Rearrangement intermolecular

R =various (see examples below), Fischer-Hepp R=various (see examples below), R =H, Hofmann-Martius OrtAo-substituted products are also formed 

As well as these traditional aqueous acid rearrangement conditions, Ratnam and coworkers have shown that the reaction can take place on solid acid catalysts, such as beta zeolite, KIO clay, sulfonated sUica, and sulfated zirconias [65]. In their work, the rearrangement of A-phenylhydrox-ylamine to para-aminophenol was investigated, in water at 80°C, using a series of these solid acid catalysts, and it was found that both the activity and selectivity are affected by the choice of the catalyst used, and the rate constant shows a linear dependence on the number of acid sites [65]. 

As well as these more traditional synthetic uses, the Bamberger rearrangement has also been observed in vitro. When studying the growth of a Pseudomonas putida strain in media enriched with 4-nitrotoluene, or its metabolite 4-nitrobenzoate, an enzymatic cascade process, which includes a Bamberger-like rearrangement, was demonstrated [68]. 

SCHEME 18.21 Mechanism of the Fischer-Hepp rearrangement (a) intermolecular mechanism and (b) intramolecular mechanism. 

FIGURE 18.9 The effect of methyl substituents on the Fischer—Hepp rearrangement. 

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A second process that occurs concurrently with the dissociation— redistribution process is an intermolecular rearrangement by which cyclohexadienone groups move along a polymer chain. The reaction maybe represented as two electrocycHc reactions analogous to a double Fries rearrangement. When the cyclohexadienone reaches a terminal position, the intermediate is the same as in equation 8, and enolization converts it to the phenol (eq. 9). 

The situation is not as clearly solved in a positive or negative sense for arenediazo phenyl ethers. Here three alternatives have to be considered, namely an intramolecular rearrangement of the arenediazo phenyl ether (Scheme 12-11, A), and two types of intermolecular rearrangement, either by heterolytic dissociation into a diazonium ion and a phenoxide ion (B) or by homolytic dissociation into a radical pair or two free radicals (C). 

However, the aminoazo product is formed via two pathways. The first is through the 1 1 addition complex (HAArNj )n as side-equilibrium and an intermolecular rearrangement involving redissociation of this complex into the reagents followed by formation of another 1 1 addition complex (HAArNJ )c and the classical C-o-complex (oc in Scheme 13-13). The second pathway starts from the first mentioned 1 1 complex (HAArNJ )N to which a second molecule of amine is added. This complex forms the aminoazo product by proton transfer to a base. The base may be the second amine molecule of the 1 2 complex. 

Each volume will be thematic, dealing with a specific and related subject that will cover fundamental, basic aspects including synthesis, isolation, purification, physical and chemical properties, stability and reactivity, reactions involving mechanisms, intra- and intermolecular transformations, intra- and intermolecular rearrangements, applications as medicinal agents, biological and biomedical studies, pharmacological aspects, applications in material science, and industrial and structural applications. 

The mechanism" of intermolecular rearrangement can involve free alkyl cations, but there is much evidence to show that this is not necessarily the case. For example, many of them occur without rearrangement within the alkyl group. The following mechanism has been proposed for intermolecular rearrangement without the involvement of carbocations that are separated from the ring." ... 

Electronic transitions in a solute take place very fast, i.e., almost immediately in comparison with the movement of the molecules as a whole and vibrations of atoms in organic molecules. Hence, absorption and fluorescence can be denoted in Fig. 5 by vertical arrows, in accordance with Franck-Condon principle. Both these processes are separated by relaxations, which are intermolecular rearrangements of the solute-solvent system after the excitation. 

In intermolecular rearrangement reactions (also referred to as apparent rearrangement reactions ), the migrating group has been completely separated from the parent species and is then attached at a new position in a subsequent step. In intramolecular rearrangements the migrating group is never separated from the parent compound. 

Reaction LI. (a) Action of Acids on the non-para substituted Hydrazo Compounds. (A., 270, 330 287, 97 B 26, 681, 688, 99.)—When hydrazobenzene is treated with mineral acids, an intermolecular rearrangement to benzidine (pj-pg-diaminodiphenyl) takes place. 

The rearrangements of several twistane derivatives to adamantyl cations under the same conditions, on the other hand, appear to involve reversible, random carbonium ion formation, at least to a limited extent. Rearrangement of 2-twistanol-2-d (38) occurs with considerable intermolecular hydrogen scrambling (Eq. (15)) 40T Similar intermolecular rearrangements are observed when a 50 50 mixture of 1-adamantanol and l-adamantanol-3,5,7-d3 in S02 is treated with SbFs 40). 

Mass spectroscopy has been often used to study the chemical behavior of silylenium ions. Intramolecular rearrangement with elimination of a neutral molecule has often been observed (e.g., Ref. 50 and references cited therein). For example, l-phenyl-2-trimethylsilylethane transforms into the more stable phenyldimethylsilylenium ion [Eq. (7)] (51). Intermolecular rearrangements may occur as well (52). 

Initially, ring expansion by tellurium leads to a four-membered heterocycle (n = 1) intermolecular rearrangements produce additional tellurium-phosphorus hctcrocycles, which are ultimately converted by extrusion of tellurium to tetra-/-butylcyclotetra-phosphane1. 

Azobenzofuroxanes undergo intermolecular rearrangement to form 2-aryl-7-nitrobenzotriazoles (Scheme 2.129) [522],... 

The synthesis and properties of 7r-allyl transition metal complexes have been discussed elsewhere (127-129). We shall draw attention to the most interesting properties of n-allyl complexes such as structure, coordinative unsaturation of the metal, and the ability of the 77-allyl ligand to enter into intermolecular rearrangements. 

Bamberger , were acid-catalysed, since many were carried out, at or near, room temperature or on a water-bath. For example, 2,4-xylyl azide was decomposed in sulphuric acid-ethanol medium at a temperature <35° , or in a 1 2 mixture of sulphuric acid and water at 65° , / -Tolyl azide was decomposed in concentrated sulphuric acid at —20° . On the other hand, it was also decomposed under conditions in which some thermolysis may well have been occurring, i.e. in a 1 3 (v/v) mixture of boiling sulphuric acid-water . More quantitative data are needed in this area, and kinetic data along with activation parameters would aid in clarifying the mode of decomposition. This type of data has been obtained for the acid-catalysed decomposition of a similar system, the arylhydroxylamines, and has helped in establishing that this is a nucleophilic intermolecular rearrangement . 

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