## 3 Discrete Cu(I/II) Complexes
### Cross-Linked Synthetic Organic Polymer Supports
After in situ reduction of the blue polymer with sodium ascorbate, the material was tested as a catalyst in three-component CuAAC of halides (Scheme 2a). Following an initial solvent screening, the reactions were performed in biphasic _t_-BuOH-H2O (1:1) medium to give various triazole products in fair to excellent isolated yields and short reaction time (69%-96% in 30-135 min). In this respect it must be noted, however, that heating to 70 \({}^{\circ}\)C was required and, more important, a higher than stoichiometric catalyst amounts was applied (225 mol % Cu, on the basis of published data). Even though the polymeric material could be used in six reaction cycles with slight reduction of the triazole yield (91% \(\rightarrow\) 87%), its catalytic performance appears therefore somewhat less impressive than for many other recoverable CuAAC systems reported to date. Following their seminal paper on CuAAC with copper catalysts on DIAION resins [158; 159], more recently [PERSON] et al. investigated the use of the highly porous styrene-divinylbenzene (DVB) material **28**, under solventless conditions [160]. Two-component catalytic runs (Scheme 1) were carried out at r.t. under Ar, with 1 mol % of the catalyst and the addition of substoichiometric amounts of Et3N to the neat reactants. Under these conditions, CuAAC between different azides and alkynes led to largely variable product yields after 4 h (47%-100%). The observation that worst results were obtained with dipolarophiles devoid of Lewis-basic heteroatoms (N, O) in the proximity of the C-C triple bond, led the Authors to conclude that neighboring-group participation may play an important role in the system under exam. Due to the fact that many triazoles are solid, isolation of the product required dilution with water and AcOEt before removal of the polymeric material by filtration. At variance with previous results with **28** in toluene [159], reuse of the resin under neat conditions showed a marked decrease of activity over four cycles (100% \(\rightarrow\) 27% triazole yield). As expected, given the lack of strongly coordinating groups in HP-20, the origin of the problem was traced back to metal leaching which was indeed found to occur to a significant extent (19%) even in a single run. Moreover, assay of the separated liquid layers from product isolation (vide supra) revealed more than twelve-fold higher metal concentration in the AcOEt phase than in the aqueous one. Although copper contamination in  
Figure 3: CuAAC catalysts and pre-catalysts **27–30**, on insoluble organic supports from commercial resins. the crude triazoles was not explicitly reported, an estimation of around 400 ppm (dry weight basis) can be made on the basis of published data. A Cu(I) chelate catalyst **29**, immobilized on polystyrene resin, was described by [PERSON] and [PERSON] [161]. The supported complex was obtained by alkylating the diazacrown macrocyclic ether Kryptofix(r) 22 with a Merrifield resin and then charging the resulting polymeric material with CuI in refluxing EtOH. Perhaps because of limited solubility of the copper salt in EtOH or scarce compatibility of the polystyrene backbone with the protic medium, the metal loading attained in the last step was considerably lower than the cryptand content of the resin. Nevertheless, **29** displayed appreciable activity in two- and three-components CuAAC reactions, especially when water was used as the reaction medium. Under air and at 0.3 mol % catalyst loading, several azides were added to acetylenic substrates (Scheme 1) to provide the corresponding triazole products in good to excellent yield after 10 h at r.t. (78%-99%). Analogous three-component CuAAC runs (Scheme 2a) with alkyl chlorides, bromides, or iodides and 0.6 mol % of supported complex were slower (56%-99% yield in 15-21 h at r.t.).