Electropositive groups

Polycationic polymers. These are macromolecules that have electropositive groups attached to the polymer chain or pendant to the chain. These materials are active against a number of bacteria and fungi. However, due to their inherent toxicity to animal species through their destructive interaction with cell membranes they are... 

Any effect which alters the density or spatial distribution of electrons around a nucleus will alter the degree of shielding and hence its chemical shift. H chemical shifts are sensitive to both the hybridisation of the atom to which the H nucleus is attached sp, sp etc.) and to electronic effects (the presence of neighbouring electronegative/electropositive groups). 

X hydrophobic group B electronegative group AX hydrophobic group or electropositive group... 

Delocalization can occur via the inductive effect, whereby an electronegative atom transmits its electron-withdrawing effect through a chain of tr bonds. Electropositive groups are electron-donating and localize more electron density on the basic site. 

The results provide a clear indication of the importance of the change in hybridization between cyclopropane ( sp2 for the external bonds) and propane ( sp3). Electropositive groups prefer to be bonded to the more electron-withdrawing cyclopropane ring, whereas electronegative substituents prefer to be bonded to C2 of propane. The one anomaly in Table 3 is found with the BH2 substituent which leads to the most exothermic group transfer reaction. This is presumably a result of the interaction of the cyclopropane ring with the empty p orbital at boron. This type of interaction will be discussed in the next section. 

Silyl groups, which are non-polar electropositive groups without lone pairs, tolerate many chemical reactions that would not be possible in presence of hydroxy groups. The Fleming-Tamao Oxidation permits silyl groups to be used as masked hydroxy groups , which has found broad application in total syntheses. In addition, enantioselective hydrosilylation of alkenes followed by Fleming-Tamao oxidation allows the preparation of chiral alcohols. 

It was also observed that an electropositive group, particularly nitrogen-containing heterocycles, at the anomeric center tends to be oriented equatorially. This phenomenon is referred to as reverse anomeric effect. However, it is accepted that the reverse anomeric effect is not generalized [1,2]. 

The three alkali-metals potassium, rubidium, and caesium form numbers 4, 6, and 8 of the even series of the most strongly electropositive group of the periodic classification, and have the atomic-sequence numbers 19, 37, and 55, differing at each step by 18 units, the atomic number of rubidium being thus exactly the mean of the atomic numbers of potassium and caesium. The increase by 18 units... 

Coordination to metals follows the usual trends. The transition metals try to achieve octahedral coordination (with a few exceptions), but the cations of the electropositive group 1-3 elements exhibit a rich variety. The coordination polyhedra are determined by radius ratios more than by topological preferences. For polyphosphides in general, all P atoms are involved in M-P interactions according to the number of lone pairs present. The anionic (lb)P and (2b)P as well as the neutral (3b)P° species adopt quasi-tetrahedral coordination, especially if main-group cations are involved. Only a few exceptions are known, for example Li3P7. With more covalent M-P bonds, the number (m + n) of available lone pairs of a polyanion P " is strongly related to the metal coordination number that is, CN(M) < m + n). If CN(M) > m + n), ion-ion and ion-dipole interactions dominate. The relation <7[M-(2b)P] > <7[M-(3b)P] is true in most cases. 

In studies of the H NMR spectra of A -glucosylimidazoles, analyses of both /4 s from the sugar ring <2001JOC1097> and chemical shifts of the imidazolyl H-2 <1999JA6911> upon protonation discount the previously proposed reverse anomeric effect of electropositive groups at C-1 of pyranoses. 

---