Butane gauche

The decalin (bicyclo[4.4.0]decane) ring system provides another important system for study of conformational effects in cyclohexane rings. Equilibration of the cis and trans isomers reveals that the trans isomer is favored by about 2.8 kcal/mol. Note that this represents a change in configuration, not conformation. The energy difference can be analyzed by noting that the cis isomer has three more gauche butane interactions that are... 

Figure 4.14 The origin of 1,3-diaxial interactions in methylcyclohexane. The steric strain between an axial methyl group and an axial hydrogen atom three carbons away is identical to the steric strain in gauche butane. Note that the -CH3 group in methylcyclohexane moves slightly away from a true axial position to minimize the strain.

Quasi-Harmonic Frequencies Calculated from Monte Carlo Trajectories on the Exact Potential Surface for Trans and Gauche Butane... 

Note that where two or more structures coexist, e.g., anti and gauche -butane, an experimental measurement can either lead to a single average stracture or a composite of stractures. 

Empirically, y-gauche effects are extremely useful in stereochemical analysis. For example, the cis- and trans-fusion of six-membered rings can be easily differentiated, since, in contrast to the trans-isomer of bicyclo[4.4.0]decane, there are gauche-butane fragments in the m-isomer (emphasized bonds) causing upheld signal shifts of the carbons involved (numbers refer to I3C chemical shifts in ppm)49-52. 

Just as conformational behavior in cyclohexane has been explained semi-quantitatively in terms of gauche-butane interactions (cf. 3 4, R = Me), corresponding gauche-propylamine (C—C—C—N) and other interactions can be used to discuss conformational equilibria involving heteroatoms.26... 

An attempt was made to extend this result to an estimation of K for the quinolizidine equilibrium. For the 2-methyl compound 116 discussed above, there is a difference of one gauche-butane (gb) interaction between the cis- and trans-fused conformers 116 and 117 compared with a difference of three such interactions between the quinolizidine conformers 118 and 119. This gives AG° at 25°C for quinolizidine as 2.8 + 2gb = 2.8 + 1.6 = 4.4 kcal mol-This value is uncertain because even groups remote from... 

LH-and 13C-NMR spectra are consistent with the twin-chair conformation 169 for trans-decahydroisoquinoline, and 13C-NMR spectroscopy gives AG° 5go of 0.37 kcal mol -1 (70% N-inside conformation 170) for the cis-decahydroisoquinoline equilibrium 170 171,82 reflecting the difference between a gauche-butane and a gauche-propylamine interaction. The 3-methyl-cis-decahydroisoquinolines prefer the equatorial methyl conformations 172 and 173.82... 

Torsional contribution cf. structure with no gauche — butane interactions... 

Actually, nonbonding interactions are already included in the torsional term (as gauche-butane interactions) we might have used an ethane-type torsional function and accounted for CH3/CH3 interactions entirely with nonbonded terms. However, in comparing calculated relative energies the torsional term will cancel out. 

The (nteraction between hydrogen atoms on the methyt groups in gauche butane. Stcric wain resuhs because the methyl groups are too close t 9ether. 

In addition to angle strain and torsional trdin. stertr Mrain is yet a third factor that cont ributes to the over l strain energy of cycloalkanes. As in gauche butane (Senonbonded atom in o molecule repel ua other if they approach tcK> closely and attoropt to occupy the same... 

Diaxial steric strain is already familiar—weX e seen it before m the stone strain between methyl groups in gauche butane (Section 4.31. Recall thjit gnitrho bot nne Is tbj n anti hiitAn > by l.ft lr>T/mo1 (0 fl krftif... 

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