Conformational analysis of 1,1,2,2-tetraaryl-disilanes

A study of the static stereochemistry of 1,1,2,2-tetraphenyldisilane (1) and 1,1,2,2-tetramesityldisilane (2) by empirical force field calculations, X-ray diffraction, and ¹H NMR reveals that the preference for the anti conformation, exhibited by the unclamped 1,1,2,2-tetraarylethanes, is sustained in the analogous disilanes, although in somewhat attenuated form. This anti preference stands in contrast to the gauche preference of 1,1,2,2-tetraalkyldisilanes. Examination of ³J_HH coupling constants for R₂HSiSiHR₂; (R = phenyl, mesityl, 2,6-dimethyl-phenyl, t-butyl, cyclohexyl) suggests the existence of a Karplus relation for H-Si-Si-H systems.     

Conformational isomerism of 1,2-bis(imidazol-1′-yl)ethane in five-coordination polymers: Influence of metal ions and m-isophthalate ligands bearing different substituents

Four novel coordination polymers, {[Zn(gauche-bime)(bdc)] · 0.5H₂O}_n (1), {[Zn(anti-bime)(HO-bdc)] · 2.5H₂O}_n (2), [Cu(gauche-bime)_0.5(anti-bime)_0.5(O₂N-bdc)]_n (4) and [Ni₂(gauche-bime)(anti-bime)(O₂N-bdc)₂(H₂O)₂]_n (5) were successfully prepared by the solvothermal reactions of 1,2-bis(imidazol-1′-yl)ethane (bime), m-isophthalic acid (H₂bdc) or its two derivatives (HO-H₂bdc = 5-hydroxyisophthalic acid, O₂N-H₂bdc = 5-nitroisophthalic acid) with different metal ions. Interestingly, bime in the four complexes exhibit different conformations owing to the synergetic influence of coexistent neutral (–H), electron-donating (–OH) or electron-withdrawing (–NO₂) groups of the dicarboxylate ligands and different metal ions. In 1 and 2, only one conformation of bime (gauche in 1 and anti in 2) is displayed, while the mixed conformations of bime (gauche:anti = 1:1) are observed in 4 and 5. At the same time, one previously reported compound {[Zn(anti-bime)(O₂N-bdc)] · H₂O}_n (3) as a comparable substance in the research system was also mentioned, in which the anti-conformation of bime was observed.     

Chirality and conformational changes in 4-phenylphenanthrenes and 1-phenylbenzo[c]phenanthrene derivatives

NMR data of several 4-phenylphenanthrenes (15, 16) have revealed that the crowding in these compounds does not lead to chirality at temperatures as low as ?90°. The easy rotation of the phenyl substituent observed by NMR implies that notwithstanding the phenanthrene moiety in average behaves as a planar part the phenyl group does not experience steric hindrance.The analysis of temperature-dependent NMR spectra of several derivatives of 1-phenylbenzo[c]phenanthrenes (17-20) indicated that in these compounds exchange processes do occur. By calculations of the free energies of activation from the NMR data two processes could be distinguished: rotation of the phenyl substituent at one side of the helical benzo[c]phenanthrene moiety, for which ΔG^XXX_rot , is ca. 13.0$\text{kcal}\text{mol}$ or slightly larger when bulky substituents are present at C₂, and racemisation by a rotation of the phenyl group around the opposite end of the benzo[c]phenanthrene skeleton with simultaneous inversion of the helical conformation. For this process ΔG^XXX_rac is ca. 16$\text{kcal}\text{mol}$. The results have been compared with comparable data of related compounds like 1.8-diphenylnaphthalene, hexahelicene, and 4-methylbenzo[c] phenanthrenes, and gave evidence for the remarkably small, space-demanding properties of the phenyl substituent in these compounds.     

Studies on organophosphorus compounds—XXXVI: Simple new routes to phosphorins from 2-hydroxy-, 2-mercapto-, and 2-aminobenzoic acids and their derivatives

2-Hydroxybenzoic acid heated with 2,4 - bis(4 - methoxyphenyl) - 1,3,2,4 - dithiadiphosphetane - 2,4 -disulfide, 1, gave 2 - (p - methoxyphenyl) - 4$\text{H}$ - 1,3,2 - benzoxathiaphosphorin - 4 - one 2 - sulfide, 3, and its thio-analogue, 4, while its ethyl or phenyl esters gave 4 as the sole product. 2 - Mercaptobenzoic acid and its ethyl ester when heated with 1 produced 3$\text{H}$ - 1,2 - benzodithiole - 3 - one, 8, 3$\text{H}$ -1,2 - benzodithiole - 3 - thione, 9, and 2- (p - methoxyphenyl) - 4$\text{H}$ - 1,3,2 - benzodithia - phosphorin - 4 - one 2 - sulfide, 10. The reaction of 2 -aminobenzoic acid with 1 gave 1,2 - dihydro - 2 - (p - methoxyphenyl) - 4$\text{H}$ - 3,1,2 - benzoxaphosphorin - 4 - one 2 - sulfide, 12. Reactions of 1 with methyl 2 - aminobenzoate and 2 - aminobenzamides are described. Mechanistic considerations for the formation of the heterocyclic phosphorus compounds are presented.     

The role of internal H-bonding in the conformational preferences of some molecules of the formula CH3CHYCH2X

The conformational energies of 1-amino-2-propanol, 2-amino-1-propanol and 1,2-diaminopropane are studied using ab initio molecular orbital theory employing minimal (STO-3G) and extended (4-31G) basis sets. Calculations at both levels of theory generally favor conformations stabilized by internal H-bonding for all molecules considered. Results are first presented for conformations employing assumed geometries. Since the conformational energy differences as found by the initial set of calculations are in some cases rather small it then becomes necessary to introduce geometry optimizations into the study at the minimal STO-3G level. In addition, to get a better estimate of the energy differences of the various conformations 4-31G calculations are performed on the STO-3G optimized structures. These latter results indicate the following, (a) For 1-amino-2-propanol only one conformation that is stabilized by intramolecular H-bonding is low in energy; this has the methyl and amino groups anti. The other H-bonded conformer, where the methyl and amino groups are gauche, is predicted to be ca. 1.2 kcal mol^?1 less stable. Similar findings for this molecule have recently been provided by micro-wave spectroscopy. (b) For 2-amino-1-propanol the two H-bonded conformers are only separated by about 0.5 kcal mol^?1, with the anti conformer being more stable. Micro-wave spectroscopy again supports these calculations. (c) For 1,2-diaminopropane the gauche conformer is predicted to be of rather high energy (ca. 2.5 kcal mol^?1) compared to the corresponding anti H-bonded conformer. The value of 2.5 kcal mol^?1should be taken as an upper limit, since the geometry optimization of the gauche conformer of 1,2-diaminopropane is incomplete compared to the optimization carried out for the anti conformer.     

Synthesis of novel macrobicyclic polyfunctional cryptands

A pathway yielding both $\text{syn}$ and $\text{anti}$-functionalized macrocycles derived from bis-tartaro-18-crown-6 (1) as well as a selective route to $\text{syn}$ compounds have been developed and applied to the synthesis of new macrobicyclic polyether cryptands (2) and (3), which may also be obtained by direct bridging of the dianhydride (4).     

Diastereoselective hydroformylation of acyclic olefins: Efficient construction of an all-anti stereotriade building block for polyketide synthesis

The hydroformylation of 2-phenyl-4-(prop-2-enyl)[1,3]dioxanes has been studied. anti-Acetals1a,b were found to give extraordinarily high diastereoselectivity on hydroformylation, giving rise to the formation of the all-anti stereotriades 2a,b. The origin of this stereoselectivity may be related to the preferred conformation of the actetals 3 in solution, as determined by 2D-NOESY NMR experiments and force field calculations (MM3). Hydroformylation of 1a afforded an intermediate for a short and efficient synthesis of a key building block for the total synthesis of the macrolide antibiotic bafilomycin A₁.     

The rotational spectrum and molecular properties of a hydrogen-bonded dimer of phosphine and hydrogen fluoride

⁵⁷Fe Mössbauer spectra have been obtained for Fe(p-CH₃C₆H₄SO₃)₂ between 2.3 and 300 K in zero field, and at 2.3 and 4.2 K in longitudinal applied magnetic fields ranging from 1.1 to 5.6 T. The complex is a fast-relaxing paramagnet under all conditions studied and there is no evidence of antiferromagnetic exchange coupling. The FeO₆ chromophore is distorted by a trigonal elongation and the orbital ground state is the [($\text{2}\text{3}$)^{$\text{1}\text{2}$}|±2〉 ? ($\text{1}\text{3}$)^{$\text{1}\text{2}$}|?1〉] doublet. The temperature dependence of the quadrupole splitting has been analysed via a crystal-field model to provide estimates of the axial field splitting parameter Ds = -93 cm^-1, spin-orbit coupling constant λ = -70 cm^-1, and fine structure constant Dσ = -28 cm^-1. The magnetic properties of the complex are described by treating the ground state as a non-Kramers doublet with fictitious spin ? = $\text{1}\text{2}$. Five separate Mössbauer-Zeeman spectra can be fitted in this spin-hamiltonian approximation with identical values of the g- and A-tensor components, viz. g_⊥ = 1.0, g_u = 9.0; A_⊥ ≈ 2.0 mm s^-1. A_u = -1.79 mm s^-1. The trigonal z axis, the z axis of the electric field gradient tensor, and the easy axis of magnetisation are collinear, and the saturation value of the internal hyperfine field along this axis is +13.0 T.     

Substituted propanes: V. The vibrational spectra,dipole moments and conformations of 1,2-dichloro-, 1-chloro-2-bromo- and 1,2-dibromopropane

The infrared and Raman spectra of 1,2-dichloro-, 1-chloro-2-bromo- and 1,2-dibromopropane were recorded as liquids, in polar and non-polar solvents and in the crystalline state at low temperatures. The infrared spectrum of a high pressure crystal of 1,2-dibromopropane was recorded at ambient temperature. Dipole measurements were carried out in CCl₄ and C₆H₆solutions.Each of the 1,2-dihalopropanes existed as a mixture of three conformers in the liquid state, the one with the halogens in the anti position was in large abundance. Spectral and dipole measurements revealed a comparatively larger concentration of the two polar gauche conformers in polar solvents. All the compounds crystallized in the anti form. The C-halogen stretching vibrations have been interpreted in detail. Vibrational analyses of the anti conformers have been carried out and a tentative assignment of the majority of the gauche bands has been proposed.     

Electronic control of stereoselectivity-21 : Stereochemical parallelism between dienophilic capture and singlet oxygenation of isodicyclopentadiene derivatives

The stereochemistry and product distribution resulting from reaction of 4',5',6',7'- tetrahydrospirol[cyclopropane-1,2']-[4,7]methano[2$\text{H}$]indene(5), endo-2-methyl(6a) and 2,2-dunethyl-4,7-dihydro-4,7-methano-2$\text{H}$-indene (6b), as well as 4',5',6',7'-tetrahydrospiro[cyclopentane-1,2']-[4,7]methano-[2$\text{H}$]indene (7) with singlet oxygen have been determined. Stereochemical assignments to the diepoxide products were readily deduced by ¹³C-NMR comparison with the spectra of the parent isomcrs of established structure (X-ray). To unravel the stereochemistry of the epoxy aldehydes, recourse was made to 2D NOE experiments The observed stereosclectivity and reaction profile of each substrate are analyzed and placed in proper mechanistic and energetic perspective.     

The gas-phase rotamers of 1,1,2,2-tetrafluoroethane - force field,vibrational amplitudes and geometry - a joint electron-diffraction and spectroscopic study

The gas-phase conformational mixture of the anti and gauche rotamers of 1,1,2,2-tetrafluoroethane has been subjected to an electron-diffraction study at 253 K. Effective least-squares refinement of the geometry and relative proportions of the conformers was achieved with vibrational amplitudes for both conformers fixed at values calculated from spectroscopic data. In order to calculate the amplitudes, a force field was deduced which reproduced the observed wave numbers for both conformers; the assignment of the modes proposed in the literature was modified slightly. At 253 K, the rotamer composition was found to be 84% anti : 16%gauche, which corresponds to an energy difference of 1170 cal mol^?1; the geometrical parameters (r_a values) and e.s.d. are C-C = 1.518 ± 0.005 Å, C-H = 1.098 ± 0.006 Å, C-F = 1.350 ± 0.002 Å. ∠CCF = 108.2 ± 0.3°, ∠FCF = 107.3 ± 0-3°, ∠ CCH = 110.3 ± 1.0δ, and the torsion angleτ _hcch in the gauche form is 78 ± 2°.     

Completely stereoselective synthesis of all four stereoisomeric 1-carbamoyloxy-1,3-alkadienes via anti-diastereoselective homoaldol reaction from aldehydes and a single carbon-three-unit

Lithiation of both ($\text{Z}$)- and ($\text{E}$)-3-trimethylsilyl-2-propenyl $\text{N}$,$\text{N}$-diisopropyl carbamate 2 affords the (2$\text{E}$) -lithium compound 3. Aluminium- or titanium-mediated addition to aldehydes, 4 gives (1$\text{E}$)-(3$\text{R}$*4$\text{S}$*)-enol carbamates 7. A stereospecific Peterson elimination (borontriflouride- or base-mediated) introduces the second double bond either with (3$\text{E}$)- or with (3$\text{Z}$)-configuration. So just by reagent selection for each of the two steps, (1$\text{E}$,3$\text{E}$)-, (1$\text{E}$,3$\text{Z}$)-, (1$\text{Z}$,3$\text{E}$)-, or (1$\text{Z}$,3$\text{Z}$)- dienes 8 - 11, respectively, are prepared with stereoselectivities up to > 99.7%     

Conformational interactions between a phenyl ring and a side-chain halide substituent: A 1H NMR and molecular mechanics study of some 2-aryl-1-halopropanes

¹H NMR data are reported for a series of 2-aryl-1-halopropanes. Vicinal coupling constants in the CH₂CH—fragment show that the rotamer populations about the CC bond are sensitive to para substituents. The ratio of anti:gauche aryl/halide conformers is greatest when the para substituent is the electron-donating ethyl group and least when it is the strongly electron-withdrawing nitro group. This points to a non-steric conformational interaction involving the ring and the sidechain heteroatom. Comparison of the empirical results with conformational preferences predicted from molecular mechanics calculations using the COSMIC force field suggests that the interaction serves to enhance the population of the anti arrangement.     

Photochemistry of dienones—X: Photochemistry of (e)-β-ionone oxime ethyl ether

(E)-β-ionone oximc ethyl ether [(E, E)-4] upon direct irradiation with λ either254or 313 nm yields the geometrical isomer (E, Z)-4 and (Z)-retro-γ-ionone oxime ethyl ether (Z,E)-5 as the sole primary products, illustrating (E)-(Z) isomerization (φ₃₁₃ =0.49) and a 1, 5-hydrogen shift (φ313 =0.15) respectively. From studies with triplet photosensitizers and with ethyl iodide (to enhance the singlet-triplet intersystem crossing) it is concluded that these two products in the direct irradiation result only from the singlet excited state, and that the inter-system crossing quantum yield is relatively low. Upon prolonged irradiation of (E,E)-4 with λ 313 nm the eventual products are (Z,E)-5 and (Z,Z)-5, whereas with λ 254 nm they are (E,E)-5 and [(Z,E)-5 and/or (E,Z)-5]. Upon triplet photosensitization (E,E)-4 undergoes only (E)-(Z) isomerization, leading to a mixture of all the four geometrical isomers of4. From the dependence of the geometrical isomer distribution in the photostationary state on the triplet energy of the sensitizer the triplet energies of (E,E)-4, (E, Z)-4, (Z, E)-4, and (Z, Z)-4 have been determined to be ca 55, < 55,57, and 57 $\text{kcal}\text{mol}$ respectively.     

Asymmetric total synthesis of stoechospermol using intramolecular (2+2) photocycloaddition reaction

The first asymmetric total synthesis of stoechospermol, a representative spatane diterpene having $\text{cis}$, $\text{anti}$, $\text{cis}$-tricyclo[5.3.0.0^2,6]decane ring system, was achieved. Using the intramolecular asymmetric (2+2) photocycloaddition reaction of the diastereomeric ester 11, the readily available butenolide 9 was transformed into the optically active dilactone 12a and 12b. Subsequent construction of tricyclic carbon ring system and introduction of substituents in a right stereochemistry gave rise to optically pure stoechospermol 1.     

Thermal rearrangements of some indole alkaloid derivatives

Under both static and flow thermolysis conditions, several compounds with an “aspidosperma” framework rearranged to “vinca” derivatives. Thus (-)1,2-dehydroaspidospermidine ($\text{4}$) rearranged to (-) gave vincane $\text{14}$. Compound $\text{6}$ rearranged to vincamine ($\text{13a}$) and 16-epi vincamine ($\text{13b}$) under either condition ; increasing the temperature resulted in formation of apovincamine ($\text{19}$) (pyrolysis) or vincamone thermolysis).     

Verbrückte und unverbrückte norbornyl-kationen in der gasphase. Berechnungen zur stabilität isomerer norbornyl-kationen mit hilfe quantenmechanischer verfahren

The geometry and the relative stability of bicyclic compounds 1–20 have been calculated by standard quantum mechanics methods.MINDO/3 yields the following stability order of isomeric norbornyl cations (relative energies in $\text{kcal}\text{mole}$): 1-norbornyl cation 9 (0.0); 1.7 σ-bridged cation 6 (0.7); 7-norbornyl cation (nonplanar) 7 (1.1); 2-norbornyl cation (classical) 2 (4.2); 7-norbornyl cation (planar) 8 (4.3); 2-norbornyl cation (bridged) 1 (6.1). The stability of the same ions calculated by ab initio methods (STO-3G, MINDO/3-geometry) leads to an order more nearly consistent with experimental results: 2-norbornyl cation (classical) 2 (0.0); 2-norbornyl cation (bridged) 1 (5.9); 7-norbornyl cation (planar) 8 (11.1); 1-norbornyl cation 9 (14.6); 7-norbornyl cation (nonplanar) 7 (21.2). For the secondary 7-norbornyl cation, MINDO/3 gives a pyramidal configuration, 3.2 $\text{kcal}\text{mole}$ more stable than the planar form. In contrast, the ab initio results (complete optimization of all geometrical parameters) indicate the planar cation to be the most stable form. The bridged structure of 2-norbornyl cation 1 is calculated (STO-3G, partly optimized) to be 4.3 $\text{kcal}\text{mole}$ less stable than the classical counterpart, 2. For the lower homologues 12 and 13 (STO-3G, complete geometry optimization), this difference is 6.4 $\text{kcal}\text{mole}$. However, more extended basis sets should favour the bridged structures. The hydrogen bridged norbornyl cations 3, 4, and 5 have been calculated (STO-3G, partly optimized) to be 14.4, 23.6 and 29.9 $\text{kcal}\text{mole}$ less stable than 2. The stability differences between the corresponding tertiary bicyclic ions 10 vs 11, and 14 vs 15 are calculated (ab initio) to be 15.3 and 19.0 kcal/mole, respectively, in favour of classical structures. The influence of methyl substitution at positions C₁ and C₆ (exo) on bridged and unbridged structure of 2-norbornyl cation is calculated. Pyramidal secondary and tertiary 2-norbornyl cations 19 (a; R=H, b; R=CH₃) and 20 (a; R=H, b; r=CH₃) have been used to model the electrical effects in the solvolysis transition states of epimeric 2-norbornyl esters. Due to more efficient hyperconjugation the pyramidal exo cation is stabilized more than the endo cation by 5.2 $\text{kcal}\text{mole}$ for the secondary series and 3.5 $\text{kcal}\text{mole}$ for the tertiary series. Bonding of endo cation 20 with a nucleophile should be stronger than bonding of exo cation 19 due to more efficient HOMO-LUMO interaction.     

Molecular structure and conformation of cis-1,3- dichloro-1-propene as determined by gas phase electron diffraction

The molecular structure and conformation of cis-1,3-dichloro-1-propene have been determined by gas phase electron diffraction at a nozzle temperature of 90°C. The molecule exists in a form in which the chlorine atom of the methyl group and the carbon-carbon double bond are gauche to one another. The results for the distance (r_g) and angle (∠_α) parameters are: r(C-H) = 1.078(10)Å, r(CC) = 1.340(5)Å, r(C-C) = 1.508(7)Å, r( =C-Cl) = 1.762(3)Å, r(C-Cl) = 1.806(3)Å, ∠Cl-C-C = 111.7°(1.8), ∠(CC-C) = 125.5°(1.5), ∠Cl-CC = 124.6°(1.6) and ∠H-C-Cl = 111°(5). The torsion-sensitive distances close to the gauche form can be approximated using a dynamic model with a quartic double minimum potential function of the form V(Φ) = V₀[1 + ($\text{Φ}\text{Φ}{}_0$⁴ - 2($\text{Φ}\text{Φ}{}_0$)²], where V_o = 1.1(8) kcal mol^?1 and Φ₀ = 56°(5) (Φ = 0 corresponds to the anti form).