Gold shuttle: Bulky groups, which will not (or only very sluggishly) undergo Stille coupling with stannanes and inexpensive ligands, can be efficiently coupled using bimetallic catalysis. A gold cocatalyst serves as an efficient shuttle to convey the bulky group from tin to palladium by reducing the steric crowding in the transition‐states (see scheme).
Gold‐Shuttle: Sperrige Gruppen, die in Stille‐Kupplungen mit Stannanen und kostengünstigen Liganden nicht oder nur langsam reagieren, können mittels Dimetall‐Katalyse effizient gekuppelt werden. Ein Gold‐Cokatalysator befördert die sperrige Gruppe vom Zinn zum Palladium und vermindert dadurch den sterischen Anspruch des Übergangszustands (siehe Schema).
Experiments on palladium catalyzed cross-coupling of [AuMe(PPh3)] with aryl iodides show that Ar-Ar homocoupling products are the main product or an abundant byproduct of the reaction. The percentage of cross-coupling product is higher for aryls with larger σp Hammet parameter. The scrambling of organic groups via bimetallic intermediates explains the formation of these products. This scrambling can be observed and the activation energies partially quantified in some cases using as aryl C6Cl2F3, which is relatively reluctant to coupling.
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