Flip process

Among all the isomers as 1 2 dichlorocycio hexane is unique in that the ring flipping process typ ical of cyclohexane derivatives (Section 3 9) converts it to its enantiomer... 

Overall, it can be envisioned that the Py-G group 47 represents an important label for the time-resolved studies of DNA dynamics and stacking interaction [123] and could be applied especially for assays in which conformational changes or base-flipping processes are essential in observation, such as in the investigation of DNA-protein complexes with DNA repair proteins. 

Note that it is not necessary to consider both forms of cyciohexane, where the methyl is either wedged (up) or dotted (down). If the cyclohexane ring were planar, the two structures would be the same, since one merely has to turn the structure over to get the other. Although the cyclohexane ring is not planar, it turns out that the two structures are still identical, because of the ring flip process. This is shown below. One set of conformers is simply the upside-down version of the other. 

In order to acquire a more quantitative understanding of the flipping process, Timothy Wong carried out a series of variable-temperature H NMR studies on 89 and 85, whose pertinent data, together with those reported previously for 92, " and 94, ° < 2 gj.g summarized in Table 1. As can be seen, the... 

Table 1. Coalescence Temperatures (Tc), Free Energies of Activation for the Conformational Flipping Process (AC ), and NMR Operating Frequencies for Measurement of Compounds 85, 89, 91-94. ...

You will understand this flipping process if you make a model of a cyclohexane ring carrying a single substituent. By manipulating the model you can discover some of the different ways the process can occur. The simplest route is simply to flip up one comer of the ring to convert the chair into a boat and then flip down the opposite carbon ... 

The stannanes 89 and 91 both generate equatorial organolithiums 90 and 92, which can interconvert only by a ring-flipping process. The interconversion does not take place at -78 °C, and the organolithium with the equatorial lithium is clearly the more stable as it quenches with no trace of other stereoisomers even at 0 °C.43 The axially-substituted 92 starts to invert at -40 °C (96% retention) but at -20 °C 35% has inverted, and at 0 °C inversion is complete. 

In the ring-flipping process, C-l flips up to give a twist boat. Then C-4 can flip down to produce another chair conformation. When opposite carbons flip like this, all axial and equatorial bonds interconvert that is, all hydrogens that were axial are converted to equa-... 

What happens when there is a substituent on the cyclohexane ring Let s consider methylcyclohexane as a simple example. As before, there are two chair conformations, which interconvert by the ring-flipping process. In this case, however, the two conformations are not identical. As shown in Figure 6.16, the methyl group is equatorial in one conformation and axial in the other. The conformation with the axial methyl is less stable than the conformation with the equatorial methyl by 1.7 kcal/mol (7.1 kJ/rnol) because of steric interactions between the methyl and the axial hydrogens on C-3 and C-5. (These are often called 1,3-diaxiaI interactions.)... 

As with any saturated six-member ring, the ring-flip process interconverts two-chair conformations. For D-glncose, these chair conformations are designated for 6 (also for 7) and 4 for its ring-flipped conformer 10. Introdnctory organic chemistry texts instruct us that equatorial substitution is favored over axial, and therefore, 6 is more stable than In fact, little computational work... 

However, two distinctly different paths for this process were considered for [260], one involving only tertiary ions and another including structures with positive charge at the secondary carbon. At low temperature only the tertiary path is of importance, since C(l) stabilizes positive charge better than C(5) in the bridge flipping process (cf. structure [257] which equilibrates C(4) with C(6), C(3) with C(7) and the 8- and S -CH, groups, but which leaves the other positions unaffected (165). When the temperature is raised an appreciable... 

Olah et al. (1975c) reported a nmr investigation of a large number of cyclobutenyl ions but only for the parent system was a dynamic process observed. The cyclobutenyl ion [266] underwent a ring flipping process interconverting the two puckered homoaromatic ions with a barrier A(7 of 8.4 kcal moh (169). 

---