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 Cu^II 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 [Cu^I(Me₄cyclen)(NCMe)]⁺ (Me₄cyclen = 1,4,7,10-tetramethyl-1,4,7,10-tetraazacyclododecane), [Cu^I(Me₄cyclam)(NCMe)]⁺ (Me₄cyclam = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane), and [Cu^I(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 [Cu^I(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.