Electron-withdrawing groups heteroatom substituent

The half-wave potentials are dependent on the nature of the substituents (R ) and of the heteroatom. Thus, electron-withdrawing groups or substituents that interfere with the coplanarity of the sulfur lone-pair electrons and the ethylene tt system tend to increase [163, 164] while electron-donating groups decrease [163, 165, 166] these values. The potentials also become higher by exchanging the sulfur atoms of TTF by Se, while the opposite effect is observed for Te [167, 168] (see Tables 4 and 5). 

Substituted isoxazoles, pyrazoles and isothiazoles can exist in two tautomeric forms (139, 140 Z = 0, N or S Table 37). Amino compounds exist as such as expected, and so do the hydroxy compounds under most conditions. The stability of the OH forms of these 3-hydroxy-l,2-azoles is explained by the weakened basicity of the ring nitrogen atom in the 2-position due to the adjacent heteroatom at the 1-position and the oxygen substituent at the 3-position. This concentration of electron-withdrawing groups near the basic nitrogen atom causes these compounds to exist mainly in the OH form. 

The incorporation of heteroatoms can result in stereoelectronic effects that have a pronounced effect on conformation and, ultimately, on reactivity. It is known from numerous examples in carbohydrate chemistry that pyranose sugars substituted with an electron-withdrawing group such as halogen or alkoxy at C-1 are often more stable when the substituent has an axial, rather than an equatorial, orientation. This tendency is not limited to carbohydrates but carries over to simpler ring systems such as 2-substituted tetrahydropyrans. The phenomenon is known as the anomeric ect, because it involves a substituent at the anomeric position in carbohydrate pyranose rings. Scheme 3.1 lists... 

This effect is generally similar to that observed in polysubstituted benzenes. Thus, groups such as N02 and CN reinforce the electron withdrawal from the substituent which is caused by the heteroatom(s). 

A different mechanism operates in the direct a-heteroatom functionalization of carbonyl compounds when chiral bases such as cinchona alkaloids are used as the catalysts. The mechanism is outlined in Scheme 2.26 for quinine as the chiral catalyst quinine can deprotonate the substrate when the substituents have strong electron-withdrawing groups. This reaction generates a nucleophile in a chiral pocket (see Fig. 2.6), and the electrophile can thus approach only one of the enantiotopic faces. 

The PPV derivatives (70-74) collated in Table 6.14 incorporate an electron-withdrawing group on the aromatic ring, either as a lateral substituent on the ring or as an heteroatom (N) within the ring, instead of on the vinyl linkage. All of these polymers exhibit a higher electron affinity than... 

Quite a number of other electron withdrawing groups containing multiple heteroatomic bonds, such as the ester carbonyl, nitrile, carboxamide, etc., can likewise stabilize carbanions. The lithium salt of tert-butyl acetate 32 is an example of an enolate anion sufficiently stable as a salt to be used as a shelf reagent. Substituents containing easily polarizable atoms, such as sulfur or selenium, are also capable of stabilizing an adjacent anionic center. 

The Michael addition of nucleophiles on oc,P-unsaturated electron withdrawing groups, often carbonyl-containing functional groups, is a widely used reaction for the formation of C—C or C—heteroatom bonds. When the Michael acceptor bears a substituent on the a-position to the carbonyl, then an asymmetric carbon is created upon protonation of the transient enolate generated by the nucleophilic addition (Scheme 7.9). 

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