Neighboring functional groups

In this second empirical approach, which has also been used for C NMR spectra, predictions are based on tabulated chemical shifts for classes of structures, and corrected with additive contributions from neighboring functional groups or substructures. Several tables have been compiled for different types of protons. Increment rules can be found in nearly any textbook on NMR spectroscopy. 

The parameter n reflects the measure of deviation of the system from the behavior of the monomeric acid where n = 1, i.e., it characterizes the degree of interaction between the neighboring functional groups of the macroion. The value of n depends on the structure of the polyelectrolyte and the nature of the counterion pK = pK0 — log (1 — a)/a is the negative decadic logarithm of the effective dissociation constant of the carboxylic CP depending on a. 

Boronic acid derivatives are able to form ring structures with other molecules having neighboring functional groups consisting of 1,2- or 1,3-diols, 1,2- or 1,3-hydroxy acids, 1,2- or 1,3-hydroxylamines, 1-2- or 1,3-hydroxyamide, 1,2- or 1,3-hydroxyoxime, as well as various... 

Selective addition of alkenes and alkynes to aromatic compounds has also been performed by ruthenium-catalyzed aromatic C-H bond activation. Carbon-carbon bond formation occurs at the ortho positions of aromatic compounds, assisted by the neighboring functional group chelation. The reaction, catalyzed by RuH2(CO)(PPh3)3, was efficient with aromatic and heteroaromatic compounds, with various functional groups, and a variety of alkenes and alkynes [ 121 ] (Eq. 90). Activation of vinylic C-H bonds can occur in a similar manner. 

A ruthenium complex such as Ru3(CO)12 can activate the C-H bond of sp carbons on the condition that a neighboring functional group can coordinate to the metal to favor intramolecular C-H bond activation [123] (Eq. 92). 

Having explored the relationships between solution pH and pKa values, we can now explore the relative acidities of various hydrogen atoms and how these values are influenced by neighboring functional groups and heteroatoms. In this arena, it is important to remember that how a reaction proceeds is largely dependent upon the relative acidities of protons (hydrogen atoms) compared to one another and not on the absolute acidity of a given proton. 

For hydrogen, the Ramsey theory is relatively easy to apply, but, as we see in Section 4.5, external perturbations from neighboring functional groups and solvent molecules are of comparable magnitude to the basic electronic terms, so that the theory is quantitatively of little practical value. For other nuclei, this theory could predict chemical shielding rather accurately if we had true wave functions and knowledge of the energies of all excited states. In practice, approximations must be made, but acceptable results can be obtained provided some fundamental aspects are kept in mind. 

Oxiranes of terminal monosubstituted and internal disubstituted olefins do not undergo the isomerization under standard conditions, but give aldehydes at elevated temperature. For the special substrates described in Sch. 35, different modes of reaction originated from intermediary carbocationic species, involve neighboring functional group participation, oxidation, etc. An improvement employing other silicon Lewis acids, for example Mc3SiI and McsSiBr, was developed by Kraus, Detty, and Sakurai [17,19f,62]. 

Because of the chemical and steric variety of neighboring functional groups on humic and fulvic acids, there may be a range of affinities (binding energies) for metal ions and for protons, that is, a range of complex formation constants and acidity constants. 

FIGURE 36.12 Cyclic protections of two neighboring functional groups. 

TABLE 16.1 THE EFFECT OF NEIGHBORING FUNCTIONAL GROUPS ON CHEMICAL SHIFT... 

Figure 1. The 15 possible nearest neighbor functional group distributions in a silicate sol-gel.

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