Chair cyclohexanes, conversion

A study of the photochemical Cope reaction of the hexadienes 40 has been carried out under photoinduced electron-transfer conditions. Evidence was gathered for the formation of a chair cyclohexane-1,4-radical cation 41 °. In snch systems, where the radical cation is formed using DCA as the sensitizer, a degenerate Cope process is operative. Thus when the tetradeuterio derivative 42 is used, rearrangement affords a (52 48) mixture of the two dienes 42 and 43. Related to this general problem, DCA-sensitized reactions of the isomeric dienes 44 and , -45 and the cyclization prodnct, the bicyclohexane 46, have been studied in considerable detail. At low conversions, the irradiation of 46 affords a mixture of the dienes 44 and , -45 in ratios that are independent of temperature. The influence of the position of the aiyl groups on the diene skeleton has also been studied. This does not appear to affect the conversion to a cyclic radical cation. Thus the SET-induced reaction of the diene 47 has shown that the open chain radical cation of the diene 48 cyclizes preferentially to the radical cation 49. ... 

When compared to chair cyclohexane, the presence of the trigonal planar carbonyl in 215 removes the 1,3-diaxial hydrogen atom interactions and, as seen in the models for Corey s calculations, partially flattens the ring. The conversion of alcohol 218 to ketone 219 illustrates this point. Although 218 is an equilibrating... 

FIGURE 5.23 Conversion of the energy-minimum chair cyclohexane into the halfchair and then the twist form. 

The strain energy minimization technique, which has previously been applied with success to conformation problems, has been modifiedin order to investigate the conformational change associated with cyclohexane chair-boat conversion attention is directed to the reaction co-ordinate as a minimum energy path. 

Stereochemistry. Cyclohexane can exist ia two molecular conformations the chair and boat forms. Conversion from one conformation to the other iavolves rotations about carbon—carbon single bonds. Energy barriers associated with this type of rotation are low and transition from one form to the other is rapid. The predominant stereochemistry of cyclohexane has no influence ia its use as a raw material for nylon manufacture or as a solvent. 

This is most readily studied with cyclohexane- /n in which 11 of the 12 protons are replaced with deuterium. The spectrum of cyclohexane- /n resembles the behavior shown in Fig. 4-8 at about — 100°C (the slow exchange regime) two sharp lines are seen these broaden as the temperature is increased, reaching coalescence at — 61.4°C, and becoming a single sharp line at higher temperatures. (The deuterium nuclei must be decoupled by rf irradiation.) Rate constants t for the conversion were measured over the temperature range — 116.7°C to — 24.0°C by Anet and Bourne. It is probable that the chair-chair inversion takes place via a boat intermediate. 

Fig. 3 summarizes calculations 23) for the conversion of the chair form of cyclohexane to the boat. The calculated difference in energy between the two isomers (5.4 kcal/mole) agrees well with experiment (5.3 kcal/mole). We have not as yet calculated the complete potential surface for the interconversion however energies have been calculated for three possible transition states (a - c) in which four, five, and six carbon atoms respectively are coplanar. In each case the energy was minimized subject to this one constraint. It will be seen that the predicted intermediate is a, in agreement with calculations by... 

Energy changes for the conversion of the chair to the boat conformation of the cyclohexane ring can be estimated from a study of the equilibrium between cis- and tra i-l,3-di-t-butylcyclohexane. Some analytical results of Allinger and Freiberg [9] are listed in Table 12.7. 

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]. 

Concerning conformation , we prefer to adopt a broader definition than has generally been used heretofore. Excluding normal molecular vibrations, relative atomic motion that breaks no bonds within a molecular framework generates conformations. A usual definition which specifies that conformations are attained by rotations about bonds is far too restrictive, considering that the conversion of one chair form of cyclohexane to another involves several complicated coupled rotations and 11... 

Ring-fiip (Section 6.5) The conversion of one chair conformation of cyclohexane to another in a process that changes all axial bonds to equatorial and vice versa. 

The comparatively strainless chair form of cyclohexane will be less influenced by conversion to the corresponding heterocyclic compounds. 

The presence of the double bond in cycloalkenes affects the conformation of the ring. The conformation of cyclohexene is a half-chair, with carbons 1, 2, 3, and 6 in the same plane, and carbons 4 and 5 above and below the plane. Conversion to the alternative half-chair occurs readily, with an energy barrier of 22.2 kJ/mol (5.3 kcal/mol), which is about one half that required for chair-to-chair interconversion in cyclohexane. Substituents at carbons 3 and 6 are tilted from their usual axial and equatorial orientations in cyclohexane and are referred to as pseudoaxial and pseudoequatorial. 

Saturated heterocyclic six-membered rings are much less flexible than their five-membered counterparts [46]. The activation energy barrier for the conversion of one chair form of cyclohexane into the alternate form is ca. 42 kJ/mol [47,48]. The activation energy for the interconversion barrier between the two puckered conformations, i.e., N- and S-type pseudorotamers, of the pentofuranose moiety in purine nucleosides is much smaller in comparison, i.e., below 20-25 kJ/mol. 

These results indicated that the E/Z isomer ratios in the Claisen rearrangements corresponded to the free-energy change for the conversion of a substituent from the equatorial to the axial position of a cyclohexane. Furthermore, using the well-established chair model for the transition state (Eq. 3.1.21), Faulkner has expected that the axial substituent r would introduce a relatively large 1,3-diaxial interaction with substituent and thus increase the stereoselectivity of the Qai-sen rearrangement [27]. 

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