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Kinetic and Mechanistic Aspects of Atom Transfer Radical Addition (ATRA) Catalyzed by Copper Complexes with Tris(2-pyridylmethyl)amine

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posted on 05.11.2012, 00:00 by William T. Eckenhoff, Ashley B. Biernesser, Tomislav Pintauer
Kinetic and mechanistic studies of atom transfer radical addition (ATRA) catalyzed by copper complexes with tris­(2-pyridylmethyl)­amine (TPMA) ligand were reported. In solution, the halide anions were found to strongly coordinate to [CuI(TPMA)]+ cations, as confirmed by kinetic, cyclic voltammetry, and conductivity measurements. The equilibrium constant for atom transfer (KATRA = ka/kd) utilizing benzyl thiocyanate was determined to be approximately 6 times larger for CuI(TPMA)­BPh4 ((1.6 ± 0.2) × 10–7) than CuI(TPMA)Cl ((2.8 ± 0.2) × 10–8) complex. This difference in reactivity between CuI(TPMA)Cl and CuI(TPMA)­BPh4 was reflected in the activation rate constants ((3.4 ± 0.4) × 10–4 M–1 s–1 and (2.2 ± 0.2) × 10–3 M–1 s–1, respectively). The fluxionality of CuI(TPMA)­X (X = Cl or Br) in solution was mainly the result of TPMA ligand exchange, which for the bromide complex was found to be very fast at ambient temperature (ΔH = 29.7 kJ mol–1, ΔS = −60.0 J K–1 mol–1, ΔG298 = 47.6 kJ mol–1, and kobs,298 = 2.9 × 104 s–1). Relatively strong coordination of halide anions in CuI(TPMA)­X prompted the possibility of activation in ATRA through partial TPMA dissociation. Indeed, no visible differences in the ATRA activity of CuI(TPMA)­BPh4 were observed in the presence of as many as 5 equiv of strongly coordinating triphenylphosphine. The possibility for arm dissociation in CuI(TPMA)­X was further confirmed by synthesizing tris­(2-(dimethylamino)­phenyl)­amine (TDAPA), a ligand that was structurally similar to currently most active TPMA and Me6TREN (tris­(2-dimethylaminoethyl)­amine), but had limited arm mobility due to the rigid backbone. Indeed, CuI(TDAPA)­Cl complex was found to be inactive in ATRA, and the activity increased only by opening the coordination site around the copper­(I) center by replacing chloride anion with less coordinating counterions such as BF4 and BPh4. The results presented in this Article are significant from the mechanistic point of view because they indicate that coordinatively saturated CuI(TPMA)­X complexes catalyze the homolytic cleavage of carbon–halogen bond during the activation step in ATRA by prior dissociation of either halide anion or TPMA arm.