and tetrahydrothiopyran C5SH9Y (Y = F, Cl, OH, and CH3) in solvents including chloroform, acetone, and water. In accord with literature, our computations show that for anomeric systems the conformational preference is reduced in solution and the magnitude of reduction increases as the solvent polarity increases. Significantly, on one hand the solute-solvent interaction diminishes the intramolecular electron delocalization in β anomers more than in α anomers, thus destabilizing β anomers relatively. But on the other hand, it reduces the steric effect in β anomers much more than α anomers and thus stabilizes β anomers relatively more, leading to the overall reduction of the anomeric effect in anomeric systems in solutions.

Probing stereoelectronic interactions in an O-N-O unit by the atomic energies: experimental and theoretical electron density study

Stereoelectronic interaction lp(O1) → σ*(N1-O2) in a O-N-O unit was analyzed by means of R. Bader's Atoms in Molecule theory on the basis of X-ray diffraction data for dimethyl-(2R,4aR,5S,7R)-2,5,7-triphenylhexahydro-4H-[1,2]oxazino[2,3-b][1,2]oxazine-4,4-dicarboxylate. Atomic energies obtained by applying this approach to both the experimental and theoretical electron densities were used to probe the energy of this strong stereoelectronic interaction, giving consistent results with the NBO analysis, although showing its destabilizing character.