A Conformational Study on Silacyclohexane. Comparison of ab initio (HF,MP2), DFT,and Molecular Mechanics Calculations. Conformational Energy Surface of Silacyclohexane

The structures and relative energies for the basic conformations of silacyclohexane 1 have been calculated using HF, RI‐MP2, RI‐DFT and MM3 methods. All methods predict the chair form to be the dominant conformation and all of them predict structures which are in good agreement with experimental data. The conformational energy surface of 1 has been calculated using MM3. It is found that there are two symmetric lowest energy pathways for the chair‐to‐chair inversion. Each of them consists of two sofa‐like transition states, two twist forms with C₁ symmetry (twist‐C₁), two boat forms with Si in a gunnel position (C₁ symmetry), and one twist form with C₂ symmetry (twist‐C₂). All methods calculate the relative energy to increase in the order chair < twist‐C₂ < twist‐C₁ < boat. At the MP2 level of theory and using TZVP and TZVPP (Si atoms) basis sets the relative energies are calculated to be 3.76, 4.80, and 5.47 kcal mol^–1 for the twist‐C₂, twist‐C₁, and boat conformations, respectively. The energy barrier from the chair to the twisted conformations of 1 is found to be 6.6 and 5.7 kcal mol^–1 from MM3 and RI‐DFT calculations, respectively. The boat form with Si at the prow (C_s symmetry) does not correspond to a local minimum nor a saddle point on the MM3 energy surface, whereas a RI‐DFT optimization under C_s symmetry constraint resulted in a local minimum. In both cases its energy is above that of the chair‐to‐twist‐C₁ transition state, however, and it is clearly not a part of the chair‐to‐chair inversion.     

Molecular mechanics conformational analysis of cyclononane using the RIPS method and comparison with quantum-mechanical calculations

The recently reported Random Incremental Pulse Search (RIPS) technique has been used to probe the conformational energy surface of cyclononane. The stochastic method permits searching of the potential energy surface for all minimum-energy conformations. The search located all previously reported structures together with three additional conformations that were not found by earlier, primitive searching techniques. Two of these structures are high-nergy skew forms, and the third is a low-energy conformer that should contribute significantly to the overall equilibrium set of cyclononane conformations. The global minimum has been found to be the D₃ symmetrical twist chair-boat (TBC) form in accordance with previous studies. The newly discovered low-energy structure, which lies only 2.2 kcal/mol above the global minimum, has been designated twist chair-twist chair (TCTC). The two higher energy conformers are skewed chair-chair (SCC) and skewed boat-boat (SBB) forms that are 5.7 kcal/mol and 10.4 kcal/mol above the global minimum, respectively. The seven reported conformations were reanalyzed quantum mechanically (AM 1), and a comparison between MM 2 and AM 1 results is presented.     

Computational study of sulfoxides of thiacyclohexane, 4‐silathiacyclohexane, 4‐fluoro‐4‐silathiacyclohexane,and 4,4‐difluoro‐4‐silathiacyclohexane: Relative energies of conformations and sulfinyl oxygen stabilized pentacoordinate silicon in boat and twist structures

Second‐order Møller‐Plesset theory (MP2) has been used to calculate the equilibrium geometries and relative energies of the chair, 1,4‐twist, 2,5‐twist, 1,4‐boat, and 2,5‐boat conformations of thiacyclohexane 1‐oxide (tetrahydro‐2H‐thiopyran 1‐oxide), 4‐silathiacyclohexane 1‐oxide, cis‐ and trans‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide, and 4,4‐difluoro‐4‐silathiacyclohexane 1‐oxide. At the MP2/6‐311+G(d,p) level of theory, the chair conformer of axial thiacyclohexane 1‐oxide is 0.99, 5.61, 5.91, 8.57, and 7.43 kcal/mol more stable (ΔE) than its respective equatorial chair, 1,4‐twist, and 2,5‐twist conformers and 1,4‐boat and 2,5‐boat transition states. The chair conformer of equatorial thiacyclohexane 1‐oxide is 4.62, 6.31, 7.56, and 7.26 kcal/mol more stable (ΔE) than its respective 1,4‐twist and 2,5‐twist conformers and 1,4‐boat and 2,5‐boat transition states. The chair conformer of axial 4‐silathiacyclohexane 1‐oxide is 1.79, 4.26, 3.85, and 5.71 kcal/mol more stable (ΔE) than its respective equatorial chair, 1,4‐twist, and 2,5‐twist conformers and 2,5‐boat transition state. The 2,5‐twist conformer of axial 4‐silathiacyclohexane 1‐oxide is stabilized by a transannular interaction between the sulfinyl oxygen and silicon, to give trigonal bipyramidal geometry at silicon. The chair conformer of equatorial 4‐silathiacyclohexane 1‐oxide is 2.47, 7.90, and 8.09 kcal/mol more stable (ΔE) than its respective 1,4‐twist, and 2,5‐twist conformers and 2,5‐boat transition state. The chair conformer of axial cis‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide is 4.18 and 5.70 kcal/mol more stable than its 1,4‐twist conformer and 2,5‐boat transition state and 1.51 kcal/mol more stable than the chair conformer of equatorial cis‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide. The chair conformer of axial trans‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide is 5.02 and 6.11 kcal/mol more stable than its respective 1,4‐twist conformer and 2,5‐boat transition state, but is less stable than its 2,5‐twist conformer (ΔE = ?1.77 kcal/mol) and 1,4‐boat transition state (ΔE = ?1.65 kcal/mol). The 2,5‐twist conformer and 1,4‐boat conformer of axial trans‐4‐fluoro‐4‐silathiacyclohexane 1‐oxide are stabilized by intramolecular coordination of the sulfinyl oxygen with silicon that results in trigonal bipyramidal geometry at silicon. The chair conformer of axial 4,4‐difluoro‐4‐silathiacyclohexane 1‐oxide is 3.02, 5.16, 0.90, and 6.21 kcal/mol more stable (ΔE) than its respective equatorial chair, 1,4‐twist, and 1,4‐boat conformers and 2,5‐boat transition state. The 1,4‐boat conformer of axial 4,4‐difluoro‐4‐silathiacyclohexane 1‐oxide is stabilized by a transannular coordination of the sulfinyl oxygen with silicon that results in a trigonal bipyramidal geometry at silicon. The relative energies of the conformers and transition states are discussed in terms of hyperconjugation, orbital interactions, nonbonded interactions, and intramolecular sulfinyl oxygen–silicon coordination. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

Conformational analysis of cycloheptane,oxacycloheptane, 1,2‐dioxacycloheptane, 1,3‐dioxacycloheptane,and 1,4‐dioxacycloheptane

The conformational analysis of cycloheptane (1), oxacycloheptane (2), 1,2‐dioxacycloheptane (3), 1,3‐dioxacycloheptane (4), and 1,4‐dioxacycloheptane (5) has been carried out using B3LYP, CCD, CCSD, and QCISD with the 6‐311+G(d,p) and cc‐pVDZ basis sets. The twist chair conformers are predicted to be lower in energy than their corresponding boat and chair conformations. All levels of theory predict (4) to be lower in energy than (3) and (5). CCSD predicts remarkably similar activation barriers for the conformational interconversion of the twist chair conformers to their corresponding boat conformers. Small barriers to pseudorotation are also predicted. © 2007 Wiley Periodicals, Inc. Int J Quantum Chem, 2008     

Konformative Beweglichkeit Flexibler Ringsysteme—XIII: Untersuchungen mit Hilfe der Protonenresonanzspektroskopie die Konformation des Ungesättigten Siebenringes im Benzocyclohepten und in symmetrisch substituierten derivaten

The confromations of the unsaturated seven membered ring in 4,4,6,6-tetradeuterium-1,2-benzocycloheptene-(1) ( 1 ) and five benzocycloheptene derivatives were determined by NMR spectroscopy. For all investigated compounds at ?80°C only one conformer was present in detectable quantity. By analysis of the NMR data – molecular symmetry, coupling constants and chemical shift – it can be shown that the conformation is always the chair form. The free conformational enthalpy of both the other conformations with boat or twist form of the ring is for all six compounds more than 1.8 kcal/mole. The experimental results agree with those from model calculations: thus for benzocycloheptene, the 5,5-dimethyl derivative ( 2 ) and the 4,4,6,6-tetramethyl derivative ( 4 ) the lowest energy was found for the chair conformation; the second most stable conformations were found to be the boat for 1 and 4 , and the twist form for 2 .     

Computational study of 4‐fluoro‐4‐chloro‐ and 4‐fluoro‐4‐bromo‐4‐silathiacyclohexane S‐oxides: Effect of halogen on the SO→Si intramolecular coordination in the boat and twist conformers

Chair, twist, and boat conformations of axial and equatorial 4‐fluoro‐4‐chloro‐ and 4‐fluoro‐4‐bromo‐4‐silathiacyclohexane S‐oxides have been calculated at the second‐order Møller–Plesset (MP2) and density functional theory (DFT) levels of theory. The 2,5‐twist and 1,4‐boat conformers of the axial sulfoxides are stabilized by transannular interaction between the sulfinyl oxygen and silicon to give trigonal bipyramidal geometry at silicon. The stability of conformers containing pentacoordinate silicon is governed by the balance of the transannular sulfinyl oxygen–silicon coordination, nonbonded H…H and O…Hlg_ax interactions, electronegativity of Hlg_ax and nucleofugality of Hlg_eq, and may approach the stability of the lowest‐energy chair conformers. The effect of pentacoordination on the energy and geometry of Si‐halogenated 4‐silathiane S‐oxides is analyzed as a function of the halogen nature and position. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2007     

Chair, boat and twist conformation of dodecamethylcyclohexasilane and undecamethylcyclohexasilane: a combined DFT and Raman spectroscopic study.

The conformations of dodecamethylcyclohexasilane Si6Me12 and undecamethylcyclohexasilane Si6Me11H have been investigated by ab initio calculations employing the B3LYP density functional with a 6-31+G(d) basis set. Local minima as well as transition structures were calculated with imposed symmetry constraints. For Si6Me12, three unique minima, which correspond to the chair, twist and boat conformations were located with relative zero-point-vibration-corrected energies of 0.0, 7.8 and 11.4 kJ mol(-1). A half-chair conformation with four coplanar silicon atoms connects the chair and twisted minima via an energy barrier of 16.0 and 8.2 kJ mol(-1), respectively. A second transition structure with a barrier of 3.9/0.3 kJ mol(-1) connects the twist with the boat structure. Solution Raman spectra of Si6(CH3)12 and Si6(CD3)12 fully corroborate these results. Below -40 degrees C, the symmetric SiSi ring breathing vibration is a single line, which develops a shoulder (originating from the twist conformer) at longer wavelengths whose intensity increases with increasing temperature. From a Van't Hoff plot, the chair/twist enthalpy difference is 6.6+/-1.5 kJ mol(-1) for Si6(CH3)12 and 6.0+/-1.5 kJ mol(-1) for Si6(CD3)12, which is in reasonable agreement with the ab initio results. Due to the low barrier, the boat conformation cannot be observed, because either the lowest torsional vibration level lies above it or a rapid interconversion between the twist and boat conformations occurs, resulting in averaged Raman spectra. For Si6Me11H, six local minima were located. The chair with the hydrogen atom in the axial position (axial chair) is the global minimum, followed by the equatorial chair (+1.9 kJ mol(-1)) and the three twist conformers (+5.3, +8.0 and +8.1 kJ mol(-1)). The highest local minimum (+11.9 kJ mol(-1)) is a C(s) symmetric boat with the hydrogen atom in the equatorial position. Two possible pathways for the chair-to-chair interconversion with barriers of 13.9 and 14.5 kJ mol(-1) have been investigated. The solution Raman spectra in the SiSi ring breathing region clearly show that below -50 degrees C only the axial and equatorial chairs are present, with an experimental deltaH-value of 0.46 kJ mol(-1). With increasing temperature a shoulder develops which is attributed to the combined twist conformers. The experimental deltaH-value is 6.9 kJ mol(-1), in good agreement with the ab initio results. Due to the low interconversion barriers, the various twist conformers cannot be detected separately.     

Ab initio conformational analysis of 1,5‐dithiacyclooctane, 1,5‐diselenacyclooctane,and 1,5‐ditelluracyclooctane

Energy‐minimum structures of 1,5‐dithiacyclooctane (1,5‐DTCO), 1,5‐diselenacyclooctane (1,5‐DSeCO), and 1,5‐ditelluracyclooctane (1,5‐DTeCO) were calculated by the ab initio molecular orbital method. Nine energy‐minimum structures were obtained for each compound. A twist‐boat–chair (TBC) structure is the most stable for 1,5‐DTCO and 1,5‐DSeCO, whereas a boat–boat (BB) structure is the most stable in 1,5‐DTeCO. The TBC conformer of 1,5‐DTCO has received little attention so far. The energy gap between HOMO and NHOMO in the TBC conformer of 1,5‐DTCO is in good agreement with the experimental data (photoelectron spectrum). For 1,5‐DTCO and 1,5‐DSeCO, the boat–chair (BC) conformer in which two chalcogen atoms face each other has the highest HOMO energy among the nine conformers, and the energy barriers between the TBC and BC conformers were calculated to be relatively low for these compounds. Therefore, a conformational change from the TBC to the BC is predicted to occur before these compounds are oxidized in solution. © 1999 John Wiley & Sons, Inc. Heteroatom Chem 10: 159–166, 1999     

3‐Fluoro‐2,4‐dioxa‐7‐aza‐3‐phosphadecalin 3‐Oxides as Rigid Acetylcholine Mimetics: Conformational Analysis and Direct Evidence of the Anomeric Effect

Conformational analyses of the P(3)‐axially and P(3)‐equatorially F‐substituted (±)‐cis‐ and (±)‐trans‐2,4‐dioxa‐7‐aza‐3‐phosphadecalin 3‐oxides (3‐fluoro‐2,4‐dioxa‐7‐aza‐3‐phosphabicyclo[4.4.0]decane 3‐oxides) were performed. The results are based on independent studies in both solution and the solid state by ¹H‐ and ³¹P‐NMR experiments and computational and X‐ray crystallographic data. As expected, the axial epimers adopt neat double‐chair conformations in solution and in the crystal. Due to the anomeric effect of the electron withdrawing F‐substituent, the 2,4‐dioxa‐3‐phospha moiety in the equatorial epimers adopts a mixture of conformations in solution, mainly chair and twist‐boat; whereas a neat twist‐boat (trans‐isomer) and the unusual envelope conformation (cis‐isomer) were detected in the solid state. This is the first report of a straight visualization of these conformations and the impact of the anomeric effect in such systems.     

Ab initio molecular orbital study of dications of 1,5‐dichalcogenacyclooctanes

The molecular and electronic structures of the dications of three homonuclear and three heteronuclear dichalcogenacyclooctanes (chalcogen = S, Se, or Te) were investigated by ab initio molecular orbital calculations. Four energy‐minimum structures were located for each dication. Three of those (chair‐chair, boat‐boat, and boat‐chair) have the cis configuration with respect to the chalcogen lone pairs, and the remaining one has the trans configuration. The cis isomers were found to be much more stable than the trans isomer. Among the three cis structures, the stability is in the order of boat‐chair > boat‐boat > chair‐chair for all dications. This order can be explained by considering the nonbonding H···H interactions. The chair‐chair structure (C_2v symmetry) of the 1,5‐dithiacyclooctane dication has a very low vibrational frequency of a₂ symmetry, and its LUMO energy is lower than those of boat‐boat and boat‐chair. These can rationalize the fact that in the crystalline state the dication adopts a distorted C₂ chair‐chair conformation. The transition states between the three conformers of the homonuclear dications were also located. The corresponding energy barriers are relatively low, which is consistent with their NMR spectra. The relative stabilities of the homonuclear and heteronuclear dications were elucidated on the basis of their energies and those of the corresponding neutral compounds. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:31–41, 2000     

rac‐5‐Diphenylacetyl‐2,2,4‐trimethyl‐2,3,4,5‐tetrahydro‐1,5‐benzothiazepine and rac‐5‐formyl‐2,2,4‐trimethyl‐2,3,4,5‐tetrahydro‐1,5‐benzothiazepine

rac‐5‐Diphenylacetyl‐2,2,4‐trimethyl‐2,3,4,5‐tetrahydro‐1,5‐benzothiazepine, C₂₆H₂₇NOS, (I), and rac‐5‐formyl‐2,2,4‐trimethyl‐2,3,4,5‐tetrahydro‐1,5‐benzothiazepine, C₁₃H₁₇NOS, (II), are both characterized by a planar configuration around the heterocyclic N atom. In contrast with the chair conformation of the parent benzothiazepine, which has no substituents at the heterocyclic N atom, the seven‐membered ring adopts a boat conformation in (I) and a conformation intermediate between boat and twist‐boat in (II). The molecules lack a symmetry plane, indicating distortions from the perfect boat or twist‐boat conformations. The supramolecular architectures are significantly different, depending in (I) on C—H...O interactions and intermolecular S...S contacts, and in (II) on a single aromatic π–π stacking interaction.     

Conformational properties of permethylcyclohexane as compared to cyclohexane: A force field study

The conformational properties of the recently synthesized highly strained permethylcyclohexane molecule 2 have been studied by empirical force field calculations using three different potentials (CFF, MM2, MM2′) and second-derivative optimization methods. A comparison of the results with the conformational behavior of parent cyclohexane 1 leads to the following conclusions: The best conformation of 2 is a chair minimum whose six-membered ring is flatter than that of 1 , due to the strong H…H repulsions introduced by the methyl groups. The twist minimum of 2 is energetically less favorable than the chair by an amount similar to 1 . A potential energy barrier Δ V^# for the chair inversion of 2 of 15.32 kcal/mol results with the CFF, only about three kcal/mol higher than for 1 . The free energy of activation ΔG^# for this process obtained with the CFF is 16.96 kcal/mol (at 333 K) and agrees well with the experimental value of 16.7(2) kcal/mol.¹ MM2 and MM2′ give substantially lower and higher potential energy inversion barriers Δ V^# of 9.03 and 20.29 kcal/mol, respectively, which is attributed to inappropriate torsional energy terms in these force fields. The characteristic difference in the conformational behavior of 2 and 1 concerns the boat forms which are substantially less favorable in the per-methyl compound than in 1 . Expectedly, strong H…H repulsions between the 1,4 diaxial flagpole–bowsprit methyl groups in 2 are responsible for this difference. The particularly high strain of the boat forms of 2 leads to flexibility differences as compared to 1 which in turn affect the relative entropies of the various statiomers (stationary point conformations); e.g., the chair ring inversion activation entropies of 2 and 1 are predicted by the CFF calculations to have opposite signs (?4.82 and 3.41 cal/mol K, respectively, at 298 K). The twist and half-twist statiomers of 2 are much more rigid than those of 1 , which is a consequence of the substantially larger boat barriers along their pseudorotational interconversion paths. The boat transition state separating two enantiomeric twist minima represents a barrier calculated to be more than tenfold higher for 2 than for 1 (CFF Δ V^# values 11.14 and 0.92 kcal/mol, respectively); likewise the half-boat chair inversion barrier of 2 is calculated 5.07 kcal/mol less favorable than the respective half-twist barrier. These statiomers are practically equienergetic in the case of 1 . Except for the axial flagpole–bowsprit CH₃ substituents of the boat forms, the methyl groups of all the relevant calculated statiomers of 2 are more or less staggered. The rotational barrier of the equatorial methyl groups of the chair minimum of 2 is computationally predicted to be 5.78 kcal/mol (ΔG^#), i.e., unusually high. Interesting vibrational effects are brought about by the strong H…H repulsions in 2 ; thus the chair minimum has a largest C? H stretching frequency estimated to be 3050 cm^?1 and involves several particularly low frequencies which have a substantial influence on its entropy. CFF calculations for the lower homologue permethylcyclopentane 5 indicate that its pseudorotational properties are similar to those of cyclopentane 4 , in contradistinction to the pair 2/1 .     

Structural properties of five- and six-layered [3.3]metacyclophanes

We performed X-ray structural analyses of the five- and six-layered [3.3]metacyclophanes (MCPs) 1 and 2 and the six-layered [3.3]MCP tetraone 3. In the solid state, the MCP moieties of 1, 2, and 3 adopt different conformations from those of the free MCPs in solution. In the five-layered [3.3]MCP 1, all the [3.3]MCP moieties adopt anti (chair/boat) conformations. In the six-layered [3.3]MCP 2, two three-layered [3.3]MCPs are connected by a [3.3]MCP in the anti conformation with completely parallel benzene rings. In the six-layered [3.3]MCP tetraone 3, the outer [3.3]MCP moieties and diones adopt general syn and anti geometries, respectively. However, the inner [3.3]MCP moiety adopts an anti geometry. Based on density functional theory (DFT) calculations, the most stable conformers of 1, 2, and 3 are syn (chair/chair) in the [3.3]MCP moieties and anti (twist boat/twist boat) in the dione moieties.     

Conformational analysis of azepane, oxepane, silepane, phosphepane, thiepane and the azepanium cation by high level quantum mechanics

A complete conformational analysis of the title compounds was performed by quantum mechanics at the B3LYP and the CCSD(T) levels of theory with the triple-zeta quality basis set 6-311+G(d,p). The results are compared with the well-established results of cycloheptane. Without exception, twist-chair conformations are found to be stable conformations with the chair forms as transition states. Second row hetero atoms lower the relative energy of the boat conformations relative to the chair forms. The barrier of interconversion between the chair and boat families of conformations is found to be around 8 kcal/mol, except for phosphepane where it is 2 kcal/mol lower and for silepane, where the barrier is only 2.7 kcal/mol.     

(24R)‐24,25‐Dihydroxycyclo­artan‐3‐one

In the title compound, C₃₀H₅₀O₃, the three six‐membered rings adopt chair, twist and twist‐boat conformations. The five‐membered ring is in a slightly distorted envelope conformation. The substituent on the five‐membered ring is in an extended conformation, with its two hydroxyl O atoms forming an intramolecular hydrogen bond. One of these O atoms also forms an intermolecular hydrogen bond with the oxy­gen of the carbonyl group in a neighbouring mol­ecule.     

Lanosterol Biosynthesis: The Critical Role of the Methyl‐29 Group of 2,3‐Oxidosqualene for the Correct Folding of this Substrate and for the Construction of the Five‐Membered D Ring

Lanosterol synthase catalyzes the polycyclization reaction of (3S)‐2,3‐oxidosqualene ( 1 ) into tetracyclic lanosterol 2 by folding 1 in a chair‐boat‐chair‐chair conformation. 27‐Nor‐ and 29‐noroxidosqaulenes ( 7 and 8 , respectively) were incubated with this enzyme to investigate the role of the methyl groups on 1 for the polycyclization cascade. Compound 7 afforded two enzymatic products, namely, 30‐norlanosterol ( 12 ) and 26‐normalabaricatriene ( 13 ; 12 / 13 9:1), which were produced through the normal chair‐boat‐chair‐chair conformation and an atypical chair‐chair‐boat conformation, respectively. Compound 8 gave two products 14 and 15 ( 14 / 15 4:5), which were generated by the normal and the unusual polycyclization pathways through a chair‐chair‐boat‐chair conformation, respectively. It is remarkable that the twist‐boat structure for the B‐ring formation was changed to an energetically favored chair structure for the generation of 15 . Surprisingly, 14 and 15 consisted of a novel 6,6,6,6‐fused tetracyclic ring system, thus differing from the 6,6,6,5‐fused lanosterol skeleton. Together with previous results, we conclude that the methyl‐29 group is critical to the correct folding of 1 , with lesser contributions from the other branched methyl groups, such as methyl‐26, ‐27, and ‐28. Furthermore, we demonstrate that the methyl‐29 group has a crucial role in the formation of the five‐membered D ring of the lanosterol scaffold.     

The Rearrangement of 2,2‐Diphenyl‐1‐[(E)‐2‐phenylethenyl]cyclopropane to 3,4,4‐Triphenylcyclopent‐1‐ene: a DFT Analysis

A computational study on the rearrangement of 2,2‐diphenyl‐1‐[(E)‐2‐phenylethenyl]cyclopropane ( 1 ) is presented, using density functional theory (DFT), (U)B3LYP with the 6‐31G* basis set (DFT1) and (U)M05‐2X with the 6‐311+G** basis set (DFT2). In agreement with a biradical character of the transition structure (TS) or intermediate, the potential‐energy hypersurface is lowered by the influence of three conjugated Ph groups. Surprisingly, two conformations of the geminal diphenyl group (different twist angles) induce two different minimum‐energy pathways for the rearrangement. Independent of the functional used, the first hypersurface harbors true biradical intermediates, whereas the second energy surface is a flat, slightly ascending slope from the starting material to the TS. The functional (U)M05‐2X with the basis set 6‐311+G** provides realistic energies which seem to be close to experiment. The activation energy for racemization of enantiomers of 1 is lower than that of rearrangement by 2.5 kcal mol^?1, in agreement with experiment.     

SYNTHESIS AND CONFORMATIONAL ANALYSIS OF 16H-DINAPHTHO AND 12 H-DIBENZO [D,G][1,3,2]DIOXASILOCINE

Abstract

The conformation of the heterocyclic eight-membered ring in 16H-dinaphtho and 12H-dibenzo [d,g][1,3,2]dioxasilocine was investigated in solution by ¹H NMR spectroscopy. The barrier to ring inversion in the 16H-dinaphtho compound 3a was found to be 8.6±0.2 Kcal/mol and for the 12 H-dibenzo compound 4a, 8±0.2 Kcal/mol. Molecular mechanics calculations show three energy minima conformations for both compounds, boat chair(BC), twist boat(TB) and twist boat boat(TBB). Twist boat form is estimated to be the global minimum for the dibenzo compounds while TBB is the global conformation of the dinaphtho compounds. The result of molecular mechanics calculations are supported by analysis of the ¹H-NMR spectra.     

Molecular structure of 1,5-diazabicyclo[3.1.0]hexane as determined by gas electron diffraction and quantum-chemical calculations

The equilibrium molecular structure and conformation of 1,5-diazabicyclo[3.1.0]hexane (DABH) has been studied by the gas-phase electron-diffraction method at 20 degrees C and quantum-chemical calculations. Three possible conformations of DABH were considered: boat, chair, and twist. According to the experimental and theoretical results, DABH exists exclusively as a boat conformation of C s symmetry at the temperature of the experiment. The MP2 calculations predict the stable chair and twist conformations to be 3.8 and 49.5 kcal mol(-1) above the boat form, respectively. The most important semi-experimental geometrical parameters of DABH (r(e), A and angle)e), deg) are (N1-N5) = 1.506(13), (N1-C6) = 1.442(2), (N1-C2) = 1.469(4), (C2-C3) = 1.524(7), (C6-N1-C2) = 114.8(8), (N5-N1-C2) = 107.7(4), (N1-C2-C3) = 106.5(9), and (C2-C3-C4) = 104.0(10). The natural bond orbital (NBO) analysis has shown that the most important stabilization factor in the boat conformation is the n(N) --> sigma*(C-C) anomeric effect. The geometry calculations and NBO analysis have been performed also for the bicyclohexane molecule.