INTRODUCTION
Azines (R-C=N-N=C-R´) have gather increasing attention since they find application in several fields. They are usually synthetized by aggressive methods frequently metal-catalysed. Furthermore, a general process for the synthesis of symmetric (R=R´) and asymmetric (R≠R´) azines is still lacking till this work, which introduces mild-oxidized multiwalled carbon nanotubes (oxMWNT) as versatile azine’s metal-free catalyst.
RESULTS AND DISCUSSION
HNO3-medidated oxidation of MWNT yielded the sample oxMWNT whose oxygen surface chemistry, especially as epoxides and carboxylic acids (25% of the sample) was deployed. oxMWNT promoted the condensation of benzaldehyde and hydrazine to diphenyl azine 3a in 3h. We detected the transformation of the self-formed hydrazone into the azine monitoring the reaction by NMR. Control sample based on non-oxidized MWNT only reached 60% conversion of 3a in 3h, while overoxidized sample oxMWNT* finished the reaction in 10 min. Therefore, the surface oxygen chemistry behaved as solid Lewis acid and catalysed the azine formation. oxMWNT also showed high selectivity for aromatic aldehydes (higher than 98%) with a fully recyclable behaviour.
With this excellent catalyst in hand, 15 examples of symmetric azines with different functionalities, including heterocycles and gram-scale performance, were achieved in yields 66-94%. We also synthetized 6 asymmetric examples under similar reaction conditions with 40-92% yields. Two antifungal and antibacterial molecules were obtained with this procedure too, and thanks to the inherent oxMWNT oxidative power able to promote redox reactions, we also developed the one-pot synthesis of azines from benzyl alcohols (7 examples, total yield 47-79%).
