Halide Effects in Transition Metal Catalysis

Abstract

Among the most common ligands found on transition metal catalysts are halide ions. Of the commercially available catalysts or pre‐catalysts, most are halo–metal complexes. In recent years, manipulation of this metal‐halide functionality has revealed that this can be used as a highly valuable method of tuning the reactivity of the complex. Variation of the halide ligand will usually not alter the nature of the system to the extent that it becomes unreactive but will impart sufficiently large changes that differences in reactivity or selectivity occur. These differences are a product of the steric and electronic properties of the halide ligand which has the ability to donate electron density to the metal occurs in a predictable manner. Despite the common perception in asymmetric catalysis that halide ligands are of secondary importance compared to chiral ligands, halide ligands have been found to exert dramatic effects on the enantioselectivity of asymmetric transformations. While the mechanism of action is known for relatively few of the cases, many intriguing and potentially synthetically useful trends are apparent. This review discusses the physical properties of the halides and their effects on stoichiometric and catalytic transition metal processes. The metal‐halide moiety thus emerges as a tunable functionality on the transition metal catalyst that can be used in the development of new catalytic systems.

Toward efficient asymmetric hydrogenation: Architectural and functional engineering of chiral molecular catalysts

“…The weak carboxylic acid is unable to generate high catalytic activity. Thus, even achiral anionic ligands in the precatalyst are not innocent in catalytic hydrogenation (39).…”

Catalysis and Organometallic Chemistry of Monometallic Species

“…A selection of reviews of catalytic reactions or processes where organometallic species are prominent include an historical perspective of 30 years of the crosscoupling reaction, 31 metal-mediated hydrodehalogenation of organic halides, 32 silicon-based cross coupling reactions, 33 halide effects in transition metal-mediated catalysis, 34 aryl-aryl bond formation, 35 development of C-C bond formations catalysed by late transition metals in air and water, 36,37 enantiomerically pure planar chiral organometallic complexes via facially selective π-complexation, 38 and inventing reactions for atom economy. 39 Organometallic catalysis performed in ionic liquids, 40 under the action of a microwave field, 41 on dendrimer 42 and solid supports 43 have also been reviewed.…”

Multiple Absolute Stereocontrol in Pd-Catalyzed [3+2] Cycloaddition of Oxazolidinones and Trisubstituted Alkenes Using Chiral Ammonium–Phosphine Hybrid Ligands

“…The insufficient reactivity can be attributed to the reluctant addition of the sulfonamide ion moiety of the zwitterionic allylpalladium to 4, probably because of the internal association of this weak nitrogen nucleophile with the cationic Pd center (Figure 2). This unproductive interaction could be suppressed by the addition of an external anion that has a strong coordinating ability to Pd such as a halide ion, 24 as previously observed by Knight and Aggarwal in related reaction systems. 16,25 Indeed, in the presence of a catalytic amount of tetrabutylammonium bromide (TBAB), the cycloaddition proceeded much faster under otherwise identical conditions, affording 5 in 79% yield (Entry 2).…”