Insights into reactivity trends for electrochemical C–N bond formations

Abstract

Electrochemical synthesis techniques are currently of great interest due to the possibility of synthesizing products while limiting reactant and energy input and providing potentially unique selectivity. Our group has previously reported the development of the “anion pool” synthesis method. As this is a new method for organic synthesis and the coupling of C–N bonds, it is important to understand the reactivity trends and limitations this method provides. In this report we explore the reactivity trends of a series of nitrogen-containing heterocycles under reductive electrochemical conditions. The results show that anionic nitrogen heterocycles are stable at room temperature in acetonitrile/electrolyte solutions up to a parent N–H pK_a value up to 23. Addition of carbon electrophiles to solutions containing the electrochemically generated anionic nitrogen heterocycles led to the C–N cross-coupling reactivity. Product yields tracked linearly with the pK_a value of the N–H bond of the heterocycles over 4 orders of acidity magnitude. Both benzylic halides and perfluorinated aromatics were found suitable for undergoing C–N cross-coupling with the anionic nitrogen heterocycles with product yields as high as 90%. It is also shown that the stability and reactivity of the anions are affected by the choice of electrolyte and temperature. Additionally, this procedure compares well to green chemistry processes in atom economy and PMI values.