Cyclohexanone Dimethyl Acetals: A 13C NMR, Thermodynamic, and Ab Initio Study

The spatial structures of a number of mono- and disubstituted 1,1-dimethoxycyclohexanes (cyclohexanone dimethyl acetals) were studied by ¹³C NMR spectroscopy. In the monosubstituted acetals, substituents (Me, Et, i-Pr, and MeO) on C-2 are axially oriented, contrary to their normal, equatorial orientation on C-3 and C-4. Besides the spectroscopic study, the relative thermodynamic stabilities of the cis-trans isomers of a few 2,X-dialkyl (X = 3, 4, 5, or 6) derivatives of the parent cyclohexanone dimethyl acetal were determined by acid-catalyzed chemical equilibration in MeOH solution. In the most stable isomeric form, the 2-substituent is axial and the other equatorial. In the less stable isomer, both substituents are equatorial, excluding the cis-2,6-dimethyl derivative, where the ¹³C NMR shift data point to a predominance of the diaxial form. In general, the enthalpy difference between the isomeric forms is ca. 9 kJ mol^–1, while the entropy term favors the less stable isomer by 4 to 16 J K^–1 mol^–1. In the 2,6-dimethyl derivatives, however, the trans form is favored by only 0.8 kJ mol^–1 in G _m at 298.15 K. The main findings of the experimental work are in good agreement with ab initio calculations.     

Asymmetric synthesis of heteroorganic analogs of natural products 5. Synthesis of pure diastereomers and enantiomers of fluoro-substituted (2S,3R)- and (2R,3R)-β-phenylserines

A convenient method is proposed for the preparative synthesis of pure diastereomers and enantiomers of fluoro-substituted (2S,3R)- and (2R,3R)--phenylserines by the hydroxyalkylation of the Ni(II) — complex of the Schiff base of glycine with (S)-2-N-(N¹-benzylpropyl)aminobenzophenone using the corresponding fluoro-substituted benzaldehydes.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya. No. 8, pp. 1906–1913, August, 1991.     

Quantum Chemical Study of the Preferred Conformations,Molecular Geometries,and Relative Thermodynamic Stabilities of 2-Substituted 4-Methylene-1,3-Dioxolanes and 4-Methyl-1,3-Dioxoles

The preferred conformations, molecular geometries, and relative stabilities of carbon–carbon double-bond exo–endo isomeric 2-substituted 4-methylene-1,3-dioxolanes (a) and 4-methyl-1,3-dioxoles (b) have been studied by DFT calculations at the B3LYP/6-31G* level of theory. The main interest of this work was devoted to the contribution of alkoxy substituents on the relative thermodynamic stabilities of these isomeric unsaturated acetals. Comparison of the computational data with previous experimental findings shows both the enthalpies and entropies of the a b isomerization to be accurately predictable by the DFT calculations. Most importantly, the good agreement between experiment and theory proved also to be applicable to the previously observed unexpectedly large effect of 2-alkoxy substitution on the isomer equilibria.     

Stereochemistry of the methylation of the (11S,4S) and (11S,4R) diastereomers of 4-methyl-1-(α-methylbenzyl)azetidin-2-one

The methylation of the lithium derivatives of the (1¹S,4S) and (1¹S,4R) diastereomers of 4-methyl-1-(-methylbenzyl)azetidin-2-one proceeds stereospecifically with the formation of only trans-(3S,4S)- and trans-(3R,4R)-dimethyl-1-[(S)--methylbenzyl] azetidin-2-one, respectively. The process is accompanied by epimerization at the asymmetric center of the N--methylbenzyl substituent.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 7, pp. 914–920, July, 1990.     

An Ab Initio Study of the Relative Stabilities and Molecular Structures of 3-Substituted 2,5-Dihydrofurans and 4-Substituted 2,3-Dihydrofurans

The geometry optimized structures and total energies of 3-substituted (R) 2,5-dihydrofurans (a) and their isomers, 4-substituted 2,3-dihydrofurans (b), have been determined by ab initio calculations at the MP2/6-31G^*//HF/6-31G^* level. The nature of the moiety R has a marked effect on the relative total energies of the isomeric forms: at the calculation level cited, the reaction enthalpies for the a b isomerization range from +4.7 kJ mol^–1 for R = MeO to –30.5 kJ mol^–1 for both R = COOMe and R = NO₂. The reaction enthalpies appear to be controlled by the electronic effect of R on the strength of p- conjugation in b. The a isomer has a planar ring, independent of R (excluding NH₂), whereas the planarity of b depends on the electronic nature of R: the 2,3-dihydrofuran ring is planar for both R = COOMe and R = NO₂, but nonplanar for less conjugation-enhancing substituents.     

Asymmetric synthesis,stereochemistry, and absolute configuration of 1,2-5-trimethyl-5-(2-cyanoethyl)- and 1,2,5-trimethyl-4-piperidinones

The asymmetric Michael alkylation of (–)-1,2,5-trimethyl-4-(1S-phenylethylimino)piperidine by electrondeficient alkenes proceeds with the formation of(+)-cis-(2S,5S)- and (–)-trans-(2R,5S) diastereomers of 1,2,5-trimethyl-5-(2-cyanoethyl)- and 1,2,5-trimethyl-5-(2-methoxycarbonylethyl)-4-piperidinones containing a chiral quaternary center C(₅). The spatial structure of these new chiral 4-piperidinones has been established on the basis of ¹H and ¹³C NMR spectroscopic data. The absolute configuration of the C(₂) and c(₅) chiral centers in the diastereomers has been determined by stereochemical correlation on the basis of circular dichroism data.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1369–1378, October, 1992.     

Cyclization and rearrangement of diterpenoids. VII. Composition of the hydrocarbon fraction of a mixture of the products of cyclization of manool and sclareol by ordinary acids

The hydrocarbon fractions of the products of the cyclization of manool and sclareol by a mixture of conc. sulfuric and formic or acetic acids have been investigated. It has been established that they contain, in addition to the ⁸/⁹-pimaradiene, ⁸(⁹)-isopimaradiene, roasadiene, and 13-epirosadiene identified previously, the tetracyclic hydrocarbons (1R, 2S, 7S, 11S, 12R, 13R)-2, 6, 6, 11, 13-pentamethyl-tetracyclo [10.2.1.0^1.10.0^2.7] pentadeca-9-ene and (1S, 2R, 11S, 12R, 1R)-2, 7, 7, 11, 15-pentamethyltetracyclo[10.2.1.0^2.11.0^3.8] pentadeca(8)-ene.Institute of Chemistry of the Moldavian SSR Academy of Sciences, Kishinev. Translated from Khimiya Prirodnykh Soedinii, No. 5, pp. 719–721, September–October, 1988.     

Stabilization of a K+-(bis-Cage-Annulated 20-Crown-6) Complex by Bidentate Picrate

Bis-cage-annulated 18-crown-6 and 20-crown-6 macrocyclic ethers (i.e., 1 and 2, respectively) have been synthesized, and their alkali metal picrate extraction profiles have been determined. Host system 1 proved to be a significantly more avid alkali metal cation complexant than 2 and somewhat more avid than 18-crown-6. Both 1 and 18-crown-6 display modest selectivity toward K⁺ and Rb⁺. A stable host–guest complex was prepared by slow evaporation of a CH₂Cl₂–hexane solution of an equimolar mixture of 2 and potassium picrate. The X-ray crystal structure of this complex reveals that picrate anion functions as a bidentate ligand therein. The gas-phase interaction energy between the 2 K⁺ complex and picrate anion was calculated to be ca. –64.9 kcal mol^–1, thereby indicating that participation of picrate anion as an additional bidentate ligand results in significant stabilization of complex 10.     

17O, 13C, and 1H NMR Studies of p–π Conjugation in 2-Substituted 4-Methylene-1,3-dioxolanes and 4-Methyl-1,3-dioxoles

The ¹⁷O NMR spectra of a number of unsaturated 5-membered cyclic acetals, 2-substituted 4-methylene-1,3-dioxolanes and their endocyclic isomers, 4-methyl-1,3-dioxoles, have been recorded. The ¹⁷O NMR chemical shifts, in comparison with those of similarly 2-substituted 1,3-dioxolanes, were used to explore the variation of the strength of p– conjugation in the unsaturated acetals as a function of the nature of substitution at C2. The ¹⁷O NMR shift data reveal that alkoxy substituents have a significantly more favorable effect on the strength of p– conjugation in 4-methyl-1,3-dioxoles than in 4-methylene-1,3-dioxolanes. This fact appears to be responsible for the previously observed unexpectedly large effect of alkoxy substitution on the relative thermodynamic stabilities of these two classes of isomeric compounds. Additional information of the unexpected charge distribution in 4-methyl-1,3-dioxoles is provided by their ¹H and ¹³C NMR spectra.     

IR spectra of diastereomeric (with respect to phosphorus) 2′, 3′-O-alkylphosphonates of nucleosides

In a study of the IR absorption spectra of diastereomeric (with respect to phosphorus) 2,3-O-alkylphosphonates of nucleosides it was shown that the IR spectra of the diastereomers differ at 820–860 (_O-P-O), 1040–1060 (_P-O-C), and 1250–1300 cm^–1 (_P=O). Higher _O=P and _P-O-C values and lower _O-P-O values are observed for the R isomers (as compared with the S isomers), and this provides a possibility for the determination of the orientation of the P=O group relative to the protons of the phospholane ring in alkylphosphonates of nucleosides by comparison of the spectra of the diastereomers.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 2, pp. 181–184, February, 1979.     

Molecular mechanics modelling of Pt(II) complexes with antitumor activity. Influence of the type and the positions of the ring substituents on the conformational energies and thermodynamic stabilities

Summary The influence of the type and the positions of the ring substituents on the conformational energies and thermodynamic stabilities of a series of Pt(II) complexes of the general formula [1,2-bis(hydroxyphenyl)ethylenediamine]PtL ₂(L ₂=2Cl^–, 2I^–, SO ₄ ^2– ) has been studied by molecular mechanics. The calculations were carried out for the ligand conformations (R,S/S,R)-, (R,S/S,R)-, (R,R)-, and (S,S)-. The obtained energies and thermodynamic stabilities are in agreement with experimental data on the reactivity and antitumor activity of the compounds.

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Molecular Modelling von Pt(II)-Komplexen mit Antitumoraktivität. Einfluß von Art und Position von Substituenten auf Konformationsenergien und thermodynamische Stabilitäten

Zusammenfassung Die konformationellen Energien und thermodynamischen Stabilitäten einer Reihe von Pt(II)-Komplexen mit der allgemeinen Formel [1,2-bis(Hydroxyphenyl) ethylendiamin]PtL ₂(L ₂=2Cl^–, 2I^–, SO ₄ ^2– ) wurden mittels molekularmechanischer Methoden in Abhängigkeit von Art und Stellung der Substituenten an den Phenylringen untersucht. Die Berechnungen wurden für die Ligandenkonformationen (R,S/S,R)-, (R,S/S,R)-, (R,R)- und (S,S)- durchgeführt. Die erhaltenen Energien und Stabilitäten stimmen mit experimentellen Daten über Reaktivität und Antitumoraktivität der Verbindungen überein.

     

Tautomerism of rhodanine

Semiempirical (MINDO/3, AM1, PM3, MNDO) and ab initio (4-31G and 4-3IG + dAO/S basis sets) calculations on the relative stabilities and structures of the five potential tautomeric forms of rhodanine are reported. It is shown that all methods (excepting PM3) predict as most stable 2-thioxo-4-thiazolidinone. These results correspond to the known experimental data. The infrared spectrum of rhodanine was recorded for the region 4000-150 cm^–1, and the characteristic bands were compared with AM1 and 4-31G + dAO/S calculated frequencies. The transition states between five pairs of all possible tautomeric forms of the rhodanine were found by the AM1 method.     

Hydrogenation of the silanone groups (≡Si-O)2Si=0. Experimental and quantum-chemical studies

The reactivity of bis(siloxy)silanone groups (Si-0)₂Si=O stabilized on a silica surface with respect to H₂ molecules was studied. The reaction was found to give the (Si-O)₂SiH(OH) groups. The rate constant for this process was determined. Its activation energy in the 300–580 K temperature range is 13.4±0.3 kcal mol^–1, and the enthalpy is 54±5 kcal mol^–1. The activation energy for the reverse reaction,viz., elimination of a hydrogen molecule, is equal to 65 kcal mol^–1. Quantum-chemical calculations of hydrogenation of F₂Si=O and (HO)₂Si=O, which are the simplest molecular models of the silanone groups, were performed. Data on the geometrical and electronic structures of transition states and on the effects of substituents at the silicon atom on the reactivity of the silanone groups in this process were obtained. The optical absorption band of the surface silanone groups was quantitatively characterized. Its maximum is located at 5.65±0.1 eV; the extinction coefficient at the maximum (220 nm) is (3±0.5) · 10^–18 cm² molec.^–1.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1951–1958, August, 1996.     

Estimation of the Relative Stabilities of the E,Z-Isomers of α,β-Dialkylsubstituted Methyl Vinyl Ethers by Various Computational Methods

The relative thermodynamic stabilities (relative enthalpies) of the E,Z-isomers of ,-dialkylsubstituted methyl vinyl ethers MeOC(R₁)=CHR₂ have been estimated by various computational methods including molecular mechanics, semiempirical, ab initio, and DFT calculations. The best performance, approaching the accuracy of the experimental method of chemical equilibration, is shown by the DFT calculations. Ab initio methods, provided that electron correlation is taken into account, are also satisfactory, but clearly less successful than the DFT calculations. The reliability of the semiempirical methods AM1 and PM3 is considerably less good, and varies in an unpredictable manner from case to case. The poorest general performance is shown by the MM2 and MM3 calculations, which may overestimate the relative stability of the Z isomer by as much as 18 kJ mol^–1.     

Addition of diazomethane to acetals of some unsaturated aldehydes

Conclusions Diazomethane adds to the acetals of crotonaldehyde and glutaconic dialdehyde in a regioselective manner, forming a mixture of both possible ¹-pyrazolines.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 3, pp. 620–623, March, 1977.     

An investigation of mononuclear and binuclear palladium(II) complexes of egta (egta = glycine,N,N′-(1,2-ethanediylbis(oxy-2,1-ethanediyl)bis[N-carboxymethyl]) by 1H-, 13C- and 15N-n.m.r. methods

1:1 and 2:1 palladium(II) complexes of egta^4– (egta^4– = glycine, N,N-(1,2-ethanediylbis)(oxy-2,1-ethanediyl)bis[N-carboxymethyl]) were prepared by 1:1 and 2:1 addition of K₂PdCl₄ to K₄egta, and examined by ¹H-, ¹³C- and ¹⁵N-n.m.r. methods. The 1:1 complex, [Pd(egta)]^2– in solution, utilizes a square-planar coordination comprised of two nitrogen and two glycinato carboxylate donors of egta^4–, leaving two glycinato carboxylates pendant. The complex has a cis-(R,S) stereochemistry which places both pendant carboxylates below the PdN₂O₂ square plane and the tether backbone of egta^4– in the up, up sense above the same plane. The cis-(R,S) assignment was assisted by computer simulations of the ¹³C-n.m.r. spectrum for four possible isomers. Only cis-(R,S) and trans-(R,R) calculated ¹³C-spectra were compatible with the observed ¹³C-n.m.r. pattern. The HH NOESY spectrum of [Pd(egta)]^2– detects long range coupling of the backbone –OCH₂CH₂O– linkage with both coordinated and pendant glycinato CH₂ moieties. The cis-(R,S) isomer's tortional movements allow such contacts whereas a trans-(R,R) isomer does not. The 2:1 complex, [Pd₂(egta)(H₂O)₂] in solution has an extended-chain structure with each palladium(II) center coordinated in the mer-iminodiacetate-like coordination with two bound glycinato-functionalities.     

Relative Stabilities and Molecular Structures of the Geometrical Isomers of α,α′-Dioxasubstituted Cycloalkylidenecycloalkanes. A DFT Study

The relative stabilities and molecular structures of the geometrical isomers of seven ,-dioxa derivatives of cycloalkylidenecycloalkanes with 3to 6-membered rings have been studied by DFT calculations at the B3LYP/6-31G* level of theory. In each case, the E form was calculated to have the lower total energy. The relative energy of the Z isomer proved to increase regularly with increasing sizes of the two heterocyclic rings, ranging from 1.1 to 13.8 kJ mol^–1 on going from the 3,3to the 6,6-membered rings. Intermediate values of the relative energy were calculated for the Z forms of compounds containing two dissimilar rings. The relative energy of the Z isomer was found to be proportional to d ^–9.3, where d = the distance separating the two O atoms of the Z compound. The molecular structures, electric dipole moments, and atomic charges on the O atoms are also discussed.     

Molecular structure,conformational mobility,vibrational spectra,and thermochemistry of peroxyacetic acid: an ab initio and density functional study

The structure of the peroxyacetic acid (PAA) molecule and its conformational mobility under rotation about the peroxide bond was studied by ab initio and density functional methods. The free rotation is hindered by the trans-barrier of height 22.3 kJ mol^–1. The equilibrium molecular structure of AcOOH (C _s symmetry) is a result of intramolecular hydrogen bond. The high energy of hydrogen bonding (46 kJ mol^–1 according to natural bonding orbital analysis) hampers formation of intermolecular associates of AcOOH in the gas and liquid phases. The standard enthalpies of formation for AcOOH (–353.2 kJ mol^–1) and products of radical decomposition of the peroxide — AcO^· (–190.2 kJ mol^–1) and AcOO^· (–153.4 kJ mol^–1) — were determined by the G2 and G2(MP2) composite methods. The O—H and O—O bonds in the PAA molecule (bond energies are 417.8 and 202.3 kJ mol^–1, respectively) are much stronger than in alkyl hydroperoxide molecules. This provides an explanation for substantial contribution of non-radical channels of the decomposition of peroxyacetic acid. The electron density distribution and gas-phase acidity of PAA were determined. The transition states of the ethylene and cyclohexene epoxidation reactions were located (E _a = 71.7 and 50.9 kJ mol^–1 respectively).     

On the energy transfer at the radiolysis of CO-H2S gas mixture

The kinetics of hydrogen formation at various amounts of H₂S /1–60%/ in the radiolysis of CO–H₂S mixture has been studied. The ratio of the reaction rate constants for reactions CO^x+COproduct and CO^x+H₂SH+SH=CO, which amounts to 5×10^–2, has been estimated. The effective activation energy of hydrogen formation /E_eff/ has been determined at various amounts of H₂S in the temperature interval 323–573 K.     

Relative Thermodynamic Stabilities of Isomeric Vinyl Ethers Derived from 2-Acetyland 2-Isobutyryltetrahydrofurans. An Experimental and DFT Study

The relative thermodynamic stabilities of four isomeric vinyl ethers obtained by acid-catalyzed cleavage of MeOH from the dimethyl acetals of both 2-acetyltetrahydrofuran and 2-isobutyryltetrahydrofuran have been determined by chemical equilibration in cyclohexane solution. In addition, the structures and relative energies of many of these compounds have also been studied theoretically by DFT calculations. In each series of compounds, the endocyclic isomers, 5-(1-methoxyethyl)-2,3-dihydrofuran and 5-(1-methoxy-2-methylpropyl)-2,3-dihydrofuran, respectively, show the highest thermodynamic stabilities. This is particularly prominent for the vinyl ethers derived from 2-isobutyryltetrahydrofuran in which the equilibrium mixture consists of 90% of the endocyclic isomer at room temperature. The geometrical (exocyclic) isomers of the latter series of compounds show unexpected relative thermodynamic stabilities, but on the basis of the geometry-optimized structures provided by the DFT calculations, a reasonable explanation can be found.