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

Abstract

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 [Cu(I)(TPMA)](+) cations, as confirmed by kinetic, cyclic voltammetry, and conductivity measurements. The equilibrium constant for atom transfer (K(ATRA) = k(a)/k(d)) utilizing benzyl thiocyanate was determined to be approximately 6 times larger for Cu(I)(TPMA)BPh(4) ((1.6 ± 0.2) × 10(-7)) than Cu(I)(TPMA)Cl ((2.8 ± 0.2) × 10(-8)) complex. This difference in reactivity between Cu(I)(TPMA)Cl and Cu(I)(TPMA)BPh(4) 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 Cu(I)(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 k(obs,298) = 2.9 × 10(4) s(-1)). Relatively strong coordination of halide anions in Cu(I)(TPMA)X prompted the possibility of activation in ATRA through partial TPMA dissociation. Indeed, no visible differences in the ATRA activity of Cu(I)(TPMA)BPh(4) were observed in the presence of as many as 5 equiv of strongly coordinating triphenylphosphine. The possibility for arm dissociation in Cu(I)(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 Me(6)TREN (tris(2-dimethylaminoethyl)amine), but had limited arm mobility due to the rigid backbone. Indeed, Cu(I)(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 BF(4)(-) and BPh(4)(-). The results presented in this Article are significant from the mechanistic point of view because they indicate that coordinatively saturated Cu(I)(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.

Towards the development of highly active copper catalysts for atom transfer radical addition (ATRA) and polymerization (ATRP)‡

“…For a given transition metal, this is typically achieved using a complexing ligand. In case of copper, neutral nitrogen based ligands are typically used, ranging from bidentate (2,2 -bipyridine (bpy) (Pirrung et al, 1993(Pirrung et al, , 1994(Pirrung et al, , 1995 and N-alkyl-2-pyridylmethanimine (NAlkPMI) (Clark, 2002;Clark et al, 2000Clark et al, , 2001aClark et al, , 2001bHaddleton et al, 1997aHaddleton et al, , 1997bHaddleton et al, , 1998Lad et al, 2003), tridentate (N,N,N ,N ,N -pentamethyldiethyelenetriamine (PMDETA) (Benedetti et al, 1997;Ghelfi et al, 1999;Ghelfi & Parsons, 2000) and trispyrazolyl borate (Tp x ) (Muñoz- Molina et al, 2007Molina et al, , 2008, tetradentate (1,1,4,7,10,10-hexamethyltriethylenetetramine (HMTETA) , tris[2-(dimethylaminoethyl]amine (Me 6 TREN) (Clark et al, 1999(Clark et al, , 2000(Clark et al, , 2001bEckenhoff & Pintauer, 2011) and TPMA (De Campo et al, 1999, 2000Eckenhoff et al, 2012;Eckenhoff & Pintauer, 2007, 2010a, 2010b to multidentate (N,N,N ,N -tetrakis(pyridin-2-ylmethyl)ethylenediamine (TPEDA) (De Campo et al, 1999, 2000Kaur et al, 2015a). Representative examples of molecular structures of copper(I) (activators) and copper(II) (deactivators) complexes commonly used in ATRA and ATRP are shown in Fig.…”

“…Clearly, the search for a better and more efficient catalyst is needed. Since the rate of alkene consumption in ATRA is directly proportional to the product of addition rate constant (k add ) and K ATRA , the reaction times and catalyst loadings for other alkyl halides to the currently most active CBr 4 , which has been extensively studied in our laboratories , 2010Eckenhoff et al, 2008Eckenhoff et al, , 2012Eckenhoff & Pintauer, 2007), can be compared. For example, the addition of CBr 4 to 1-octene in the presence of reducing agents using 5 mg L −1 of the catalyst is completed within 3 h at 60 • C, yielding the monoadduct in higher than 96 % yield (Eckenhoff et al, 2008).…”

“…Structural modifications of the most active TPMA and Me 6 TREN ligands , 2010Eckenhoff et al, 2008Eckenhoff et al, , 2012Eckenhoff & Pintauer, 2007, 2010a, 2010bTaylor et al, 2010) in copper catalyzed atom transfer radical addition and cyclization that employ reducing agents is perhaps the best approach towards the development of highly active complexes for activation of monohalogenated substrates. One way to decrease the reduction potential of copper(II) complexes is the systematic incorporation of electron donating groups (EDGs) to both ligands.…”

Polymerization and degradation of aliphatic polyesters synthesized by atom transfer radical polyaddition

“…Among the established RDRP techniques, atom transfer radical polymerization (ATRP), [23][24][25][26] extended from atom transfer radical addition (ATRA), 27 is one of the most widely used controlled radical polymerization methods because it can be applied to a wide range of functionalized monomers. Thanks to detailed mechanistic studies, a vast family of atom transfer radical reactions can be gathered, including ATRP 28 and its variants, 29 ATRA, 30 Cu(0)-mediated controlled radical polymerization, [31][32][33] atom transfer radical coupling (ATRC), 34 atom transfer nitroxide radical cross coupling (ATNRC), 35 atom transfer self-condensing vinyl polymerization (ATSCVP), [36][37][38] atom transfer radical polyaddition (ATRPA), 39,40 and simultaneous chainand step-growth radical polymerization. 41,42 These methods facilitate highly efficient "divergent" and "convergent" approaches for the synthesis of unique macromolecules, in terms of their composition, topology, and functionality.…”

Tris(2-pyridylmethyl)amine Based Ligands in Copper Catalyzed Atom Transfer Radical Addition (ATRA) and Polymerization (ATRP)

“…Reproduced with permission from reference (10). Copyright 2010, AmericanChemical Society.In [Cu I (TPMA)(CH 3 CN)][BPh 4 ], a singlet for acetonitrile was shifted downfield by approximately 1.6 ppm in acetone-d 6 upon cooling from 298 K to 180 K, indicating a deshielding effect as a result of coordination(10,87). On the other hand, [Cu I (TPMA)] 2 [ClO 4 ]*CH 3 OH, exhibited four broad resonances at room temperature similar to Cu I (TPMA)Br, suggesting the structure was also monomeric(Figure 6b).…”