The tautomerism of some substituted 2-hydroxypyridines is investigated by UV/Vis-and 1 H-, and 13 C-NMR spectroscopic methods, with the aid of the N-Me and O-Me fixed parents. NMR spectroscopic data do not allow discrimination between the two tautomeric forms (with the exception of the unsubstituted 2-hydroxypyridine), while UV/Vis-data permit the quantitative determination, in different solvents, of the amounts of the two forms. The electronic substituent effect and the change of solvent are discussed. An X-Ray diffraction study carried out on a crystal of 2-hydroxy-5-nitropyridine (7) reveals that this compound, in the solid state, is in the oxo-form.

Proposal accepted: NMR experiments have shown unequivocally that the previously only proposed zwitterionic carbon–carbon Meisenheimer–Wheland complexes are formed in the reaction of 4,6‐dinitrobenzofuroxan (DNBF) with 1,3,5‐tris(N,N‐dialkylamino)benzenes. Increasing the temperature results in a rapid exchange between three homomeric forms of the complex (see scheme; NR2=piperidyl, morpholinyl, pyrrolidinyl).

We have investigated reactions between 1,3,5‐tris(1‐piperidyl)benzene and 1,3,5‐tris(4‐morpholinyl)benzene and 4‐substituted arenediazonium tetrafluoroborate salts, mainly by NMR spectroscopic measurements. The main products are σ complexes (Wheland‐like intermediates) which are formed in almost quantitative yields and are moderately stable compounds. From these covalent complexes, products of the azo‐coupling reaction may be obtained as salts or as free bases. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

The reactions between strongly electron-rich aromatic substrates (1,3,5-tris(N,N-dialkylamino)benzenes, neutral carbon super nucleophiles) and diazonium salts produce moderately stable sigma complexes (Wheland complexes). The reactivity of Wheland complexes with electrophiles (other diazonium salts, or 4,7-dinitrobenzofuroxan) produces exchange reactions in the electrophilic part: the better electrophile replaces the less powerful electrophile. In the same way, in Wheland complexes with the 1,3,5-tris(morpholinyl)benzene, the 1,3,5-tris(piperidinyl)benzene replaces the less powerful nucleophile 1,3,5-tris(morpholinyl)benzene. Evidence is reported here indicating that for the title system the reaction of the attack of the electrophilic reagent producing Wheland complexes is a reversible process. The final products of the diazo-coupling reactions undergo a further attack of some diazonium salts. From the final products of the double diazo-coupling reactions (diazo compounds), we collected evidence that is a clear instance of complete reversibility of the diazo-coupling reaction.

No abstract

Reactions of DNBF with a series of 2-aminothiazoles (1 a-f) to afford thermodynamically stable C-bonded sigma-adducts have been investigated in acetonitrile. A most significant finding emerged on recording NMR spectra immediately after mixing of equimolar amounts of DNBF and the unsubstituted 2-aminothiazole (1 a) in Me2SO: namely, that the formation of 9 a is preceded by that of a short-lived intermediate species X. From the 1H NMR parameters characterizing this intermediate, as well as the dependence of its lifetime on the experimental conditions-the presence of excess DNBF over 1 a increases the lifetime of X while an excess of base (1 a) accelerates its conversion into 9 a--it is convincingly demonstrated that the structure of X combines the presence of a positively charged Wheland complex moiety (with regard to the thiazole ring) with that of a negatively charged Meisenheimer complex moiety (with regard to the benzofuroxan system). So far, only one intermediate of this type (noted WM) has been successfully characterized, in the reactions of DNBF with 1,3,5-tris(N,N-dialkylamino)benzenes. Among the key features supporting the intermediacy of X along the reaction coordinate leading to 9 a is the fact that the reactions of DNBF with 1 a in the presence of an alcohol (MeOH, EtOH, nPrOH) produce new adducts arising from the addition of an alcohol molecule to the thiazole moiety of WM-1 a. Reflecting the presence of three chiral centres, these species are formed as mixtures of several diastereomers that could all be characterized in their racemic forms in ethanol. These findings generalize the previous report on the formation of Wheland-Meisenheimer carbon-carbon complexes in homocyclic series.

Vorschlag akzeptiert: NMR‐Experimente belegen eindeutig, dass die bisher nur vorgeschlagenen zwitterionischen Kohlenstoff‐Kohlenstoff‐Meisenheimer‐Wheland‐Komplexe bei der Reaktion von 4,6‐Dinitrobenzofuroxan (DNBF) mit 1,3,5‐Tris(N,N‐dialkylamino)benzolderivaten entstehen. Bei Temperaturerhöhung tritt ein schneller Austausch zwischen drei homomeren Formen des Komplexes auf (siehe Schema; NR2=Piperidyl, Morpholinyl, Pyrrolidinyl).

The reactions of 2-aminothiazole (1a), 4-methyl-2-aminothiazole (1b), and 4,5-dimethyl-2-aminothiazole (1c) with superelectrophilic 4,6-dinitrobenzofuroxan (DNBF) have been studied in acetonitrile and a 70/30 (v/v) H2O/Me2SO mixture. While exhibiting a somewhat higher nitrogen basicity than that of anilines, 1a and 1b do not react as nitrogen nucleophiles, affording exclusively anionic C-bonded sigma-adducts (C-1a and C-1b) through electrophilic S(E)Ar substitution of the thiazole ring by DNBF. Only in the case of the 4,5-dimethyl derivative 1c a N-adduct, N-1c, was obtained. On the basis of 1H-15N correlations, it is demonstrated that this adduct, N-1c;1c,H+, is derived from DNBF addition at the exocyclic amino group and not at the endocyclic nitrogen center of 1c. Rate constants have been determined in the two solvents for the formation of the adducts, revealing a reactivity sequence which accounts well for the finding that 1a and 1b behave preferentially as carbon rather than nitrogen nucleophiles. The enaminic character of these thiazoles is assessed through an estimation of the pKa values for their C-protonation in aqueous solution as well as through a positioning of their reactivity on the nucleophilicity scale recently developed by Mayr et al. (Acc. Chem. Res. 2003, 36, 66). With N values of the order of 6.80 and 5.56, 1b and 1a have a carbon nucleophilicity comparable to that of N-methylindole and indole, respectively.