Thermal or photodecomposition of the classical free radical generating diazo reagent 2,2′-azobis(2-methylpropionitrile) (AIBN) was used as a source of cyanide anions in the synthesis of copper(I)–cyanide frameworks. The reported methodology utilizes the direct reduction of Cu^II(aa)(NN)X (aa = deprotonated amino acid, NN = bidentate nitrogen-based ligand, X = Cl or Br) complexes by AIBN/ascorbic acid to yield seven novel coordination networks. Aromatic amines were directly incorporated into 1D Cu^I–CN chains. In the case of the aliphatic amine tetramethylethylenediamine (TMEDA), a 3D Cu^I–CN framework was obtained. This novel procedure is mild, applicable to a variety of nitrogen-based ligands, and represents an efficient alternative to currently used hydrothermal or solvothermal methods.

As a continuing effort of expanding the scope of catalyst regeneration in the presence of environmentally benign reducing agents, one-pot sequential azide–alkyne [3+2] cycloaddition and atom transfer radical addition (ATRA) reactions were performed via in situ reduction of copper(II) by ascorbic acid. The formation of functionalized triazoles was achieved utilizing a ligand-free catalytic system for the cycloaddition between tripropargylamine and vinylbenzyl azide and subsequent addition of tris(2-pyridyl)methylamine (TPMA) ligand in the ATRA step. With this strategy, reactions with carbon tetrachloride and carbon tetrabromide proceeded efficiently providing the desired triazoles in nearly quantitative yields (>90 %) using 10 mol % of copper. Sequential azide–alkyne [3+2] cycloaddition and ATRA reactions were also extended to less active alkyl halides such as methyl trichloroacetate, methyl dichloroacetate and dichloroacetonitrile. The corresponding products were obtained in modest yields (50–80 %). The presented methodology enables efficient synthesis of functionalized polytriazoles, which could have a potential use as chelating agents for a variety of transition metals.

One-pot sequential reactions involving azide–alkyne [3+2] cycloaddition and atom transfer radical addition (ATRA) catalyzed by [Cu^II(TPMA)X][X] {X = Br^– or Cl^–, TPMA = tris(2-pyridylmethyl)amine} in the presence of ascorbic acid as a reducing agent are reported. Reactions with azidopropyl methacrylate and 1-(azidomethyl)-4-vinylbenzene in the presence of a variety of alkynes [phenylacetylene, (3,4-difluorophenyl)acetylene, propargyl alcohol, 2-methyl-3-butyn-2-ol, methyl propiolate and ethyl propiolate] and alkyl halides (carbon tetrachloride, carbon tetrabromide, ethyl trichloroacetate, methyl trichloroacetate, ethyl dichloroacetate, methyl dichloroacetate, dichloroacetonitrile and 2-bromopropionitrile) proceeded efficiently to yield highly functionalized (poly)halogenated esters and aryl compounds containing a triazolyl group in the presence of as low as 0.5 mol-% of the catalyst. It is envisioned that the presented methodology could have further implications in the organic synthesis of functionalized triazoles, which have recently been identified as the lead targets for the screening of potential pharmaceutical drugs.

Transition-metal-catalyzed atom-transfer radical addition (ATRA) and cyclization (ATRC) are considered fundamental reactions in organic chemistry for the formation of C–C bonds using free-radical means. Until recently, both processes were plagued by the large amounts of catalysts needed to achieve high selectivity towards the desired target compound (as high as 30 mol-%). The principal problem was the accumulation of the transition metal complex in the higher oxidation state as a result of unavoidable radical-radical termination reactions. In this article, recent advanced in the area of catalyst regeneration in transition-metal-mediated ATRA and ATRC reactions in the presence of free-radical diazo initiators or magnesium as reducing agents are reviewed. The role of the reducing agent in both systems is to continuously regenerate the activator (transition metal complex in the lower oxidation state) from the deactivator (transition metal complex in the higher oxidation state). As a result, ATRA and ATRC reactions can be conducted using very small concentrations of metal catalysts, making this methodology a “greener” alternative to currently available synthetic processes for such organic transformations.

The synthesis, characterization and exceptional activity of Cu^I(TPMA)Br [TPMA = tris(2-pyridylmethyl)amine] and [Cu^II(TPMA)Br][Br] complexes in ATRA reactions of polybrominated compounds to alkenes in the presence of reducing agent (AIBN) was reported. [Cu^II(TPMA)Br][Br], in conjunction with AIBN, effectively catalyzed ATRA reactions of CBr₄ and CHBr₃ to alkenes with concentrations between 5 and 100 ppm, which is the lowest number achieved in copper-mediated ATRA. The molecular structure of Cu^I(TPMA)Br indicated that the complex was pseudo-pentacoordinate in the solid state due to the coordination of TPMA [Cu^I–N: 2.1024(15), 2.0753(15), 2.0709(15) and 2.4397(14) Å] and bromide anion to the copper(I) center [Cu^I–Br 2.5088(3) Å]. Variable temperature ¹H NMR and cyclic voltammetry studies confirmed the equilibrium between Cu^I(TPMA)Br and [Cu^I(TPMA)(CH₃CN)][Br], indicating some degree of halide anion dissociation in solution. The coordination of the bromide anion to the [Cu^I(TPMA)]⁺ cation resulted in a formation of much more reducing Cu^I(TPMA)Br complex (E_1/2 = –720 mV vs. Fc/Fc⁺) than the corresponding ClO₄^– (E_1/2 = –422 mV vs. Fc/Fc⁺) and PF₆^– (E_1/2 = –421 mV vs. Fc/Fc⁺) analogues. In [Cu^II(TPMA)Br][Br], the Cu^II atom was coordinated by four nitrogen atoms [Cu^II–N_eq 2.073(2) Å and Cu^II–N_ax 2.040(3) Å] from TPMA ligand and a bromine atom [Cu^II–Br 2.3836(6) Å]. The overall geometry of the complex was distorted trigonal bipyramidal. Cu^I(TPMA)Br and [Cu^II(TPMA)Br][Br] complexes showed similar structural features from the point of view of TPMA coordination. The only more pronounced difference in the TPMA coordination to the copper center was observed in the shortening of Cu–N_ax bond length by approximately 0.400 Å on going from Cu^I(TPMA)Br to [Cu^II(TPMA)Br][Br]. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)