Mechanism acid catalyzed rearrangement

Mechanism of the formation of phenol by acid-catalyzed rearrangement of cumene hydroperoxide. 

Refer to Figure 26.2 and propose a mechanism for the final step in the Edrnan degradation—the acid-catalyzed rearrangement of the ATZ derivative to the PTH derivative. 

Probtem 19.7 Give a mechanism for the acid-catalyzed rearrangement of cumene hydroperoxide involving an intermediate with an electron-deficient O (like R ). ... 

In the case of tricyclo[4.4.2.01-6]dodecan-2-one, acid-catalyzed rearrangement gave tricy-clo[4.3.3.01,6]dodecan-7-one (8), the propellane-like skeleton of which is otherwise difficult to construct synthetically. The reaction mechanism requires two consecutive Wagner-Meerwein... 

In one case, however, the carbonyl group did migrate in the acetic acid catalyzed rearrangement of cyclobutanone 5 in refluxing toluene.141 This is probably due to a different mechanism involving the second carbonyl and the methoxy group present in 5. 

The principle of the Lewis acid catalyzed rearrangements of hydrocarbons is well documented 4,81. Lewis acids react with a promotor deliberately added or present as an impurity in the reaction mixture to form carbonium ions which initiate intermolecular hydride transfers involving the hydrocarbon. These hydride transfers appear to be fairly unselective processes. While the expected tertiary > secondary > primary selectivity order is observed, the differences are significantly reduced relative to typical carbonium ion reactions. Possibly this is due to a hydride transfer mechanism which involves a pentaco-ordinate carbon transition state in which charge development on carbon would be minimized 38dh... 

N-monosubstituted 1,2-diaminobenzenes react with Appel salt 20 in dichloromethane to give the corresponding 2-cyanobenzimidazoles 87 either directly or through thermal or acid-catalyzed rearrangement of the intermediate imino-l,2,3-dithiazoles 88, which can be isolated (Scheme 9). The reaction mechanism includes reversible attack of the o-amino group on the dithiazole ring <1998T9639>. 

Scheme 1. Top formation of the spiro-bisperoxide 2 from cyclohexanone (1) and hydrogen peroxide. Bottom suggested mechanism of the acid-catalyzed rearrangement of the spiro-bisperoxide 2 to two molecules of the product lactone 3...

Unlike Lewis-acid-catalyzed rearrangements of organic compounds, ionic intermediates are not important in the mechanism. No evidence has been found to support the existence of silylenium ions, RgSi, and careful studies have ruled out carbocations as well. 

Dihydrodiols have not been obtained from hydrolysis reactions of benzene oxides however, evidence for the formation of 1,4-dihydrodiols has been obtained from acid-catalysis studies on 1,4-dimethylbenzene oxide 63 ° and 8,9-indan oxide 7. The mechanism of acid-catalyzed rearrangement and hydrolysis of benzene oxide has been investigated theoretically using the semiempirical allvalence electron MINDO/3 method and the perturbational MO method, both from the viewpoint of product stabilities and reaction pathways. The aromatization reaction was found to be much more exothermic than the hydrolysis and thus would be the preferred reaction, as was found experimentally. 

Problem 28.4 When a-phcnylcthyl hydroperoxide, CcHsCHCCH.OO-OH, undergoes acid-catalyzed rearrangement in H2 0, recovered unrearranged hydroperoxide is found to contain no oxygen-18. Taken with the other evidence, what does this finding tell us about the mechanism of reaction ... 

Bhatia, K., Fry, A. Mechanism of the sulfuric acid-catalyzed rearrangement of methyl and carbonyl carbon-14-labeled 3,3-dimethyl-2-butanone. J. Org. Chem. 1969, 34, 806-811. 

Different mechanisms for thermal and acid-catalyzed rearrangements of steroidal a-acetoxy epoxides have also been proposed." On heating (55 equation 24), intramolecular migration of the acetate group occurs, with stereospecific inversion of the cleaved epoxide center leading to (56). Treatment of (55)... 

The mechanism of the thermal and acid-catalyzed rearrangement of A-alkyl-iV-chloro-anilines 223 to ortho- and para-chloroanilines was studied intensively for many years, because this reaction is a part of a large series of related isomerizations that can be characterized by moving a migrant group from the nitrogen atom to the aromatic nucleus310,311 (equation 86). 

SECS proved to be useful in mechanistic as well as synthetic research. The program was used to systematically generate all Wagner-Meerwein products, with elimination of duplicate structures and calculation of strain energies, in order to find the most likely mechanism for acid-catalyzed rearrangement of tetrahydro-Binor-S to diamantane 24... 

The kinetic characteristics were measured for the rearrangements of arene oxides of benzo[a]anthracene, its methyl-substituted derivatives as well as for other polycyclic arene oxides (for transformations of arene oxides into phenols, see Section VII.C). The mechanism of these acid-catalyzed rearrangements and the isotope effects in these reactions were discussed. ... 

The origin of oxygen in cleavage products-Cl and XIII) of the hydroperoxide lyase reaction presumably occurs in a manner (Fig. 4) similar to the mechanism for acid-catalyzed rearrangement of the 13-... 

Cycloalkanes also undergo related acid-catalyzed rearrangements. Higher coordinate carbocations again play a key role in the mechanism as shown in the cyclohexane-methylcyclopentane rearrangement. ... 

---