Stabilization, functional group

The role of the solvent in determining equilibrium solution conformation can best be understood in terms of functional group stabilization. In polar protic media the equilibrium conformation of the uncomplexed anionic ionophore is determined by the solvation of the carboxylate anion and the polar llgandlng groups. 

ACS Symposium Series American Chemical Society Washington, DC, 1980. 

CD was utilized to obtain the solvent dependency of the conformation of the cation-lonophore complex as well as Kp s. Saturation Isotherms were plotted from linear computer fits of 1/[cation] versus 1/ArJ the slopes yielded Kjj s while extrapolation of Rq to infinite cation concentration provided the rJ s of the cation-saturated lonophore. It Is Important to note that the cation Itself is a significant vlnclnal moiety, which by virtue of Its charge, polarizability and location with respect to the chromophore of concern, can modify the rotational strength of the chromophore. 

X-ray crystallographic studies confirm that all cationic complexes of carboxylic lonophores have their llgandlng atoms oriented toward a central cavity. The extent to which this conformation would be altered in the absence of a bound cation due to the mutual electrostatic repulsion of the dipolar oxygen atoms would, in turn, be modulated by the mobility of the backbone supporting the ligands. 

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The fundamental aspects of the structure and stability of carbanions were discussed in Chapter 6 of Part A. In the present chapter we relate the properties and reactivity of carbanions stabilized by carbonyl and other EWG substituents to their application as nucleophiles in synthesis. As discussed in Section 6.3 of Part A, there is a fundamental relationship between the stabilizing functional group and the acidity of the C-H groups, as illustrated by the pK data summarized in Table 6.7 in Part A. These pK data provide a basis for assessing the stability and reactivity of carbanions. The acidity of the reactant determines which bases can be used for generation of the anion. Another crucial factor is the distinction between kinetic or thermodynamic control of enolate formation by deprotonation (Part A, Section 6.3), which determines the enolate composition. Fundamental mechanisms of Sw2 alkylation reactions of carbanions are discussed in Section 6.5 of Part A. A review of this material may prove helpful. 

It is possible to achieve some selectivity using this method. As illustrated above, tertiary C—H sites are much more reactive than primary C—H sites. Radical-stabilizing functional groups also impart selectivity, as illustrated by the regioselective functionalization of (9 equation 4). ... 

It should be noted that only terminal alkynes can be deprotonated in this fashion, as the resulting anion is stabilized by the sp hybrid orbital. Because hydrocarbons contain no stabilizing functional groups, sp and sp C-Hs of these compounds are NOT acidic enough to be deprotonated by NaNH . When carbons such as these are needed as nucleophiles, the corresponding Grignard reagents are employed. 

Stabilizing functional groups are carried out in dimethyl sulfoxide solution... 

With acyclic compounds, help from neighboring, conformation-stabilizing functional groups is needed. These certainly work particularly well, whenever rigidization [91] by complex formation is possible. Quite remarkable results are... 

Another unusual situation is encountered in epoxides that carry a charge-stabilizing functional group close to the epoxide centers. 

In the synthesis of molecules without functional groups the application of the usual polar synthetic reactions may be cumbersome, since the final elimination of hetero atoms can be difficult. Two solutions for this problem have been given in the previous sections, namely alkylation with nucleophilic carbanions and alkenylation with ylides. Another direct approach is to combine radical synthons in a non-polar reaction. Carbon radicals are. however, inherently short-lived and tend to undergo complex secondary reactions. Escheirmoser s principle (p. 34f) again provides a way out. If one connects both carbon atoms via a metal atom which (i) forms and stabilizes the carbon radicals and (ii) can be easily eliminated, the intermolecular reaction is made intramolecular, and good yields may be obtained. 

Carbocations stabilized by functional groups can also effect 3-alkylalion of indoles. From a synthetic point of view the most important are jV.jV-dialkyl-methyleneiminium ions which can be generated under Mannich conditions from formaldehyde and secondary amines[13]. The products, 3-(A/,A-dialkyl-aminornethyl)indoles, are useful synthetic intermediates (see Chapter 12). 

In the preceding chapter the special stability of benzene was described along with reac tions in which an aromatic ring was present as a substituent Now we 11 examine the aromatic ring as a functional group What kind of reactions are available to benzene and Its derivatives What sort of reagents react with arenes and what products are formed m those reactions ... 

Fig. 3. (a) Chemical stmcture of a synthetic cycHc peptide composed of an alternating sequence of D- and L-amino acids. The side chains of the amino acids have been chosen such that the peripheral functional groups of the dat rings are hydrophobic and allow insertion into Hpid bilayers, (b) Proposed stmcture of a self-assembled transmembrane pore comprised of hydrogen bonded cycHc peptides. The channel is stabilized by hydrogen bonds between the peptide backbones of the individual molecules. These synthetic pores have been demonstrated to form ion channels in Hpid bilayers (71). 

Fig.3 Illustration of functional groups appearing in stabilized PAN fiber [9].

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