With equatorial halogen

Alcohols, conversion to alkyl halides with triphcnylphosphine-halogen adducts, 48, 53 synthesis of equatorial, 47, 19 Aldehydes, aromatic, synthesis of, 47, 1... 

NMR has been used to show that the anomeric effect operates in 2-bromo- and 2-chloro-tetrahydropyrans, which exist predominantly with the halogen atom axial (66JOC544), whilst the equatorial preference of a 4-halogen atom is less pronounced than in cyclohexane and decreases with decreasing electronegativity (78SA(A)297). 

Until the present we have become accustomed to the idea that a bulky substituent imposes a six-membered ring conformation where this substituent is equatorial. The tendency is thus opposite in the a-position of an ether. This phenomenon can be observed on sugar derivatives with a halogen or, more generally, an oxygen atom at C-1. We will try to evaluate the anomeric effect starting from an equilibrium such as (2.5), relative, this time, to the most common pyranose. 

In contrast to a thoroughly performed conformational analysis of 1,4-dioxanes and 1,4-oxa-thianes, the data on the conformational behavior of 1,4-dithianes are quite limited. According to NMR ( H, C) spectroscopy and molecular mechanics calculations, 2-alkyl-substituted 1,4-dithianes exist preferentially in the conformation with equatorially oriented substituents. For 1,4-dithianes with substituents of tbe CHjX type (X = aeetoxy, halogen), the predominance of axial conformers is observed originating from an intramolecular electrostatic attraction <91T9455). 

In order to determine the conformational equilibrium of a-halocyclohexanones, Corey used infrared spectroscopy, since the substitution of one a-hydrogen in a cyclohexanone with a halogen produced a frequency shift in the absorption of the carbonyl group, where the frequency shift magnitudes depended upon whether or not the a-halogen atom was axial or equatorial (Table 1.1). 

In both molecules the coordination polyhedron of the P atom is a trigonal bipyramid or tbp. The phosphorus and the two axial halogen atoms define a vertical threefold symmetry axis, the three equatorial halogen atoms a horizontal symmetry plane. In addition to the threefold symmetry axis and the horizontal symmetry plane, each molecule has a twofold symmetry axes through each of the three equatorial P-X bonds. The symmetry of a molecules with one n-fold symmetry axis and n twofold symmetry axes perpendicular to it are denoted by D . The presence of a horizontal symmetry plane is indicated by the additional subscript h. The symmetry of PF5 or PCI5 then, is 1)3. ... 

Two i.r. methods have shown marked conformational differences in the juilibria of 2-chIoro- and 2-bromo-cyclohexanone ethylene acetal. The former has 72-80 % equatorial halogen whereas the latter is 70-80% axial. In a u.v. and i.r. study of variously substituted 2,6-diarylidene-cyclopentanones and -cyclohexanones extended conjugation effects are apparent in that the v(C=0) frequency is linearly correlated with Hammett o substituent values. 

When the cyclic substrate has an equatorial halogen (Table 2, step A), the experiment shows that the enolization at the a position does not lead to epimerization. This again is in perfect agreement with the stereoelectronic control which favors fixation of the deuterium by the enolate in the axial direction. This enolization does not seem to have any further influence on the Favorskii rearrangement. ... 

The behavior of derivatives with an equatorial halogen is described in... 

The results obtained with cyclohexane derivatives with an equatorial halogen reported in Table 3 show that the rearranged esters are obtained whatever the nature of the solvent Compounds 16, 17, and 19e do not react by an S y2 mechanism, as they yield esters with retention of configuration at the carbon initially bearing the halogen. It is only 3 -bromo-8,13-epoxy-2-labdanone (18e) which gives products with inversion of configuration. It... 

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