Ipso attack migration

A-Acyl-l,3,4,5-tetrahydro-2,1 -benzoxazepines (268) are synthesized from the A-chloro derivatives (267). The reaction proceeds via initial ipso attack followed by 1,2-carbon migration (84JCS(P1)2255, 87T2577, 90T7247). 

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. 

Recently, it was shown that the attack of CN on [FeCp(C6H5Cl)]+ PFortho-position. In the intermediate cyclohexadienyl complex, the CN group migrates to the ipso-carbon, whereas Cl is displaced. The monosubstituted benzonitrile complex is subjected to a second ortho-CN- attack but hydride is not removed spontaneously to give back an arene complex (Scheme XIX). Removal of the hydride is achieved by oxidation using DDQ (2,3-dichloro-... 

Path c. The electrophilic group (in this case NOj) can undergo a 1,2 migration, followed by loss of the proton. The product in this case is the same as that obtained by direct attack of NOj at the ortho position of PhZ. It is not always easy to tell how much of the ortho product in any individual case arises from this pathway, though there is evidence that it can be a considerable proportion. Because of this possibility, many of the reported conclusions about the relative reactivity of the ortho, meta, and para positions are cast into doubt, since some of the product may have arisen not from direct attack at the ortho position, but from attack at the ipso position followed by rearrange-... 

Another hydroxylation reaction is the Elbs reaction In this method, phenols can be oxidized to p-diphenols with K2S20g in alkaline solution. Primary, secondary, or tertiary aromatic amines give predominant or exclusive ortho substitution unless both ortho positions are blocked, in which case para substitution is found. The reaction with amines is called the Boyland-Sims oxidation. Yields are low with either phenols or amines, generally under 50%. The mechanisms are not clear, but for the Boyland-Sims oxidation there is evidence that the S20 ion attacks at the ipso position, and then a migration follows. ... 

The first step of the reaction is the formation of the nucleophile by deprotonation. The substrate then has to adopt the reactive conformation in which the plane of the migrating ring is perpendicular to the Z-Z bond. The nucleophile attacks the ring in an Ipso fashion to form a five-membered transition state that affords the product. 

Because of the enhanced reactivity of the 3-position, ipso substitution is sometimes observed with 3-substituted indoles. The mechanism is initial attack at C-3, followed by migration to C-2. If the original C-3 substituent is capable of migration, this opens the possibility for a rearrangement. 

---