Butane gauche form

Xhe anti form of butane itself is also more stable than the gauche form Schrumpf, G. Angew. Chem. Int. Ed. Engl., 1982, 21, 146. 

Figure 5-8 Conformational energies and rotational barriers in butane, the difference in energy between the anti and gauche forms is 0.8-0.9 kcal mole-1. The energies are relative to conformation 7 as zero.

Figure 5-10 Sawhorse and Newman conventions for showing the staggered conformations of butane. Only one gauche form is shown. Cyclohexane is shown to emphasize the resemblance of its stable conformation to the gauche conformation of butane.

Exercise 12-3 Figure 5-8 indicates that the difference in energy between the conformation of butane with eclipsed methyls and the gauche form is about 5 kcal mole-1. Use this number to estimate the contribution of eclipsing to the instability of planar cyclohexane. Then calculate the instability of planar cyclohexane by including the angle strain from Exercise 12-2 in your estimate. 

Figure 12-16 Favored conformation of cyclotetradecane as proposed by Dale. For comparison, the trans and gauche forms of butane are shown by the same convention. (The convention implies that the wedged lines are C-C or C-H bonds projecting out of the plane of the paper, with the wide end closest to you, and the broken lines are C-H bonds projecting behind the plane of the paper. The result is an aerial" view of the molecule in the most stable staggered conformation.)...

Figure 12-19 Boat-boat conformation of cyclooctane, based on two gauche forms of butane (see Figure 12-16 and Exercise 12-23).

The hemiacetal thiol 75 (Fig. 13) gave on acid cyclization an equilibrium mixture of cis and trans monothioacetals 76 and 77 in a 1 1 ratio (63). If a value of 1.7 kcal/mol is accepted for the steric effects in 76 (two gauche forms of SCl XCHg = 0.8 kcal/mol and one gauche form of butane= 0.9 kcal/mol), the anomeric effect for the ether oxygen in 76 must also be equal to 1.7. 

The first precise evaluation of the anomeric effect was realized by Descotes and co-workers in 1968 (22). These authors have studied the acid catalyzed isomerization of the cis and trans bicyclic acetals 6 and 6 and found that, at equilibrium, the mixture contains 57% ci s and 43% trans at 80°C. The cis isomer is therefore more stable than the trans by 0.17 kcal/mol. The cis isomer 5 has one (stabilizing) anomeric effect whereas the trans isomer 6 has none. Steric interactions in cis acetal 5 were estimated tobel.65 kcal/mol (one gauche form of ri-butane, 0.85 kcal/mol and an OR group axial to cyclohexane, 0.8 kcal/mol). By subtracting an entropy factor (0.42 kcal/ mol at 80°C) caused by the fact that the cis acetal S exists as a mixture of two conformations (cis decalin system), they arrived at a value of 1.4 kcal/mol for the anomeric effect. 

This last study is quite interesting because it permits an evaluation of the anomeric effect for the nitrogen atom. Conformer 91 with the axial N-methyl group should be less stable than conformer 92 by approximately 1.3 kcal/mol on the basis of steric effects (one gauche form of n-butane, =0.9 kcal/mol and one gauche form of CHj — N - CHj-0, =0.4 kcal/mol). The second anomeric effect caused by the equatorial orientation of the nitrogen electron pair in 91 must compensate for the steric effect. An approximate value of 1.3 kcal/mol must therefore be taken for that electronic effect, a value close to that estimated for the oxygen atom of the acetal function. 

Recently, Crabb, Turner, and Newton (68) have observed that perhydropyrido-[1.3]oxazine exists as a =9 1 mixture of the trans and the cis forms 97 and 98. The cis form 98 has two anomeric effects (-2.8 kcal/mol), two gauche forms of butane (1.8 kcal/mol) and one gauche form of -propyl ether (0.4 kcal/mol) whereas the trans form 97 has only one anomeric effect (-1.4 kcal/ mol). On that basis, the trans form 97 should be more stable than the cis form 98 by about 0.8 kcal/mol, in agreement with the experimental result. Katritzky and co-workers (69) have also shown that 1-oxa-3,5-diaza and 1,3-dioxa-5-aza cyclohexane derivatives exist respectively in the conformations 99 and 100. With an alkyl group in the axial orientation, both conformations gain two anomeric effects. 

Kibby et al compared the behaviour of a hydroxyapatite (Ca/P = 1.58) and an alumina catalyst for the dehydration of threo- and erythro- 3- x -butan-2-ol. Both catalysts gave but-2-enes preferentially, but water can be eliminated by a syn- or anft -mechanism, the former occurring, when groups are removed from the same side of the molecule and the latter, when they are removed from opposite side. Furthermore, syn- and fl/ift -eliminations occur respectively with eclipsed and gauche conformations. In the case of homogeneous reactions, anti-elimination is preferred since the gauche form is thermodynamically more stable. 

For butane and most compounds of the form YCH2CH2X, antiperiplanar conformers are the most stable ones. The long alkyl chains of fatty acids are usually also all-anti or all-trans conformers in molecular assemblies (Fig. 1.2.3). If the substituents are, however, electronegative atoms, in particular oxygen, the synclinal (= gauche) form usually dominates. Important gauche effects (= the tendency to adopt conformations with the maximum number of gauche interactions between adjacent electron pairs and/or polar bonds) are found in peroxides. 

It is rather surprising that conformer 2 cannot compete with conformer 1 because they differ by only one gauche form of butane (0.9 kcal/mol). The experimental results suggest that . and 2 should be separated by over 1 kcal/mol. It was suggested (36) that this unexpected situation could be mainly the result of a short Ci — 0, bond which would amplify the steric... 

Dithiaspiro[5.5]undecane can exist in the three conformations 64A, 64B, and 64C (Fig. 11). Conformer 64A which has the two sulfurs axially oriented can have two anomeric effects, conformer 64C which has the two sulfurs equa-torially oriented has no anomeric effects and conformer 64B with one sulfur axially oriented has only one anomeric effect. Taking Into account the ster-1c effects (0.9 kcal/mol for a gauche form of butane and 0.4 kcal/raol for a gauche form of S-CH2 X-CH2) and an anomeric effect of 1.4 kcal/mol. the relative stabilities 64A, 64B, and 64C are 0, 2.4, and 4.8 kcal/mol. On that basis, this compound must be essentially conformationally rigid existing in the conformation 64A only. This prediction was verified experimentally (25, 62), and further confirmation was found by the study of the acid catalyzed cyclization of ketone dithiol 65. This compound can give either isomer 66 or 67 which can take two different conformations each. 

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