Organometallic nucleophiles and Pd: What Makes ZnMe2 Different? Is Au like Zn?

Abstract

The cis/trans isomerization of [PdMeAr(PR3)2] complexes (Ar = C6F5, C6F3Cl2) can take place spontaneously (via dissociation and topomerization, studied experimentally) or catalyzed by ZnMe2. 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 ZnMe2 (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 [PdArMeL2] or [PdMe2L2]. 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.