Isomers diequatorial

The more stable diastereomer in each case is the one having both methyl groups equatorial. The free-energy difference favoring the diequatorial isomer is about the same for each case (about 1.9 kcal/mol) and is close to the — A(j value of the methyl group (1.8 kcal/mol). This implies that there are no important interactions present that are not also present in methylcyclohexane. This is reasonable since in each case the axial methyl group interacts only with the 3,5-diaxial hydrogens, just as in methylcyclohexane. 

According to an X-ray diffraction study bis-2-spirocyclohexyl-l-boraadamantane dipyridine adduct 65 is a trans-diequatorial isomer. The presence of two molecules of benzene per one complex molecule in the crystal has also been revealed <20040L313> (see Table 1 entries 3 and 4 and Schemes 26 and 29). 

Chart II summarizes the reactions of these epoxides with nucleophilic reagents. In all but one reaction, the product is very preponderantly, if not exclusively, that derived by diaxial opening of the epoxide ring. The absence of the alternative, diequatorial isomer argues against the likelihood, in the general case, of a mechanism... 

The versatility of host 1 allows discrimination not only between isomeric planar, aromatic compounds but also between quite bulky derivatives of cyclohexane. For example, host 1 will include selectively the diequatorial isomer of 3,5-dimethylcyclohexanone (65), but not 66 or isomer 67 from a mixture of 67-69 [67],... 

The two-layered ONIOM(B3LYP HF) method describing the whole system at quantum mechanical level, without the support of a molecular mechanics method, was employed by Landis and Uddin to explore the hydroformylation of 1-alkenes by a Rh-xantphos complex [119]. The authors state that their results dispel the assumption that only phosphine diequatorial isomers... 

With a more powerful donor than an H—C bond, cyclohexyl esters carrying a [3 silyl group demonstrate the preference for the donor and the acceptor bonds to be anti. The equilibrium proportion of the alcohol 2.89 (R = H) is in favour of the diequatorial isomer, but with esters (R = acyl) the equilibrium shifts to favour the diaxial conformation 2.90. Furthermore, the equilibrium constant correlates... 

Other substituted piperidines, particularly piperidinols, have been exploited as benzomorphan precursors. The first 6,7-benzomorphan lacking a 6-alkyl substituent, the parent heterocycle, was reported by May et al. in 1968<43,44) in a synthesis from 2-cyano-4-phenylpyridine (102) (Scheme 4.15). The 2-carbomethoxypiperidine (103) was prepared readily, but it proved resistant to direct cyclization to 3-methylbenzomorphan-l-one (105) with polyphosphoric acid, presumably because the more stable 2,4-diequatorial isomer is not favorable for ring closure for geometric reasons. Hydrolysis to the corresponding acid (104), however, gives an intermediate that closes to 105 in 94% yield. The parent heterocycle 106 is produced by standard techniques. 

In the case of diaxial/diequatorial equilibria of trans -l,2-dihalocyclohexanes (with and without a 4-ter+butyl group), AG° [aa ee) also shows a pronounced dependence on the medium, varying from about 4 kJ/mol in apolar solvents to about —2 kJ/mol in polar solvents [90-94, 192-194]. This is mainly due to the very different dipole moments of the two conformers. In general, the more dipolar diequatorial isomer is favoured in polar solvents. 

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. 

The conditions necessary for rearrangement of 2/3,3a-(diaxial) iodohydrin esters [e.g. (28)] to the transposed diequatorial isomers [e.g. (29)] have been further... 

One explanation for the regioselectivity is related to the stereochemistry of the diphosphine-Rh catalyst complexed with the alkene. We have seen earlier that high linear-to-branch ratios in Rh-catalyzed hydroformylation, as with Co-catalyzed hydroformylation, stem from the alkene complexation-insertion step of the catalytic cycle, so differences in steric and electronic factors as a function of stereochemistry could play a significant role in the regiochemical result of the reaction. There are two trigonal bipyramidal possibilities for the diphosphine-Rh-alkene complex, which are denoted a-e (apical-equatorial isomer 18) and e-e (diequatorial isomer 19). 

The Fiirst Plattner rule can be used only for predicting the regioselectivity of kinetically controlled cleavage reactions. If thermodynamic control takes place and the initially formed diaxial isomer is able reversibly to close back to the aziridine ring, the diequatorial isomer can begin to accumulate in the reaction mixture and is isolated either as the sole product or in a mixture " with the diaxial isomer. 

The diaxial isomer 189 was the main product at short times of reaction, whereas the diequatorial product 190 was formed exclusively after prolonged heating. Application of the same conditions to the diaxial isomer alone led to its conversion into the diequatorial isomer through the equilibrium between the diaxial isomer and the epimine. Rate constants for each reaction involved in the cleavage have been estimated. ... 

Exclusive formation of the diequatorial isomers 193 194 has been observed in the reactions of l,6-anhydro-4-0-benzyl-2,3-(A-benzylepimino)-2,3-dideoxy-p-D-allopyranose 160 with a mixture of ammonium and tetrabutylammonium bromide or iodide on heating in toluene at high temperature. In these instances, the corresponding diaxial isomers 191-192 were unstable and were formed only in situ (see Table X for details). ... 

The observed ratio of diaxial/diequatorial isomers is very little influenced by the presence of ammonium chloride in the reaction mixture in contrast to the marked effect of the A -substituent. The diaxial isomers predominate in most reactions, although the amounts of diequatorial isomers are sometimes significant. In the... 

The claimed diequatorial cleavage of A -benzoylepimine 45 reported in Ref. 8 was later corrected. Guthrie and Williams in their contribution failed to reproduce the claim to the sole formation of the diequatorial isomer 229 in 70% yield. Instead, the cleavage of the A -benzoylepimine with NaN3/NH4Cl in DMF afforded the altro (224) and the gluco (229) products in 56 and 18% yields, respectively. The authors therefore considered the previously claimed exclusive formation of the diequatorial isomer as an erroneous result. 

Ring cleavage of the p anomer of A -benzoyl a//o-epimine 67 gave preferential formation of diequatorial isomers as shown by Meyer zu Reckendorf (see Table III). 

Cleavage reactions of epimino derivatives of methyl 4,6-0-benzylidene-2,3-epimino-2,3-dideoxy-a-D-allopyranosides with ammonium halides led either to diaxial or diequatorial isomers, or mixtures of both (Table VI). 

Free epimino derivative 44 afforded products of trara-diequatorial cleavage only, whereas A-substitution with an electron-withdrawing group led to the formation of mixtures of both stereoisomers, with the diaxial isomer predominant. In the cleavage of benzyl glycosides 53 and 255, the authors proved the existence of an equilibrium between the diaxial isomer and the starting epimine, which led to preponderant formation of the diequatorial isomer. 

Mixture of diaxial and diequatorial isomer without benzylidene group. 

The final example in Table 7.13 shows that two elimination reactions can occur on the same substrate. Although it appears that elimination occurs from the 1,2-diequatorial isomer, it is almost certainly true that conformational change to the diaxial isomer (prepared, e.g.,by addition of bromine [Br2] to cyclohexene) precedes the elimination as shown in Scheme 7.41. 

As usual, Newman projections are vital, and in Figure 5.41 we sight along the carbon-carbon bond attaching the two methyl-substituted carbons. In the diequatorial isomer, there is one gauche methyl-methyl interaction costing 0.6 kcal/mol. 

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