Substitution, electrophilic ipso position

Table 4.4 helps us to understand why 1,4- and 1,2-xylenes undergo substitution in the ipso positions (the positions where the methyl groups are located). The electrophilic reagents obviously have to attack the substrate at the position with maximal electron density. These are ipso positions for the cation-radicals of 1,4- and 1,2-isomers. (For the neutral isomers, these are positions where the methyl groups are absent ) The substitution is directed into the ipso positions of 1,4-xylene and ring by up to 76 and 24% only, respectively. In case of 1,2-xylene, the snbstitntion takes place for 60 and 40% at ipso-positions and into the ring, respectively. In 1,3-xylene cation-radical, spin density is maximal in the nonmethylated positions 4 and 6, which are involved in the substitution for 84% (Fisher and Wright 1974). 

Tanaka et al. (2000) reported that N02 nitration of 1,8-dimethylnaphthalene leads to 2-nitro and 4-nitro products. For 2-nitro prodnct, the reaction proceeds as electrophilic substitution—the nitro group comes into the ipso position and then migrates to the position 2, thus giving the final product. For 4-nitro product, the process develops according to electron-transfer ronte. The spin density of 1,8-dimethylnaphthalene cation-radical is highest at position 4 (or, the same, at position 5). It is the para nitration that takes place in the experiment. 

The X-substituted benzene (aniline, Figure 11.2) is activated toward electrophilic attack since the HOMO is raised significantly. The electrophile would be directed to the ortho, para, and ipso positions of the ring and to the X substituent itself. The ipso channel is usually nonproductive since the common heteroatom-based X substituents are not easily displaced as Lewis acids. Loss of substituent is frequently observed with tertiary alkyl-substituted benzenes. Attachment of the electrophile to the X substituent is most likely if... 

This chapter covers reactions in which coordination of a transition metal to the ir-system of an arene ring activates the ring toward addition of nucleophiles, to give V-cyclohexadienyl-metal complexes (1 Scheme 1). If an electronegative atom is present in the ipso position, elimination of that atom (X in 1) leads to nucleophilic aromatic substitution (path a). Reaction of the intermediate with an electrophile (E+) can give disubstituted 1,3-cyclohexadiene derivatives (path b). If a hydrogen occupies the ipso posi-... 

Hydroxylation of arylamines with persulfate ion, or Boyland-Sims oxidation, gives ortho-substituted aminophenols in good yields [29]. As with the Elbs oxidation, the procedure is also carried out in two steps - first, treatment with the oxidant to obtain an aminophenyl sulfate ester and, second, hydrolysis to obtain the final product. Primary, secondary and tertiary amines can all be used in this reaction. The ortho product is formed, except when no ortho-positions are available, which leads to para-substitution. Electrophilic attack on the ipso-carbon is believed to be the most likely mechanism, although minor radical pathways also seem to be present. 

According to Section 5.1.1 electrophilic ipso substitutions via Wheland complexes occur, for example, when a proton attacks at the substructure Csp2—ferf-Bu or Csp2—S03H of appropriately substituted aromatic compounds. After expulsion of a ferf-butyl cation or an HSO3 ion, an aromatic compound is obtained, which has been defunctionalized in the respective position. 

Electrophilic attack can also occur at a position already occupied by a substituent, the ipso position. Such ipso substitutions are not common, but they are industrially useful. An example is ipso nitration by... 

In this section three main aspects will be considered. Firstly, the basic strengths of the principal heterocyclic systems under review and the effects of structural modification on this parameter will be discussed. For reference some pK values are collected in Table 3. Secondly, the position of protonation in these carbon-protonating systems will be considered. Thirdly, the reactivity aspects of protonation are mentioned. Protonation yields in most cases highly reactive electrophilic species. Under conditions in which both protonated and non-protonated base co-exist, polymerization frequently occurs. Further ipso protonation of substituted derivatives may induce rearrangement, and also the protonated heterocycles are found to be subject to ring-opening attack by nucleophilic reagents. 

This type of addition process is particularly likely to be observed when the electrophile attacks a position that is already substituted, since facile rearomatization by deprotonation is then blocked. Reaction at a substituted position is called ipso attack. Addition products have also been isolated, however, when initial electrophilic attack has occurred at an unsubstituted position. The extent of addition in competition with substitution tends to increase on going to naphthalene and the larger polycyclic aromatic ring systems. ... 

The general mechanism for electrophilic substitution suggests that groups other than hydrogen could be displaced, provided the electrophile attacked at the substituted carbon. Substitution at a site already having a substituent is called ipso substitution and has been observed in a number of circumstances. The ease of removal of a substituent depends on its ability to accommodate a positive charge. This fector determines whether the newly attached electrophile or the substituent is eliminated from the [Pg.588]

The silyl group directs electrophiles to the substituted position. That is, it is an ipso-directing group. Because of the polarity of the carbon-silicon bond, the substituted position is relatively electron-rich. The ability of silicon substituents to stabilize carboca-tion character at )9-carbon atoms (see Section 6.10, p. 393) also promotes ipso substitution. The silicon substituent is easily removed from the c-complex by reaction with a nucleophile. The desilylation step probably occurs through a pentavalent silicon species ... 

Ipso substitution, in which the electrophile attacks a position already carrying a substituent, is relatively rare in electrophilic aromatic substitution and was not explicitly covered in Section 10.2 in the discussion of substituent effects on reactivity and selectivity Using qualitative MO cOTicepts, discuss the effect of the following types of substituents on the energy of the transition state for ipso substitution. 

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