Ipso anion

Lithium tris(phenylthio)methide. Ipso anion precursor. Nuck... 

Ipso anion precursor. Nucleophilic homologation of the reagent generates products that readily submit a PhS unit to another nucleophile (e.g., 5ec-BuLi). The resulting lithio compounds are acyl anion or methylene anion equivalents. 

In the first mechanism (equation 74) the nucleophile function attacks the aromatic ring in an ipso-type displacement involving a Meisenheimer complex intermediate243,244, and leads to the rearranged product after expulsion of sulfinate anion (X-). This mechanism should be favoured by the presence of an electron-withdrawing substituent in conjugation with the anion. The second mechanism (equation 75) involves a direct displacement of sulfinate anion (X ) by Y-, without involvement of the aromatic n electrons. 

With Roustan et al. using the sodium salt of the Hieber anion 76-Na, the procedure was improved by Xu and Zhou in 1987 when they introduced the corresponding shelf-stable tetrabutylammonium salt 76-[Bu4N] which is available from Fe(CO)5 78, NaN02 and Bu4NBr (Scheme 17) [61,62]. As well as discovered by Roustan they obtained the substitution products with an ipso-preference (Scheme 16) albeit in a significantly lower yield. In order to maintain the catalytic activity of the product, the reactions were performed under CO-gas atmosphere. 

An abnormal (tele) substitution of chlorine in both 2,3- and 2,6-dichloropyrazines 91, 92 occurred on reaction with dithiane anion, while morpholine gave the normal //wo-substitution <06TL31>. Another paper described the highly selective ipso monosubstitution of the 2,3-dichloro compound by enolates in toluene <06T9919>. 

The electrochemical generation of the germyl anion has been the subject of a recent paper105. Evidence for its formation by SET reduction of Ph3GeH on Pt in DMF with tetrabutylammonium tetrafluoroborate is based on 13C NMR which shows a strong down-field shift of Ca 30 ppm for the ipso carbon of the anion. The anion tends to yield Ph3GeGePh3 above 20 °C, but is also trapped by reactions at —40°C with O2 and CH3I ... 

While donor substituents assist in ortho and meta protonation, acceptor substituents direct protonation of the primary anion-radicals to the ipso and para positions. It should be emphasized that water treatment of the naphthalene anion-radical in THF leads to 1,4-dihydronaphthalene. Notably, the same treatment of this anion-radical, but o-bound to rhodium, leads to strikingly different results. In the rhodium-naphthalene compound, an unpaired electron is localized in the naphthalene, but no protonation of the naphthalene part takes places on addition of water. Only evolution of hydrogen was observed (Freeh et al. 2006). Being a-bound to rhodium, naphthalene acts as an electron reservoir. The naphthalene anion-radical part reacts with a proton according to the electron-transfer scheme similar to the anion-radicals of aromatic nitro compounds (see Scheme 1.14). 

In these reaetions, the most reliable mechanism is considered to involve the initial metal-coordination at the nitrogen atom of the pyridine ring and the subsequent attack of an alkyl or aryl anion at the most probable cationic sites on the ring, namely, the 2- and/or 4-position of the ring. If a 2-halogen substituted pyridine is used, the nucleophilic anion attacks the 6-position. Thus, the addition is a more prefered reaction than the ipso-substitution as shown in reaction (29). The substitution of amide or phenyl-... 

Figure 4.9 The structure of (a) (dppe)2Au3lri3CI6(THF)3 21 [48], and (b) of the anion in [(dppe)2Au][(dppe)2Au3ln3Br7(THF)] 22 (the phenyl rings, less ipso carbons, on phosphorus have been omitted for clarity) [49].

The radical anion from 2,3-diphenylquinoxaline has been isolated in the dark blue sodium or potassium salts, 77 and 78, respectively.131 The crystal structures show that the metal is coordinated (Fig. 50) to DME, to the nitrogen atoms of the heterocycle where the negative charge is concentrated, and to ortho and ipso carbon atoms of the adjacent phenyl rings (Na-C 302.2(6)-307.6(6) pm and K-C 312.1(3)-333.3(3) pm). The M-Ph interactions link the ion pairs into infinite chains. The formation and stability of the radical anion have been studied by ESR spectroscopy and cyclic voltammetry. 

Diethyl sodiomalonate is an example of type (i). Reaction with [(fluorobenzene)Cr(CO)3] proceeds to completion after 20 h at 50 C in HMPA to give the diethyl phenylmalonate complex in over 95% yield. Monitoring the reaction by NMR gave no evidence for an intermediate (e.g. the cyclohexadienyl anion complex) interruption of the reaction by addition of iodine at less than 20 h gave significant amounts of unreacted fluorobenzene. A satisfactory picture is the simple one, that the anion adds reversibly and unfavorably (k < k-i, as in Scheme 3), slowly finding itself at the ipso position then irreversible loss of fluoride gives the substitution product.5152... 

Chloro-l,7-naphthyridine (110 X = Cl) gives on animation with KNH2/ NH3 the tele product 2-amino-1,7-naphthyridine (53) in addition to the ipso product 8-amino-l,7-naphthyridine (54).10 25 The formation of 53 involves as intermediates anionic cr-adduct 111 (X = Cl) (its existence has been proved by NMR spectroscopy see Section I1,B,1) and probably 2-amino-2,8-dihydro-8-chloro-1,7-naphthyridine (112). The latter undergoes a base-catalyzed dehydrochlorination, yielding 53. Because there are four atoms between position 2 and 8. the reaction is called an even tele substitution. 

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