Conformationally equilibrated

For substituted cyclohexanes, the slow-exchange condition is met at temperatures below about —50 C. Table 3.5 presents data for the half-life for conformational equilibration of cyclohexyl chloride as a function of temperature. From these data, it can be seen that conformationally pure solutions of equatorial cyclohexyl chloride could be maintained at low temperature. This has been accomplished experimentally. Crystallization of cyclohexyl chloride at low temperature affords crystals containing only the... 

These results can be interpreted in terms of competition between recombination of the diradical intermediate and conformational equilibration, which would destroy the stereochemical relationships present in the azo compound. The main synthetic application of azo compound decomposition is in the synthesis of cyclopropanes and other strained-ring systems. Some of the required azo compounds can be made by 1,3-dipolar cycloadditions of diazo compounds (see Section 6.2). 

Cycloalkanes can be oxygenated when irradiated in the presence of nitrobenzene.196 A 50% yield of cyclohexanol and cyclohexanone is achieved from cyclohexane. Since the product ratio is independent of reaction time, the alcohol is not an intermediate in ketone formation. Isomeric 1,2-dimethylcyclohexanes give an identical mixture of the isomeric tertiary alcohols, indicative of conformational equilibration and the presence of a radical intermediate. 

Perhaps you are curious as to why absorptions are observed in the infrared spectrum of alcohols that correspond both to free and bydrogen-borided hydroxyl groups, whereas only one OH resonance is observed in their proton nmr spectra. The explanation is that the lifetime of any molecule in either the free or the associated state is long enough to be detected by infrared absorption but much too short to be detected by nmr. Consequendy, in the nmr one sees only the average OH resonance of the nonhydrogen-bonded and hydrogen-bonded species present. The situation here is very much like that observed for conformational equilibration (Section 9-IOC). 

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. 

These considerable solvent-dependent shifts demonstrate that for molecules with such pronounced conformational ambiguity, the equilibration of conformations has to be taken into account in the calculation of thermodynamic phase-equilibrium data. In order to enable a consistent treatment, we have implemented an automated conformation equilibration scheme in COSMO therm. A compound X can be represented by a set of COSMO-files for the conformers, and a multiplicity ojx(i) can be assigned to each con-former based on geometrical degeneration aspects. Then the population of a conformer, i, in a solvent S is calculated as... 

Antioquine (466) has been studied by two-dimensional H- H (COSY 45) NMR and by two-dimensional H-13C NMR. O,0-Diacetylantioquine and O, O-dimethylantioquine show doubling of all proton signals at room temperature due to slow conformational equilibration, but at 90°C in pyridine-[Pg.197]

During this reaction an aldehyde functionality is introduced selectively at C-6 of the piperidine. The advantages of the BOC group as activator and director of lithiation are its applicability to carbanion formation at otherwise inactivated or-positions and its convenience for addition and removal.6 Equatorially substituted 2-piperidines are well recognized to be less stable than their axially substituted isomers because of allylic 1,3-strain.7 Therefore 4 would be expected to undergo conformational equilibration to 4 . Directed lithiation via 16 and subsequent reaction with dimethylformamide finally yields 5 as an 8 1 mixture of trans and cis isomers. They can be separated by careful silica gel chromatography. 

The observation of stereospecific cyclopropane formation requires not only that both reaction steps be stereospecific but also that there be no conformational equilibration in the intermediate adduct. Further, more subtle tests of equation (18) need to be devised. 

This applies to triplet diradicals only, not to singlet diradicals whose lifetimes are too short for conformational equilibration. This may be used to conserve chirality information of the reactant as depicted in Scheme 5b. Accordingly, the singlet excited carbonyl group of a ketone 20 abstracts a hydrogen atom from a chirality center whose chirality information is preserved in the planar chirality of the extremely short-lived singlet diradical 21. On cyclization to 22 this information is... 

A carbonyl group adjacent to a ring junction bearing a hydrogen atom allows conformational equilibration to take place at the ring junction through enolisation (p. 154). For the 4-oxocholestane system (7), optical rotatory dispersion measurements [47] have indicated the presence of 99% A/B trans-isomer (5a) at equilibrium. In contrast, similar data [47] for... 

Photodimerization of simple 1,3-dienes in a 4 1 + 4 r cycloaddition process is typically an inefficient process . This is not surprising, given the highly ordered transition state for [4+4]-cycloadditions, and the predominance of the unreactive s-trans conformation . As a result, as noted above [2 + 2]-cycloadducts are often the major product, accompanied by varying amounts of vinylcyclohexenes and cyclooctadienes. Crossed photocycloadditions employing 1,3-dienes with substituents at the 2- or 3-positions can furnish greater amounts of cyclooctadiene products (equation This presumably results from a perturbation of the diene conformational equilibration to provide a higher proportion of the s-cis conformer. 

The outcome of this reaction depends on the relative magnitude of the various rate constants (activation barriers). If the conformational equilibration is rapid compared with product formation, essentially all of the reaction will be channelled through the more reactive conformer (Fig. 6a). For that reason the relative population of the conformers is irrelevant in the reactions of conformationally mobile systems... 

AG <5 kcal/mole, AG 2 >10 kcal/mole). If the rate of reaction is fast with respect to conformational equilibration the relative populations of the conformers will be of major importance (Fig. 6b). This situation is often encountered with conformationally rigid systems (e.g., polycyclic compounds). Obviously, conformational control can also be brought about by increasing the rate of reaction. We have seen (Section 3) that some 1,2 shifts in carbocations are extremely fast (AG <5 kcal/mole). We expect, therefore, that rearrangements of carbocations may be competitive, at least, with conformational equilibration even in acyclic systems. 

---