Gauche attractive effects

The other halocyclohexanones show similar behavior but the free energy differences in the gas state are larger [AG°(C1) = —1.05 kcal/mol, AG°(Br) = —1.5 kcal/mol and AG°(I) = —1.9 kcal/mol)] and the axial conformer is still the predominating conformer in solution. The experimental results are corroborated by DFT calculations. For the fluoro and the chloro compounds, there is a clear attractive gauche effect between the halogen and oxygen atoms in the equatorial conformer. 

Eliel has been involved for many years in heterocyclic conformational analysis, and has made important contributions to investigations related to the anomeric effect. In this volume, Eliel and Juaristi present important data for systems of the type X-C-C-Y (X, Y = heteroatoms), and discuss repulsive gauche effects. Attractive gauche effects have long been known, and a theoretical connection of the gauche effect with the anomeric effect has been made. However, the repulsive interactions were not well recognized until recently. 

The attractive gauche effect has long been known in the X-C-C-Y systems, as for example when the X-Y-OCH3 system [113], and the relationship between the gauche and anomeric effect was made some years ago [114]. However, the repulsive interactions were not initially recognized. In 1979 Zefirov et al. [114] studied... 

If AG atpt = (AEsteric + AEpoisf) calc d, there is a repulsive gauche effect, and If AG°atpt = (AEsteric + AEpoisf) calc d, there is an attractive gauche effect, and... 

By equilibrating (BF3) the cis/trans isomers of 2-Ph-5-OR-l,3-dioxanes, the conformational free energy differences were determined [-AG° = 0.24 kcal mol- (OMe), 0.01 (OTs), -0.34 (OSO2C6H4-PNO2), -0.48 (OSO2 CH3)] and discussed with respect to the steric and the attractive gauche O-C-C-O effect (92T5941). 

In this study, the dipole moments at 298 K and the corresponding temperature coefficient of PDTC are calculated in the RIS approximation. The results are compared to the values determined by experiment. The results indicate that an extra stabilization energy of about 3.8 kj mol-1 must be added to these gauche states relative to trans state. This provides evidence of a large attractive sulfur gauche effect in the polysulfides, about equal in magnitude to that found for the analogous POM. 

The experimental value of the dihedral angle25 in HOOH is 111°. The interactions between adjacent lone pairs are repulsive, but the other gauche effects result from attractive influences (see below). Had we use the rabbit ear structure 22, with equivalent lone pairs, the experimental structure would be hard to explain. 

The main subject of this paper is the gauche effect (relating to X-C-C-Y interaction). Referring to Scheme 3 we may consider the gauche effect to be attractive when the gauche conformation... 

Substituent effects in organoselenium compounds (e.g., in Ph-Se-R with various R) are qualitatively similar as compared to homologous carbon analogues (Ph-CH2-R) but they are 3-5 times larger on the ppm-scale. The diagnostically valuable /-gauche effect in olefins has attracted some attention. 

Thus, the gauche effect will be attractive when the gauche conformatitMi is the favored one of two equilibrium conformations, determined by the calculations of known steric and polar interactions. However, if the anti-conformation is favored more than the calculations suggest, the gauche effect is repulsive. 

Oxathiolans and 1,3- and 1,4-Oxathians.— The attractive and repulsive gauche effects, including effects observed in 2-substituted 1,4-oxathians and 5-substituted... 

Similar conclusions were reached for sulfoxides 157. Conformation 158 was preferred for (RS/SR)-157 but with some contribution from conformer 159. The (RR/SS) dias-tereomers preferred the reverse conformer 161 was preferred to 160161. An attractive force between Ph/Ar and Ph/R was thought to be the primary factor in determining the conformational preference of sulfoxides 152 and 157. MM2 calculations were carried out on a series of molecules of general structure PhCHR—X—R with X equal to CHOH, C=0, S and S=0151. The main conformers of these molecules have the Ph (or aryl) and R (alkyl) groups gauche. The calculations supported the existence of CH-tr attractive interactions with minor contributions from other effects. 

R (alkyl) groups gauche. The calculations supported the existence of CH-jt attractive interactions with minor contributions from other effects. 

On the basis of the above discussion, we are led to the conclusion that sigma nonbonded attractive interaction in the form of a hydrogen bond will tend to favor a syn conformation opposing the inherent preference of ethane molecules for a staggered conformation. A compromise is expected to be reached in the gauche conformation. However, severe steric effects may force an anti conformational preference. 

In other words, the apparent order of stability of 1,3-butadiene is trans > gauche > cis. We conclude, therefore, that the preferred conformation of 1,3-butadiene is dictated by steric effects which overwhelm attractive pi and sigma nonbonded interactions. 

Since the C—Br bond is a much better donor and a much better acceptor than the C—H bond, the prediction is that the gauche conformation will be stabilized more than the cis conformation by a—it interactions. These interactions may then dominate nonbonded attractive effects present in the cis isomer. 

Microwave spectroscopic studies of l-fluoro-2-propene show that the cis conformation is more stable than the gauche conformation by approximately 306 cal/mol293 However, when fluorine is replaced by chlorine, bromine, or iodine, the gauche conformation becomes more stable294. These results confirm our expectations that the conformational preferences of allyl halides may depend on a balance of nonbonded attractive effects and a—it interaction effects. 

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