Catalysis of Organic..

Durán Pachón and Rothenberg 50157. Electroreductive Pd-Catalyzed UllmannReactions in Ionic LiquidsAbstractLaura Durán Pachón and Gadi Rothenberg*Van ’t Hoff Institute for Molecular Sciences, University of Amsterdam,Nieuwe Achtergracht 166, 1018 WV Amsterdam, The Netherlands.*gadi@science.uva.nlA catalytic alternative to the Ullmann reaction is presented, based on reductivehomocoupling catalyzed by palladium nanoparticles. The particles are generated insitu in an electrochemical cell, and electrons are used to close the catalytic cycle andprovide the motivating force for the reaction. This system gives good yields usingiodo- and bromoaryls, and requires only electricity and water. Using an ionic liquidsolvent combines the advantages of excellent conductivity and cluster stabilising.IntroductionSymmetrical biaryls are important intermediates for synthesising agrochemicals,pharmaceuticals and natural products (1). One of the simplest protocols to makethem is the Ullmann reaction (2), the thermal homocoupling of aryl chlorides in thepresence of copper iodide. This reaction, though over a century old, it still usedtoday. It has two main disadvantages, however: First, it uses stoichiometric amountsof copper and generates stoichiometric amounts of CuI 2 waste (Figure 1, left).Second, it only works with aryl iodides. This is a problem because chemicals reactby their molarity, but are quantified by their mass. One tonne of iodobenzene, forexample, contains 620 kg of ‘iodo’ and only 380 kg of ‘benzene’.In the past five years, we showed that heterogeneous Pd/C can catalyseUllmann-type reactions of aryl iodides, bromides, and chlorides. Two reactionpathways are possible: Reductive coupling, where Pd 2+ is generated and reducedback to Pd 0 , and oxidative coupling, which starts with Pd 2+ and needs an oxidisingagent. Different reagents can be used for closing the reductive coupling cycle,including HCO 2–(3), H 2 gas (4), Zn/H 2 O (5), and alcohols (6). The two pathways caneven be joined, giving a tandem system that converges on one product (7). All ofthese examples, however, require an extra chemical reagent. In this shortcommunication, we present a different approach, using electrochemistry to close thecatalytic cycle (Figure 1, right).