Introduction of Electron-Withdrawing Group

According to Yagpolskii s principle, Koppel et cd. reported that introduction of very strong electron-acceptor substituents such as =NTf and =NS02P into an acid system instead of double bonded sp oxygen or sulfur atom increased the stability of counter anions and their acidity [4]. Representative results are shown in Table 2.1 and Table 2.2 

The catalytic activities of polystyrene-bound tetrafluorophenylbis(triflyl)methane in several reactions (direct esterification, Friedel-Crafts acylation, acetalization, the Mukaiyama aldol reaction, and Sakurai-Hosomi allylation) were superior to those of Nafion SAC-13 which is perfiuororesinsulfonic acid (Table 2.3). 

At the same time, they developed fluorous super Brousted acid by nucleophilic para substitution of the lithium salt of pentafluorophenylbis(triflyl)methane with 

Entry r r = o Diol Solvent Flow rate (mL/min.) Yield (%f 

Reactants Organic solvent OOOOCXXJ Heat Homogeneous reaction Product Organic solvent OOQOOOO 

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This is, however, a weak electrophile compared with species such as N02 and will normally only attack highly reactive aromatic compounds such as phenols and amines it is thus without effect on the otherwise highly reactive PhOMe. Introduction of electron-withdrawing groups into the o- or p-positions of the diazonium cation enhances its electrophilic character, however, by increasing the positive charge on the diazo group ... 

As we saw above, the introduction of electron-withdrawing groups into an aromatic nucleus tended to inhibit electrophilic substitution... 

It has to be taken into account that these catalysts are not yet fully developed. Introduction of electron-withdrawing groups is suspected to increase Lewis acidity, which influences activity, and the optimum spacer length may enable formation of chiral polymers, but such variations are synthetically challenging. 

A portion of these data is summarized in Table I, in which the acidities of a number of heteroannular ferrocenecarboxylic acids are compared. Introduction of an electron-releasing alkyl group into the unsubstituted ring lowers the acidity of the corresponding acid, while the introduction of electron-withdrawing groups increases it. 

Hydrolysis of 8-acetoxyindolizine did not allow isolation of the corresponding indolizinol owing to spontaneous decomposition. A stabilization could be effected by introduction of electron withdrawing groups in position 3 of the indolizine ring (81JCR(S)366). 

It may also be concluded from the interaction scheme that the properties of [3.3.3]cyc-lazines are very sensitive to inductive perturbations. Aza substitution as well as introduction of electron withdrawing groups in the active positions 1, 3, 4, 6, 7 and/or 9 lowers the energy of the HOMO, leaving that of the LUMO unchanged. A stabilizing effect by these perturbations is to be expected together with a dramatic influence on the NMR and UV spectra. 

Metal complexes with fluoro-/3-diketones have been comprehensively reviewed.1585 The introduction of electron-withdrawing groups in the chelate ring increases the Lewis acidity strength of the ML2 complexes, and consequently the bis adducts of the fluoro-/3-diketonato complexes are more stable than the corresponding complexes with /3-diketones. As an example of a nickel complex with 1,1,1-trifluoroacetylacetone which does not have a counterpart in the nickel acetylacetonate complexes we can mention the hexanuclear complex Ni6Lio(OH)2(H20)2.1586... 

The central bond of [l.l.l]propellanes (1) is the center of their reactivity, and in many ways, it is useful to think of it as somewhat akin to the n bond in an alkene. The strengths of the two are comparable, and both are susceptible to electrophilic and radical attack. The main difference is that the central bond in la is apparently somewhat susceptible to nucleophilic attack as well, whereas the n bond in unsubstituted ethylene is not. In both cases, introduction of electron-withdrawing groups enhances reactivity towards nucleophiles. 

Accordingly, the consecutive introduction of electron withdrawing groups in Fe(TPP)Cl 214 = > Fe(TF5PF8P)Cl 218 has a dramatic effect with respect to the stability both towards oxidative degradation and /x-oxo dimer formation. This... 

A section of this chapter considered the effect of electron-withdrawing groups on pifaH and showed that they reduce the pKa and make a base less basic. This was the approach now adopted— the introduction of electron-withdrawing groups on to the guanidine to lower its pKTaH. Table 8.7 shows the p CaHS of various substituted guanidines. 

To drive the reaction over to the sulfonic acid, an excess of sulfuric acid is generally employed which also assists the forward reaction by removing the water formed. Sulfonation is a bimolecular electrophilic substitution (Se2) reaction and is therefore facilitated by the presence of electron-donating substituents on the aromatic nucleus and is made more difficult by the introduction of electron withdrawing groups (see chapter 7, p. 98). 

The results indicate that the introduction of electron-withdrawing groups lowers the numerical value of Ev. This also implies that the main interaction of phenyl groups at the 5(7)-positions is inductive, since conjugative interaction of such groups is of necessity electron-donating (see Lloyd et cd.37). 

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