Catalysis of Organic..

504 Electroreductive Catalytic Ullmann ReactionsWe then tested several other iodo-, bromo- and chloroaryl substrates. Table 1 showsthe conversions, yields and corresponding turnover number (TON). PhI and PhBrgave biphenyl in good yields, and the p-nitrophenyls were also active. Thecorresponding p-nitrophenylchloride was much less active.An interesting question is what closes here the catalytic cycle? Although we donot have a full mechanistic picture at this stage, we think that the complementaryhalf-reaction of the oxidation of the aryl halide is the oxidation of water, i.e. 2H 2 O →O 2 + 4H + + 4e – (E 0 = –1.229 V). Ionic liquids are notoriously hygroscopic, and asmall water impurity is enough to close the cycle. Indeed, control experiments in thepresence of 1 molar equivalent of water gave a faster reaction (complete conversionafter 6 h, cf. with 8 h for the ‘dry’ system). No difference was found when excesswater was added.In summary, palladium nanoparticles generated in situ catalyse Ullmann-typereaction. Using electrochemistry is a simple and efficient way to perform thiscatalytic reductive homocoupling, and the reaction gives good yields with arylbromides and iodides. To the best of our knowledge, this is the first electro-reductivepalladium-catalysed demonstration of a Ullmann reaction (12). Moreover, thisreaction proceeds smoothly at room temperature. Further work in our laboratory willinclude kinetic and mechanistic studies to gain further understanding into thisinteresting system.Experimental SectionMaterials and instrumentation. Experiments were performed using a specialhome-made cell coupled to a dual current supply with a maximum output of 10 V/40mA. A detailed technical description of this system is published elsewhere (10). 1 HNMR spectra were recorded on a Varian Mercury vx300 instrument at 25 °C. GCanalysis was performed on an Interscience GC-8000 gas chromatograph with a 100%dimethylpolysiloxane capillary column (DB-1, 30 m × 0.325 mm). GC conditions:isotherm at 105 ºC (2 min); ramp at 30 ºC min –1 to 280 ºC; isotherm at 280 ºC (5min). Pentadecane was used as internal standard. The ionic liquid [omim] + [BF 4 ] –was prepared following a published procedure and dried prior to use (8).All otherchemicals were purchased from commercial sources (> 98% pure).Procedure for Pd clusters-catalysed Ullmann homocoupling. Example:biphenyl from PhI. The electrochemical cell was charged with PhI (4.09 gr, 20.0mmol) and 50 mL [omim][BF 4 ]. After stirring 5 min, a constant current (10 mA, 1.6V) was applied and the mixture was further stirred for 8 h at 25 ºC. Reactionprogress was monitored by GC. After 8 h, the product was extracted with ether (3 ×50 mL). The ether phases were combined and evaporated under vacuum to give 1.12g (75 mol% based on PhI) as a colourless crystalline solid. The solvent can berecycled by washing with aqueous NaBF 4 . δ H (ppm, Me 4 Si): 7.36–7.42 (m, 2H),7.45–7.51 (m, 4H), 7.62–7.68 (m, 4H). Good agreement was found with theliterature values (13).