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

Abstract

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.

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

“…To remedy this problem, a competition titration with bromide and chloride present gave a continuous transition in the spectral properties with chloride addition (Figure S5). A similar approach was taken in the halide binding constant determinations of [Cu II (Me 3 pyclen)X] + complexes …”

“…The CV data in Figure were simulated with the model comprising eqs – with DigiSim . The methodology has been described previously . Experiments were conducted on 1 mM solutions of [Cu II (Me 2 py 2 clen)Br 2 ] over a range of BrCH 2 CN concentrations (0–4 mM) and scan rates (20–1000 mV s –1 ).…”

“…In previous work, we have shown that electrochemical generation of the carbanion complexes [Cu II (Me 6 tren)(CH 2 CN)] + and [Cu II (Me 3 pyclen)(CH 2 CN)] + in situ provides an effective route to electrochemical atom transfer radical addition ( e ATRA) to alkenes, both aromatic and aliphatic (eq ). R C H  normalC normalH 2 + normalBr normalC H 2 normalC normalN → [ Cu L ] + normalR normalC normalH normalBr normalC H 2 normalC normalH 2 normalC normalN …”

“…The kinetic data in Table may be compared with those obtained for five-coordinate [Cu II (Me 3 pyclen)(NCMe)] 2+ and [Cu II (Me 6 tren)Br] + , which were successfully employed as catalysts in e ATRA. The Cu II/I redox potential (Table ) again looms large as a discriminator of radical activation kinetics ( k act ).…”

“…Once the spectra exhibited no further changes with successive halide additions, the titration was stopped. Competition titrations (comprising mixtures of bromide and chloride) were carried out when the Cu II –Cl complex formation constants were too large, as described previously . The halide complex formation constants K Cu(II)–X (Table ) were obtained by global analysis (350–1100 nm) of each spectrum in the titration with the program ReactLab EQUILIBRIA .…”