Equatorial directions

This equation is the Porod law for the large-angle tail of the scattering curve along the equatorial direction, which indicates that the equatorial scattered intensity I q is proportional to in the Porod... 

There is another aspect to the question of the reactivity of the carbonyl group in r ck)hexanone. This has to do with the preference for approach of reactants from the axial ir equatorial direction. The chair conformation of cyclohexanone places the carbonyl coup in an unsynunetrical environment. It is observed that small nucleophiles prefer to roach the carbonyl group of cyclohexanone from the axial direction even though this is 1 more sterically restricted approach than from the equatorial side." How do the ctfcnaices in the C—C bonds (on the axial side) as opposed to the C—H bonds (on the equatorial side) influence the reactivity of cyclohexanone ... 

More bulky nucleophiles usually approach the cyclohexanone carbonyl from the equatorial direction. This is called steric approach control and is the result of van der Waals type repulsions. Larger nucleophiles encounter the 3,5-axial hydrogens on the axial approach trajectory. 

Another difference between dimethylsulfonium methylide and dimethylsulfoxonium methylide concerns the stereoselectivity in formation of epoxides from cyclohexanones. Dimethylsulfonium methylide usually adds from the axial direction whereas dimethylsulfoxonium methylide favors the equatorial direction. This result may also be due to reversibility of addition in the case of the sulfoxonium methylide.92 The product from the sulfonium ylide is the result the kinetic preference for axial addition by small nucleophiles (see Part A, Section 2.4.1.2). In the case of reversible addition of the sulfoxonium ylide, product structure is determined by the rate of displacement and this may be faster for the more stable epoxide. 

When n electron donors are involved, the XB is preferentially along the axis of the donated lone pair on D. For instance, XBs around ethers, thioethers, and amines feature a tetrahedral arrangement with preferential axial directions for the XBs around hexacyclic amines, and equatorial directions for hexacyclic thioethers [16,17,124,139-142] (Fig. 6). 

In addition to the crystal forms, X-ray scattering studies indicate that when unoriented PEN fiber was drawn at 120 °C ( 7 g), a mesophase is generated. In this form, the molecular chains are in registry with each other in the meridional direction but not fully crystallized in the equatorial direction. This conclusion was based on the presence of additional meridional peaks not accounted for by the crystal structure obtained by X-ray scattering. The mesophase is a intermediate phase and its existence is strongly dependent upon the processing conditions consequently, it could have implications with respect to the properties of commercially produced fibers and films, since it appears to be stable and not easily converted to the crystalline form, even at elevated temperature [25, 26], The mesophase structures of PET, PEN and poly(ethylene naphthalate bibenzoate) were compared by Carr et al. [27], The phase behavior of PEN and PEN blends with other polymers has also been studied [28-32],... 

As seen previously, the Cl -C3 bond of bicyclo[1.1.0]butane behaves as a rc-bond in addition reactions. According to SCF MO calculations, the hybridization in the two central carbons is equivalent to 0% p character.19 As a result, this central bond is easily cleaved by a nucleophilic attack when activated by an electron-withdrawing group. The course of the nucleophilic addition of a hydride ion to bicyclofl.l. 0]butane has been studied by ab initio calculations, the results of which suggested that the hydride ion should approach bicyclofl. 1.0]butane from an equatorial direction.20 An example depicting these features is shown by the attack of an alkoxide on bicyclo[1.1.0]butane-l-carbonitrile, giving the isomeric 3-alkoxycyclobutane-l-carbonitriles 10 in 35% yield.21... 

The structural anisotropy in crystalline or structurally ordered BPDA-PFMB films was studied in this laboratory with wide-angle X-ray diffraction (WAXD) methods. In brief, WAXD experiments were designed to examine both the reflection and transmission modes of thin-fihn samples. In addition, uniaxially oriented polyimide fiber WAXD patterns were obtained to aid in the identification of the film structure. The film WAXD pattern obtained from the reflection mode corresponded well to the fiber pattern scanned along the equatorial direction (Figure 16.3), " which indicates that the reflection mode pattern represents the (hkQ) diffractions. On the other hand, as shown in Figure 16.4, the (001) diffractions were predominant in the film WAXD pattern obtained via the transmission mode. This pattern corresponded to the fiber pattern scanned along the meridian direction. These experimental observations clearly indicate that the c-axes of the crystals are preferentially oriented parallel to the film surface however, within the film, they are randomly oriented. 4.2 5 j( should be pointed out that the WAXD experiments are only sensitive to crystalline or ordered structures in polyimide films. They do not provide any information on the amorphous regions. 

Figure 16.3. Comparison of BPDA-PFMB WAXD film patterns obtained with a reflection mode and fiber patterns scanned along the equatorial direction.

On the other hand, WAXS measurements of PE melt clearly indicate a range of intermolecular distance correlations of about 25 A [3]. Together with the relatively high density of polymer melts, the fact that the first interchain halo in WAXS patterns of oriented amorphous polymers tends to lie in the equatorial direction and the relatively high WAXS intensity of the interchain halo support the idea of parallel chain segments on the short range scale. 

The oxidation of imines derived from substituted cyclohexanones occurs predominantly from the equatorial direction. However, the product oxaziridines can undergo subsequent equilibration to favor a more stable conformation which places the bulkier nitrogen substituent in an equatorial conformation (equation 44)219. 

This observation tells us that the predominant mode of hydrogen addition to the double bond is from the equatorial direction. Equatorial addition is the less hindered approach and thus occurs faster. 

The major product is the stereoisomer that corresponds to transfer of oxygen from the equatorial direction. 

The allylation of 4-/ r/-butyl-2-hydroxycyclohexanone shows a high diastereoselectivity. When the hydroxyl substituent is oriented in the equatorial plane, kinetic acceleration accompanies exclusive entry of the allyl group from the equatorial direction (Equation (15)). The chelation in water has been revealed by competition experiments (Scheme 37).164 165... 

R erne m ber th e gu Iddl i rie 1 ri the, surjimary baxon p. 4 70 uniike ydohesiBne abides, cyclohexanones are already chairs, so thejf can be attacked from the axial orthe equatorial direction. 

P-Hydroxy sulfoximines are thermally labile and revert to their starting carbonyl compound and sulfoximine on mild thermolysis. This property has been exploited effectively as a method for the resolution of racemic chiral cyclic ketones.65 For example, the addition of the lithium salt of (+)-(S)-2b (99% ee) under kinetically controlled conditions (-78 °C) to racemic menthone gave three of the four possible diastereomeric adducts. The major two adducts resulted from attack on the menthone from the equatorial direction. These diastereomeric adducts could be readily separated by column chromatography. Thermolysis of the individual two major diastereomeric carbinols at 140 °C gave d- and /-menthone, respectively, in high enantiomeric purities (90-93% ee). This methodology has been successfully applied to the resolution of other 2-substituted cyclohexanones as well as other chiral ketones that have served as advanced synthetic intermediates for the synthesis of natural products.66-69... 

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