STUDIO DELLA REAZIONE DI CICLOADDIZIONE AZIDE-ALCHINO CATALIZZATA DA RAME(I) IN LIQUIDI IONICI

The copper(I)-catalyzed azide-alkyne cycloaddition (CuAAC) reaction constitutes a substantial improvement of the classical Huisgen-type thermal 1,3-dipolar cycloaddition. This reaction affords only 1,4-disubstituted triazole and it’s a powerful ligation tool of the most disparate molecular fragments. While it is well known that the CuAAC can be carried out in water, the use of ionic liquids (ILs) is another way to meet the principles of green chemistry. ILs are special molten salts with low melting points (<100 °C) and are typically constituted of organic cations (pyrrolidinium, pyridinium, tetralkylammonium and tetralkylphosphonium ions) and inorganic anions. We selected the unreported coupling of ethynyl C-galactoside with methyl 6-azido-glucopyranoside to give the triazole-linked disaccharide: three types of ILs as solvents were considered by cation variation, structural diversity was also introduced by the anion change. The cycloaddition was performed overnight in the presence of CuI and i-Pr2EtN using the ionic liquids as the solvents. A yield of 91% comparable to that in toluene (96%) was obtained in the N-octyl dabco-cation based dicyanamide ([C8dabco][N(CN)2]. The recycling of this IL was considered: the yields in four subsequent runs were almost identical. Moreover, ILs have been proved to be good microwave absorbers and are well suited in reactions using microwave (MW) dielectric heating. So we focused on the multiple glycosylation of a calixarene to give structurally defined glycoclusters. We studied in some details the model click reaction of ethynyl C-galactoside with a tetraazido- calixarene. The reaction was performed in three different ILs and carried out by thermal heating (oil bath for 16 h) and MW dipolar heating (for 2 h). The most performing IL was N-octyl dabco-cation based dicyanamide ([C8dabco][N(CN)2]. The optimized conditions in [C8dabco][N(CN)2] were employed to perform the reaction of monosaccharidic and disaccharidic alkynes. In addition we focused on the copper(I)-catalyzed azide-alkyne cycloaddition under base-free conditions. The model reaction carried out in a range of basic ILs turned out to afford mixtures of 1,4- and 1,5-disubstituted triazoles with the exception of the non-basic tetraalkyl ammonium salt Ammoeng 110. In this solvent, the CuI-catalyzed cycloaddition in the absence of i-Pr2EtN afforded the 1,4-disubstituted sugar triazole as main product and in fair yield. Then we demonstrated that CuI-catalyzed reactions of structurally different azides and alkynes occur readily in the densely oxygenated nonbasic ionic liquid Ammoeng 100 by MW dielectric heating and in the absence of any added amine, to give exclusively 1,4-disubstituted triazole derivatives in high yields.