Conformational equilibration and

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. 

Thus, a complete description requires knowledge of the rate constants for formation and dissociation of four exciplexes, three equilibrium constants for conformational equilibration, and tiie rate constants and k . We can avoid... 

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). 

As for any desymmetrization of meso compounds, enantioselectivity comes from the ability of a homochiral base to distinguish between two enantiotopic protons, in this particular case, to discriminate between the two pseudo-axial protons of the rapidly equilibrating enantiomeric half-chair conformations 51 and 52 (Scheme 25). 

The mode of the elimination can be recognised by the composition of the products if the olefin formed has such substituents on the double bond that cis and trans stereoisomers may be distinguished. The question is of interest with respect to the concerted E2 mechanism, because in the pure El and ElcB processes, the intermediate carbonium ion or carbanion, respectively, usually have enough time to rotate around the Ca—Cp bond, equilibrate and give the same cis/trans ratio from different conformers. 

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. 

Other factors (charge repulsion, solvation factors, etc.) could influence the position of the equilibrium in favor of enolate dianion 216. It is also possible that there is a kinetic preference for the formation of dianion 216 and that this species would undergo protonation more rapidly than equilibration. This rule of axial protonation" of 216 has been found to be widely applicable in many cases. However, in systems in which a significant amount of strain must be introduced in order for protonation to occur axially on 216, protonation of conformer 217 (and even conformer 218) becomes important (60). 

Cyclization of a mixture of dl and meso dihydroxyketones T3 and 14 under mild acid conditions gave a mixture of the three isomeric spiroketals 15, 16, and 17. Low temperature C nmr analysis confirmed that isomers lj> and 16 are conformationally rigid and that they exist in the conformations 15A and 16A respectively. Using the same technique, isomer V7 was shown to exist as a mixture of conformers 17A and 17B as predicted. Furthermore, acid equilibration of lj> (or 16) gave a =97 3 mixture of isomers 15 and 16, and when isomer V7 was treated under the same conditions it was converted into a 97 3 mixture of 2j> and 26. These results are completely consistent with the analysis made above. 

The general equation relating the product ratio at infinite time, P, to the rate constants (kt, k 3 = rates of equilibration of conformers X and Y, k2 = rate of formation of product A from X, k3 = rate of formation of B from Y) for the conformation-ally mobile system described by Eq. (9.2) is given by Eq. (9.3), with K=k1/k 1 (Zefirov, 1977). 

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]

S. A. S. A1 Janabi, J. G. Buchanan, and A. R. Edgar, Base catalysed equilibration and conformational analysis of some methyl 2,3- and 3,4-anhydro-6-dcoxy-/fi)-hcxopyrano-sides, Carbohydr. Res., 35 (1974) 151-164. 

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. 

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