Hybrid-DFT Study and NBO Analysis of the Stereoelectronic Interaction Effects (Associated with the Anomeric Effects) on the Conformational Properties of 2,3,5,6-Tetrahalo-1,4-dioxanes and Their Analogs Containing S and Se Atoms

NBO analysis and hybrid density functional theory–based method (B3LYP/6-311+G**) was used to study the anomeric effects (AE), dipole–dipole interactions, and steric repulsion effects on the conformational properties of 2,3,5,6-tetrahalo-1,4-dioxane [halo = F (1), Cl (2), Br (3)], 2,3,5,6-tetrahalo-1,4-dithiane [halo = F (4), Cl (5), Br (6)], and 2,3,5,6-etrahalo-1,4-diselenane [halo = F (7), Cl (8), Br (9)]. B3LYP/6-311+G** results revealed a strong axial preference in compounds 1–3. Gibbs free energy difference (G _eq–G _ax) values (e.g., ΔG _eq-ax) between the axial and equatorial conformations of compound 1 to compound 3 are 8.19, 3.86, and 3.13 kcal mol^?1, respectively, as calculated by the B3LYP/6-311+G** level of theory. On the other hand, the NBO analysis of donor–acceptor (bond–antibond) interactions revealed that the AE for compounds 1–3 are ?12.26, ?16.46, and ?18.11 kcal mol^?1, respectively. Contrary to the increase of the AE values from compound 1 to compound 3, the increase of the steric repulsions (e.g., 1,3-syn-axial repulsions) could fairly explain the decrease of the axial conformation stability in compounds 1–3 compared to their equatorial conformations. Further, the correlations between the AE, structural parameters, and conformational behavior of compounds 4–9 have been investigated.     

Conformational Behaviors of 2-Substituted Cyclohexanone Oximes: An AB Initio,Hybrid Dft Study,and NBO Interpretation

Abstract

The impacts of the generalized anomeric effect (GAE) and gauche effect (GE) associated with donor–acceptor electron delocalizations and dipole–dipole interactions on the conformational properties of 2-methoxy- (1), 2-methylthio- (2), 2-methylseleno- (3), 2-fluoro- (4), 2-chloro- (5), and 2-bromocyclohexanone oxime (6) have been studied by means of hybrid density functional theory (B3LYP/6–311+G**) and ab initio molecular orbital (HF/6–311+G**)-based methods and natural bond orbital (NBO) interpretation. Both methods used showed that the above compounds exist predominantly in the axial chair conformation and the axial conformation stability increased from 2-methoxy- (1) to 2-methylselenocyclohexanone oxime (3) and also from 2-fluoro- (4) to 2-bromocyclohexanone oxime (6). The NBO analysis showed that the GAE increases from compound 1 to compound 3 and also from compound 4 to compound 6. GE does not have significant impact on the conformational behaviors of compounds 1–6 and GAE succeeds in accounting qualitatively for the increase of the axial preferences in both series of compounds.

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements for the following free supplemental resource: Table S1.

Supporting Information Available. The structures optimized and thermodynamic functions of the axial and equatorial conformations of compounds 1–6. This material is available free of charge via the Internet.     

Molecular structure and conformational preferences of 1-bromo-1-silacyclohexane, CH2(CH2CH2)2SiH-Br, as studies by gas-phase electron diffraction and quantum chemistry

The molecular structure of axial and equatorial conformer of the 1-bromo-1-silacyclohexane molecule, CH₂(CH₂CH₂)₂SiH-Br, as well as thermodynamic equilibrium between these species are investigated by means of gas-phase electron diffraction and quantum chemistry on the MP2(full)/SDB-AUG-cc-PVTZ level of theory. It is revealed that according to electron diffraction data, the compound exists in the gasphase as a mixture of conformers possessing the chair conformation of the six-membered ring and C _s symmetry and differing in the axial and equatorial position of the Si-Br bond (ax. = 80(5) mol %, eq. = 20(7) mol %) at 352 K, that corresponds to the value of A = (G _ax ^?? ? G _eq ^?? ) = ?0.82(32) kcal/mol. It is found that observed data agree well with theoretical ones. Using Natural Bond Orbital (NBO) analysis it is revealed that axial conformer of 1-bromo-1-silacyclohexane molecule is an example of the stabilization of the form that is unfavorable from the point of view of steric effects and effects of conjugations. It is concluded that stabilization is achieved due to electrostatic interactions.     

Molecular conformations of jet-cooled 2-methylindan and 2-phenylindan

The molecular conformations of jet-cooled 2-methylindan (2MI) and 2-phenylindan (2PI) have been studied using resonant-enhanced two-photon ionization spectroscopy in combination with ab initio calculations. Both axial (2MI_ax) and equatorial (2MI_eq) conformers of 2MI have been observed. A 2MI_eq/2MI_ax conformer ratio of 2.3 was estimated at 298 K, leading to the energy difference, \Updelta E = E _{2 \textMI\textax} - E _{2 \textMI\texteq} \Updelta E = E_{{ 2 {\text{MI}}_{\text{ax}} }} - E_{{ 2 {\text{MI}}_{\text{eq}} }} , of 0.49 kcal/mol. Ab initio calculations predicted three stable conformers of 2PI: two equatorial conformers (2PI_eq0 and 2PI_eq90), and one axial conformer (2PI_ax). Only the axial conformer of 2PI (2PI_ax) was experimentally observed. The indan ring of 2PI_ax is slightly more planar than the indan rings of the two equatorial conformers of 2PI because of the intramolecular C_sp2–H/π interactions in 2PI_ax. The equatorial conformers of 2PI relax to the more stable axial conformer because of the high pre-expansion temperature (383 K), and relatively low barrier (1.68 kcal/mol) to axial–equatorial interconversion. The barrier (2.33 kcal/mol) to axial–equatorial interconversion in 2MI is high enough to prevent conformational relaxation at the pre-expansion temperature of 298 K. Intramolecular C–H/π interactions are found to be more important in determining the conformational preference of 2PI than 2MI; this can be attributed to the higher acidity of the C_sp2–H bond than that of C_sp3–H bond.     

Complete basis set,hybrid-DFT study,and NBO interpretations of the conformational behavior of 1,2-dihaloethanes

The conformational behavior of 1,2-difluoroethane (1), 1,2-dichloroethane (2), 1,2-dibromoethane (3), and 1,2-diiodoethane (4) have been analyzed by means of complete basis set CBS-QB3, hybrid-density functional theory (B3LYP/Def2-TZVPP) based methods and natural bond orbital (NBO) interpretation. Both methods showed the expected greater stability of the gauche conformation of compound 1 compared to its anti conformation. Contrary to compound 1, the anti conformations of compounds 2–4 are more stable than their gauche conformation. The stability of the anti conformation compared to the gauche conformation increases from compound 1 to compound 4. The NBO analysis of donor–acceptor (σ → σ*) interactions showed that the generalized anomeric effect (GAE) is in favor of the gauche conformation of compound 1. Contrary to compound 1, GAE is in favor of the anti conformations of compounds 2–4. The GAE values calculated (i.e., GAE_anti − GAE_gauche) increase from compound 1 to compound 4. On the other hand, the calculated dipole moment values for the gauche conformations decrease from compound 1 to compound 4. In the conflict between the GAE and dipole moments, the former succeeded in accounting for the increase of the anti conformation stability from compound 1 to compound 4. There is a direct correlation between the calculated GAE, ∆[r _c–c(G) − r _c–c(A)] and ∆[r _c–x(A) − r _c–x(G)] parameters. The correlations between the GAE, bond orders, total steric exchange energies (TSEEs), ΔG _{Anti–Gauche}, ΔG ^‡(Gauche → Gauche′, C _2v), ΔG ^‡(Anti → Gauche, C ₂), dipole–dipole interactions, structural parameters, and conformational behaviors of compounds 1–4 have been investigated.     

Conformational preferences of Si–Ph,H and Si–Ph,Me silacyclohexanes and 1,3-thiasilacyclohexanes. Additivity of conformational energies in 1,1-disubstituted heterocyclohexanes

The conformational equilibria of 1-phenyl-1-silacyclohexane 1, 3-phenyl-1,3-thiasilacyclohexane 2, 1-methyl-1-phenyl-1-silacyclohexane 3, and 3-methyl-3-phenyl-1,3-thiasilacyclohexane 4 have been studied for the first time by low temperature ¹³C NMR spectroscopy at 103 K. Predominance of the equatorial conformer of compound 1 (Ph_eq/Ph_ax=78%:22%) is much less than in its carbon analog, phenylcyclohexane (nearly 100% of Ph_eq). And in contrast to 1-methyl-1-phenylcyclohexane, the conformers with the equatorial Ph group are predominant for compounds 3 and 4: at 103 K, Ph_eq/Ph_ax ratios are 63%:37% (3) and 68%:32% (4). As the Si–C bonds are elongated with respect to C–C bonds, the barriers to ring inversion are only between 5.2–6.0 (ax→eq) and 5.4–6.0 (eq→ax) kcal mol^?1. Parallel calculations at the DFT and MP2 level of theory (as well as the G2 calculations for compound 1) show qualitative agreement with the experiment. The additivity/nonadditivity of conformational energies of substituents on cyclohexane and silacyclohexane derivatives is analyzed. The geminally disubstituted cyclohexanes containing a phenyl group show large deviations from additivity, whereas in 1-methyl-1-phenyl-1-silacyclohexane and 3-methyl-3-phenyl-1,3-thiasilacyclohexane the effects of the methyl and phenyl groups are almost additive. The reasons for the different conformational preferences in carbocyclic and heterocyclic compounds are analyzed using the homodesmotic reactions approach.     

An Ab Initio Study and NBO Analysis of the Stability and Conformational Properties of Hexakis(trimethylelementhyl)benzene (Element = C,Si, Ge,and Sn)

Ab initio molecular orbital and density functional theory were used to investigate energetic and structural properties of the various conformations of hexa-tertbutylbenzene (1), hexakis(trimethylsilyl)benzene (2), hexakis (trimethylgermyl)benzene (3), and hexakis(trimethylstannyl)benzene (4). HF/3-21G//HF/3-21G and B3LYP/3-21G//HF/3-21G results revealed that the Twist-Boat (TB) conformer of compound 1 is more stable than the 1-Chair (C), 1-Boat (B), and 1-Planar (P) conformers. B3LYP/3-21G//HF/3-21G results show that the 1- TB conformer is more stable than 1- C, 1- B, and 1- P conformers of about 1.13, 4.34, and 99.94 kcal mol^?1 , respectively. Contrary to the stability order of compound 1 conformers, the C conformer of compounds 2–4 is more stable than TB, B, and P conformations, as calculated by B3LYP/3-21G//HF/3-21G and HF/3-21G//HF/3-21G levels of theory. The energy gap between the C and P conformers in compounds 1–4 is decreased in the following order: ΔE(4: C, P) < ΔE (3: C, P) < ΔE(2: C, P) < ΔE (1: C, P). This fact can be explained in terms of the increase of C _aromatic -M (M═C, Si, Ge, and Sn) bond lengths and the decrease of steric (van der Waals) repulsions in the previously discussed compounds. For compounds 1–3, the calculations were also performed at the B3LYP/ 6-31G*//HF/3-21G level of theory. However, the comparison showed that the results at B3LYP/3-21G//HF/3-21G methods correlated well with those obtained at the B3LYP/6-31G*// HF/6-31G method. Further, NBO analysis revealed that in compounds 1–4, the resonance energy associated with the σ_M-C1 to σ*_C2-C3 delocalization is 5.20, 9.68, 11.15, and 12.27 kcal mol^?1, respectively. These resonance energy values could explain the easiness of the ring flipping processes of C, B, and TB conformers of compounds 4 to 1. Also, the NBO results showed that by an increase of the σ_M-C1 → σ *_C2-C3 resonance energies in compounds 1–4, the σ_M-C1 bonding orbital occupancies decrease. This fact could fairly explain the increase of the C_aryl-M bond length from compound 1 to 4. The NBO results are also in good agreement with the calculated energy barriers for the ring flipping of the chair conformations in compounds 1–4, as calculated by B3LYP and HF methods.     

Stereoelectronic Properties,Stereospecificity and Stabilization of α-Seleno Carbanions. An ab initio Study

An ab initio study of α-seleno carbanions show that they are subject to appreciable polarization and stereoelectronic effects. Like in α-thia carbanions, the equatorial e forms are more stable than the axial a forms, one of the stabilizing contributions being the conformation dependent (C-lone pair, σ* Se? Z) interaction. The carbanion stabilizing effect of the α-Se atom is about 3 kcal/mol larger than that of the sulfur analog. As in the case of the sulfur no specific effect of the d orbitals is found.     

Microwave spectrum, centrifugal distortion, dipole moment and conformation of 2-methyl-1,3-dioxane

Microwave spectrum of 2-methyl-1,3-dioxane has been investigated in the frequency range 8–40 GHz. Rotational a-type and c-type transitions with J≤40 have been identified. Rotational constants A = 4658.122(2) MHz, B = 2503.221(1) MHz, C = 1783.950(1) MHz and centrifugal distortion constants for the ground vibrational state have been found. Dipole moment components μ_a = 1.43 ± 0.01 D, μ_c = 1.15±0.01 D and overall dipole moment μ = 1.84±0.02 D have been determined. The data obtained are in accord with the chair conformation of the molecule having equatorial arrangement of the methyl group. Original Russian Text Copyright ? 2006 by A. Kh. Mamleev, R. V. Galeev, L. N. Gunderova, M. G. Faizullin, and A. A. Shapkin __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 47, No.2, pp. 373–375, March–April, 2006.     

Synthesis,characterization, and solid state electrical conductivity of coordination polymers with copper and zinc

Heterobimetallic complexes [Cu _x Zn_{1? x} (dadb)?·?yH₂O] _n {where dadb?=?2,5-diamino-3,6-dichloro-1,4-benzoquinone (1); x?=?1 (2), 0.5 (4), 0.25 (5), 0.125 (6), 0.0625 (7), and 0 (3); y?=?2; n?=?degree of polymerization} were synthesized and characterized. All metal complexes are stable at room temperature but weakly absorb moisture on exposure to air. Monometallic 2 exhibits subnormal magnetic moment whereas 3 exhibits diamagnetism. Heterobimetallic complexes exhibit normal magnetic moments. Heterobimetallic complexes are characterized from powder X-ray diffraction, thermal analysis, and electron spin resonance (ESR) spectral studies. Delocalization of unpaired electron from metal to ligand has been inferred from ESR and natural bond orbital (NBO) analysis. Greater delocalization of unpaired electron of Cu(II) on ligand of 4 as compared to that of 2 is reflected from NBO analysis. Heterobimetallic complexes show higher conductivity than monometallic complexes; all the complexes exhibit semiconductor behavior.     

Ab Initio Calculations of the Conformational Preferences of 1,3-Oxathiane S-Oxide and its Analogs Containing S and SE Atoms—Evidence for Stereoelectronic Interactions Associated with the Anomeric Effects

Abstract

Stereoelectronic interactions associated with the AE and also the conformational and structural properties of 1,3-oxathiane S-oxide (1), 1,3-dithiane S-oxide (2), 1,3-thiaselenane S-oxide (3), 1,3-oxaselenane Se-oxide (4), 1,3-thiaselenane Se-oxide (5), and 1,3-diselenane Se-oxide (6) were investigated using quantum mechanical methods. These compounds were fully optimized at the B3LYP/6 – 311 + G** and HF/6 – 311 + G** levels of theory. The Gibbs Free Energy, Enthalpy, and Entropy differences (i.e., ΔG, ΔH, and ΔS) between the axial and equatorial conformations were calculated at the B3LYP/6 – 311 + G** and HF/6 – 311 + G** levels of theory. The decrease of the AE is corresponding to the decreases of calculated Δ(G _ax – G _eq) value of the above mentioned compounds. The calculated AE values are more significant for the justification of the conformational dominances of the compounds than steric effects. In this work, the relations between the Anomeric Effects, donor and acceptor orbital energies, occupancies, structural parameters, dipole–dipole interactions, and conformational behavior of the compounds have been studied.     

Conformational analysis of 2-OAryl-2-oxo-4,6-dimethyl- and -4-methyl-1,3,2λ5-dioxaphosphorinanes. Spectroscopic,X-Ray,and solid-state 13C and 31P studies

Results of IR and ¹H, ¹³C, and ³¹P NMR studies of the anancomeric title compounds ( 2–5 ) and compound 1 (Scheme 1) are analyzed to search for the existence of high-energy boat or twist-boat conformations in the equatorial epimers. While the difference in frequencies (Δν)_P=O between the axial and equatorial compounds and ¹³C NMR J_POCC and anti J_POCCH3 values suggest the participation of twist-boat conformations for the equatorial isomers, coupling constants in ¹H NMR J_H4H5a or J_H6H5a and J_H4H5e or J_H6H5e of the equatorial isomers 2e–4e along with the lack of a large ³J_PH in ³¹P NMR are consistent with predominant chair conformations. In addition, an X-ray structure of the equatorial 2-p-nitrophenoxy-2-oxo-cis-4,6-dimethyl-1,3,2-dioxaphosphorinane ( 4e ) showed that the molecule adopts a chair conformation with no severe ring flattening in the OPO region in the solid state. X-ray structures of trans- 4 and trans- 5 displayed chair conformations with mild ring flattening especially in the axial methyl region, presumably as a result of the steric methyl-oxygen interaction. CPMAS ¹³C and ³¹P NMR spectra of 4a and 4e provide evidence against the presence of a significant contribution of a twist-boat conformation in solid equatorial 4e . The NMR spectral analysis of 1e , on the other hand, suggests a substantial contribution of a twist conformation as well as, possibly, some contribution of the inverted chair. © 1997 John Wiley & Sons, Inc. Heteroatom Chem 8: 509–516, 1997     

Molecular Structure,Vibrational Analysis,Hyperpolarizability and NBO Analysis of 3-Methyl-Picolinic Acid Using SQM Calculations

In this study, FT-IR and FT-Raman spectra of 3-methyl picolinic acid (MPA) are recorded in the ranges 4000–450 cm^–1 and 4000–50 cm^–1, respectively. The optimized geometry is obtained by scaled quantum mechanical calculations using density functional theory employing the B3LYP functional with the 6–311++G(d,p) basis set. Vibrational assignments are suggested for all the fundamental vibrations unambiguously, using the potential energy distribution obtained in the computations. The rms error between the observed and calculated frequencies is found to be 8.48 cm^–1. The dipole moment, polarizability, and hyperpolarizability values are computed to study the NLO behavior of the molecule. The NBO analysis is made to study the stability of the molecule arising from hyperconjugative interactions and charge delocalization.     

DFT Calculations of the Elimination Kinetics of Silacyclobutanes and its Methyl Derivatives in the Gas-Phase

Abstract

The gas-phase thermal decomposition kinetics of silacyclobutane (1), 1-methyl- silacyclobutane (2), and 1,1-dimethyl-1-silacyclobutane (3) has been theoretically studied at the B3LYP/6-311G**, B3PW91/6-311G**, and MPW1PW91/6-311G** levels. The B3LYP/6-311G** method was found to give a reasonable good agreement with the experimental kinetics and thermodynamic parameters. The decomposition reaction of compounds 1–3 yields ethylene and the corresponding silene. Based on the optimized ground state geometries using B3LYP/6-311G** method, the natural bond orbital (NBO) analysis of donor-acceptor (bonding–antibonding) interactions revealed that the perturbation energies (E₂) associated with the electronic delocalization from σ_Si1–C2 to σ*_C4–Si1 orbitals increase from compounds 1 to 3. The σ_Si1–C2→σ*_C4–Si1 resonance energies for compounds 1–3 are 1.17, 1.26, and 1.43 kcal/mol, respectively. Also, the decomposition process in these compounds is controlled by σ→σ* resonance energies. Moreover, the obtained order of energy barriers could be explained by the number of electron-releasing methyl groups substituted to the Si_sp2 atom. NBO analysis shows that the occupancies of σ_Si1–C2 bonds decrease for compounds 1–3 as 3 < 2 < 1, and the occupancies of σ*_Si1–C2 bonds increase in the opposite order (3 > 2 > 1). Moreover, these results can fairly explain the decrease of the energy barriers (ΔE_o) of the decomposition reaction of compounds 1 to 3. The calculated data demonstrate that in the decomposition process of the studied compounds, the polarization of the C₃–C₄ bond is the rate determining factor. Analysis of bond orders, NBO charges, bond indexes, synchronicity parameters, and IRC calculations indicate that these reactions are occurring through a concerted and asynchronous four-membered cyclic transition state type of mechanism.     

Random-Coil configurations of the polymormals [CH2?O?(CH2)y?O?]. II. Dipole moments of poly(1,3–dioxepane) [CH2?O?(CH2)4?O?;]

Dielectric constant measurements were carried out on a sample of poly(1,3–dioxepane) [CH₂? O? (CH₂)₄? O? ], in benzene over the range 25—60°C. This chain molecule was found to be very similar to poly(1,3–dioxolane) [CH₂? O? (CH₂)₂? O? ] in having a relatively small dipole moment, which increases markedly with increasing temperature. Rotational isomeric-state calculations show that these two characteristics are due to very strong preferences for gauche states in the two central bonds in the sequence CH₂? O? CH₂? O? CH₂. Such pairs of gauche states, necessarily of the same sign, place neighboring-group dipoles in essentially antiparallel orientations. The resulting attenuation of dipole vectors is the origin of the small dipole moment, and the disruption of these preferred conformations with increasing temperature gives rise to the large positive temperature coefficient of the dipole moment.     

Conformational analysis 29: Preparation and1H and13C NMR,FTIR, MS,and crystallographic conformational and configurational study of 3-Oxo-1,3-oxathiane and its monomethyl derivatives

3-Oxo-1,3-oxathiane (1) and its monomethyl derivatives were prepared by oxidation of the corresponding 1,3-oxathianes. The structural analysis was carried out by¹H and¹³C NMR, FTIR, and mass spectrometry. At 298 K compound1 was a 1 1 (at 173 K a 3 1) mixture of the SO(ax) and SO(eq) chair forms. The major oxidation products of methyl 1,3-oxathianes attained exclusively the SO(ax), Me(eq) chair forms except that of the 5-methyl derivative, which consisted of 7% of the SO(eq), Me(ax) chair conformation in CDCl₃ solution. The minor products of oxidation existed in anancomeric SO(eq), Me(eq) chair conformations. The oxidation of 2-methyl- 1,3-oxathiane, however, led to 3,3-dioxo derivative (6) in addition to thetrans [SO(eq)] monoxide. The crystal structures of6 andtrans-3-oxo-5-methyl-1,3-oxathiane were solved by X-ray diffractometry.     

Chiral Lithiated Allylic α‐Sulfonyl Carbanions: Experimental and Computational Study of Their Structure,Configurational Stability,and Enantioselective Synthesis

X‐ray crystal structure analysis of the lithiated allylic α‐sulfonyl carbanions [CH₂?CHC(Me)SO₂Ph]Li ? diglyme, [cC₆H₈SO₂tBu]Li ? PMDETA and [cC₇H₁₀SO₂tBu]Li ? PMDETA showed dimeric and monomeric CIPs, having nearly planar anionic C atoms, only O?Li bonds, almost planar allylic units with strong C?C bond length alternation and the s‐trans conformation around C1?C2. They adopt a C1?S conformation, which is similar to the one generally found for alkyl and aryl substituted α‐sulfonyl carbanions. Cryoscopy of [EtCH?CHC(Et)SO₂tBu]Li in THF at 164 K revealed an equilibrium between monomers and dimers in a ratio of 83:17, which is similar to the one found by low temperature NMR spectroscopy. According to NMR spectroscopy the lone‐pair orbital at C1 strongly interacts with the C?C double bond. Low temperature ⁶Li,¹H NOE experiments of [EtCH?CHC(Et)SO₂tBu]Li in THF point to an equilibrium between monomeric CIPs having only O?Li bonds and CIPs having both O?Li and C1?Li bonds. Ab initio calculation of [MeCH?CHC(Me)SO₂Me]Li ? (Me₂O)₂ gave three isomeric CIPs having the s‐trans conformation and three isomeric CIPs having the s‐cis conformation around the C1?C2 bond. All s‐trans isomers are more stable than the s‐cis isomers. At all levels of theory the s‐trans isomer having O?Li and C1?Li bonds is the most stable one followed by the isomer which has two O?Li bonds. The allylic unit of the C,O,Li isomer shows strong bond length alternation and the C1 atom is in contrast to the O,Li isomer significantly pyramidalized. According to NBO analysis of the s‐trans and s‐cis isomers, the interaction of the lone pair at C1 with the π* orbital of the CC double bond is energetically much more favorable than that with the “empty” orbitals at the Li atom. The C1?S and C1?C2 conformations are determined by the stereoelectronic effects n_C–σ_SR* interaction and allylic conjugation. ¹H DNMR spectroscopy of racemic [EtCH?CHC(Et)SO₂tBu]Li, [iPrCH?CHC(iPr)SO₂tBu]Li and [EtCH?C(Me)C(Et)SO₂tBu]Li in [D₈]THF gave estimated barriers of enantiomerization of ΔG^≠=13.2 kcal mol^?1 (270 K), 14.2 kcal mol^?1 (291 K) and 14.2 kcal mol^?1 (295 K), respectively. Deprotonation of sulfone (R)‐EtCH?CHCH(Et)SO₂tBu (94 % ee) with nBuLi in THF at ?105 °C occurred with a calculated enantioselectivity of 93 % ee and gave carbanion (M)‐[EtCH?CHC(Et)SO₂tBu]Li, the deuteration and alkylation of which with CF₃CO₂D and MeOCH₂I, respectively, proceeded with high enantioselectivities. Time‐dependent deuteration of the enantioenriched carbanion (M)‐[EtCH?CHC(Et)SO₂tBu]Li in THF gave a racemization barrier of ΔG^≠=12.5 kcal mol^?1 (168 K), which translates to a calculated half‐time of racemization of t_1/2=12 min at ?105 °C.     

Steric control in the base catalysed H-D exchange of an α,β-unsaturated ketone

For the base catalysed H-D exchange of the equatorial hydrogen and the axial hydrogen in 4-t-butyl-1-acetylcyclohex-1-ene (1) in CH₃OD containing CH₃ONa, k_eq/k_ax is 3.8:1. This is clearly a result of steric rather than stereoelectronic factors.     

Synthesis and physicochemical studies of some novel picrate derivatives

(1)H and (13)C NMR spectra were recorded for some novel picrate derivatives derived from some 3,3-methyl-2,6-diarylpiperidin-4-ones and 3-benzyl-2,6-diarylpiperidin-4-one. The difference in the chemical shift of equatorial methylene proton and axial methylene proton at C(5) [Δ = δ(eq)-δ(ax)] is highly negative which is in contrast to the value observed in the corresponding parent piperidin-4-ones and this is attributed to the syn 1,3-diaxial interaction between the axial N-H bond and axial hydrogen at C-5. The effect of protonation on the chemical shifts was studied in detail. The chemical shifts of the heterocyclic ring protons are influenced by the picrate anion. Solvatochromism of picrates were studied in detail. DFT calculations were carried out in order to find out the NBO analysis, HOMO-LUMO energies, MEP studies and hyperpolarisability behaviour. The electric dipole moment (μ) and the first-hyperpolarisability (β) value of the investigated molecules have been studied theoretically which reveal that the synthesized molecules have microscopic non-linear optical (NLO) behaviour with non-zero values.     

Structural and Conformational Aspects of Equatorial and Axial Trifluoromethyl,Difluoromethyl, and Monofluoromethyl Groups

The X‐ray crystal structures of cis‐ and trans‐1‐(indol‐3‐yl)‐4‐methyl cyclohexane and its congeners with stepwise fluorination of the methyl group are reported. The trans‐configured compounds adopted diequatorial conformations, whereas the cis analogues adopted regular cyclohexane chair conformations with the methyl group preferentially assuming the axial position, even in the case of the CF₃ group. Surprisingly, although the axial CF₃ derivative displayed distinct valence deformations in the cyclohexane moiety, the observed structural changes were relatively modest. The cis derivatives with axial mono‐ and difluorinated methyl groups exhibited conformational disorder in the crystals with significant population levels for the staggered conformations that had one fluorine atom in the endo position; their respective trans counterparts adopted unique conformations, but again with one fluorine atom in the endo position. Theoretical calculations for a series of cis‐ and trans‐1,4‐dimethyl cyclohexane model compounds with stepwise fluorination of one equatorial or axial methyl group reproduced the experimentally observed structural response patterns very well, reproduced the experimentally determined nonlinear correlation of the axial–equatorial energy difference with the degree of methyl fluorination in a satisfactory manner, and provided further insights into important conformational aspects of partially fluorinated methyl groups.