Gauche Interactions Research Articles

The Conformation of a Diastereoisomeric Pair of 2,2-Dimethyl-4-phenyl-3-pentanols

Abstract The conformations of a diastereoisomeric pair of 2,2-dimethyl-4-phenyl-3-pentanols were studied by means of 1H-NMR, 13C-NMR, IR, GC, and MS analyses. All of the results point to an unambiguous conclusion that these molecules are present dominantly in conformers where the t-butyl and the phenyl group lie close to each other. The generality of the phenomenon (favored gauche interaction) as well as the nature of the interactions involved were discussed in light of the conformational problems of the structurally related molecules. The presence of an attractive interaction (CH/π interaction) is suggested between an alkyl and a phenyl group, the possible importance of which in some dynamic phenomena (e.g., selectivities in certain reactions) has then been discussed on this basis.

Intramolecular interactions. 33. Conformational and geometrical adaptations to strong gauche interactions: analysis of the x-ray crystallographic data of six congested 3-tert-butylcyclohexene derivatives

Intramolecular interactions. 33. Conformational and geometrical adaptations to strong gauche interactions: analysis of the x-ray crystallographic data of six congested 3-tert-butylcyclohexene derivatives

1,4-Interactions in molecular mechanics calculations of Ethers

Van der Waals and electrostatic interactions are found to be insufficient for the calculation of conformational energies of ethers by molecular mechanics. Low order torsional potential functions must be added for the potential about C-O bonds. A onefold term necessary for the CCOC-fragment is interpreted to be a substitute for gauche interactions present in CCCC-, but missing in CCOC-fragments. For the COCO fragment the anomeric effect must be included explicitly as another torsional energy term, but no such term is required to stabilize the gauche conformation for OCCO. With the resulting ether force field the geometries and energies of model compounds, many of them 1,3-dioxanes, are calculated with good accuracy.

13C nuclear magnetic resonance spectra. Part 10. Substituent effects on the 13C chemical shifts of adamantanes, diamantanes, and triamantanes

The 13C n.m.r. data of a variety of substituted adamantanes, diamantanes, and triamantanes are presented. The α-substituted chemical shifts (SCS) can be described by gauche XC interactions, the magnitudes of which decrease in the sequence BrC > HC > OC ≳ CC. Similarly, gauche interactions of the types HX and CX contribute to the β-SCS, but additionally the β-SCS are increased, if the β-carbon atoms possess antiperplanar γ′-carbon atoms. Steric interaction of the substituent with gauche γ-carbon atoms seem to cause rather downfield rather than upfield shifts of the γ-signals. The hyperconjugative interaction of parallel Cα–Cβ- and Cβ′–Cα′-bond orbitals increase the γanii-SCS of the hydroxy and, to a smaller extent, the bromine substituent.

Determination of the conformational preference of the halogens in 4-halo-oxacyclohexanes

The equatorial conformational preferences for the halogens in 4-oxacyclohexanes are found to be less pronounced than in cyclohexanes, the magnitude depending on the electronegativity (F > Cl > Br > I). The results were obtained from 1H-NMR and 13C-NMR spectral data, extracted below and above coalescence. The 13C shift method was also tested on the cyclohexylhalides for which accurate data were already available. The observed phenomenon confirms the existence of an attractive gauche interaction between a positivated carbon fragment and an electronegative moiety.

Determination of gauche and trans proton vicinal coupling constant Jg and Jt for 1,2-dichloropropane as a model compound for 1,2-disubstituted and 1,2,3-trisubstituted profanes

Variations in temperature and nature of solvent lead to changes in the population of the three rotamers of the title compounds in solution. The medium can be adjusted in this way so that either equality in the population of the three rotamers is reached, or the population of the conformer with two gauche interactions is decreased to near zero. These two limits are used to calculate J t and J g, for 1,2-dichloropropane.

Interactions intramoléculaires: Partie XXVIII. Études structurales en série oxa-7 bicyclo[4.1.0]heptanique: identification des stéréoisoméres, paramétres RMN et équilibr

Proton chemical shifts and coupling constants are obtained for 11 compounds containing the 7-oxa bicyclo-[4.1.0]-heptane system (substituents CH 3, COOCH 3, CN). Diastereoisomers are identified by means of various methods (shift reagent, chemicalshift increments). Conformational equilibria are determined from vicinal coupling constants and conformational free energies of methyl group (position 3) and methoxycarbonyl (position 4) are obtained; gauche interactions between CH 3, and COOCH 3 groups are also discussed.

Interpretation of monomer‐polymer equilibria in the polymerization of alkyl substituted 1,3‐dioxolanes

AbstractThe thermodynamic parameters for the monomer‐polymer equilibria in the polymerization of 4‐methyl‐, 4‐ethyl‐, 4‐isopropyl‐, 4,4‐dimethyl‐, and cis‐ and trans‐4,5‐dimethyl‐1,3‐dioxolanes were calculated from the number of conformational isomers. The number of conformational isomers and the gauche interactions involved were enumerated for the monomeric unit of each polymer, whilst the number of conformational isomers and their energies due to the gauche interactions, bond opposition strain (“Pitzer strain”) between the cis neighbouring bonds of the ethylene part of the 1,3‐dioxolane ring, and 1,3‐nonbonded interaction were estimated for each monomer. On the basis of these enumerations, the enthalpy and entropy values for the polymerization of the alkyl substituted 1,3‐dioxolanes were computed by using the observed values for 1,3‐dioxolane as the standard for the calculation. From the comparison of the calculated values with the experimental results, it is concluded that the effects of alkyl substituents on the monomer‐polymer equilibria can arise principally from the various conformational isomers in the monomeric and polymeric states.

The shielding effect of heterosubstituents in 13C nuclear magnetic resonance

The effect of a heterosubstituent X on 13C chemical shifts, in conformations characterized by dihedral angles of 60°, is analyzed in terms of α-shifts, heteroatom–carbon (XC), hydrogen–heteroatom (HX) and carbon–heteroatom (CX) gauche interactions, and nonbonded γ(X) and δ(X) interactions. The more general parameters [Formula: see text] describe the β-heteroatom shifts in conformations in which the dihedral angles [Formula: see text] differ from 60°. The shielding effects of chlorine, exocyclic oxygen, and endocyclic nitrogen are analyzed.

β-Carbon Shielding Effects in Carbon-13 Nuclear Magnetic Resonance Spectroscopy

Carbon-13 chemical shifts in saturated hydrocarbons of staggered conformation may be derived from the corresponding resonances in ethane, propane, isobutane, and neopentane in the following way: every H—C—C(α)—C(β) gauche interaction between an α-hydrogen and a 3-carbon deshields a carbon nucleus by ca. 4.7 p.p.m. Every exocyclic carbon–carbon gauche configuration H—C(α)—C—C(α)—C(β)—H has a deshielding effect of ca. 1.7 p.p.m., as the result of nonbonded hydrogen–hydrogen γ-interactions. Every nonbonded interaction between an α-hydrogen and a hydrogen on a γ-carbon makes a shielding contribution of ca.−4.5 p.p.m.

Lanthanide induced 1H and 13C NMR shifts and their use for geometry analysis with alicyclic compounds

Based on computations with conformationally rigid substrates the ambiguities involved in the geometrical analysis of pseudocontact shifts are demonstrated. The minimal agreement factors R and the corresponding lanthanide positions (usually more than one) are extremely dependent upon the chosen structural data for the model substrate and on the errors of the experimental shift values. It is observed, that a difference or improvement of R factors of less than 3% is not significant in most cases. This is so, even on the basis of Ytterbium induced 13C shifts, which are found to be more accurate than 1H shifts and free from contact contributions within the experimental error. Using lanthanide induced shifts some 13C signal assignment problems are discussed. The computed minima for five substituted norbornanols indicate a lanthanide position with an L . O distance of d = 2·5 Å for the secondary alcohols and an orientation avoiding gauche interactions with the two neighbouring CC bonds. Similar computed results are obtained with five bicyclic ketones, except for an L . O distance from 3 to 4 Å increasing with steric hindrance. The particular problems with the analysis of symmetrical compounds like cyclohexyl derivatives are pointed out.

Heats of formation of C1C4 alkyl nitrites (RONO) and their RO‐NO bond dissociation energies

AbstractThe heats of formation of C3 and C4 alkyl nitrites (RONO) have been determined via their heats of combustion by bomb calorimetry, thereby providing a complete set of values of ΔHºf for C1‐C4 alkyl nitrites. The experimental values are in excellent agreement with values derived from group additivity rules. For branched compounds these calculations involve corrections for gauche interactions. In these cases, the gauche interactions are reflected in the activation energies E1 determined by recent kinetic studies, required for breaking the RO‐NO bond. equation image The heats of formation of the alkoxy radicals involved together with ΔHºf(NO) = 21.6 kcal/mole leads to the result D(RO‐NO) = 41.5 ± 1 kcal/mole. The concordance between D(thermochemical) and D(kinetic), unlike previous kinetic studies, implies that E2 = 0 ± 1 kcal/mole.

Conformational analysis—CI : The gauche-hydrogen interaction as the basis of conformational analysis

It is concluded that the gauche-butane interaction which is one of the cornerstones of conformational analysis has been incorrectly interpreted as to origin, and that the gauche relationship between methyl (or methylene) groups is not the cause of the relative instability of gauche conformations. Rather, the gauche interactions between vicinal-2,3-hydrogens are mainly responsible. By the same taken, a substituent on a cyclohexane ring should not be said to have a preference for the equatorial position. Rather, the tertiary hydrogen has an energetic preference for an axial position, which leaves the substituent in the equatorial position by default. Several poorly understood phenomena are better interpreted on this basis.

The gauche Effect. A Theoretical Study of the Topomerization (Degenerate Racemization) and Tautomerization of Methoxide Ion Tautomer

Non-empirical double zeta quality molecular orbital calculations on −CH2OH as a function of the C—O bond length (r), the rotational angle about the C—O bond (θ), and the pyramidal angle at carbon [Formula: see text] are described. From the stretching potential curve, E(r), it is shown that dissociation of −CH2OH proceeds to give CH2 and OH−. The rotation–inversion surface, [Formula: see text], has two types of minima; in both cases the most favorable pyramidal angle at carbon is 105°. The lower minimum corresponds to a structure (the Y conformation) having the hydroxyl proton on the external bisector of the HCH angle. The higher minimum is 6.67 kcal/mol less stable and corresponds to a structure (the W conformation) having the hydroxyl proton on the internal bisector of the HCH angle. The relationship of these results to the gauche effect is discussed and it is noted that at certain internuclear distances the nuclear–nuclear repulsion term (Enucl) may overcome the tendency of adjacent electron pairs and polar bonds to exist preferentially in that conformation which has the maximum number of gauche interactions between these electron pairs or polar bonds.The topomerization of −CH2OH, i.e., the conformational transformation from one Y conformation into another, proceeds, via the W conformation as an intermediate, by two separate events, viz. rotation about the C—O bond, having a barrier of 10.58 kcal/mol, and pyramidal inversion at carbon, with a barrier of 20.52 kcal/mol. Some factors governing the relative importance of rotation and inversion in degenerate racemization are discussed.In its ground electronic state CH3O− is 22.18 kcal/mol more stable than −CH2OH. However, in the low-lying excited states all conformations of −CH2OH are stabilized relative to CH3O−. The most stable excited state structure of −CH2OH corresponds to the energy maximum for rotation–inversion of the ground electronic state.

Dipole moments and conformation of thiosulphinates

The conformation of some S-alkyl and -aryl thiosulphinates has been evaluated from the electric dipole moments measured in benzene solution. The synclinal conformation (A) here observed is the same as for sulphinates. From comparison with disulphides and their S-oxides, it appears that the stereochemistry of all these compounds is controlled by the gauche interaction of the two C–S bonds.

Conformational analysis—LXXXIII : Calculation of the structures and energies of silanes by the method of molecular mechanics

The structures and energies of a number of silanes have been studied by the method of molecular mechanics, and the results are compared with the available experimental data, and with the hydrocarbon analogs. Torsional barriers are relatively small and gauche interactions are unimportant in the silanes, which leads to several unusual conformational effects. Heats of formation are calculated.

Conformational analysis of dibromocyclohexanes by nuclear magnetic resonance spectroscopy: Evidence for the participation of a twist form to the conformational equilibrium of r-1, t-2-dibromo- c-3-t-butylcyclohexane

The NMR parameters of several substituted trans-1,2-dibromocyclohexanes, trans-2-bromocyclohexanols and their derivatives have been measured in order to investigate their conformational features. An analysis of these data and the temperature dependence of the coupling constants of r-1, t-2-dibromo- c-3-t-butylcyclohexane demonstrate that an appreciable population of the twist conformer 1t contributes to the conformational equilibrium of this dibromide. The destabilization of its triequatorial chair conformation 1e with respect to the twist one 1t is attributed to astrong gauche interaction between equatorial t-butyl and bromine and dipole-dipole repulsion between the two halogen atoms.

Influence de la structure du prépolymère sur la température de transition vitreuse de polyuréthanes réticulés

The influence of prepolymer structure on glass transition temperatures (Tg) of cross-linked polyurethanes has been studied with polysebacates from a homologous family of N-substituted bis(2-hydroxyalkyl)amines. The tertiary amine segments have made it possible to study the effect of lengthening the side chains (C1–C18) and inserting gauche interactions in the polymeric chain. The results show that in this system, adding gauche interactions has more effect on Tg than lengthening the side chains. Each time a gauche interaction is introduced in the diol segments, Tg increases by 9 °C. Comparisons with other systems are also presented.

Studies on conformation and reactivity—1 : Sugars with potential antiviral activity—4 The conformations of some acyclic sugar derivatives

An examination of the conformations adopted by acyclic sugar derivatives has been made. It is shown that 1,3-eclipsed rather than 1,2- gauche interactions are important in determining relative conformer population and that the observed conformer distribution, as determined by analysis of PMR spectra, is close to that calculated from consideration of non-bonded interactions.

Conformational studies XIV. Destabilization of chair conformations of cyclohexanones by [formula omitted]-butyl-hydroxyl and [formula omitted]-butyl-methoxyl gauche interactions.

Conformational studies XIV. Destabilization of chair conformations of cyclohexanones by [formula omitted]-butyl-hydroxyl and [formula omitted]-butyl-methoxyl gauche interactions.