Aromatic compounds, addition ring isomerization

Addition of carbenes to Jt-electron excessive aromatic compounds, or those which possess a high degree of bond fixation, is well established. Dihalocarbenes react with naphthalenes with ring expansion to produce benztropylium systems (Scheme 7.8). Loss of hydrogen halide from the initially formed product leads to an alkene which reacts with a second equivalent of the carbene to yield the spirocyclopropyl derivatives in high yield (>95%) [14, 50]. Insertion into the alkyl side chain (see Section 7.2) also occurs, but to a lesser extent [14]. Not unexpectedly, dichlorocarbene adds to phenanthrenes across the 9,10-bond [9, 10, 14], but it is remarkable that the three possible isomeric spiro compounds could be isolated (in an overall yield of 0.05% ) from the corresponding reaction with toluene [14]. 

The first characteristic reaction of arynes to be discovered was the addition of polar species, especially nucleophiles, to the triple bond. Since arynes are bidentate intermediates, such additions could lead to two different products in the event of unsymmetrical substitution in the aryne ring. If, as is often the case for such polar additions, the aryne was generated by elimination of HX from a monosubstituted aromatic compound 126, then the product 127 with the entering group in the position previously occupied by the substituent is called the product of normal or ipso-substitution while the rearranged product 128 is referred to as that of cine-substitution. Should the aryne be generated from the isomeric precursor 129 then the designation of which is the normal... 

Polycyclic aromatic compounds, such as naphthalene and anthracene, are weU known to participate in Diels-Alder and related transformations as these reactions typically result in loss of aromaticity in only one of several arene rings [38]. Such cycloadditions are rarely observed in simpler monocyclic arenes. Several examples of intramolecular [3+2] cycloaddition between an arene and an attached nitrile oxide moiety have been reported [39]. Additionally, 4+2 cycloadditions (also intramolecular) between phenyl groups and attached aUenes have been examined [40]. Allene substrates can be conveniently genwated by base-induced isomerization of alkynyl amides under conditions also suitable for cycloaddition, thweby affording a one-pot method to convert substrates such as 42 to tricyclic products 43 (Schane 15.16) [41]. Elaboration of 43 and structurally related... 

Position isomerism in aromatic compounds occurs when the substituents are positioned on different parts of the benzene (or other aromatic) ring. Only one isomer of phenol (or hydroxybenzene) exists, but cresol (or methylphenol) has three isomers where the additional methyl group can be placed on three different positions on the ring. 

This type of isomerism is possible not only in biphenyls, but also in compounds in which rotation about an sp2-sp2 bond is restricted and the two planes involving the sp2 center are noncoincident and substituted unsymmetrically. In addition to enantiomers, diastereomers are possible. There are some examples reported of restricted rotation about an aromatic ring-to-carbonyl bond or aromatic ring-to-nitrogen bond (28). Since these reports make no mention of diastereomers but only of enantiomers, they will receive no further mention here. 

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