Arynes monosubstitution

We have pictured in Scheme 25 the whole mechanism we could expect for monosubstitutions on 69 and 70. Fortunately all those reactions have not the same probability. Indeed, according to hydrogen acidities of halogenobenzenes7 48, and chlorine elimination probabilities49 arynes are formed following the decreasing order ... 

Here mechanisms are essentially arynic with a very strong amino meta-directing effect. Taking into account what we saw about monosubstitutions and the two reactions of Scheme 26, it is easy to understand the results described in Scheme 2747 ... 

Another family that can participate in metal-catalyzed cocyclotrimerization with arynes are the terminal allenes (e.g. 162). This cocycloaddition is best catalyzed by Ni(0), and leads in the case of 162 to 9-cyclohexyl-10-methylene-9,10-dihydrophenanthrene (163). For monosubstituted allenes the reaction appears to be highly selective, since only the internal C-C double bond of the allene participates in the cocyclotrimerization. With disubstituted allenes, mixtures of the two possible regioisomers are obtained as the result of the participation of both C-C double bonds in the cycloaddition reaction [79,80] (Scheme 46). 

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... 

Accordingly, the photolysis of the dihalomaleic anhydrides was examined. In the solid state the dichloro compound 211 gave the same polymer 212 as was obtained from thermolysis,a result which permits-but does not require-an aryne interpretation. In dilute cyclohexane solution, however, a monosubstitution product (220) (and no polymer) was isolated, indicating a stepwise decomposition. Irradiation of the dibromo compound 209... 

Sato and coworkers have reported the [2+2+2] cyclization of a diene and two arynes to generate 9,10-dihydrophenanthrenes (Figure 13.37) [84]. Interestingly, when the linker length between the alkenes was extended or a monosubstituted alkene was employed, at best a low yield of the products was observed, suggesting that the diene plays a key role in the reaction. 

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