Reductive Elimination of d8-Organotransition Metal Complexes

Abstract

A theoretical analysis of two aspects of the mechanism of reductive elimination is presented—how the choice of central metal and peripheral ligands affects the activation energy for reductive elimination from a four-coordinate MR2(PR3)2 complex and how ligand asymmetry controls cis-trans rearrangements and elimination pathways proceeding through three-coordinate intermediates. The following conclusions emerge: (1) In the four-coordinate complex, the better the σ-donating capability of the leaving groups, the more facile the elimination; (2) Stronger donor ligands trans to the leaving groups will increase the barrier to elimination; (3) The reductive elimination barrier in four-coordinate complexes is controlled by the energy of an antisymmetric b2 orbital, which in turn depends on the energy of the metal levels. The activation energy for such direct reductive elimination should be, and is, substantially lower for Ni than for Pt or Pd; (4) T-shaped trans PdLR2, arising from dissociation of L in PdL2R2, will encounter a substantial barrier to polytopal rearrangement to cis PdLR2, which in turn has an open channel for reductive elimination of R2; (5) If the leaving groups are poor donors, cis-trans isomerization in the three-coordinate manifold should be easier than elimination.

Theoretical and Computational Study of a Complex System Consisting of Transition Metal Element(s): How to Understand and Predict Its Geometry, Bonding Nature, Molecular Property, and Reaction Behavior

“…), the transition state is planar. This planar transition state is consistent with the orbital interaction diagram proposed by Tatsumi and co-workers; 47 because the CT interaction from the doubly occupied d orbital to the antibonding σ*-MO of the σ-bond plays crucial roles in the oxidative addition, the σ*-MO wants to overlap with the HOMO. In the oxidative addition to the two-coordinate ML 2 complex of a d 10 metal, the d xz orbital is the HOMO of ML 2 with a bending LML angle (Scheme 3B), and thereby, the planar structure is favorable for the overlap between the σ*-MO and the d xz (Scheme 10A).…”

A Density Functional Theory Study of the Stille Cross‐Coupling via Associative Transmetalation. The Role of Ligands and Coordinating Solvents

“…These two geometries have been considered more stable than the ideal trigonal planar structure, and the T more stable than the Y. [35] Our computations confirm that these equilibrating geometries are easily attained, and indeed the Y geometry with a PMe 3 ligand is placed 10.5 kcal mol À1 above the T geometry xv with the vinyl groups in trans (for AsMe 3 ), becoming the transition state interconverting both trans and cis T-shaped structures. Geometric features of xv are the P À Pd À Ca and P À Pd À Ca' bond angles of 98.1 and 92.18, respectively, and a Pd À P bond distance of 2.20 .…”

“…Yamashita and Hartwig [34] reported the irreversible reductive elimination of the arylamido product [in contrast to the reversible nature of the reductive elimination from PA C H T U N G T R E N N U N G (t-Bu) 3 Pd(Ar)(X) [33] ] from these T complexes but obviously no structural and mechanistic details are avalaible for the irreversible processes. Early theoretical [35] studies on the reductive elimination of d 8 metal complexes confirmed the experimental findings that trans-dialkyl-Pd complexes are quite stable and instead of reductive elimination they often undergo b-elimination whenever possible. Should reductive elimination occur, it would take place through the corresponding T-cis complexes.…”

“…Should reductive elimination occur, it would take place through the corresponding T-cis complexes. Without the intervention of polar coordinating solvents or ligands assisting the isomerization mechanism as suggested, [35] or complex intermolecular ex- Table 4. Gibbs free energies (in kcal mol À1 ) for the species along the cyclic transmetalation mechanism starting from cis-PdL 2 A C H T U N G T R E N N U N G (vinyl)Br ii (L = PMe 3 or PH 3 ) and depicted in Figure 10.…”

Mechanistic Aspects of Metal‐Catalyzed C—C and C—X Bond‐Forming Reactions

“…In addition, theoretical studies and experimental results were in contradiction with several aspects of the mechanistic model of Scheme 1-2. In effect, intermediates of the type trans-[PdR 1 R 2 L 2 ] (2) might be expected to be quite longlived, as trans-to-cis isomerizations in this type of complexes are not facile processes [23][24][25]. However, complexes 2 have never been detected under catalytic conditions [26].…”