Rearrangement reaction with aromatic compounds

The reaction of dichlorocarbene with aromatic compounds, with a few exceptions, lacks preparative value the adducts rearrange undergoing addition of another molecule of carbene and the final yield of complex product mixture rarely exceeds more than a few percent. Of synthetic importance are the reactions with indenes, phenanthrene (its 9-substituted derivatives 9-ethoxy-,9-trimethylsilyl-, " 9-arylthio-, 9-arylseleno- ), pyrene and various alkyl-naphthalenes. 120 Reactions with aromatic compounds are discussed in Houben-Weyl, Vol. E19b, pp 1551-1553. 

The photochemical cycloadditions of alkenes and alkynes with aromatic compounds have received by far the most attention. Yields of [2+2] cydoadducts can be good, but reaction times are often long and secondary rearrangement products are common [139, 140, 141,142, 143,144, 145,146] (equations 63-65). The pioneering mechanistic and synthetic work on aromatic photocycloadditions has been reviewed [147],... 

Arynes are intermediates in certain reactions of aromatic compounds, especially in some nucleophilic substitution reactions. They are generated by abstraction of atoms or atomic groups from adjacent positions in the nucleus and react as strong electrophiles and as dienophiles in fast addition reactions. An example of a reaction occurring via an aryne is the amination of o-chlorotoluene (1) with potassium amide in liquid ammonia. According to the mechanism given, the intermediate 3-methylbenzyne (2) is first formed and subsequent addition of ammonia to the triple bond yields o-amino-toluene (3) and m-aminotoluene (4). It was found that partial rearrangement of the ortho to the meta isomer actually occurs. 

The highly reactive species methylene inserts into C—H bonds,both aliphatic and aromatic,though with aromatic compounds ring expansion is also possible (see 15-62). This version of the reaction is useless for synthetic purposes because of its nonselectivity (see p. 248). This contrasts with the metal carbene insertion reaction, which can be highly selective, and is very useful in synthesis. Alkylcarbenes usually rearrange rather than give insertion (p. 249), but, when this is impossible. 

This type of duality of action is presumably present in other situations, such as the Fries rearrangement (78), the Friedel-Crafts reaction with acid chlorides (65) or acid anhydrides (21), and the catalytic chlorination of nitrobenzene (17). In these reactions it appears that the uncoordinated Lewis acid is the effective catalyst. The same situation is illustrated by recent work on aromatic amination (32, 33) and halogenation (57, 58, 71) and seems to be general feature of Lewis acid-catalyzed electrophilic reactions of aromatic compounds containing suitable donor groups. 

Side Reactions during Sulfonation. A major object in choosing a suitable sulfonation reagent and process is the avoidance of undesired side reactions the nature and extent of which will depend upon the structure of the compound being sulfonated, the sulfonating agent, and the physical conditions used. The principal side reactions encountered with aromatic compounds are sulfone formation, polysulfonation, formation of undesired isomers, and oxidation. Anhydride formation, dealkylation, and rearrangement have also been noted. 

In recent years, however, large evidence indicates that the orientation effects in the reactions of aromatic compounds with electrophiles are not only determined by the thermodynamic characteristics of c-complexes. This is evidenced, in particular, by kinetic control in the protonation of aromatic compounds, i.e. cases of the primary formation of less stable arenium ions which are then rearranged into thermodynamically more favourable ions (sec the preceding sec-... 

The amino-triazole (615) undergoes a reversible photo-Dimroth rearrangement to the anilino-derivative (616) the equilibrium mixture contains more of the former isomer. In the thermal reaction, on the other hand, the second isomer is favoured. 1-Acetoxybenzotriazole (617) exists in equilibrium with the iV-oxide (618). Benzotriazolyl oxide (619), generated by the action of lead dioxide on the corresponding hydroxy-compound, reacts with aromatic compounds ArR to yield carbazolyl oxides (621) via o-nitrosophenyl radicals (620). ... 

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