Evidence of Reversibility in Azo-Coupling Reactions between 1,3,5-Tris(N,N-dialkylamino)benzenes and Arenediazonium Salts

Abstract

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.

Unusual Reactions Between Aromatic Carbon Supernucleophiles and 1,2‐Diazabuta‐1,3‐dienes: Useful Routes to New Pyrazolone and Cinnoline Derivatives

“…Indeed, in view of our previous work, [2,3] evidence of Wheland intermediates (W, Scheme 4) might be expected at low temperatures. Scheme 4. The reactions between compounds 1 or 2 and DBD 3 were carried out directly in NMR tubes in different solvents (CDCl 3 , CD 2 Cl 2 , CD 3 CN) and at different temperatures (from -70 to +25°C), but no evidence of the presence of possible W complex-like intermediates in detectable amounts was obtained.…”

“…Recently, we isolated σ-complexes (the so-called Wheland intermediates, W) from electrophilic aromatic reactions between these nucleophiles and diazonium salts. [2] Furthermore, reactions between 1,3,5-tris(dialkylamino)benzene derivatives and a more electrophilic reagent -namely 1,4-dinitrobenzofuroxan (DNBF) -permitted us to obtain the first evidence of a carbon-carbon Wheland-Meisenheimer complex (WM, Scheme 1), which was studied by NMR spectroscopy. [3a] This observation is consistent with the strong electrophilic power of DNBF, as quantitatively indicated by Terrier [4] on the Mayr [5] electrophilicity scale.…”

Ring Closure of Azo Compounds to 1,2-Annulated Benzimidazole Derivatives and Further Evidence of Reversibility of the Azo-Coupling Reaction

“…We had previously determined that the benzenediazonium salt bearing the electron-donating group is replaced by the more powerful electrophile. 3 When the two different groups bound to the benzenediazonium salt moiety of dicationic species are both electronwithdrawing groups, such as in the case of compound 7e, in the reaction mixture both benzimidazole derivatives 9a and 9b are present in 1:1 relative amount together with the respective released substituted anilines 11a and 11b, as reported in Scheme 7.…”

“…3,4 Contained in the older literature, benzenediazonium cations have generally not been considered strong electrophilic reagents. In the recent Mayr classification, they are considered very strong electrophilic reagents; for example, Mayr's electrophilicity parameter for the 4-nitrobenzenediazonium cation (E = −5.1) 9 is very close to that of 4,6-dinitrobenzofuroxan (E = −5.06), 10 a carbon neutral superelectrophile that, when coupled with triaminobenzenes 1−3, permitted us to characterize, for the first time, Wheland−Meisenheimer intermediates.…”

The first isolation of a Wheland complex in azo-coupling reaction, X-ray diffraction analysis and products from its evolution

“…Since in the azo-coupling reaction between compounds 1 and 2 and a series of p-substituted benzenediazonium salts the presence of an electron-donating substituent on the benzene ring of the latter causes the deceleration of all the processes 32 including the back reaction 34 and the proton abstraction, we selected 4-methoxybenzenediazonium tetrafluoroborate as a suitable candidate to obtain single crystals of W.…”