Lone-pair orientation and the magnitude of one-bond CHcoupling adjacent to oxygen and sulfur

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.

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.

Investigation of the role of stereoelectronic effects in the conformation of piperidones by NMR spectroscopy and X-ray diffraction

This paper reports the synthesis of a series of piperidones 1-8 by the Mannich reaction and analysis of their structures and conformations in solution by NMR and mass spectrometry. The six-membered rings in 2,4,6,8-tetraphenyl-3,7-diazabicyclo[3.3.1]nonan-9-ones, compounds 1 and 2, adopt a chair-boat conformation, while those in 2,4-diphenyl-3-azabicyclo[3.3.1]nonan-9-ones, compounds 3-8, adopt a chair-chair conformation because of stereoelectronic effects. These stereoelectronic effects were analyzed by the (1) J C-H coupling constants, which were measured in the (13)C satellites of the (1)H NMR spectra obtained with the hetero-dqf pulse sequence. In the solid state, these stereoelectronic effects were investigated by measurement of X-ray diffraction data, the molecular geometry (torsional bond angles and bond distances), and inter- and intramolecular interactions, and by natural bond orbital analysis, which was performed using density functional theory at the ωB97XD/6311++G(d,p) level. We found that one of the main factors influencing the conformational stability of 3-8 is the interaction between the lone-pair electrons of nitrogen and the antibonding sigma orbital of C(7)-Heq (nN→σ*C-H(7)eq), a type of hyperconjugative interaction.

Manifestations of stereoelectronic interactions in 1JC-H one-bond coupling constants

Two-electron/two-orbital hyperconjugative interactions depend on the relative orientation of bonds and lone pairs in a molecule and are also inversely proportional to the energy difference between the interacting orbitals. Spectroscopic manifestations of stereoelectronic interactions are particularly useful experimental signatures of these effects which can be utilized for testing molecular models. Empirical observations together with theoretical interpretations in cyclohexane and six-membered heterocycles confirm the relevance of sigma C-H ax --> sigma* C-H ax , n X --> sigma* C-H ax (X = O or N), sigma C-S --> sigma* C-H eq , beta-n O --> sigma* C-H eq , sigma C(2)-H ax --> pi* CY (Y = O, S, or CH 2), and sigma C(2)-H ax --> sigma* S-O ax two-electron/two-orbital stereoelectronic interactions that weaken the acceptor (or donor) C-H bonds and attenuate the Fermi contribution to the one-bond (13)C- (1)H coupling constants.

Stereoelectronic Effects in Cyclic Sulfoxides, Sulfones, and Sulfilimines: Application of the Perlin Effect to Conformational Analysis

The 1JC--H coupling constants in conformationally constrained sulfoxides, bissulfoxides, sulfoxide-sulfones, and sulfilimines derived from 2-benzylidene-1,3-dithiane and 2-(2,2-dimethylpropylidene)-1,3-dithiolane were measured by means of HMQC and HSQC NMR experiments and the Perlin effects were calculated. The type and the relative configuration of S==X groups (X= O, NTos) in these compounds have a strong influence on the magnitude of coupling constants for axial and equatorial C--H bonds, respectively. Axial S==O bonds give rise to a stereoelectronic effect on antiperiplanar axial C--H bonds. The resultant weakening of the respective C--H bonds leads to a smaller coupling constant than for a respective equatorial C--H bond. Equatorial S==O groups have an influence on beta-C--H bonds through a homoanomeric effect. Here, the axial C--H bond is weakened and a smaller coupling constant is measured. Sulfilimine groups show similar effects to sulfoxide groups.

Rehybridization as a general mechanism for maximizing chemical and supramolecular bonding and a driving force for chemical reactions

Dynamic variations in hybridization patterns (rehybridization) were analyzed at B3LYP/6-31G** and MP2/6-31+G* levels. Computations clearly illustrate the generality of rehybridization in a variety of chemical phenomena, which involve structural reorganization in hydrogen-bonded complexes, nonhyperconjugative stereoelectronic effects in saturated heterocycles, Mills-Nixon effect, and contrasting substituent effects in cycloaromatization reactions.

Manifestation of Stereoelectronic Effects on the Calculated Carbon−Hydrogen Bond Lengths and One-Bond 1JC-H NMR Coupling Constants. Relative Acceptor Ability of the Carbonyl (CO), Thiocarbonyl (CS), and Methylidene (CCH2) Groups toward C−H Donor Bonds

Theoretical examination [B3LYP/6-31G(d,p), PP/IGLO-III//B3LYP/6-31G(d,p), and NBO methods] of six-membered cyclohexane 1 and carbonyl-, thiocarbonyl-, or methylidene-containing derivatives 2-27 afforded precise structural (in particular, C-H bond distances) and spectroscopic (specifically, one-bond (1)J(C)(-)(H) NMR coupling constants) data that show the consequences of stereoelectronic hyperconjugative effects in these systems. Major observations include the following. (1) sigma(C)(-)(H)(ax)() -->(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() (Y = O, S, or CH(2)) hyperconjugation leads to a shortening (strengthening) of the equatorial C-H bonds adjacent to the pi group. This effect is reflected in smaller (1)J(C)(-)(H)(ax)() coupling constants relative to (1)J(C)(-)(H)(eq)(). (2) Comparison of the structural and spectroscopic consequences of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) hyperconjugation in cyclohexanone 2, thiocyclohexanone 3, and methylenecyclohexane 4 suggests a relative order of acceptor orbital ability C=S > C=O > C=CH(2), which is in line with available pK(a) data. (3) Analysis of the structural and spectroscopic data gathered for heterocyclic derivatives 5-12 reveals some additivity of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y), pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)(), n(X) --> sigma(C)(-)(H)(ax)(), n(beta)(O) --> sigma(C)(-)(H)(eq)(), and sigma(S)(-)(C) --> sigma(C)(-)(H)(eq)() stereoelectronic effects that is, nevertheless, attenuated by saturation effects. (4) Modulation of the C=Y acceptor character of the exocyclic pigroup by conjugation with alpha-heteroatoms O, N, and S in lactones, lactams, and methylidenic analogues 13-24 results in decreased sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() hyperconjugation. (5) Additivity of sigma(C)(-)(H)(ax)() --> pi(C)(=)(Y) and pi(C)(=)(Y) --> sigma(C)(-)(H)(ax)() hyperconjugative effects is also apparent in 1,3-dicarbonyl derivative 25 (C=Y equal to C=O), 1,3-dithiocarbonyl derivative 26 (C=Y equal to C=S), and 1,3-dimethylidenic analogue 27 (C=Y equal to C=CH(2)).

Homoanomeric Effects in Six-Membered Heterocycles

Structural and energetic consequences of homoanomeric n(X) --> beta-sigma(C-Y) interactions in saturated six-membered heterocycles where X = O, N, S, Se and Y = H, Cl were studied computationally using a combination of density functional theory (B3LYP) and Natural Bond Orbital (NBO) analysis. Unlike the classic anomeric effect where the interacting donor and acceptor orbitals are parallel and overlap sidewise in a pi-fashion, orbital interactions responsible for homoanomeric effects can follow different patterns imposed by the geometric restraints of the respective cyclic moieties. For the equatorial beta-C-Y bonds in oxa-, thia- and selena-cyclohexanes, only the homoanomeric n(X)(ax) --> sigma(C-Y)(eq) interaction (the Plough effect) with the axial lone pair of X is important, whereas the n(X)(eq) --> sigma(C-Y)(eq) interaction (the W-effect) is negligible. On the other hand, the W-effect is noticeably larger than the n(X)(ax) --> sigma(C-Y)(eq) interaction in azacyclohexanes. Hyperconjugation is a controlling factor which determines relative trends in the equatorial beta-C-H bonds in heterocycloxanes. In contrast, all homoanomeric interactions are weak for the respective axial bonds where relative lengths are determined by intramolecular electron transfer through exchange interactions and polarization-induced rehybridization. Although the homoanomeric effects are considerably weaker than the classic vicinal anomeric n(X)(ax)-->alpha-sigma(C-Y)(ax) interactions, their importance increases significantly when the acceptor ability of sigmaorbitals increases as a result of bond stretching and/or polarization. Depending on the number of electrons and the topology of interactions, homoconjugation interactions can be cooperative (enhance each other) or anticooperative (compete with each other). Such effects reflect symmetry of the wave function and can be considered as weak manifestations of sigma homoaromaticity or homoantiaromaticity.

Manifestation of stereoelectronic effects on the calculated carbon-hydrogen bond lengths and one bond (1)J(C-H) NMR coupling constants in cyclohexane, six-membered heterocycles, and cyclohexanone derivatives

Cyclohexane (1), oxygen-, sulfur-, and/or nitrogen-containing six-membered heterocycles 2-5, cyclohexanone (6), and cyclohexanone derivatives 7-16 were studied theoretically [B3LYP/6-31G(d,p) and PP/IGLO-III//B3LYP/6-31G(d,p) methods] to determine the structural (in particular C-H bond distances) and spectroscopic (specifically, one bond (1)J(C-H) NMR coupling constants) consequences of stereoelectronic hyperconjugative effects. The results confirm the importance of n(X) --> sigma*(C-H)(app) (where X = O, N), sigma(C-H)(ax) --> pi*(C=O), sigma(S-C) --> sigma*(C-H)(app), sigma(C-S)-->sigma*(C-H)(app), beta-n(O) --> sigma*(C-H), and sigma(C-H) --> sigma*(C-H)(app) hyperconjugation, as advanced in previous theoretical models. Calculated r(C-H) bond lengths and (1)J(C-H) coupling constants for C-H bonds participating in more than one hyperconjugative interaction show additivity of the effects.

Stereoelectronic interactions in cyclohexane, 1,3-dioxane, 1, 3-oxathiane, and 1,3-dithiane: W-effect, sigma(C)(-)(X) <--> sigma(C)(-)(H) interactions, anomeric effect-what is really important?

Stereoelectronic effects proposed for C-H bonds in cyclohexane, 1, 3-dioxane, 1,3-oxathiane, and 1,3-dithiane were studied computationally. The balance of three effects, namely, sigma(C)(-)(X) --> sigma(C)(-)(H)()eq, sigma(C)(-)(H)()eq --> sigma(C)(-)(X), and n(p)(X) --> sigma(C)(-)(H)()eq interactions, was necessary to explain the relative elongation of equatorial C(5)-H bonds. The role of homoanomeric n(p) --> sigma(C(5))(-)(H)()eq interaction is especially important in dioxane. In dithiane, distortion of the ring by long C-S bonds dramatically increases overlap of sigma(C(5))(-)(H)()eq and sigma(C)(-)(S) orbitals and energy of the corresponding hyperconjugative interaction. Anomeric n(p)(X) --> sigma(C)(-)(H)()ax interactions with participation of axial C-H bonds dominate at C(2), C(4), and C(6). The balance of hyperconjugative interactions involving C-H(ax) and C-H(eq) bonds agrees well with the relative bond lengths for all C-H(ax)/C-H(eq) pairs in all studied compounds. At the same time, the order of one-bond spin-spin coupling constants does not correlate with the balance of stereoelectronic effects in dithiane and oxathiane displaying genuine reverse Perlin effect.