## 3 Carbon-Nitrogen Forming Reactions  
### Copper(I)-Catalysed Azide Alkyne Cycloaddition  
Beside the reactions leading to carbon-carbon bond formation, the reactions providing new carbon-nitrogen bond also play a relevant role in organic synthesis. Particularly, this includes the Huisgen 1,3-dipolar cycloaddition of alkynes with azides providing 1,4- and 1,5- isomers of 1,2,3-triazole [43]. Copper(I)-catalysed azide alkyne cycloaddition (CuAAC) represents a current variant, which converse to the original thermic version, proceeds quickly, regiospecifically (1,4-), and with high yields [44; 45]. These properties rank this reaction among the most significant \"near-perfect\" bond-forming reactions known as \"click\" reactions [46]. The importance of this reaction consists not only in the efficient preparation of substituted 1,2,3-triazole, but especially in the possibility for connecting different molecules and macromolecules, or to functionalize surfaces in a controlled way. The first polystyrene-supported copper(I) catalyst for CuAAC was based on the Amberlyst A21-Cul complex and was published in 2006 [47]. This and many further papers are discussed in a recently published review concerning immobilized copper complexes anchored on polystyrene and other heterogeneous carriers [48]. Also, two other papers deal with CuAAC [49; 50].  
The authors [49] prepared the catalyst **18** based on copper(I) iodide complex with cryptand-22 immobilized on commercial chloromethylated polystyrene (1% DVB) (Scheme 11). This heterogeneous heat- and air-stable catalyst **18** was used for a two-component CuAAC reaction of substituted organic azides with substituted alkynes, and this provided excellent yields of 1,4-disubstituted 1,2,3-triazolees (78-99%). The second three-component variant of this reaction involving organic halides, sodium azide and alkynes, also resulted in the production of 1,4-disubstituted 1,2,3-triazole with good yields (45-99%). Both variants of the reaction were performed under eco-friendly conditions (0.6 mol % Cu, aerobic, water, room temperature). The reusing of catalyst **18** was successfully tested for the reaction of benzyl azide with phenylacetylene (Cycle 1: 99%; Cycle 4: 87%) and for the variant of the reaction of benzyl bromide, sodium azide and phenylacetylene (Cycle 1: 99%; Cycle 4: 70%) (Scheme 11) [49]. The catalyst **18** was also successfully used for the formation of aryl-sulphur bonds in aqueous media [50]. In the next work [51], the heterogeneous catalyst **19** based on a copper(II) complex of 1,2-bis(4-pyridylthio)ethane anchored on chloromethylated polystyrene was prepared (Scheme 12). Catalyst **19** was tested for three-component reaction of organic halides, sodium azide and alkynes, which was performed in the presence of sodium ascorbate as a reduction agent in poly(ethylene glycol) (PEG-400), (0.2 mol % Cu), at 65 \({}^{\circ}\)C, 10-25 min. The yields of isolated 1,4-disubstituted 1,2,3-triazole were excellent, ranging from 87-99%. For the reaction of 4-bromobenzylbromide, sodium azide and phenylacetylene catalysed by **19** were further tested different solvents (0.1-0.3 mol % Cu, 15-30 min): water (78%), methanol (70%), toluene (40%), DMF (65%), MeCN (28%), PEG-water 1:1 (89%), ethanol-water 1:1 (75%).  
The recovery and reuse of catalyst **19** was successfully confirmed with the same three-component reaction (Cycle 1: 99%; Cycle 4: 95%; Cycle 7: 93%) (Scheme 12). This catalytic system also showed excellent activity for the synthesis of _bis_-1,4-disubstituted 1,2,3-triazoles (66-88%) [51].  
Figure 12: Huisgen 1,3-dipolar cycloaddition catalysed by polystyrene supported complex **19**.