Energy barriers, to ring inversions

Although planar structures for 111 and 112 were not attained, it is still likely that these novel diazabiaryls can serve as chiral ligands with C2 symmetry. In support of this likelihood, the resolution of 112 was accomplished recently on swollen, microcrystalline triacetylcellulose by Jan Sandstrom. The free energy barrier to ring inversion of 112 was found to be about 101 kJ/mol, through a thermal racemization process using chiral 112. ... 

Semi-empirical models do not provide good descriptions of the energy barrier to ring inversion in cyclohexane. The MNDO model underestimates the barrier by a factor of three, and the AMI and PM3 models by almost a factor of two. This behavior is consistent with previous experience in dealing with single-bond rotation barriers. 

Of special interest are results of the dynamic NMR study of the conformational mobility of type-1 1-heteracyclohexanes (73JA4634) and their 3,5-naphtho analogs of type 4 (82CC333). Free energy barriers to ring inversion of these compounds are given in Table II. Values for compounds 1 decrease in the order O > S > Se > Te, whereas in their naphtho analogs, they increase in the same order. 

Free Energy Barriers to Ring Inversion of Compounds 1 and 4... 

An axial preference of the aryl moiety of spirocyclic metacyclophanes (155) has been deduced from H n.m.r. data. The energy barriers to ring inversion were determined for the complexes (156) and (157). The coalescence phenomenon observed in the spectra of the dihalogen complexes (157) was found to be due not to reversal of the 1,4-dithian ring but rather to inversion of the configuration at the sulphur atom. ... 

The four-membered rings in compounds 30 (R = C02Me) are not planar and Table 13.2 gives angles d determined by X-ray diffraction. The energy barriers to ring inversion determined by nmr methods decrease with decreasing... 

The effect of introducing -hybridized atoms into open-chain molecules was discussed earlier, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are noted when sp centers are incorporated into six-membered rings. Whereas the fiee-energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecyclohexane and to 4.9 kcal/mol in cyclohexanone. ... 

For each molecule, calculate the overall energy barrier for ring inversion in each direction. Use this barrier to calculate the half-life (t./,) of an individual molecule at 298 K (use equation 2). Which molecule inverts most rapidly Most slowly Why (Hint What geometrical changes are required for inversion )... 

The overall barrier to ring inversion is well established experimentally, and is believed to correspond to the energy difference between chair and half-chair structures. Less certain are the relative energies of chair and twist-boat conformers and the energy of the boat transition structure, although a small range of values for each of these quantities has been established experimentally. Comparison of the experimental data with the results of calculations is provided in Table 8-5. The... 

In cyclohexene itself, the barrier to ring inversion is certciinly low at 5.3 kcal/mol m.ns) and it appears that the preferred conformation is a half-chair 39 in equilibrium with its mirror image conformation 40. There have been calculations of the energies of cyclohexene conformations by Bucourt and Hainaut and by AUinger and his colleagues as well as suggestions on the basis of experimental evidence These... 

The effect of introducing /j -hybridized atoms into acyclic molecules was discussed in Section 2.2.1, and it was noted that torsional barriers in 1-alkenes and aldehydes are somewhat smaller than in alkanes. Similar effects are seen when sp centers are incorporated into six-membered rings. Whereas the energy barrier for ring inversion in cyclohexane is 10.3 kcal/mol, it is reduced to 7.7 kcal/mol in methylenecy-clohexane ° and to 4.9 kcal/mol in cyclohexanone. The conformation of cyclohexene is described as a half-chair. Structural parameters determined on the basis of electron diffraction and microwave spectroscopy reveal that the double bond can be accommodated into the ring without serious distortion. The C(l)—C(2) bond length is 1.335 A, and the C(l)-C(2)-C(3) bond angle is 123°. The substituents at C(3) and C(6) are tilted from the usual axial and equatorial directions and are referred to as pseudoaxial and pseudoequatorial. 

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