Equilibrium axial/equatorial

Examination of the NMR spectrum of thiane 3,3,5,5-d4 oxide enabled the estimation of the axial/equatorial equilibrium constant317. The value was found to be 1.62 (at — 90 °C), corresponding to a free-energy difference of0.175 kcal mol - , which is in good agreement with field force calculations315. 

Since the steric requirement of the /erf-butyl group was known to be much larger than that of the isopropyl group, as exemplified by the free energy difference involved in the axial-equatorial equilibrium of substituted cyclohexanes56), 2,7-di-terf-butylthiepin derivatives seemed to be more promising candidates for the synthesis. 

A popular measure of steric requirements is A, the free energy difference involved in the axial-equatorial equilibrium of substituted cyclohexane. Recommended d-values for i-C3H9 and tert-C4H9 are 2.1 and >4.4, respectively. Eliel, E. L. Chem. Educ. 37,126(1960) Angew. Chem. Int. Ed. Engl. 4, 761 (1965)... 

In the case of BDPP with a bite angle of 90°, the high-pressure NMR and high-pressure IR studies showed the structures of the hydrido dicarbonyl diphosphine resting state as an axial-equatorial BPT. Similar behavior was observed for the furanoside diphosphines. Dinuclear rhodium species in equilibrium with the mononuclear pentacoordinate rhodium hydride carbonyl diphosphines have been found for these ligands. The position of this equilibrium depends on the hydrogen concentration and the ligands. The rate... 

The axial equatorial equilibrium of 5-OH-l, 3-dioxane was strongly solvent dependent alcohols favor the axial position, other solvents, more polar than CCI4, the equatorial orientation of the 5-hydroxy substituent, both effects being corroborated by PM3 calculations [92MI3 93JST(287)185],... 

The axial equatorial isomer ratio for cyclohexanols obtained by reduction of cyclohexanones depends upon the solution pH (see Table 10.6). In alkaline or initially neutral but unbuffered solutions [59, 60], this ratio approaches the value at thermodynamic equilibrium The equatorial-aicohol, for exanq>le 9, is generated [61] from reduction of steroid ketones. In acid buffers, the thermodynamically less... 

Taft steric values. A, values from the axial-equatorial conformational equilibrium in cyclohexane v, Charton steric parameters. ... 

Table 12-2 shows the contribution made by various substituents to the free-energy change from the axial to the equatorial orientations of the substituent. Thus, for bromine, the free-energy change, AG°, is —0.5 kcal mole, which means that at 25°, the equilibrium constant, K, for the axial <= equatorial equilibrium is about 2.3 (from —2.303 RT log K = AG° see Section 4-4A). 

The stereochemistry of azacyclohexanes is complicated by the fact that there is a conformational change in the ring as well as inversion at the pyramidal nitrogen. Therefore it is difficult to say whether the axial-equatorial equilibrium of, for example, 1-methylazacyclohexane is achieved by ring inversion, or by nitrogen inversion, or both ... 

Another example is l,3,5-cycloheptatriene-7-d, where the equilibrium constant [equatorial]/[axial] is 1.25 favoring the conformer 42 with deuterium equatorial.113 Since deprotonation of either conformer would produce the same cycloheptatrienyW anion, the axial conformer must be more acidic, with a Apifa = -logi01.25. This is a case where the separate acidity constants would not be measurable, because the barrier to conformational equilibration is only 6kcalmol 1 and because the anion is antiaromatic and very unstable. 

The homoleptic complex exists in the solid state as a dimer (CN of Th is 5), whose structure can be considered as two ThOs units (in a trigonal bipyramidal geometry) joined along a common axial-equatorial edge [281], At room temperature, in noncoordinating solvents, there is a monomer-dimer equilibrium (5.51) [308,311] ... 

A detailed ab initio and modified neglect of diatomic overlap (MNDO) study of structural parameters and pyramidal phosphorus atom inversion and also enthalpy differences between the participants in the conformational equilibrium (axial and equatorial) of 1-R-phosphorinanes (R = H, CH3, C2H5) has been investigated and is in good agreement with the observed NMR and X-ray values <1999IJB660>. 

In most cases, steric effects control the axial-equatorial equilibrium and favor the equatorial position of the substituents. Indeed, an axial substituent experiences gauche interactions with C-3 and C-5, while the equatorial substituent is trans to C-3 and C-5 and thus relieves the strain of these gauche, interactions. Furthermore, the two axial hydrogens on C-3 and C-5 experience steric interactions with the hydrogen atom (LXXXa) or the substituent (LXXXb) occupying the C-1 axial position. These diaxial 1 /3 interactions are larger with a substituent than with H. 

A compilation of classical substituents and the free energy difference associated with their axial-equatorial equilibrium in monosubstituted cyclohexanes is given in Table 8 [103]. All these substituents prefer the equatorial position, but a 20-fold variation in —AG° values is apparent. However, a very few substituents are known to prefer the axial position. This is the case for the deuterium atom [104], and for the -HgCl substituent (0.3 kcal/mol [105]). 

Standard free energy change for the axial-equatorial equilibrium in monosubstituted cyclohexane derivatives (from [103])... 

Fig. 6.—Axial-Equatorial Equilibrium in (a) Complex Aldohexopyranoses, (b) a Simple Derivative of Oxane, and (c and d) Corresponding Conformational Equilibria in Acyclic Model...

Finally, a less conventional explanation of the origin of the anomeric effect may be mentioned. In a communication, " it was stated that, contrary to the results from measurements and theoretical calculations, the equilibrium coitstant for axial-equatorial equilibrium in... 

Fig. (10). Axial-equatorial equilibrium of V-methyl group in tropane alkaloids.

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