Density Functional Theory Computational Reexamination of the Anomeric Effect in 2-Methoxy- and 2-Cyano-1,3-dioxanes and 1,3-Dithianes. Stereoelectronic Interactions Involving the Cyano (C≡N:) Group Revealed by Natural Bond Orbital (NBO) Analysis

Conformational Analysis Explores the Role of Electrostatic Nonclassical CF···HC Hydrogen Bonding Interactions in Selectively Halogenated Cyclohexanes

The conformational equilibria of selectively halogenated cyclohexanes are explored both experimentally (VT-NMR) for 1,1,4,-trifluorocyclohexane 7 and by computational analysis (M06-2X/aug-cc-pVTZ level), with the latter approach extending to a wider range of more highly fluorinated cyclohexanes. Perhaps unexpectedly, 7ax is preferred over the 7eq conformation by ΔG = 1.06 kcal mol-1, contradicting the accepted norm for substituents on cyclohexanes. The axial preference is stronger again in 1,1,3,3,4,5,5,-heptafluorocyclohexane 9 (ΔG = 2.73 kcal mol-1) as the CF2 groups further polarize the isolated CH2 hydrogens. Theoretical decomposition of electrostatic and hyperconjugative effects by natural bond orbital analysis indicated that nonclassical hydrogen bonding (NCHB) between the C-4 fluorine and the diaxial hydrogens at C-2 and C-6 in cyclohexane 7 and 9 largely accounts for the observed bias. The study extended to changing fluorine (F) for chlorine (Cl) and bromine (Br) at the pseudoanomeric position in the cyclohexanes. Although these halogens do not become involved in NCHBs, they polarize the geminal -CHX- hydrogen at the pseudoanomeric position to a greater extent than fluorine, and consequent electrostatic interactions influence conformer stabilities.

Anomeric and Perlin Effect Ladders for 2-Substituted 2-Fluorotetrahydro-2H-pyrans Using Sensitive Structural, Energetic, and NMR Probes

A comprehensive exploration of the anomeric and Perlin effects in a series of 2-substituted-2-fluorotetrahydro-2H-pyrans employing sensitive structural, energetic, and NMR probes calculated by density functional theory (DFT) and natural bond orbital (NBO) approaches is undertaken. We used a wide variety of X substituents exhibiting diverse electronic features (σ-donors, σ-donors/π-donors, and σ-donors/π-acceptors). It is noteworthy that a group of 8 substituents (NHMe, SCN, OBr, NO₃, OCl, Cl, OF, and Br) favor the axial conformations, while a group of 13 substituents (NO₂, NF₂, NH₂, CN, SH, H, N₃, B(OH)₂, OMe, BH₂, OH, Me, and Ph) favor the equatorial conformations. Interestingly, a group of 6 substituents (NH₂, NHMe, NF₂, OF, OCl, and OBr) are responsible for the observed normal Perlin effect while the remaining 16 substituents induce the reverse Perlin effect. An exhaustive investigation of possible correlations of anomeric and Perlin effect descriptors with structural, energetic, one-bond ¹J_C-F couplings and the nucleophilicity of X descriptors demonstrated the general relationship of the Perlin and anomeric effects and manifested their common stereoelectronic origin.

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.

Computational Study of the Donor-Acceptor Interactions Underlying the Variable Oxygen Probe

The variable oxygen probe (VOP) is a crystallographic technique that has been used to explore the relative donor abilities of various filled orbitals ranging from vicinal lone pairs to polarized heteroatom-carbon bonds, remote π functionalities, and strained carbon-carbon (CC) bonds. In this study, the donor-acceptor interactions which underlie the VOP have been explored in the gas phase using density functional theory on the model systems 1-13 with natural bond orbital analysis of the various donor-acceptor interactions involving both neutral and charged σ* antibonding orbitals as the acceptor probes. Updated values for the VOP slopes of 1-13 were shown to relate qualitatively with the sum of all significant donor-acceptor interactions present in these derivatives. Application of the VOP to calculated structures of 1-13 with various -OR substituents revealed a similar relationship between the C-OR bond distance to pK_a (ROH). However, the VOP slopes in the gas phase were significantly smaller in magnitude than those obtained from crystal structural data, likely due to the valence form (C⁺-OR) being disfavored in the former, highlighting the advantage of the VOP as an experimental technique to discriminate donor ability more effectively than calculated structures.

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.

Gold-catalyzed domino cyclization enabling construction of diverse fused azaspiro tetracyclic scaffolds: a cascade catalysis mechanism due to a substrate and counterion

The detailed mechanism and origins of gold-catalyzed domino cyclization to diverse fused azaspiro tetracyclic scaffolds by cooperative dual catalysis and cascade catalysis are systematically studied.

Electronic Donation or Steric Contraction: A Spectroscopic and Structural Analysis of Medium-Sized Constrained Rings for Potential Long-Range Hyperconjugation

Herein, we report the ¹J_CH analyses, natural bond orbital analyses, and X-ray crystal structures of a number of C, O, and N constrained tricyclic cycles. These experiments provide access into the nature of the apparent Perlin effect previously reported in constrained tricyclic cycles, as well as evidence suggesting both steric contraction and long-range hyperconjugation account for the observed ¹J_CH perturbations. We report a true Perlin effect of 10.9 Hz in an azocane and large steric effect resulting in Δ¹J_C-H = 10.9 Hz in a cyclooctane.