Rearrangements intermolecular aromatic

The retrosynthetic concept of the Nicolaou group is shown in Scheme 22. The target molecule 36 is disconnected via an IMDA cyclization of the diene quinone precursor 138, which would be generated from the tetraline derivative 139 using Wittig chemistry followed by aromatic oxidation. A Claisen-type rearrangement would provide access to 139 whereby the side chain required for the rearrangement of 140 would be introduced by 0-acylation. The core of 141 would be formed via an intermolecular Diels-Alder reaction between diene 142 andp-benzoquinone 130 [42]. 

The rearrangement has been shown under these conditions to be an intermolecular process, i.e. the diazonium cation becomes free, for the latter may be transferred to phenols, aromatic amines or other suitable species added to the solution. It is indeed found that the rearrangement proceeds most readily with an acid catalyst plus an excess of the amine that initially underwent coupling to yield the diazoamino compound (33). It may then be that this amine attacks the protonated diazoamino compound (39) directly with expulsion of PhNH2 and loss of a proton ... 

The aromatic aza-C.laisen rearrangements of fluorinated A -(l -methylbut-2-enyl(anilines 473 and b have been carried out in refluxing n-xylene in the presence of zinc(II) chloride 39 mixtures of ortho- and /wrn-Claisen products are obtained. In the case of 3,4-difluoro-/V-(1-methylbut-2-enyl)aniline, the product corresponding to 48 predominates, but 4,5-difluoro-yV,2-bis(l -methyl-but-2-enyl)aniline is also formed, which led to the proposal of an intermolecular mechanism. 

Exercise 23-35 Some of the rearrangements of arenamines, ArNHY, to Y—Ar—NH2 shown above proceed by an intermolecular mechanism involving acid-catalyzed cleavage of the N-Y bond followed by a normal electrophilic substitution of the aromatic ring. Show the steps in this mechanism for Y = NO and Cl. 

Halogen dance is more common in aromatic systems. Thus 1,2,4-tribromobenzene is rearranged in an intermolecular mechanism, by potassium anilide in liquid ammonia to give 40-60% of 1,3,5-tribromobenzene (equation 29)229. 

They also applied this method to the intermolecular ene reactions of aliphatic and aromatic aldehydes with alkenes containing a disubstituted vinylic carbon, a potentially valuable route to homoallylic alcohols [50]. Proton-initiated rearrangements do not take place, because the alcohol-Lewis acid complex formed in the ene reaction reacts readily to give methane and a non-acidic aluminum alkoxide. Formaldehyde and excess Me2AlCl gave good yield of ene adducts with all types of alkene, as exemplified in Sch. 26. 

In these reactions, a group is detached from a side chain and then attacks the ring, but in other aspects they resemble the reactions already treated in this chapter. Since a group moves from one position to another in a molecule, these are rearrangements. In all these reactions, the question arises as to whether the group that cleaves from a given molecule attacks the same molecule or another one, that is is the reaction intramolecular or intermolecular For intermolecular reactions the mechanism is the same as ordinary aromatic substitution, but for intramolecular... 

The exact mechanism has still not been completely worked out. " Opinions have been expressed that it is completely intermolecular, ° completely intramolecular, and partially inter- and intramolecular. One way to decide between inter- and intramolecular processes is to run the reaction of the phenolic ester in the presence of another aromatic compound, say, toluene. If some of the toluene is acylated, the reaction must be, at least in part, intermolecular. If the toluene is not acylated, the presumption is that the reaction is intramolecular, though this is not certain, for it may be that the toluene is not attacked because it is less active than the other. A number of such experiments (called crossover experiments) have been carried out sometimes crossover products have been found and sometimes not. As in 11-17, an initial complex (68) is formed between the substrate and the catalyst, so that a catalyst/substrate molar ratio of at least 1 1 is required. In the presence of aluminum chloride, the Fries rearrangement can be induced with micro-wave irradiationSimply heating phenyl acetate with microwave irradiation gives the Fries rearrangement. " The Fries rearrangement has been carried out in ionic melts. 

It has been found that benzo[c ]-l,3-azasilolines (141) can be prepared by the intermolecular reaction of A,A-dimethyl-2-triorganosilylethylamines (137) with benzyne in the presence of excess -butyllithium <76JOCi962>. It is postulated that deprotonation of the first betaine intermediate (138) with butyllithium affords the second intermediate (139) which undergoes silyl rearrangement with loss of ethylene. Cyclization of the resultant aromatic anion (140) onto silicon with concomitant loss of an R group then gives the heterocycle (141) (Scheme 15). 

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