Normal halogen dependence

Mason and Grieves reported measurements for both mononuclear and dinuclear Mo(II) compounds. The chemical shifts for [Mo2X4(PMe3)4] (X = Cl, Br, I) show a normal halogen dependence and, for all the dinuclear species, shielding increases with the Mo-Mo bond order as quadruple < triple < double < single bond, which they note is opposite to the sequence for NMR. 

Nevertheless, for a given oxidation state, the Mo chemical shift is sensitive to ligand environment and overall stereochemistry. The normal halogen dependence,i.e., increased shielding in the order Cl
other transition metal... 

Bromine substituents by comparison were much less effective, and carbons bearing chlorine appeared at even lower fields, thereby establishing the shielding order Cl < Br I as the normal halogen dependence (NHD). This shielding order is followed by the other atoms in Group IV, by all the atoms in Group III, and by many of the transition metals. 

Whereas the Cl, Br, and I compounds of other atoms span at least 50% of the total shielding range, these halides of phos-phorusQlI) are separated by only 45 ppm, barely 6% of the total p range of 720 ppm. Even more frustrating is the fact that there is no halogen sequence that can be used to establish either normal or inverse halogen dependence. 

Enolization is the rate-determining step in the halogenation of normal ketones. Where alternate directions for enolization exist, the preferred direction (and hence the position of kinetic bromination) depends on the substituents and stereochemistry. Furthermore, the orientation of the bromine introduced depends on stereochemical and stereoelectronic factors. 

Phase changes are characteristic of all substances. The normal phases displayed by the halogens appear in Section II-L where we also show that a gas liquefies or a liquid freezes at low enough temperatures. Vapor pressure, which results from molecules escaping from a condensed phase into the gas phase, is one of the liquid properties described in Section II-I. Phase changes depends on temperature, pressure, and the magnitudes of intermolecular forces. 

The tungsten-halogen lamp operates in a very specific mode, dependent on the detailed chemistry of the atmosphere within the lamp enclosure. The lamp envelope is normally a thin quartz bulb, filled with an inert... 

An unexpected deshielding is found in 2-halogenoaryl furans such as (112) where the adjacent proton is noticeably affected (Table 6). No other substituent behaves like this, and halogens in other positions are likewise devoid of effect. Perhaps for some reason the conformation is preferred that has the halogen atom close to the proton. This would help to explain the fact that a second halogen atom at the aryl 6-position restores the resonance to its normal position, and that the effect is temperature dependent (71NKK1206). 

A variety of ring halogenated compounds has been prepared, especially chlorinated derivatives. The latter are obtained using chlorine, normally in carbon tetrachloride, and the products are the 2,2- or 2,3-dichloro or 2,3,5,6-tetrachloro derivatives depending upon conditions . 

Stereocontrol in intermolecular cyclopropanation also depends on the structure of the unsaturated substrate. Early work concerning the influence of substrate on stereoselectivity has been summarized by Doyle2. In general, cyclopropanation of ciy-disubstituted alkenes results in higher stereoselectivity than with monosubstituted alkenes and the steric bulk of the olefinic substituent enhances the stereoselectivity. However, the stereocontrol appears not simply to be caused by a steric factor. In comparable cases, the presence of halogen as an alkene substituent may cause a reversal of the normal stereoselectivity. A few examples which illustrate these effects are shown in equations 124167 172, 74. 

---