Polarity axial

Figure 7 Synthesis of a silicon(IV) phthalocyanine with defined polar axial ligands.65 Pc = phthalocyanine nucleus the centrally coordinated atom (here Si) immediately precedes the Pc acronym.

In the case of a cyclohexanone this implies that in enolization a "polar" (i,e. axial) a-hydrogen is lost in preference to an equatorial a-hydrogen (cf. 441 442). Furthermore, it follows that in the ketoniza-tion of an enolized cyclohexanone (e.g. by bromination or protonation) the incoming substituent should adopt preferentially the polar (axial) orientation."... 

The magnitude of the anomeric effect depends on the nature of the substituent and decreases with increasing dielectric constant of the medium. The effect of the substituent can be seen by comparing the related 2-chloro- and 2-methoxy-substituted tetrahydropy-rans in entries 2 apd 3. The 2-chloro compound exhibits a significantly greater preference for the axial orientation than the 2-methoxy compound. Entry 3 also provides data relative to the effect of solvent polarity it is observed that the equilibrium constant is larger in carbon tetrachloride (e = 2.2) than in acetonitrile (e = 37.5). 

Several structural factors have been considered as possible causes of the anomeric effect. In localized valence bond terminology, it can be recognized that there will be a dipole-dipole repulsion between the polar bonds at the anomeric carbon in the equatorial conformation. This dipole-dipole interaction is reduced in the axial conformation, and this factor probably contributes to the solvent dependence of the anomeric effect. 

The axial-equatorial conformational equilibria for 2-fluoro- and 2-chlorotetrahydropyran have been investigated with several MO calculations, including calculations at the MP2/6-31G level. The MP2/6-31G calculations give values of 3.47 and 2.84kcal/mol, respectively, for the energy favoring the axial conformer. Solvent effects were also explored computationally and show the usual trend of reduced stability for the axial conformation as solvent polarity increases. 

Although the nature of the general polar effect suggested by Kamernitzsky and Akhrem " to account for axial attack in unhindered ketones is not clear, several groups have reported electrostatic interactions affect the course of borohydride reductions. Thus the keto acid (5a) is not reduced by boro-hydride but its ester (5b) is reduced rapidly further, the reduction of the ester (6b) takes place much more rapidly than that of the acid (6a). Spectroscopic data eliminate the possibility that in (5a) there is an interaction between the acid and ketone groups (e.g. formation of a lactol). The results have been attributed to a direct repulsion by the carboxylate ion as the borohydride ion approaches. " By contrast, House and co-workers observed no electrostatic effect on the stereochemistry of reduction of the keto acid (7). However, in this compound the acid group may occupy conformations in which it does not shield the ketone. Henbest reported that substituting chlorine... 

The 1,3-dianions formed across the sulfone330 of /J-ketosulfones may be selectively dialkylated331 with an a, w-dihalide and thus cy clize to give 2-ketothiane dioxides332. Due to its polarity, the 2-keto-substituent (or other polar group in the 2-position) adopts the axial orientation332 (equation 124). 

Fig. 32. (continued)—(b) An axial projection of the unit cell shows that there is sufficient room for guest molecules in the middle. Calcium ions also bridge adjacent chains of the same polarity along the a-direction (pointing down). 

An alkyl group located on a carbon a to a heteroatom prefers the equatorial position, which is of course the normally expected behavior, but a polar group in such a location prefers the axial position, An example of this phenomenon, known as the anomeric is the greater stability of a-... 

Direct conversion of the 6-sulfonates of D-galactopyranosides into their 6-deoxy-6-fluoro derivatives is usually hindered, because of the polar field-effect exerted by the lone electron pairs of the axial 0-4 (Ref. 158). Thus, the 6-0-mesylgalactoside 294 gave ° the 3,6-anhydro derivative 297 (23%) as the only isolated product on reaction with CsF in boiling ethanediol. How-... 

Surface Polarization in TFF The simplified model of polarization shown in Fig. 20-47 is used as a basis for analyzing more complex systems. Consider a single component with no reaction in a thin, two-dimensional boundary layer near the membrane surface. Axial diffusion is negligible along the membrane surface compared to convection. 

To substitute the strongly bound axial CO ligand of the ruthenium or osmium center, it is necessary to employ more drastic conditions than simple stirring at room temperature. Imamura (11,20) used photolysis to synthesize porphyrin trimers on the basis of simultaneous coordination of two 4-pyridyl porphyrins to the same ruthenium porphyrin (12, Fig. 3). Some interesting photophysical behavior was observed for these systems. The trimers have an extra UV-Vis absorption band at about 450 nm which is ascribed to metal-ligand charge transfer (MLCT), a d7r(Ru(II))-7r (OEP) transition. This band shows a batho-chromic shift in more polar solvents, and decreased in intensity when... 

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