Hydroxy‐ and Methoxybenzene Derivatives with Benzenediazonium Salts ― Chemical Behavior and Tautomeric Problems

Abstract

The azo‐coupling reactions between 4‐nitrobenzenediazonium tetrafluoroborate and a series of benzene derivatives bearing at least one electron‐donating hydroxy or methoxy substituent have been studied. Depending on the nucleophile and its relative ratio with the diazonium salt, it was possible to obtain mono‐ and disubstituted products as well as a trisubstituted product for phloroglucinol. The reactions between 3,5‐diaminoanisole or 3,5‐diaminophenol derivatives and 2 equiv. of diazonium salt gave the monosubstituted product in the first case and a diadduct in the second case; X‐ray diffraction analysis revealed a highly symmetric structure for the latter. The different behaviors of hydroxy‐ and methoxy‐substituted compounds was particularly evident for phloroglucinol and 1,3,5‐trimethoxybenzene and may be rationalized on the basis of the different electronic effects of the substituents. The propensities of the aromatic rings to undergo H/D exchange reactions were investigated for a series of substrates, and a comparison of the results obtained provided new insights into this phenomenon and permitted a better explanation of the findings reported here; a new particular interaction competes with the formation of a proton–phloroglucinol σ complex.

Highly conjugated architectures and labile reaction intermediates from coupling between 10π electron-deficient heteroaromatics andsym-trihydroxy- or triamino-benzene derivatives

“…This observation can be due to higher electron donor effect of hydroxyl, with respect the methoxy group (s OH + ¼ À0.92, s OMe + ¼ À0.78) 50 and to the occurrence of tautomeric equilibria involving phloroglucinol, as recently investigated. 51 Further, the reaction carried out between 12 and 4-chloro-7-nitrobenzofurazan (ClNBZ) did not occur, even if in reuxing acetonitrile, in contrast with the already reported results between ClNBZ and 1-3; these ndings indicate that the lower nucleophilicity of trihydroxybenzene derivatives with respect to triaminobenzenes requires the presence of a further nitro group to permit the aromatic substitution reactions.…”

Synthesis of thia-Michael-Type Adducts between Naphthoquinones and N-Acetyl-L-Cysteine and Their Biological Activity

“…The reaction with lawsone ( 6 ) did not occur, possibly because of the presence, not in a negligible amount, of the tautomeric keto form [ 33 ] ( Figure 3 ), which deactivates the nucleophilic attack on 6 .…”