Aryl transpositions

Moreover, a comparison of the results achieved using the catalyst containing La " with the other metal Ion exchanged zeolites indicates that hard Lewis acids also do not catalyze the 1,2-aryl transposition. This finding fits well with the previous reported data for scilts of these metal ions which shows that only soft and borderline Lewis acids can catalyze this rearrangement. 

The consecutive formation of o-hydroxybenzophenone (Figure 3) occurred by Fries transposition over phenylbenzoate. In the Fries reaction catalyzed by Lewis-type systems, aimed at the synthesis of hydroxyarylketones starting from aryl esters, the mechanism can be either (i) intermolecular, in which the benzoyl cation acylates phenylbenzoate with formation of benzoylphenylbenzoate, while the Ph-O-AfCL complex generates phenol (in this case, hydroxybenzophenone is a consecutive product of phenylbenzoate transformation), or (ii) intramolecular, in which phenylbenzoate directly transforms into hydroxybenzophenone, or (iii) again intermolecular, in which however the benzoyl cation acylates the Ph-O-AfCL complex, with formation of another complex which then decomposes to yield hydroxybenzophenone (mechanism of monomolecular deacylation-acylation). Mechanisms (i) and (iii) lead preferentially to the formation of p-hydroxybenzophenone (especially at low temperature), while mechanism (ii) to the ortho isomer. In the case of the Bronsted-type catalysis with zeolites, shape-selectivity effects may favor the formation of the para isomer with respect to the ortho one (11,12). 

The arylation reaction can be extended to bis- and tris-(dialkylamino)phosphines73, but in the case of iV-ethylanilinophosphine the reaction takes place with a rearrangement of the phosphasemidine type483. A structural transposition also occurs in the alkylation of some N-silylaminophosphines484-487 (reaction 141). Finally, it is noteworthy that, as far as the tris(dimethylamino)phosphine is concerned, many polyhalomethyltris-(dimethylamino)phosphonium salts have been prepared by the action of the appropriate polyhalomethanes (reaction 142). 

The only comparable sulfur-containing system investigated is that found in thiabenzene187 and thianaphthalene.188 In both cases, 1,2-aryl migrations have been observed on irradiation, as shown, for example, in the triphenyl-thianaphthalene (237). No evidence for any ring transposition reaction in these compounds has been reported. 

Umezawa and co-workers9 have reported a new synthesis of the tetracyclic lactam (19), which is a key intermediate in Torssell s synthesis of lycorine (cf. Vol. 9, p. 139) the Japanese work (Scheme 2), therefore, represents a formal synthesis of the alkaloid. The cyclohexyl isocyanate (15) (trans-diequatorial aryl and isocyanate groups) cyclized to a tricyclic lactam, which by reduction with a hydride and hydrolysis gave the ketone (18). The tetracyclic ketone (16) was converted into the 2,3-ene (17) by a Cope elimination reaction, and the synthesis of compound (19) was completed by transposition of the lactam carbonyl group from C-5 to C-7. 

This transposition explains why the condensation in alkaline media of 4-aryl-l,2-dithiole-3-thiones with acenaphthenone gives a y-dithiopyrone instead of a dithiolylidene ketone.24 A possible mechanism is as in Scheme 19. 

Examples of alkylation, dealkylation, homologation, isomerization, and transposition are found in Sections 1, 17, 33, and so on, lying close to a diagonal of the index. These sections correspond to such topics as the preparation of acetylenes from acetylenes carboxylic acids from carboxylic acids and alcohols, thiols, and phenols from alcohols, thiols, and phenols. Alkylations that involve conjugate additions across a double bond are found in Section 74 (alkyls, methylenes, and aryls from olefins). 

Allylic substitution with borates. Arylation of allylic acetates without transposition using B-alkyl-B-aryl-l,3,2-dioxoborolidines occurs in the presence of (Ph3P)2NiCl2. Note the exclusive transfer of the aryl group. 

The rearrangements of phenols which are accompanied by hydroxy group transpositions are called the Wessely-Moser reaction (equations 50 and 51). In essence, these rearrangements are recyclizations of flavonoides 114 via the ring-opened form 115 to give the novel structures 116. Compounds that can participate in these rearrangements are flavones (114, R = H, R = Aryl), flavonoles (114, R = OH, R = Aryl), isoflavones (114, R = Aryl, R = H), chromones (114, R = H, R = Alkyl), chromonoles (114, R = OH, R = Alkyl), xanthones (114, R R = benzo) as well as benzopyrylium salts (e.g. see Reference 95). 

Initially, one of the geometrical isomers, presumably Z, forms faster. The competing thermal E-Z interconversion complicates the situation sufficiently that it is not yet clear whether the transposition reaction forms this geometrical isomer exclusively. If so, the mechanism is likely to be of the dyoiropic kind, involving a shift of both migrating aryl groups and a biradicaloid transition state with a bent or planar bicyclobutane-like structure 24. An alternative that cannot yet be ruled out with confidence is a disilene-to-silylsilylene shift followed by a silylsilylene-to-disilene shift (cf. Sections II.A.3 and II.B.l.b). Both possibilities are displayed in equation 19. 

This section contains alkylations of ketones and protected ketones, ketone transpositions and annulations, ring expansions and ring openings, and dimerizations. Conjugate reductions and Michael alkylations of enone are listed in Section 74 (Alkyls, Methylenes, and Aryls from Alkenes). For the preparation of enamines or imines from ketones, see Section 356 (Amine-Alkene). 

Although it is known that the presence of electron-donor substituents on the aromatic ring greatly enhances the efficiency of the transposition leading to 2-arylpropanoates, we have chosen for our study the unsubstituted phenyl group, which shows a low migratory aptitude. This will provide us a deeper understanding of the influence of the physicochemical parameters of the zeolite on the side reactions competitive to the 1,2-aryl shift. 

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