Equatorial-axial conversion

Chiral diphosphites based on (2R,3R)-butane-2,3-diol, (2R,4R)-pentane-2,4-diol, (25, 5S)-hexane-2,5-diol, (lS -diphenylpropane-hS-diol, and tV-benzyltartarimide as chiral bridges have been used in the Rh-catalyzed asymmetric hydroformylation of styrene. Enantioselectivities up to 76%, at 50% conversion, have been obtained with stable hydridorhodium diphosphite catalysts. The solution structures of [RhH(L)(CO)2] complexes have been studied NMR and IR spectroscopic data revealed fluxional behavior. Depending on the structure of the bridge, the diphosphite adopts equatorial-equatorial or equatorial-axial coordination to the rhodium. The structure and the stability of the catalysts play a role in the asymmetric induction.218... 

There are two chair forms and two types of valences (axial and equatorial). The conversion of one chair form to the other interconverts the axial and equatorial valences (i.e., a valence which is axial in one chan form is equatorial in the other chair form and vice versa). In the structures below one of the carbons is indexed with a star ( ) to help keep track of it. 

No values are given for c/s-1,2-axial-equatorial interactions because inversion leads again to a 1,2-equatorial-axial interaction. This is not the case for the trans-, 2-diequatorial isomer, since inversion affords a diaxial conformer. For conversion of kcal to kilojoules, use 1 kcal/mol = 4.184 kJ/mol. 

Similar synthetic approach were presented by Anandhan and Rajakumar." They synthesized and subsequently characterized novel gly-codendrimers and JV-allq ldendrimers with aromatic core using NMR techniques. De Castro et al. presented conversion of mannan-polysaccharides in mannose oligosaccharides with a thiopropargyl linker. Leino et al. ° performed synthesis and conformational analysis of phos-phorylated (3-(l->2) linked mannosides. The stereochemistiy at the anomeric centre was determined by NMR, which was a challenging task due to the fact that the H-1 and H-2 protons always displayed either axial-equatorial or equatorial-axial orientation. 

Conversely, when A-alkyl tryptophan methyl esters were condensed with aldehydes, the trans diastereomers were observed as the major products." X-ray-crystal structures of 1,2,3-trisubstituted tetrahydro-P-carbolines revealed that the Cl substituent preferentially adopted a pseudo-axial position, forcing the C3 substituent into a pseudo-equatorial orientation to give the kinetically and thermodynamically preferred trans isomer." As the steric size of the Cl and N2 substituents increased, the selectivity for the trans isomer became greater. A-alkyl-L-tryptophan methyl ester 42 was condensed with various aliphatic aldehydes in the presence of trifluoroacetic acid to give predominantly the trans isomers. ... 

Another possibility to increase the diastereoselectivity in an asymmetric synthesis can arise from different thermodynamic stabilities of the diasteieoisomeric products. If the thermodynamic stabilities of these are different enough, then, under conditions of equilibrium, a complete conversion of the less stable into the more stable can be achieved. For example, the diastereoselective hydrogenation of naphthalene derivates over Pd/C catalyst leads to a mixture of dihydronaphtalenes in which the cA-isomer predominates. The conversion of this isomer into the tram occurs by changing the properties of the reaction medium, namely by equilibration with a base. For such a purpose, NaOMe in IHF can be used [263], Generally, such an increase in stability in the six-membered rings can result from a rearrangement of the substituents from an axial to an equatorial position. 

After having found the peaks due to axial and equatorial protons, the proportions of the conformers can also be established by measuring the area under the peaks. The rate of conversion from one conformer to another has been obtained from the shape of the peaks. 

Correspondingly, the interconversion of the twist-boat intermediate into the other chair form can be viewed as rotation about the opposite ring bond. Overall, two independent bond rotations , pausing at the high-energy (but stable) twist-boat intermediate, effect conversion of one chair structure into another equivalent chair, and at the same time switch axial and equatorial hydrogens. 

The acetoxyl-proton resonances were also in accordance with the 1C (d) conformation, namely, one equatorial and two axial acetoxyl resonances for 29, and the converse for 30 (see Table III), since methyl groups of axially oriented acetoxyl substituents of a pyranoid ring generally resonate at lower field than those of similar, equatori-ally attached groups. 

On each carbon, one bond is directed up or down and the other more or less in the plane of the ring. The up or down bonds are called axial and the others equatorial. The axial bonds point alternately up and down. If a molecule were frozen into a chair form, there would be isomerism in monosubstituted cyclohexanes. For example, there would be an equatorial methylcyclohexane and an axial isomer. However, it has never been possible to isolate isomers of this type at room temperature.219 This proves the transient existence of the boat or twist form, since in order for the two types of methylcyclohexane to be non-separable, there must be rapid interconversion of one chair form to another (in which all axial bonds become equatorial and vice versa) and this is possible only through a boat or twist conformation. Conversion of one chair form to another requires an activation energy of about 10 kcal/mol (42 kJ/mol)220 and is very rapid at room temperature.221 However, by... 

The six axial bonds are directed upward or downward from the plane of the ring, while the other six equatorial bonds are more within the plane. Conversion of one chair form into another converts all axial bonds into equatorial bonds and vice versa. In monosubstituted cyclohexanes, for electronic reasons, the more stable form is usually the one with the substituent in the equatorial position. If there is more than one substituent, the situation is more complicated since we have to consider more combinations of substituents which may interact. Often the more stable form is the one with more substituents in the equatorial positions. For example, in ct-1,2,3,4,5,6-hexachlorocyclohexane (see above) four chlorines are equatorial (aaeeee), and in the /Tisomer all substituents are equatorial. The structural arrangement of the /3-isomer also greatly inhibits degradation reactions [the steric arrangement of the chlorine atoms is unfavorable for dehydrochlorination (see Chapter 13) or reductive dechlorination see Bachmann et al. 1988]. 

In the course of the reaction, the nitrite ion leaves the primary anion radical. This produces the cyclohexyl radical in a pyramidal configuration. The vicinal methyl group steri-cally hinders the conversion of the pyramidal radical into the planar one. With a high concentration of the nucleophile, the rate of addition exceeds the rate of conversion i.e. radd > rconv Then the entering PhS group occupies the axial position. With a low concentration of the nucleophile, the conversion occurs earlier than the addition (radd rconv) and the planar radical center is attacked from both the axial and equatorial sides. This results in the formation of the isomer mixture. 

The NMR spectrum of the cis-fused quinolizidol (64a) and its derivatives showed the presence of an equilibrium mixture of conformers where the form with an axial OH and equatorial phenyl group was only a minor component. The signal of H-l in 64b-d appears at <5 4.0-4.1 ppm as a triplet with a coupling constant of 4-5 Hz and W1/4 of 13-14 Hz. H-3 was in part unresolved from H-l in 64a,e and in 64c,d was a broad multiplet (a triplet of triplets with J 8-9 and 4.5 Hz) with W1/4 equal to 25-28 Hz. These couplings of H-l and H-3 favor that conformation with an axial phenyl and an equatorial hydroxyl. Compound 64a was synthesized by reduction of cis-fused quinolizidone (65b) with NaBH4 to yield mixture of 64a and its epimer in 1.3 1 ratio or with lithium tri-sec-butylborohydride, where 65% conversion to axial alcohol was observed. 

Unless otherwise stated, all the conversions are quantitative. rfThe oxidation was performed on a 80/20 mixture of borneol and iso-borneol. The oxidation was effected on a 30/70 mixture of axial and equatorial isomers. The conversion amounted to 95% in this case. 

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