Rates of ring inversion

The rate of ring inversion of pentafluoro-l,2-oxazetidine as determined by 19FNMR is very fast ( a=4.2 kJ mol-1), with the nitrogen inversion barrier much higher (70JOC927, 1607). 

Table IV. Measurement of Conformational Equilibrium and Rate of Ring Inversion...

In ( )-cyclooctene systems the rate of ring inversion is lower than of epoxidation diastereoselectivities therefore strongly depend on the reaction conditions49. 

To illustrate the role of quantum energy flow on rates of conformational isomerization, we calculate the rate of ring inversion of cyclohexane. Measurements of rates of ring inversion by NMR in the vapor phase and in nonpolar liquids reveal an interesting pattern in the variation of the rate over a broad range of collision frequency with solvent. In the vapor phase, the rate increases with... 

C -l,2-dimethylcyclohexane represents an interesting case. The rapid rate of ring inversion means that it is impossible to separate the enantiomers. There is a further relevant point about ds-l,2-dimethylcyclo-hexane. The configurations of C(l) and C(2) in 20 and 21 are opposite, (IS,2R), and these configurations stay with the carbons whatever the conformation. Since the substituents at 20 and 21 are the same, cw-1,2-dimethylcyclohexane is meso. One can see this in another way the boat conformation 22, readily obtainable with models from either 20 or 21, clearly has a plane of symmetry between the stereogenic carbons. 

In 1,1,4,4-tetramethylcyclohexane (50) the rate of ring inversion has been studied by line shape analysis of the methyl resonance at various temperatures. The results are compared with those obtained for the structurally similar acetone peroxide (51). In these two systems (50) and (51), the equilibria are between two identical chair conformers and single coalescence temperatures are observed ( —61°C and — 66°C at 60 Mc./sec., respectively). In duplodithioacetone (52) the more stable twist-boat conformer enters into the equilibrium, and... 

The rates of ring-inversion of perfluoro- and 1,2-dichloro-octafluoro-cyclohexenes have been studied by n.m.r. spectroscopy. The measured rate parameters for the perfluoro-compound are (at —111-5 °C) 6-83 0-10, 7-2 0-2 kcal mol- . As +2-6 1-6 e.u. Other studies of... 

Substitution on a cyclohexane ring does not greatly affect the rate of conformational inversion but does change the equilibrium distribution between alternative chair forms. All substituents that are axial in one chair conformation become equatorial on ring inversion, and vice versa. For methylcyclohexane, AG for the equilibrium... 

Pressure-dependent studies of ring inversion of the six-membered rings previously described in the temperature-dependent section (see Table 6 for their structures and code names) have also been reported. Table 8 lists threshold energies and state densities for these molecules. In all cases, the pressure-dependent ring inversion rate constants could be adequately modeled using RRKM theory. 

Measurements of ring inversion rates in a wide range of environments [29-31] and the consistent picture presented by these calculations lead to a... 

Also the size (five- or six-membered) and flexibility of the chelate ring has an important influence on the rate of the inversion of configuration process at tin. The latter effect was nicely illustrated by a recent study of the triorganotin bromides 58-61 (see Fig. 15) containing the chiral (—)-menthyl group as a chiral probe (105,106,124). 

Biradical intermediates have a rate of internal rotation comparable, particularly at low temperatures, with the rate of spin inversion and ring closure. 

The consequence is that although the two possible pyramids are nonidentical mirror images, their lifetime at room temperature is so short that they cannot be detected by conventional means, and amines are never optically active. However, if structural features can retard the rate of pyramidal inversion, then the two forms could be detectable, which is exactly what happens in aziridines. Stability of the pyramid in a cyclic amine means that cis,trans isomers can exist if another substituent is present on a ring carbon. In 1,2-dimethylaziridine, the pyramidal inversion rate is slow enough at —40°C that both the cis and trans forms can be detected by NMR spectroscopy. 

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