Aryl derivatives substitution mechanics

The ring cleavage of 3-aryl-2-substituted-2//-azirines by molybdenum hexacarbonyl has been described earlier in regard to the synthesis of pyrroles, pyrazoles and isoxazoles. In contrast to this behavior, analogous reactions of 2-unsubstituted derivatives lead to the formation of mixtures of 2,5-diarylpyrazines (139) and isomeric 3,6- and 1,6-dihydropyrazine derivatives (140,141) (Scheme 163).47,53 It is possible that the pyrazine products are formed by an intermolecular nitrene mechanism akin to the intramolecular processes described earlier (see Scheme 22 in Section IV,A,1). 

Results on the intramolecular arylation on a 5H-indeno[l,2-b]pyridine derivative, which proceeded selectively at the pyridine ring, are also inconsistent with an electrophilic aromatic substitution mechanism for this reaction [37]. 

The first examples of cine-substitution in which the anionic a -adduct is quenched by electrophiles (other than a proton) before elimination takes place have been reported (Scheme 9). Insight into the reaction mechanism enabled the direct transformation of 2-benzylpyridazin-3(2i/)-one and 2-benzyl-6-chloropyridazin-3(2//)-one into the corresponding C(4) alkyl and aryl derivatives (when bromine was used as the electrophile). 

The effects of structural modifications on the mechanism of substitution at tetrahedral boron have been probed by a series of kinetic investigations in which the determination of rate laws and activation parameters, of deuterium isotope effects, and of Hammett correlations, have featured. The reactions were of 7V-alkyl and of B-alkyl and J -aryl derivatives of HsBjNHa with tri-n-butyl phosphine or with 2,2 -diphenylethylamine. The results can all be accommodated by the two-term rate law ... 

Coumarincarboxylate derivatives are versatile, efficient, low molecular weight, nonpeptidic protease inhibitors. Both esters and amides behave as time-dependent inhibitors of a-chymotrypsin but the esters are clearly more efficient than the corresponding amides. The criteria for a suicide mechanism are met. The presence of a latent alkylating function at the 6-position (chloromethyl group) is required to produce to inactivation by a suicide mechanism (Scheme 11.3, pathway a). Aryl esters, in particular the meta-substituted phenyl esters are the best inhibitors. Thus, m-chlorophenyl 6-(chloromethyl)-2-oxo-27/-l-benzopyran-3-carboxylate is one of the well-known inactivator of a-chymotrypsin (kJK, = 76(),000M s 1 at pH 7.5 and 25 °C, Table 11.1). 

The reactions represented by (191) are all nucleophilic substitutions occurring at a sulfonyl sulfur. Besides cpdisulfones substitutions of this kind are also of frequent occurrence in the chemistry of many other types of sulfonyl derivatives such as sulfonyl halides, aryl esters of sulfonic acids, etc., and many of the general aspects of their behaviour and mechanism have been examined in considerable detail. Most of the remainder of this section will be devoted to consideration of the results of such studies. 

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