Mapping the Pathway to Organocopper(II) Complexes Relevant to Atom Transfer Radical Polymerization

Macrocyclic Copper(II) Complexes as Catalysts for Electrochemically Mediated Atom Transfer

Copper-catalyzed electrochemical atom transfer radical addition (eATRA) is a new method for the creation of new C-C bonds under mild conditions. In this work, we have explored the reactivity of an analogous series of N4 macrocyclic CuII complexes as eATRA precatalysts, which are primed by reduction to their monovalent oxidation state. These complexes were fully characterized structurally, spectroscopically, and electrochemically. A spectrum of radical activation reactivity was found across the series [CuI(Me4cyclen)(NCMe)]+ (Me4cyclen = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane), [CuI(Me4cyclam)(NCMe)]+ (Me4cyclam = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), and [CuI(Me2py2clen)(NCMe)]+ (Me2py2clen = 3,7-dimethyl-3,7-diaza-1,5(2,6)-dipyridinacyclo-octaphane). The rate of radical production by [Cu(Me2py2clen)(NCMe)]+ was modest, but rapid radical capture to form the organocopper complex [CuI(Me2py2clen)(CH2CN)] led to a dramatic acceleration in catalysis, greater than seen in any comparable Cu complex, but this led to rapid radical self-termination instead of radical addition.

Electrocatalytic Atom Transfer Radical Addition with Turbocharged Organocopper(II) Complexes

The utility and scope of Cu-catalyzed halogen atom transfer chemistry have been exploited in the fields of atom transfer radical polymerization and atom transfer radical addition, where the metal plays a key role in radical formation and minimizing unwanted side reactions. We have shown that electrochemistry can be employed to modulate the reactivity of the Cu catalyst between its active (CuI) and dormant (CuII) states in a variety of ligand systems. In this work, a macrocyclic pyridinophane ligand (L1) was utilized, which can break the C-Br bond of BrCH2CN to release •CH2CN radicals when in complex with CuI. Moreover, the [CuI(L1)]+ complex can capture the •CH2CN radical to form a new species [CuII(L1)(CH2CN)]+ in situ that, on reduction, exhibits halogen atom transfer reactivity 3 orders of magnitude greater than its parent complex [CuI(L1)]+. This unprecedented rate acceleration has been identified by electrochemistry, successfully reproduced by simulation, and exploited in a Cu-catalyzed bulk electrosynthesis where [CuII(L1)(CH2CN)]+ participates as a radical donor in the atom transfer radical addition of BrCH2CN to a selection of styrenes. The formation of these turbocharged catalysts in situ during electrosynthesis offers a new approach to the Cu-catalyzed organic reaction methodology.

Kinetico-Mechanistic Studies on a Reactive Organocopper(II) Complex: Cu-C Bond Homolysis versus Heterolysis

Organocopper(II) reagents are an unexplored frontier of copper catalysis. Despite being proposed as reactive intermediates, an understanding of the stability and reactivity of the Cu^II-C bond has remained elusive. Two main pathways can be considered for the cleavage mode of a Cu^II-C bond: homolysis and heterolysis. We recently showed how organocopper(II) reagents can react with alkenes via radical addition, a homolytic pathway. In this work, the decomposition of the complex [Cu^IILR]⁺ [L = tris(2- dimethylaminoethyl)amine, Me₆tren, R = NCCH₂^-] in the absence and presence of an initiator (RX, X = Cl, Br) was evaluated. When no initiator was present, first-order Cu^II-C bond homolysis occurred producing [Cu^IL]⁺ and succinonitrile, via radical termination. When an excess of the initiator was present, a subsequent formation of [Cu^IILX]⁺ via a second-order reaction was found, which results from the reaction of [Cu^IL]⁺ with RX following homolysis. However, when Brønsted acids (R'-OH: R' = H, Me, Ph, PhCO) were present, heterolytic cleavage of the Cu^II-C bond produced [Cu^IIL(OR')]⁺ and MeCN. Kinetic studies were undertaken to obtain the thermal (ΔH^⧧, ΔS^⧧) and pressure (ΔV^⧧) activation parameters and deuterium kinetic isotopic effects, which provided an understanding of the strength of the Cu^II-C bond and the nature of the transition state for the reactions involved. These results reveal possible reaction pathways for organocopper(II) complexes relevant to their applications as catalysts in C-C bond forming reactions.

Organocopper(ii) complexes: new catalysts for carbon–carbon bond formation via electrochemical atom transfer radical addition (eATRA)

Electrochemical generation of a novel organocopper(ii) complex offers a new way to carry out atom transfer radical addition to alkenes under mild conditions with high yields and low catalyst loadings.