Axial rotamer

It is often the solvent effect fliat is flie only method of radical change of relative contents of different conformer forms. Thus, with flie help of the isochore equation of chemical reaction, flie data on equilibrium constants and enthalpies of dichloroacetaldehyde conformer transformation allow us to calculate that, to reach the equilibrium constant of axial rotamer formation in cyclohexane as solvent (it is equal to 0.79) to magnitude K=0.075 (as it is reached in DMSO as solvent), it is necessary to cool the cyclohexane solution to 64K (-209"C). At the same time, it is not possible because cyclohexane freezing point is -l-6.5"C. By analogy, to reach flie dimefliylsulfoxide constant to value of cyclohexane , DMSO solution must be heated to 435K (162"C). 

Adducts of M(CH-f-Bu)(NAr)(OR)2 complexes were prepared and studied as models for the initial olefin adduct [66] in an olefin metathesis reaction [67]. PMe3 was found to attack the CNO face of yy -M(CH-f-Bu)(NAr)(OR)2 rotamers to give TBP species in which the phosphine is bound in an axial position... 

In following the temperature dependence of AG°, Booth et al. [85JC-S(CC)467 87T4699]also determined A/f° and AS° for the conformational equilibria of 2-C1-, 2-OMe-, 2-OH-, and 2-NHMe-oxanes (see Table II) and discussed the results in terms of exo- or endo-anomeric effects (Section III,C,8). Employing the NOE and a number of H,H- and C,H-coupling constants as a means of analysis, the preferred rotamers of axial/equatorial-2-OMe-oxane were found to be in the conformations az and 2, respectively, as given in Scheme 1 (90T1525). 

Eliel et al. (72JA8587 76JA956) and Juaristi et al. (87JOC3806 90JOC33 92JA2157) studied the conformational behavior of 5-sulfur-substituted 1,3-dioxanes (see Table V) and discussed the results in terms of the rotamer population of the axial conformer in which steric, electrostatic, and stere-oelectronic effects are dominant. The sulfoxides place both the sulfinyl oxygen and the substituent outside the 1,3-dioxane ring in the sulfone, the position of the tBu substituent is similarly corroborated by an X-ray structure in the solid state. 

In the solid state of cis-l,4-dibenzoyl-2,5-dimethylpiperazine, the Z,Z-form is present, with the phenyl groups twisted (77AX(B)3568) with respect to the amide plane. In solution (77TL2895), the cis-isomer of the 2,5-dimethyl and 2,5-diethyl derivatives shows the presence of only one conformer (Z,Z, with the piperazine ring in the twist-boat conformation), while the tra s-form consists of an equilibrium mixture of nearly equal amounts of the four axial alkyl rotamers. 

Fig. 10) has been carried out (52) and the result was compared with those of the monomethyl acetals 50 and 1, and the gem-dimethyl acetals 52 and 53. Each pair of isomers gave essentially an identical result, i,e. 33% of the equatorial and 77% of the axial isomer ( 2%) after equilibration in benzene at 70°C. This corresponds to a aG of =0.8 kcal/mol, a value close to that predicted for the relative stability of rotamer A over E (0.6 kcal/mol). 

Axial C(4) group interferes with incoming nucleophile in the preferred rotamer for nucleophilic attack that experiences no dipolar interactions... 

Thiane (00JMS(T)225), 2-, 3-, 4-alkyl substituted thianes (98JPOC831, 99JPOC176), thiane-1-oxide and a number of 3-substituted thiane-1-oxides (01JMS(T)287, 01JMS(T)203) were calculated with HF and DFT theories employing extended basis sets, and the results were compared with experimentally determined conformational equilibria. The conformational free energies (AG°) of conformers and rotamers are discussed in terms of repulsive non-bonded interactions in the axial)equatorial conformers. 

The diastereomerically pure thiazolium salt 509 which bears a 2-/i t7-butylphenyl substituent at the nitrogen atom was converted into a mixture of 510 and its atropisomer 510 (dr = 75 25) upon treatment with base (Scheme 128). The stereogenic center in the intermediate carbene favors one rotamer 510. Upon reaction with benzaldehyde, it accounts in a similar fashion for the formation of the major enol diastereoisomer 511 over 511, which, in turn, leads to the major enantiomer 512 rather than 512 observed in the benzoin condensation catalyzed by 509. The concept of axial chirality was proven to be viable for an efficient chirality transfer. Replacement of the isopropyl group at C-4 by the bulkier 2-phenyl-2-propyl substituent using 8-phenylmenthone is likely thought to increase the ee <2004EJ02025>. 

Compd Ring type > Rotamers of the side-chain and hydroxy Ratio of the axial/equatorial side-... 

The anomeric effect the axial conformer is more stable than predicted based on the k value of the methoxy group. The exoanomeric effect the preferred rotamers about the exocyclic C-0 bond are those with the methyl gauche to the endocyclic oxygen atom and to H-1... 

Fig. 4. Top, Newman projection of the three rotamers about C-5-C-6 of a D-glycopyranose. Middle, schematic representations of the 0-6-0-4 interactions in the various C-5-C-6 rotamers for axial substituents at C-4. Bottom is the same as the middle one but for a equatorial substituent at C-4.

SC,+sc) conformation of the a isomer and either the ap,—sc) or ap,+sc) conformation of the equatorial isomer is present. Because the dipole moment calculated for both of the latter conformations had the same value, their relative abundance could not be determined. Lemieux and co-workers did not detect any appreciable amount of the ap, + jc) con-former in a number of structures examined, and concluded that the (ap,—sc) and ap,+sc) rotamers are separated by over 8 kJ.mor. On the other hand, the changes in optical rotation for methyl 2,3-dideoxy-a-D-glycero-pento-pyranoside and its 4,6-ethylidene acetal indicated the presence of the (+ sc, ap) conformer for the axial isomer. ... 

The rotamer with ( h=180° is favoured by the exo-anomeric effect but disfavoured by ordinary steric interactions the clashes between R and the axial substituent at C2 (even if only hydrogen) are what they would be if R were an axial substituent at C4. 

NMR analysis revealed that this material exists as a complex mixture of amide rotamers as well as diastereomers due to the new chiral center created by the protonated amine. Shown in Scheme 5.18 are the major and minor diastereomers, la and lb, with rotation indicated about the amide linkage. The stereochemistry of the major diastereomer, la, was determined based on the observed NOEs shown with the structure, hi characterizing this compound, it was observed that the dynamic equihbrium of diastereomers was significantly affected by solvent polarity. A crystalline perchlorate salt was obtained, and the single-crystal X-ray of this salt revealed that the piperidine ring is positioned directly over the aryl ring which results in a significant upfield shift of the axial protons. 

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