Rearrangement reactions from direct

Rearrangement Reactions from Direct Irradiation of Diarylhalonium Salts... 

Wolff rearrangement is the main carbenc rearrangement reaction from the synthetic point of view in some cases, migration occurs through the formation of an oxjrene intermediate and sometime the ketcnc is directly formed f33]. 

The aromatization of triazolines can take place either by rearrangement reactions, by direct oxidation or by an elimination reaction. A recent review discusses this topic (78BSF(2)485>. From the numerous reactions some illustrative examples are described below. 

The elimination from sulfonates of secondary alcohols is frequently easier than more direct methods applied to the free alcohols. As with the latter, there are the possibilities of isomeric olefin formation and rearrangement reactions. In addition, displacement and hydrolysis may occur, but these side reactions can usually be suppressed. 

In the course of studying the bromination reactions of the bicyclic systems we noticed that the reaction temperature has a dramatic influence on the product distribution. Increasing of the temperature gives non-rearranged reaction products (refs. 1,2). For this reason, we submitted 1 to high temperature bromination. To a solution of 1 in decalin at 150 C was added a hot solution of bromine in decalin in one portion. The colour of bromine disappeared immediately. After silica gel chromatography followed by fractional crystallization we isolated four products 2-6 in yields 8, 35, 37, and 9 % respectively. The structure of these compounds has been elucidated on the basis of spectral data by iH NMR and NMR experiments and by comparison with those reported in the literature. Symmetrical endo-c/5-isomer 6 has been observed for the first time. Studies concerning the mechanism of syn-addition show that the syn-adduct can arise either from direct... 

From what has been said thus far, it is evident that the electrophile in Friedel-Crafts alkylation is a carbocation, at least in most cases. This is in accord with the knowledge that carbocations rearrange in the direction primary — secondary —> tertiary (see Chapter 18). In each case, the cation is formed from the attacking reagent and the catalyst. For the three most important types of reagent these reactions are... 

Some doubt is thrown on B strain as the sole explanation of the branching effect by the observation that the tri-te -butylboron-ammonia complex is actually less dissociated than the trimethylboron-ammonia complex.227 Since the products of the ionization of these highly branched compounds contain large amounts of rearranged material, another effect may be operating. As will be discussed in the next section, many ionization reactions produce directly an ion of structure different from that of the covalent parent compound. The transition state presumably resembles the new ion or a non-classical... 

The rearrangement reaction continues to be of synthetic utility, often involved in industrial processes. Patent references (e.g. Reference 48) refer to the formation of 4-amino phenols. Often the reactant nitro compound is reduced (to the hydroxylamine) in an acid environment so that the two-stage reaction can be accomplished as a one-pot synthesis. 4-Amino phenol itself 45 can be made in high yield directly from nitrobenzene49 and the 4-methoxy aniline derivative 46 similarly from 2-methylnitrobenzene by hydrogenation in MeOH/H2S0450. 

The present review is comprehensive Chemical Abstracts has been searched by indexes up to mid 1981 and by a computer on-line substructure search up to Issue 26 of Volume 96. A few more recent references are included directly from the commoner international journals. This review covers 3//-pyrazoles that have been isolated or characterized spectroscopically, although some examples that are only transient intermediates in rearrangement reactions are also mentioned. Compounds having exocyclic double bonds and the benz-fused derivatives, the 3//-indazoles, are considered to be outside its scope. 

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