Highly efficient copper-mediated atom-transfer radical addition (ATRA) in the presence of reducing agent

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) angstrom] and bromide anion to the copper(I) center [Cu-I-Br 2.5088(3) angstrom]. Variable temperature H-1 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) angstrom and Cu-II-N-ax 2,040(3) angstrom] from TPMA ligand and a bromine atom [Cu-II-Br 2.3836(6) angstrom]. 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 angstrom on going from Cu-I(TPMA)Br to [Cu-II(TPMA)Br][Br]. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008).