Aryne intermediate

These reactions follow first-order kinetics and proceed with racemisalion if the reaction site is an optically active centre. For alkyl halides nucleophilic substitution proceeds easily primary halides favour Sn2 mechanisms and tertiary halides favour S 1 mechanisms. Aryl halides undergo nucleophilic substitution with difficulty and sometimes involve aryne intermediates. 

Once the intermediacy of an aryne intermediate was established the reason for the observed regioselectivity of substitution mo m and p chlorotoluene became evident Only a single aryne intermediate may be formed from o chlorotoluene but this aryne yields a mixture containing comparable amounts of o and m methylanilme... 

Elimination-addition mechanism (Section 23.8) Two-stage mechanism for nucleophilic aromatic substitution. In the first stage, an aryl halide undergoes elimination to form an aryne intermediate. In the second stage, nucleophilic addition to the aryne yields the product of the reaction. 

There is sound evidence for the existence of aryne intermediates as indicated by the following experiments, which also illustrate their reactivity. 

Most of the subsequent work on this reagent was concerned with the formation of aryl radicals (see review by Cadogan, 1971). However, 2-terf-butyl-A-nitrosoacet-anilide was found to decompose in benzene to give, instead of 2-tert-butylbiphenyl, as expected for a substitution of benzene by a 2-tert-butylphenyl radical, a mixture of isomeric tert-butylphenyl acetates. A careful reexamination (Cadogan and Hib-bert, 1964) suggested that the ratio of 2- and 3-tert-butylphenyl acetates was consistent with the involvement of 2-tert-butylbenzyne, i.e., the product of an ionic dediazoniation, as an intermediate. This was later confirmed by trapping experiments designed to detect aryne intermediates. 

By reaction with the appropropriate aryl halides can be prepared a variety of aryltin compounds that are not accessible from the reactions involving arylmagnesium halides and organotin halides (88,89) there is evidence that an aryne intermediate may be involved (90). However, for some purposes, such as the addition to carbonyl compounds, ox-iranes, and oxetanes, to give hydroxyalkyltin compounds, the Sn-Mg reagents may have advantages (see Section II,E) (91-93). 

The loss of proton from (93) could be concerted with, or followed by, loss of Cle to yield the aryne intermediate (97). The latter has two alternative positions (cf. 101 b/c) which eNH2 could attack, product formation then being completed by abstraction of a proton from the solvent NH3 the net effect is of the formal addition of NH3 in two alternative ways round. We should not expect the relative proportions of the two alternative products to be the same because (97) is... 

Furan was distilled over anhydrous potassium carbonate. Excess furan is necessary to trap the aryne intermediates. 

Via aryne intermediates The aryne is formed in two steps, the first being a het-erolytic dediazoniation, followed by a heterolytic dissociation of a substituent (H, COO etc.) in one of the o-positions relative to the diazonio group (I)n + DN/). The (metastable) aryne reacts rapidly by addition of a Brpnsted acid (H2O, HC1 etc.). 

The procedure described here is based on a method outlined by Cram, Rickborn, and Knox. The method is not a general one for the preparation of a substituted phenyl /-butyl ether because an aryne intermediate is involved. It appears that aryl fluorides undergo direct substitution to yield unrearranged aryl... 

This reaction sequence illustrates how the rates of many base-catalyzed reactions can be enhanced greatly by substitution of dimethylsulfoxide for the usual hydroxylic solvents. Other examples of the enhanced reactivity of anions in dimethylsulfoxide are found in Wolff-Kishner reductions and Cope elimination reactions. The present reaction illustrates the generation of an aryne intermediate from bromobenzene. ... 

The intermediacy of aryne intermediates generated by dehydrohalogenation of the corresponding aryl bromide is useful in preparing polycyclic isoquinolines with the phenanthridine skeleton <99T5195, 990L985>. 

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