Alguns aspectos de reatividade nucleofilica nas reações de substituição nucleofilica aromatica

Abstract

In extension of earlier research on the strength of nucleophiles in SNAr reactions, studies have been made of: (i) the reactivity in MeOH of the more important pseudohalide ions, using methoxide ion as reference nucleophile; (ii) the scope of the occurrence of the a-effect, and its magnitude; (iii) the quantitative effects of solvent changes. Thermochemical calculations based on fundamental data have been used to clarify and explain many of the results obtained. The reactivity order: MeO > y -Hal > Hal, predictable by the thermochemical calculations, has been amply verified. Within the y -Hal series, the order SeCN > SCN was demonstrated. It was shown that OCN and SCN have approximately equal nucleophilicity in their reactions with a cationic substrate; whereas with a less reactive neutra substrate the OCN was mildly more reactive. AlI these relationships were discussed in relation to fundamental thermochemical properties. On the basis of the exploratory studies of the a-effect in SNAr reactions, the following conclusions were drawn: (a) SNAr reactions are not well suited to demonstrate the a-effec: which is thus evident only in favourable circumstances; (b) the a-effect is favoured by a negative charge on the nucleophilic reagente [Nu-X]; (e) the a-effect is inhibited when electronegativity factors result in an effective increase of attraction of the nucleus for the unshared pairs of electrons involved in a potential a -effect. Thus, for example, there is no a -effect in the reactions of either hydroxylamine ar acetoximate; (d) specific differences in the a -effect of perbenzoate and benzo hydroxamate ions can be related to greater conjugation of the a atom with the carbonyl group in the latter ion. The explanations offerred for these conclusions relate to one of the theories of the origin of the a -effect, viz. that it is due to unfavourable interactions in the initial state, which are relieved in forming the rate limiting transition state. It is not implied, however, that the a -effect is due only to this factor. The inferior ability of aromatic substrates to demonstrate the a -effect probably links with the basically homopolar character of the aromatic ring. The a -effect appears to be more important in reactions of nucleophiles at heteropolar unsaturated centres. In earlier research, studies were made of differences in solvent effects of protonic and aprotonic dipolar media on reactions of anionic nucleophiles with neutral substrates. These studies have been extended, on an exploratory basis, to some reactions of anionic and neutral nucleophiles with a cationic substrate. The inversion of the solvent effect when a heavy anionic nucleophile reacts with a cationic instead of a neutral substrate was demonstrated and discussed. It was predicted that with light anionic nucleophiles a reduction instead of inversion of the solvent effect would occur. The small solvent effect observed in the reactions of a neutral nucleophile with a cationic substrate was also discussed.