2-Chlorocyclohexanone, conformers

The reaction of tin enolate with ct-chlorocyclohexanone gives chlorohydrins bearing chloro and hydroxyl groups in the t7r-conformation in the presence of catalytic amount of Bu3SnBr-Bu4NBr complex or Ph4SbBr. No selectivity is obtained in the reaction without those catalysts (Equation (73)).226... 

The preferred conformations of 2-bromo- and 2-chlorocyclohexanones depend on the polarity of the solvent. In the diequatorial conformer there is considerable electrostatic repulsion. Note that parallel dipoles are destabilizing in a nonpolar solvent. 

In 1953, Corey [1] studied the conformational equilibrium of a-halocyclohexanones (a-bromo- and a-chlorocyclohexanones) since the C=0 and the C-X (X = halogen) bonds are both strongly polarized, mutually repulsive, and next to each other. The conformer having the halogen atom equatorially oriented should be destabilized due to dipolar interactions between the C-X and the C=0 dipoles which are almost coplanar and equatorially oriented, whereas the conformer having the halogen atom... 

In order to compare the shifts of flie conformational equilibrium position with solvent effects it is advisable to select species exhibiting negligible cavity effects on the equilibrium position. Two firm candidates in this respect are the conformational equilibria of 1,2,2-trichloroethane and the equilibrium between the equatorial and axial forms of 2-chlorocyclohexanone both are accurately described by our scales. ... 

The preferred conformation of 2-bromo- and 2-chlorocyclohexanones depends on the polarity of the medium. In solvents of low dielectric constant, the bromine or chlorine substituent is more stable in the axial orientation. In carbon tetrachloride, the axial conformation of 2-bromocyclohexanone is favored by 3 1 over the... 

In 1955, Corey and Burke published an article discussing conformational studies with 2-chlorocyclohexanone, similar to those reported earlier for the corresponding bromoketone. It had been anticipated that the amount of axial isomer would be greater in the chloro compound than in the bromo. The C-Cl and C-Br dipoles are similar, so the electrostatics in the two cases should be similar. But because the axial position is crowded in a cyclohexane ring, the smaller chlorine atom was expected to fit there more easily than the bromine. However, the reverse was found to be true. Subsequent experimental studies showed that this is part of a steady progression from fluorine through bromine, where the percentage axial conformer in the equiUbrium increases steadily and appreciably as we go down the periodic table. 

A series of 3-alkyl-2-chlorocyclohexanone silyloximes (methyl, ethyl, i-propyl) have been prepared and their conformations extensively examined. By analysis of the vicinal interproton coupling, these compounds were shown to exist predominantly in the diaxial chair conformation (5) while the precursor ketones prefer the diequatorial conformation (4). This observation was corroborated by an X-ray crystal structure of fran5-3-methyl-2-chlorocyclo-hexanone t-butyldiphenylsilyloxime, which showed that the chair... 

---