Ipso attack

When the preferred substitution position (cf. 110 and 111) is occupied, activating substituents can facilitate substitution in other positions (cf. examples in Sections 4.02.1.4.2 and 4.02.1.4.5) ipso attack can also occur if the substituent is itself easily displaced. 

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. ... 

Several products resulted when 4-HMP was allowed to condense under the same conditions. These are provided in Scheme 6. These products indicate that a p-hydroxymethyl group can react with either an unsubstituted ortho position or through an ipso-attack on a hydroxymethyl-substituted para position. The two condensation products were formed at approximately equal rates. Since there were two unoccupied orthos, it seems that the specific reaction rate for the ortho site is about half that of the occupied para position in this situation. The condensation rate for 4-HMP was about 5 times the rate for 2-HMP overall. 

An ipso attack on the fluorine carbon position of 4-fIuorophenol at -40 °C affords 4-fluoro-4-nitrocyclohexa-2 5-dienone in addtion to 2-nitrophenol The cyclodienone slowly isomenzes to the 2-nitrophenol Although ipso nitration on 4-fluorophenyl acetate furnishes the same cyclodienone the major by-product is 4 fluoro-2,6-dinitrophenol [25] Under similar conditions, 4-fluoroanisole pnmar ily yields the 2-nitro isomer and 6% of the cyclodienone The isolated 2 nitro isomer IS postulated to form by attack of the nitromum ion ipso to the fluorine with concomitant capture of the incipient carbocation by acetic acid Loss of the elements of methyl acetate follows The nitrodienone, being the keto tautomer of the nitrophenol, aromatizes to the isolated product [26] (equation 20) Intramolecular capture of the intermediate carbocation occurs in nitration of 2-(4-fluorophenoxy)-2-methyIpropanoic acid at low temperature to give the spiro products 3 3-di-methyl-8 fluoro 8 nitro-1,4 dioxaspiro[4 5]deca 6,9 dien 2 one and the 10-nitro isomer [2d] (equation 21)... 

We have discussed orientation in the case of monosubstituted benzenes entirely in terms of attack at the ortho, meta, and para positions, but attack at the position bearing the substituent (called the ipsoposition ) can also be important. Ipso attack has mostly been studied for nitration. When NOj attacks at the ipso position there are at least five possible fates for the resulting arenium ion (13). 

No doubt ipso attack is also promoted here, and in some other of the dealkylations above, through the inhibition of normal electrophilic attack at positions in the ring which are flanked by massive alkyl groups. Perhaps the most important point to note about ipso... 

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. 

Bromoanisole. The competition between ortho and ipso attack (83) is also pertinent to the simultaneous nitration and transbromination of 4-bromoani-sole (Perrin and Skimer, 1971). Charge-transfer nitration leads to mixture of 2-nitro-4-bromoanisole and 2,4-dibromoanisole (and 4-nitroanisole), the relative amounts of which are equivalent to those obtained in the electrophilic nitration of 4-bromoanisole. 

Methylanisole. The competition between ortho and ipso attack [analogous to that depicted in (83)] applies to the simultaneous nitration and demethyla-tion of 4-methylanisole. The identification of 4-nitro-4-methylcyclohexa-2,5-dienone as the metastable intermediate in charge-transfer nitration (Kim et al., 1993) is particularly diagnostic of the ipso adduct (84) that is also apparent in the electrophilic nitration of 4-methylanisole (Sankararaman and Kochi, 1991). The common bifurcation of nitration pathways resulting from para (ortho) and ipso attack on the various aromatic donors, as noted above, indicates that the activation step leading to the Wheland intermediate and... 

Nitro group rearrangements have also been postulated during aromatic nitration reactions, particularly those associated with reaction pathways involving ipso attack by the reagent (N02+ or NO2). This topic is more fully discussed elsewhere in this volume and so just a few examples will be presented here. 

The example outlined above is a particularly clean reaction most ipso-attacks in amines and phenols are rarely as simple as this. Two other substrates might be used as examples... 

Of necessity, only a few of the many papers published in this area have been examined in detail in this review. Clearly, the area of aromatic nitration is still full of surprises38 and ipso-attack of nitrating species on substituted aromatics is proving useful in elucidating many of the subtle mechanistic details37. 

The reaction of durene with H NOj under the conditions just mentioned is another specific but principally important case. The reaction is accompanied with a strong enhanced absorption in N NMR spectrum with respect to a signal belonging to the product, that is, nitrodurene. Durene and naphthalene have very similar standard potentials (2.07 and 2.08 V, respectively, in AN see Ridd 1991). A significant difference between them is that, with durene, much of the nitration supposedly arises from ipso attack followed by rearrangement according to Scheme 4.41. 

Partial rate factors have proven quite useful in predicting isomer distributions from related aromatic reactants under the same reaction conditions. If two or more substituents are present on the aromatic ring, the effect of those substituents are usually considered to be additive. However, the procedure is less effective in the presence of steric effects, ipso attack, and resonance interactions between substituents. 

MEMBRANE TRANSPORT BINDING CHANGE MECHANISM Ipso-attack,... 

The most strained benzocycloalkene studied, cyclobutabenzene, undergoes extensive ipso attack upon sulphonation. In 95% sulfuric acid, cyclobutabenzene forms polyxylylene and ring cleavage products exclusively. The extent of ipso attack can be decreased in sulfur trioxide media where moderate -selectivity is observed. ... 

The reaction of o-xylene showed the formation of dimethylphenols in about 63% yield together with 21% of the dimer of 6,6-dimethyl-2,4-cyclohexadien-l-one, which involves ipso attack followed by a methyl shift and cycloadditive dimerization of the intermediate 6,6-dimethyl-2,4-cyclohexadien-l-one catalyzed by the acid (Scheme 5). 

Electrophilic hydroxylation of ortho- andpara-y[ 2Ceil4 by peroxides including BTSP in the presence of Lewis acids gave mixtures of phenols, including those derived from ipso attack followed by rearrangement ipso attack by HO leads to dealkylation. For... 

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