Halogen axial

The a-haloketone effect, as this phenomenon is known, is believed to be a result of dipolar interactions between the carbonyl group and the carbon-halogen bond dipole. The conformation with the smaller dipole moment is the one with the halogen axial, and is favored in solvents of low dielectric constant. 

Double substitutions of two halogen axial ligands in phosphorous porphyrins by an alkoxy group were reported to proceed in a single step [326,327], whereas the similar disubstitution of axial ligands of [SbTPP(Br)2]Br suffered a stepwise reaction and monosubstituted complex was actually isolated [328]. 

The incorporation of heteroatoms can result in stereoelectronic effects that have a pronounced effect on conformation and, ultimately, on reactivity. It is known from numerous examples in carbohydrate chemistry that pyranose sugars substituted with an electron-withdrawing group such as halogen or alkoxy at C-1 are often more stable when the substituent has an axial, rather than an equatorial, orientation. This tendency is not limited to carbohydrates but carries over to simpler ring systems such as 2-substituted tetrahydropyrans. The phenomenon is known as the anomeric ect, because it involves a substituent at the anomeric position in carbohydrate pyranose rings. Scheme 3.1 lists... 

The formation of derivatives of this type by free-radical attack has been mentioned previously (see section E above). The most common route to vinylogous halo ketones is by halogenation of dienol acetates or ethers. Both free halogen and A -halo compounds may be employed, and this approach has frequently been used to obtain 6 (axial) halo compounds ... 

The six-membered rings 8.12a and 8.12b adopt chair conformations with all three halogen atoms in axial positions. This arrangement is stabilized by the delocalization of the nitrogen lone pair into an S-X a bond (the anomeric effect) All the S-N distances are equal within experimental error [ld(S-N)l = 1.60 (8.12a)/ 1.59 A (8.12b) ]. 

C09-0133. Among the halogens, only one known molecule has the formula X 7. It has pentagonal bipyramidal geometry, with five Y atoms in a pentagon around the central atom X. The other two Y atoms are in axial positions. Draw a ball-and-stick model of this compound. Based on electron-electron repulsion and atomic size, determine the identities of atoms X and Y. Explain your reasoning. (Astatine is not involved. This element is radioactive and highly unstable.)... 

The chloro- and bromo-cyclohexane inclusion compounds have been extensively examined by infrared (4000-30 cm-1) spectroscopy65-68 and by Raman (< 1000 cm-1) spectroscopy 69). In the canals both guests are found to exist exclusively in the chair conformation with an axial halogen substituent, while iodocyclohexane 65, 68,69) acj0pts both axial and equatorial conformations in the canal. These results should be contrasted with the familiar situation in the liquid phase where the equatorial arrangement is the lowest energy conformer and is present to the extent of about 65-70% at room temperature. 

On the other hand, P-halogen substituents appear not to affect the axial fluorine much, but deshield the four equatorial fluorines (Scheme 7.25). 

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