Organometallic Nucleophiles and Pd: What Makes ZnMe₂ Different? Is Au Like Zn?

Abstract

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.

The Negishi Catalysis: Full Study of the Complications in the Transmetalation Step and Consequences for the Coupling Products

“…15 It yields directly cis- [PdMe2L2]. The activation energy is very similar to the phosphine independent pathway and this explains the -0.3 dependence order on PPh3 concentration.…”

“…15 Starting with cis-[PdArMe(L)2] (3) and following the ligand substitution pathway in Figure 2, the calculations propose first a very weak interaction between ZnMe2, which was commented on in a previous paper and has no kinetic significance. 15 In the transition state TSI1 the Zn-Me 2 bond acts as incoming ligand releasing one PPh3 (L 2 ) from the palladium, while the zinc takes electron density from the Rf-Pd bond, affording intermediate I1, with the exchanging Me involved in a 3c-2e bond. The second substitution takes place so that the incoming ligand is PPh3 and the leaving ligand is the RfZn bond (TSI2).…”

“…This further complicates the reaction scheme during the cross-coupling reaction. 15 From a positive point of view, all these model systems with high coupling barriers to the desired product create a landscape where the undesired side processes can be more easily studied.…”

“…We have shown in a previous study that the isomerization of 3 to 2 is phosphine dependent and can be catalyzed by ZnMe2. 15 For ZnRfMe the situation is even more complex since ZnRfMe is formed by two ways: i. through the secondary transmetalations between 2 or 3 and ZnMe2; and ii. by direct Me/Rf exchange between 1 and ZnMe2.…”

An unexpected transmetalation intermediate: isolation and structural characterization of a solely CH3 bridged di-copper(i) complex

“…10 bonds has been proposed mainly on the basis of kinetic and DFT studies for Sonogashira and Stille cross-coupling reactions co-catalyzed by Cu(I) or Au(I), 18 and the Negishi coupling reaction is also thought to benefit from Pd(II)-Zn(II) bond formation. 19 Very recently, coupling of alkynes mediated by dual gold catalysis has been proposed to involve a di-gold(I) key complex exhibiting d 10 …”