When does ipso attack occur in electrophilic aromatic substitution?
In ipso substitution first step is usually attack of the nucleophilic species at the ipso carbon of the aromatic
ring (the carbon bearing the leaving group in this case).

Scheme 1


The first step is followed by elimination of the leaving group and regeneration of the aromatic ring.

Scheme 2



This mechanism greatly resembles the tetrahedral mechanism . The attacking species forms a bond
with the substrate, giving an intermediate, such as 1, and then the leaving group
departs. We refer to this mechanism as the SNAr mechanism.The reaction between 2,4,6-trinitrophenetole and methoxide ion involves ipso substitution.Intermediates of this type are stable salts, called Meisenheimer or Meisenheimer–Jackson salts,7 and many more have been isolated .

Scheme 3



In the reaction of dinitro compound  with piperidine,same ipso substitution operates.

Scheme 4

In the reaction between picryl chloride and hydroxyl ions same ipso substitution operates.



Ref

1.MARCH’S ADVANCED ORGANIC CHEMISTRY REACTIONS, MECHANISMS,AND STRUCTURE
SIXTH EDITION  ,Michael B. Smith ,Jerry March

2.Organic Chemistry, Volume 1, 6/E By Finar