Surfactant-free emulsion electrosynthesis via power ultrasound: electrocatalytic formation of carbon–carbon bonds Trevor J. Davies, a Craig E. Banks, a Bharathi Nuthakki, b James F. Rusling, b Robert R. France, c Jay D. Wadhawan a and Richard G. Compton* a a Physical and Theoretical Chemistry Laboratory, Oxford University, South Parks Road, Oxford, UK OX1 3QZ. E-mail: compton@ermine.ox.ac.uk; Fax: +44 (0) 1865 275 410; Tel: +44 (0) 1865 275 413 b Department of Chemistry, Box U-60, University of Connecticut, Storrs, Connecticut 06269-3060, USA c Dyson Perrins Laboratory, Oxford University, South Parks Road, Oxford, UK OX1 3QY Received 12th June 2002 First published as an Advance Article on the web 25th September 2002 Proof-of-concept of the mediated electrosynthesis of carbon–carbon bonds in totally ‘green’ surfactant-free emulsion media generated by application of power ultrasound to a biphasic water–organic mixture is illustrated by reference to three systems, some requiring further activation by light, and each catalysed by vitamin B 12 . The voltammetry of aqueous vitamin B 12 solutions at an electrode modified with microdroplets of the organic reactant is employed to gain an insight into the electrocatalytic pathway and readily permits the identification of optimum reaction parameters, such as starting material ratios and wavelength of light. The latter are employed in proof-of-concept emulsion electrosynthesis under conditions of triple activation (electron transfer, ultrasound and light). 1. Introduction Over the last decade, the environment has become a major focus for electrochemists offering ‘green’ and socially responsible solutions in a plethora of areas notably the clean and efficient production of fine chemicals required for modern society with a minimisation or elimination of the resulting industrial waste. 1,2 Electrosynthetic processes conducted in emulsion media as opposed to conventional non-aqueous solvents are thus attrac- tive, since the formation of an oil-in-water emulsion in which the reactant is solubilised, provides a significant ‘greening’ of an otherwise potentially environmentally unfriendly synthetic route; conventional non-aqueous solvents are problematic since they can be immiscible with water and can yield poisons on degradation. Furthermore, since the major component of the emulsion is water, the electrochemical reaction benefits from this near-ideal reaction environment; water has both a high conductivity and polarity. Additionally, product isolation by extraction or filtration is straight-forward. 1 Following earlier work by Eberson and Helgee, 3–7 Rusling et al. have pioneered the use of microemulsions as viable and ‘green’ media in which to conduct electrosynthetic processes for the last fifteen years, 8–14 as they are less toxic and less expensive replacements for organic solvents. Product yield and selectivity in these optically clear and stable microheteroge- neous mixtures of surfactants, oils and water has been shown to be, inter alia, a function of the type of emulsion used; bicontinuous microemulsions, in which both the oil and water phases are continuous, with surfactant residing at the extended oil–water interfaces, have been shown to be superior in terms of selectivity and reaction efficiency compared with continuous oil-in-water microemulsions (where the oil droplets are typi- cally between 0.01 and 0.1 mm in diameter). By the early 1990s, efficient dehalogenation of toxic organics such as PCBs and DDT was shown, and has subsequently been extended to other organic pollutants. 8,9 Mediated electrosynthesis, in which electrons are delivered from the electrode to reactants by employing a chemical mediator, in emulsion media is addition- ally attractive as the exact composition of the emulsion can influence the reaction pathway. 9 For carbon–carbon bond formation with alkyl bromides (RX) as reactants, vitamin B 12 has proved to be useful as a chemical mediator, 15,16 in which a Co(III) species is ligated to a tetraazamacrocyclic (corrin) ring. 15–17 For organic synthesis, the seemingly ‘special’ ability of vitamin B 12 and its derivatives to form alkyl–cobalt bonds, together with the ease of their homolysis, producing alkyl radicals, has lead to their use as ‘radical-in-a-bottle’ reagents, with applications in the synthesis of, inter alia, prostaglandins, pheromones, and chiral alcohols. 15,18–22 The mediated electro- synthetic pathway involves the electrochemical generation of a ‘supernucleophile’ intermediate, which in the case of vitamin B 12 , Co(III)L, is the Co(I)L complex: 9–13,15–17 Green Context Electrosynthesis has considerable potential for greener organic chemistry including the manufacture of speciality chemicals. Electrosynthetic processes conducted in emul- sion media where the major component is water are particularly attractive. Here we see described the extension of this approach through the use of an acoustically- emulsified biphasic system. The total absence of surfactants further adds to its green credentials. This preliminary study suggests that the surfactant-free electrocatalytic synthesis of carbon–carbon bonds can be achieved in yields of at least 50%. This is believed to be the first paper dealing with electrosynthesis under triple activation conditions (light, ultrasound and electron-transfer) nicely illustrating how the combination of cleaner technologies is often required to achieve the greenest chemistry. JHC This journal is © The Royal Society of Chemistry 2002 570 Green Chemistry, 2002, 4, 570–577 DOI: 10.1039/b205706a