Mechanisms intermolecular

Bromination can be conveniently effected by transfer of bromine from one nucleus to another. As the Friedel-Crafts isomerization of bromoaromatic compounds generally takes place through an intermolecular mechanism, the migrating bromine atom serves as a source of positive bromine, thus effecting ring brominations (161,162). 2,4,6-Tribromophenol, for example, has been prepared by bromination of phenol with dibromobenzene. 

N Alkylation of 16 with benzyl bromide also occurs, and further heating of the reaction mixture leads to the C-alkylated product (25), probably by an intermolecular mechanism (33). 

An intramolecular 8 Ar2 mechanism (19) is possible in the replacement of 0X0 substituents with chloro or mercapto groups by means of phosphorus oxychloride or pentasulfide, respectively, via the intermediates shown. In the former, an intermolecular mechanism (20) is perhaps more likely. Frequently a base is not added in such reactions. 

As to the question of intra- or intermolecularity of the rearrangement there are three opinions, one which states that the reaction is completely intermolecular148, another supports a concurrent intra- and intermolecular mechanism149, whilst a third claims that the reaction is completely intramolecular150. Evidence for the intermolecular mechanism is based on trapping experiments such as the reaction of meta-tolyl acetate (CVIII) in the presence of we/a-chlorobenzoyl chloride (CIX) when the acylation product (CX) is formed rather than the products of... 

These results, although suggestive of an intermolecular mechanism do not necessarily demand it. 

These results may easily be rationalized by assuming that the formation of hydroxy sulphoxides 91, 92 and 93 from hydroperoxysulphides 89 and 90 is an intramolecular oxidation-reduction reaction proceeding through a five-membered transition state 94. However, an alternative intermolecular mechanism in which the approach of the oxidant is directed by the hydroperoxy or the hydroxy function in the reductant cannot be excluded. 

Thermally induced intra-intermolecular criss-cross cycloaddition of nonsymmetrical azines 363 in the presence of phenyl isocyanate provides the corresponding products of the mixed criss-cross cycloaddition 364 (Scheme 55) <2002TL6431>. Two different reaction mechanisms, intra-intermolecular and inter-intramolecular, of the mixed criss-cross cycloaddition with opposite sequence of reaction steps are possible. Quantum chemistry calculations suggest the intra-intermolecular mechanism as the most probable mechanism of this reaction <2004CCC231>. 

There are basically two types of mechanisms which can explain rearrangement of linkage isomers. In the intermolecular mechanism, the ligand giving rise to linkage isomerism (L-L, with two donor centers) dissociates from and recombines with the metal M ... 

This mechanism is supported by identical dissociation and racemization rate constants. This further implies either that the bis species M(AA)2 is racemic as formed, or that it may racemize (by a cis-trans change, or by a dissociative or intramolecular path) more rapidly than it re-forms iris in the dynamic equilibrium (7.23). Identical activation parameters for the dissociation (to the bis species) and racemization in aqueous acid (Table 7.5) and other solvents of Nifphen) " and Ni(bpy)3 indicate that these ions racemize by an intermolecular mechanism. This is the only such example for an M(phen)"+ or M(bpy) + species (see Table 7.5) although recently it has been observed that Fe(bps)3 (bps is the disulfonated phenanthroline ligand shown in 13, Chap. 1) but not Fe(phen)3+ also racemizes predominantly by a dissociative mechanism in water. For the other tr/s-phenanthroline complexes (and for Fe(bps)3 in MeOH rich, MeOH/HjO mixtures ) an intramolecular mechanism pertains since the racemization rate constant is larger than that for complete dissociation of one ligand, Table 7.5. 

A study of the relationship between the rates of catalyzed reactions and the concentrations of reactants may clear up the intermolecular mechanism and such studies may help in finding out which molecules are adsorbed, whether the adsorption is weak or strong, and which fraction of the surface is active. Every theory of the intermolecular mechanism can and must be checked by a calculation of the absolute velocity. 

Reactivity tests were carried out by directly feeding phenylbenzoate, in order to confirm the hypothesis formulated about the reaction network between phenol and benzoic acid. Surprisingly, phenylbenzoate yielded benzoylphenylbenzoate and phenol as the only primary products of reaction, indicating the exclusive presence of the intermolecular mechanism of acylation. Therefore, when a high concentration of phenylbenzoate is adsorbed on the catalyst, the bimolecular mechanism is quicker that the intramolecular one. 

Structure can be described as a four-membered ring (TS in Scheme 8). The inclusion of a second equivalent of CHsMgCl, corresponding to an intermolecular mechanism, decreases the barrier height, and the process can be considered as an assisted intermolecular mechanism the first equivalent forms the chelate structure and the second CHsMgCl carries out the nucleophilic addition to the carbonyl group. The most favorable pathway corresponds to an intermolecular mechanism via an anti attack. Analysis of the results reveals that... 

Schotten-Baumann benzoylation or use of an acid chloride in pyridine rapidly causes O-acylalion giving 4-acyloxythiocoumarins (37) in high yield. These derivatives undergo the Fries rearrangement to give the 3-acyl derivatives (38), either by the action of hot pyridine (quantitatively), aluminum chloride, polyphosphoric acid, or even partially by sublimation. Cross-over experiments indicate that a mixture of intra- and intermolecular mechanisms is involved, the products 38 also being available directly from 4-hydroxythiocoumarin. 

M-butane proceeds via an intermolecular mechanism with 2-butene involved intermediately.300-303 The role of the transition metal promoters such as Fe and Mn was shown to increase the surface concentration of the intermediate butene 304 The formation of butene is speculated to occur through an oxidative dehydrogenation on the metal site305 or by one-electron oxidation.306... 

Studies with sulfated zirconia promoted with Pt309 and industrial chlorinated Pt on AI2O3 isomerization catalysts310 led to the same conclusion, namely, the intermolecular mechanism operative for M-butane isomerization. A significant difference, however, is that on the industrial catalysts extensive hydride and methyl shifts taking place in the intermediates prior to P scission do not lead to a random distribution of the labels. Instead, a binomial distribution with one and three 13C atoms is observed.310 This is indicative of the involvement of the 31 carbocationic intermediate. 

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. 

Indeed, isomerization of 1-AMN into 2-AMN was found to be much faster in the presence of 2-MN than in its absence. Furthermore, this intermolecular mechanism was confirmed by investigating the transformation of a mixture of 1-AMN with a deuterated methoxy group (OCD3) and of nondeuterated 2-MN.26... 

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