Highly Efficient Copper Mediated Atom Transfer Radical Addition (ATRA) in the Presence of Reducing Agent

Tomislav Pintauer*

Department of Chemistry and Biochemistry, Duquesne University, 600 Forbes Avenue, 308 Mellon Hall, Pittsburgh, PA 15282

Abstract: 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 pentacoordinated 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) �) and bromide anion to the copper(I) center (Cu^I-Br = 2.5088(3) �). 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 bromide anion to [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) � and Cu^II-N_ax = 2.040(3) �) from TPMA ligand and a bromine atom (Cu^II-Br = 2.3836(6) �).  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 � on going from Cu^I(TPMA)Br to [Cu^II(TPMA)Br][Br].