Transient and stable interm ediates in cross-coupling reactions

• Published in: Philosophical transactions of the Royal Society of London. Series A: Mathematical and physical sciences 1988-11, Vol.326 (1592), p.587-594
• Main Authors: Brown, J. M., Cooley, N. A.
• Format: Article
• Language: English
• ISSN: 0080-4614; 2054-0272
• DOI: 10.1098/rsta.1988.0110

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 η2-olefin complex and an η1-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 η1-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.