Electron-withdrawing groups salts

Pyrrole is soluble in alcohol, benzene, and diethyl ether, but is only sparingly soluble in water and in aqueous alkaUes. It dissolves with decomposition in dilute acids. Pyrroles with substituents in the -position are usually less soluble in polar solvents than the corresponding a-substituted pyrroles. Pyrroles that have no substituent on nitrogen readily lose a proton to form the resonance-stabilized pyrrolyl anion, and alkaU metals react with it in hquid ammonia to form salts. However, pyrrole pK = ca 17.5) is a weaker acid than methanol (11). The acidity of the pyrrole hydrogen is gready increased by electron-withdrawing groups, eg, the pK of 2,5-dinitropyrrole [32602-96-3] is 3.6 (12,13). 

Aromatic haUdes do not react easily with phenoxide ions to produce diaryl ethers unless the aromatic haUde is substituted with one or more electron-withdrawing groups, eg, nitro or carboxyl groups. The Ullmann reaction uses finely divided copper or copper salts to cataly2e the reaction of phenoxides with aromatic haUdes to give diaryl ethers. 

Azole iV-oxides, iV-imides and iV-ylides are formally betaines derived from iV-hydroxy-, iV-amino- and iV-alkyl-azolium compounds. Whereas iV-oxides (Section 4.02.3.12.6) are usually stable as such, in most cases theiV-imides (Section 4.02.3.12.5) andiV-ylides (Section 4.02.3.12.3) are found as salts which deprotonate readily only if the exocyclic nitrogen or carbon atom carries strongly electron-withdrawing groups. 

Pyrazoles are weak acids unless they carry powerful electron-withdrawing groups (Section 4.04.2.1.3(v)). They form metallic salts which are readily hydrolyzed by water (Section 4.04.2.1.3(vi)). 

In contrast to the previous method (equation 70), reaction 72 made possible the preparation of iodonium triflates from functionalized acetylenes bearing an electron-withdrawing group such as tosyl, cyano, or carbonyl [138]. Of special interest is the application of this method to the synthesis of the bisiodonium acetylenic salt [139, 140] (equation 73). 

Doyle et al. (1977 c) and Oae et al. (1980) reported modified Meerwein arylations with significant improvements in the yield by the use of aryl amines and alkyl nitrites in place of arenediazonium salts. However, good yields are only achieved if alkenes activated by electron-withdrawing groups are present. 

The new reaction appears to be a simple one-step procedure, which is particularly suitable for tertiary alkyl-aryldiazenes for which alternative synthetic routes are less convenient. However, aryl radicals or alkyl radicals in which the carbon-centered radical is bonded to an electron-withdrawing group (COOR, COR, CONR2, CN, S02R, etc.) do not add to diazonium salts or give only poor results (Citterio et al., 1982 c). This indicates that the radical must be a relatively strong nucleophile in order to be able to react with a diazonium ion. 

Iminium ions bearing an electron-withdrawing group bonded to the sp carbon of the iminium function are very reactive dienophiles. Thus, iminium ions 26 generated from phenylglyoxal (Scheme 6.15, R = Ph) or pyruvic aldehyde (R = Me) with methylamine hydrochloride, react with cyclopenta-diene in water at room temperature with good diastereoselectivity [25] (Scheme 6.15). If glyoxylic acid is used, the formation of iminium salt requires the free amine rather than the amine hydrochloride. 

Alkenes activated by an electron-withdrawing group (Z may be C=C, halogen, C=0, Ar, CN, etc.) can be arylated by treatment with a diazonium salt and a cupric chloride catalyst. This is called the Meerwein arylation reaction Addition of... 

The extent of restricted rotation about the amide band of (38) was used to compare the electron-withdrawing process of phosphonium salts (38, Y = alkyl) and chalcogenides (38, Y = O or S) with the more conventional electron-withdrawing groups. These phosphorus groups were found to exert a — A7 effect comparable with that of a nitro-group. 

The cycloaddition-isomerization procedure can be accomplished in the presence of a catalytic amount of a transition metal salt. The reactions proceed at room temperature, neither air nor water needed to be excluded. The presence of an electron-withdrawing group is not necessary to activate the dienophile as the example below shows that gold coordination increases the electrophilicity of the triple bond. The presence of a terminal alkyne should also be important. In the case of a disubstituted alkyne no reaction can be observed <00JA11553>. 

Potassium nitroacetate 53a reacts with Me3SiCl in aprotic solvents to give SENA (51a) in moderate yield. At the same time, the introduction of yet another electron-withdrawing group (N02 or CC>2Me) stabilizes the anion of salt (53) to an extent that it does not react with Me SiCl by the Sm mechanism without electrophilic assistance. Hence, K or Na salts 53b, C are inert with respect to halosilanes, and silver or mercury salts are required for the preparation of the corresponding nitronates. The latter salts are much safer to use as diox-anate complexes. These complexes react with halosilanes in inert aprotic solvents... 

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