Abstract
We report an efficient and sustainable electrochemical synthesis of copper(i) acetylides using simultaneous copper oxidation and Hofmann elimination of quaternary ammonium salts. The electrochemically-generated base was also regenerated electrochemically, making it catalytic. A ‘Click test’ (CuAAC reaction) was performed to assess product purity and an electrochemically-promoted, one-pot CuAAC reaction was performed, which serves as a promising initial demonstration of this approach in a pharmaceutically-relevant reaction.
Highlights
Copper has great potential utility in electro-organic chemistry due to its readily accessible redox states
We hypothesised that we could carry out this reaction in an undivided cell by incorporating the reduction reaction of the tetrabutylammonium (TBA) electrolyte salt used in our previous conditions to produce Bu3N in situ and obviate the requirement for any added base, such as DABCO, in our protocol
We began by using similar reagents to our previous method,[2] Bu4NPF6/MeCN was used as the electrolyte solution, causing CuI to be produced from the Cu0 working electrode (WE) and Bu3N to be formed directly at the Pt counter electrode (CE)
Summary
Copper has great potential utility in electro-organic chemistry due to its readily accessible redox states. The application of mild electrical potentials to exert control over oxidation states of copper catalysts introduced to solutions has previously been exploited to select for either Glaser–Hay (CuII pathway) or CuAAC (CuI pathway) reactions.[1] the use of elemental copper as an electrode material to produce CuI ions in situ for reactions has only recently been published by our group.[2] This work represented an electrochemical synthesis and isolation of copper(I) acetylides, which are valuable intermediates in many synthetic processes, such as Huisgen-type/Click,[3] Castro–Stephens,[4] halogenation,[3] Sonogashira,[5] ynamide-formation[6] and phosphorussubstitution reactions,[6] as well as for the formation of a variety of products via photochemical protocols.[7] Traditionally prepared by reacting a terminal alkyne with a copper halide in aqueous ammonia with EtOH or in DMF with K2CO3,8 we found that in a divided cell, applying a positive potential whilst using a Cu0 working-electrode, having DABCO present as a base and using Bu4NPF6/MeCN as an electrolyte solution, we could efficiently produce CuI ions that were used to form the desired copper(I) acetylides in excellent yields. We hoped to demonstrate and make use of a catalytic base cycle by electrochemically reducing protonated base species, releasing only H2 gas as a clean by-product,[9] improving the efficiency of this method further still (Fig. 1)
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