Aromatic substitution, ipso

Nucleophilic aromatic substitution Ipso subsiiiuiion, giving phenols, henzen-amines (anilenes), or alkoxybenzcnes. 

Trialkyltin substituents are also powerful ipso-directing groups. The overall electronic effects are similar to those in silanes, but the tin substituent is a better electron donor. The electron density at carbon is increased, as is the stabilization of /S-carbocation character. Acidic cleavage of arylstannanes is formulated as an electrophilic aromatic substitution proceeding through an ipso-oriented c-complex. ... 

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. 

He is, in contrast to H , a very poor leaving group indeed, with the result that in simple aromatic nucleophilic substitution ipso attack (cf. p. 161) is the rule rather than the exception. Cl , Bre, N2, S03, NR2, etc., are found to be among the more effective leaving groups and, with them, certain analogies to nucleophilic substitution at a saturated carbon atom (p. 77) may now be observed. 

Ipso adducts. Ipso adducts as reactive intermediates in aromatic substitution are diagnostic of the electrophilic reactivity of the nitrating agent, particularly towards the following prototypical aromatic donors (Schofield, 1980 Olah et al., 1989). 

The first product is derived from a normal electrophilic aromatic substitution reaction of the kind described in the text. The second product is derived from ipso electrophilic aromatic substitution. The mechanism is exactly the same, but in the last step z-Pr+ is lost instead of H+. 

Remarkable regioselectivity is exhibited in aromatic substitutions via aryne intermediates. When there is a donor group at the meta position, apparent ipso substitution occurs [24], The polarity alternation sequence indicates an electronic cause. 

Interestingly, exclusive ipso substitution was observed in the reaction of trialkylarylstannanes with arenesulfonyl chlorides [118]. Diaryl sulfones, with substitution patterns different from those available via electrophilic aromatic substitution, were obtained in good to excellent yields. The... 

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-... 

The chemistry involved in nucleophilic aromatic substitution is well reflected in the reactions of a variety of nucleophiles with methyl penta-fluorophenyl ether (Ingemann et al 1982a). For most of the nucleophiles such as alkoxide, thiolate, enolate and (un)substituted allyl anions, the dominant reaction channel is the attack upon the fluoro-substituted carbon atoms, as is the case for OH-. The latter ion reacts approximately 75% by attack upon the fluoro-substituted carbon atoms and the remaining 25% by Sn2 (20%) and ipso (5%) substitution as summarized in (41). In the attack upon the fluorinated carbon atoms, the interesting observation is made that a F- ion is displaced via an anionic o-complex to form a F- ion/molecule complex, which is not observed to dissociate into F- as a free ionic product. 

Ipso, Cine, Tele Nucleophilic Aromatic Substitutions... 

Tab. 1. Ipso nucleophilic aromatic substitution of arenetricarbonylchromium complexes C-O, C-S and C-Se bond formation.

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