Transient and stable interm ediates in cross-coupling reactions

Abstract

The mechanism of catalytic cross coupling mediated by palladium complexes has been examined. This has involved the successful in situ generation of a biphosphine palladium(0) species, and then observation of the sequential formation of an η 2 -olefin complex and an η 1 -alkenyl bromide complex on addition of an alkene. The latter is a stable isolable species, which reacts rapidly with added arylmagnesium halides; only the η2-olefin complex is observed after this step at —80 °C. The contribution of these intermediates to the catalytic cycle of cross coupling was tested, employing a 13C-labelled alkenyl iodide, and determ ining the label distribution in both palladium complex and product. On this basis it was dem onstrated that the η 1 -alkenyl iodide is a true interm ediate in cross coupling, and the reaction cycle occurs w ithout the intervention of a second molecule of substrate. Platinum complexes are more stable than their isostructural palladium analogues. All the interm ediates in a slow but sustainable catalytic cycle have been characterised in solution. Oxidative addition of the alkenyl bromide is profoundly accelerated by one-electron oxidizing agents.