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

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, ΔG⧧298 = 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.