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).

AuClL(1)/PdCl2L(2)2 cocatalyzed coupling of Ar(1)X and Ar(2)SnBu3 is feasible for bulky Ar(1), provided that at least one ligand on Pd is not strongly coordinating. This can be achieved in situ by Au/Pd ligand exchange if L(1) is weakly coordinating. When the exchange is slow, independent preparation of appropriate catalysts is recommended.

ABSTRACT:The cis/trans isomerization of [PdMeAr(PR 3 ) 2 ] complexes (Ar = C 6 F 5 , C 6 F 3 Cl 2 ) can take place spontaneously (via dissociation and topomerization, studied experimentally) or catalyzed by ZnMe 2 . The later mechanism, studied by DFT methods, involves methyl exchange between Pd and Zn. The study of this catalyzed isomerization shows that, in contrast with the typical acidic behavior of Zn in ZnMeCl, Zn in ZnMe 2 (or, more exactly, the ZnMe bond) behaves as a strong basic center, able to attack the relatively high in energy acceptor orbital at Pd in fairly electron rich Pd complexes such as [PdArMeL 2 ] or [PdMe 2 L 2 ]. This makes the two reagents very different in Negishi couplings. The catalyzed isomerization occurs via transmetalation, thus both processes are connected. A comparison of the Pd/Zn intermediates and transition states with those found previously for Pd/Au transmetalations reveals very similar structures with intermetallic distances in the order of or noticeably shorter than the sum of the vdW radii, regardless of the nature of the metal (metallophilic Au or non metallophillic Zn). These short distances are associated to the involvement of the metals in 3c2e electron deficient bonds during R group transmetalation. In this respect there is a remarkable similarity with the structurally known behavior of main-group electron-deficient compound, which supports a unified view of the transmetalation processes.

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).

A bimetallic system Pd/CuF2, catalytic in Pd and stoichiometric in Cu, is very efficient and selective for the coupling of fairly hindered aryl silanes with aryl, anisyl, phenylaldehide, or pyridyl iodides of conventional size. The reaction involves the activation of the silane by Cu II followed by disproportionation and transmetalation from the Cu I (aryl) to Pd II , on which coupling takes place. The Cu III formed in the disproportionation is reduced to Cu I (aryl) by the aryl silane in excess, so that the CuF2 used is fully converted into Cu I (aryl) and used in the coupling. Moreover, no extra source of fluoride is needed. Interesting size selectivity towards coupling is found in competitive reactions of hindered aryl silanes. Easily accessible [PdCl2(IDM)(AsPh3)] is by far the best catalyst, and the isolated products are esentially free of As or Pd (< 1 ppm). The mechanistic aspects of the process are experimentally examined and discussed.