Ipso attack substitution

In this reaction, ipso-attack (substitution) of a carboxyl group by a nitro group takes place. 

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. 

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. 

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. 

From this it is possible to calculate the overall theoretical rate ratio for acetylation of m-xylene relative to benzene, since this is one-sixth the sum of the partial rate factors (in this case 1130), and the isomer distribution if the reaction is kinetically controlled. The overall rate ratio actually is 34769 and the calculated and observed isomer distributions are listed in Table 11.2.69 In this case, and in many others, agreement is fairly good, but many cases are known where the effects are not additive.70 For example, the treatment predicts that for 1,2,3-trimethylbenzene there should be 35% 5 substitution and 65% 4 substitution, but acetylation gave 79% 5 substitution and 21% of the 4 isomer. The treatment is thrown off by steric effects, such as those mentioned earlier (p. 511), by products arising from ipso attack (p. 512) and by resonance interaction between groups (for example, 24), which must make the results deviate from simple additivity of the effects of the groups. 

Several factors have been considered to play a role in determining the stability of a geminally substituted dimethoxy adduct and/or its rate of formation by ipso attack on a methoxy substituted substrate provided with two nitro groups flanking the methoxy group. Such factors include... 

Phenyl attachment at position 2 becomes more important when position 4(6) is substituted. The reaction of 4-phenylpyrimidine yields adducts 98 and 99, in a 1 7.2 ratio. This ratio is significantly altered by complexing agents in the presence of TMEDA it becomes 1 1.8. Apparently, the phenyl group has a relatively rate-depressing effect toward ipso attack, so that the reactions at positions 2 and 6 become more favored. This behavior is similar to that... 

Ipso attack has been detected at both a-positions in the reaction of methyl and adamantyl radicals with methyl 5-nitrofuran-2-carboxylate and 5-nitrofuran-2-carbaldehyde. A typical example is shown in Scheme 43. Ipso attack at the carbon bearing the nitro group leads to substitution via the [Pg.617]

This refers to attack by the radical R at a position in the ring already occupied by another substituent X. The consequence of such an ipso attack can be one of the following (Scheme 63) (i) ipso substitution by loss of the original substituent X from the ipso intermediate (229) to give (230) (ii) reversion to the starting material (228) or (iii) decay of the intermediate (229) by other processes (80ACR51). 

The second situation in which ipso attack is favoured is when the addition at the ipso position affords a cr-complex intermediate of considerably greater stability than those arising from the addition at the unsubstituted positions. This can happen with electroneutral radicals also. An example is the reaction of the dinitrothiophene (236) with methyl radicals. Here the intermediate (237) is the most stable radical which can be formed from (236) this therefore leads to the ipso substituted product (238) in 35% yield. 

In general, orientation effects in the substitution of alkylbenzenes are complicated by ipso attack. For example, in the nitration of 4-methylisopropylbenzene (para-cymene) about 10% of the nitration product is 4-nitromethylbenzene ... 

The 4-nitromethy (benzene arises from ipso attack of NOW at the isopropyl-substituted ring carbon. Unlike methyl, the isopropyl group is eliminated rapidly as propene. Can we say that the other products, 3 and 4, arise by direct substitution Evidently not, because nitration at 0° gives two other products, 5 and 6, which must be formed by ipso attack at the inelhyl-heanng carbon. At low temperatures, intermediate ion 7 is attacked by the weakly nucleophilic ethanoate ion to give 5 and 6. Both of these adducts solvolyze rapidly in 78% sulfuric acid to give 3 only ... 

Nitration is widely applicable, can be carried out under a variety of conditions, can usually be stopped cleanly after mononitration, is usually effected by the nitronium ion, can take place on a neutral molecule or a cation, and in many cases can be considered as the standard aromatic electrophilic substitution. However, this last point must be treated with caution. Depending on the reaction conditions and reagents, the mechanism of the reaction does vary, and accompanying reactions such as oxidation (due to the oxidative action of nitric acid), acetoxylation (by acetyl nitrate), and migration of nitro groups following ipso attack (80MI1) can occur. Ipso nitration processes have been extensively studied by Fischer and co-workers. 

In the case of 5-bromo-3,4-dimethyl-l-phenyl-l/f, 6//-pyrano[2,3-c]pyrazol-6-one (59), mixed acid nitration results not only in para substitution in the phenyl ring, but also in ipso attack at the 5-position to yield... 

Treatment of symmetrically substituted bis(arylmethylthio)alkynes with either IC1 or Br2 and in the strict absence of water promotes cyclization to 1//-2-benzothiopyrans (Scheme 129). Initial formation of a cyclic halonium species induces the electrophilic ring closure. In the presence of nucleophiles, ipso attack is favored and a spirocyclohexa-dienone results. The alkynes are accessible from arylmethyl thiocyanates and sodium acetylide <1998JOC4626, 2003EJ047>. 

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. 

---