Substituents equatorial

Reactions such as catalytic hydrogenation that take place at the less hindered side of a reactant are common m organic chemistry and are examples of steric effects on reactivity Previously we saw steric effects on structure and stability m the case of CIS and trans stereoisomers and m the preference for equatorial substituents on cyclo hexane rings... 

Energy differences between conformations of substituted cyclohexanes can be measured by several physical methods, as can the kinetics of the ring inversion processes. NMR spectroscopy has been especially valuable for both thermodynamic and kinetic studies. In NMR terminology, the transformation of an equatorial substituent to axial and vice versa is called a site exchange process. Depending on the rate of the process, the difference between the chemical shifts of the nucleus at the two sites, and the field strength... 

Ring inversion (Section 3.9) Process by which a chair conformation of cyclohexane is converted to a mirror-image chair. All of the equatorial substituents become axial, and vice versa. Also called ring flipping, or chair-chair interconversion. 

The bromide (5) failed to couple with dialkoxypyrimidines, even at elevated temperatures the failure is probably because of steric conditions imposed by the axially oriented C-4 p-nitrobenzoyloxyl group (see structure 5) this same group in the other halides (la, lb, 2a, 2b, 4a, and 4b) discussed in the foregoing is an equatorial substituent. Accordingly, the preparation of a differently constituted halide of 2-deoxy-D-Zt/xo-hexose is being investigated for the synthesis of pyrimidine nucleosides. 

Draw the possible conformations, and calculate the strain energy in each.. Remember that equatorial substituents cause less strain than axial substituents. 

Ring-flip (Section 4.6) A molecular motion that converts one chair conformation of cyclohexane into another chair conformation. The effect of a ring-flip is to convert an axial substituent into an equatorial substituent. 

Since compounds with alkyl equatorial substituents are generally more stable, trans-1,2 compounds, which can adopt the ee conformation, are thermodynamically more stable than their cis-1,2 isomers, which must exist in the ae conformation. For the 1,2-dimethylcyclohexanes, the difference in stability is about 2kcal moP (8 kJ mol" ). Similarly, trans-1,4 and cis-1,3 compounds are more stable than their stereoisomers. 

Next we need to learn how to draw the equatorial substituents. These are the substituents pointing out toward the sides. There are also six of them. Each one is rawn so at it is parallel to the two bonds from which it is once removed ... 

It is believed that equatorial substituents such as chlorine or bromine would increase the guest diameter beyond the allowed values (assuming that the guest molecules stack roughly parallel to the canal68)). Support for this comes from the study of fluorocyclohexane where the population of the axial conformer is not enhanced to any major extent70. Nitro-71) and cyano-cyclohexane, trans-l,2-dichloro-, trans-1,2-dibromo-, tram-1,4-dichloro-, trans-1,4-dibromo-, and trans-l-bromo-4-chloro-cyclohexane all pack most efficiently in the thiourea canals as the axial or diaxial conformer 68,72. Tram-2,3-dichloro-1,4-dioxane behaves similarly73. In contrast isocyanato-, tram-1,4-diiodo-, trans-1 -bromo-4-iodo-, and tram-1 -chloro-4-iodo-cyclohexane are present as mixtures of the axial/equatorial or diaxial/diequatorial conformations as appropriate 68,72). The reason for this anomalous behaviour of the iodosubstituted cyclohexanes is not clear. 

Phenylthio)nitroalkenes are also excellent intermediates for the synthesis of other heterocyclic ring systems. For example, tetrahydropyran carboxylic acid derivatives are formed by the intramolecular addition of oxygen nucleophile to l-(phenylthio)nitroalkene predominantly as the m-isomer (9.1 1) (see Eq. 4.40). The reaction may proceed via the chair-like transition state with two pseudo-equatorial substituents.50... 

Stereoisomers adopt a chair conformation with equatorial substituents at carbon (forms A and B, respectively), dioxaphosphorinane oxide, and... 

Structural investigations of individual stereoisomers and equilibrium mixtures revealed some general features (83M11). A phenyl group at phosphorus is predominantly in an axial position when there is no steric hindrance. The introduction of bulky substituents onto carbon atoms of the ring shifts the equilibrium toward an equatorial form, and one more twist conformer appears. The influence of equatorial substituents on the equilibrium position is clearly pronounced in the case of P(III) derivatives as well as for a P(IV) compound. The substitution of a phenyl group by a... 

In this scheme, a highly puckered metallocycle was envisioned, possessing pseudoaxial and equatorial substituents, and reaction pathways were said to be favored which minimized the following effects (a) 1,3-diaxial interactions of substituents on the two a-carbons (b) axial substituent interactions with juxtaposed ring carbons and (c) 1,2-diequatorial interactions. This scheme predicts relatively nonstereospecific metathesis of rra/j.v-olefins but highly stereospecific metathesis of ra-olefins. For example, the following pathways for reactions of m-olefins were proposed ... 

With these guidelines, the results from the metathesis of 4-methyl-2-pentene now appear to be reasonably accountable for. Most importantly, the lack of formation of c7s-2,5-dimethyl-3-hexene indicates that cis- 1,2-diisopropyl-substitution on the ring is highly unfavored, and trans-1,2-substitution leading to adjacent equatorial substituents is acceptable ... 

In chair form of cyclohexane ring, there are two possible kinds of bonds, those pointing up and down are called axial and those pointing sideways are called equatorial. Since they are commonly represented by letters a or e, this is why the substituents attached to these bonds are called axial or equatorial substituents respectively. 

The study of S -1 reaction in cyclohexane becomes difficult because of the ease with which the elimination reactions occur at the same time. However it has been shown that SN reaction will be sterically accelerated for an axial substituent, because the formation of carbocation will relieve the steric strain due to 1, 3 interactions which is absent with equatorial substituent. 

Since the equatorial substituents are more stable than axial substituent, the product development control gives mainly the equatorial alcohol unless the steric hindrance to the approach of the reagent is very severe. 

Since Eliel and Pietrusiewicz (142) have published a comprehensive review on l3C chemical shifts in saturated heterocycles, the discussion in the present report may be confined to a few basic features for the sake of completeness. It stands to reason that there is an electronegativity dependence of a-carbon chemical shifts. This was demonstrated by Lambert and co-workers (143) for a great variety of pentamethylene heterocycles, and holds also for positively charged species, such as X = (CH3)2N+, (CH3)2P+, CH3S +, Br+, and I+. The same is suggested by the good correlation of a-carbon chemical shifts in some heterocyclohexanes (143) and the a-SCS values in cyclohexanes with equatorial substituents (Figure 2)(114). 

Carbohydrates have been included in the wide range of molecules used in the parameterization of MM2 and of MM3. Alcohol and ether parameters have usually been determined from simple alcohols and ethers themselves. However, carbohydrates contain some unusual features in the acetal linkages, and in the many vicinal hydrogen-bonded hydroxyl groups. The "anomeric effect", first discovered by Edward (15) and popularized by Lemieux (16.), is best known in carbohydrates, although, of course, it occurs in other classes of compounds as well. One apparent result of this effect is that an axial alkoxy substituent is often more stable than the corresponding equatorial substituent when attached at the Cl position of a tetrahydropyranyl ring. This effect can be... 

Add three pairs of equatorial substituent bonds ensuring they are parallel to existing bonds... 

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