Repulsive gauche effect

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. 

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... 

AG° = -1.82 kcal mol in solvent cyclohexane) than that measured for (methylthio)cyclohexane, this being the result of a repulsive interaction of the unshared electrons of sulfur with the unshared electrons of the ring oxygens (the so-called repulsive gauche effect [73]) (Scheme 3.52). 

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... 

In connection with a series of studies of the so-called repulsive gauche effect , chemical equilibration of 5-f-butylsulfonyl-2-isopropyl-1,3-dioxane (cis-3 trans-3) was carried out, and the results were compared with the methyl analog (cis-4 trans-4). The effect of changing from methyl to r-butyl is quite dramatic the large preference in the methyl analog 4 for the axial isomer (AC° =-1-1.19 kcal mol". Scheme 1) is reversed in 3, where the equatorial isomer is more stable by 1.14 kcal mol". Thus the r-butyl group on sulfur displaces the equilibrium toward the equatorial isomer by 2.33 kcal mol" (Scheme 1). 

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. 

A related stereoelectronic effect with particular relevance to fluoroaliphatic compounds is the gauche effect [38]. Contrary to expectations based solely on the steric repulsion of fluorine, 1,2-difluorocthanc prefers the gauche rather than antiparallel alignment of the fluorine substituents, by 0.5-0.9 kcal mol [39]. The same is observed for related compounds in which one fluorine atom is replaced... 

Figure 2.1 Preferred conformations of butane and 1,2-difluoroethane, illustrating the gauche effect. In the gauche conformation each C-F a orbital can overlap with a C-H a orbital on the vicinal carbon, corresponding to no-bond resonance as shown. Such no-bond resonance would be disfavoured if the C-F bonds were trans, since it would remove electron density from an electronegative element. The effect is strong enough in this case to overcome the electrostatic repulsion between the two C-F dipoles, which favours the trans form.

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. 

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... 

The existence of a repulsive S/S or S/O gauche effect has been demonstrated by cis/trans equilibration studies with 1,3-dithians (158). Carbon-13 n.m.r. data have been reported for 2-X-substituted (X = Me, SiMej, GeMej, SnMej, or PbMe3) and a series of methyl-substituted 1,3-dithians. Incremental shifts of the carbon atoms of the ring have been used for assignments of the stereochemistry of S—O of anancomeric compounds (159). Other spectral studies have dealt with... 

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