N-Heterocyclic Carbenes in Late Transition Metal Catalysis

Catalytic activities of Schiff base transition metal complexes

Ligand design is becoming an increasingly important part of the synthetic activity in chemistry. This is of course because of the subtle control that ligands exert on the metal center to which they are coordinated. Ligands which contain significantly different chemical functionalities, such as hard and soft donors, are often called hybrid ligands and find increasing use in molecular chemistry. Although the interplay between electronic and steric properties has long been recognized as essential in determining the chemical or physical properties of a complex, predictions remain very difficult, not only because of the considerable diversity encountered within the Periodic Table-different metal centers will behave differently towards the same ligand and different ligands can completely modify the chemistry of a given metal-but also because of the small energy differences involved. New systems may-even through serendipity-allow the emergence of useful concepts that can gain general acceptance and help design molecular structures orientated towards a given property. The concept of ligand hemilability, which finds numerous illustrations with hybrid ligands, has gained increased acceptance and been found to be very useful in explaining the properties of metal complexes and in designing new systems for molecular activation, homogeneous catalysis, functional materials, or small-molecule sensing. In the field of homogeneous enantioselective catalysis, in which steric and/or electronic control of a metal-mediated process must occur in such a way that one stereoisomer is preferentially formed, ligands containing one or more chiral oxazoline units have been found to be very valuable for a wide range of metal-catalyzed reactions. The incorporation of oxazoline moieties in multifunctional ligands of increasing complexity makes such ligands good candidates to display hemilabile properties, which until recently, had not been documented in oxazoline chemistry. Herein, we briefly recall the definition and scope of hemilabile ligands, present the main classes of ligands containing one or more oxazoline moieties, with an emphasis on hybrid ligands, and finally explain why the combination of these two facets of ligand design appears particularly promising.

Hemilability of Hybrid Ligands and the Coordination Chemistry of Oxazoline‐Based Systems

The last 15 years of hydroformylation and carbonylation chemistry are reviewed, including technical and commercial aspects.

Progress in hydroformylation and carbonylation

Cooperativity in Noncovalent Interactions.

The racemic and meso diastereomers of an electron-rich binucleating tetraphosphine ligand have been used to prepare homobimetallic rhodium norbornadiene complexes. The racemic bimetallic Rh complex is an excellent hydroformylation catalyst for 1-alkenes, giving both a high rate of reaction and high regioselectivity for linear aldehydes, whereas the meso complex is considerably slower and less selective. A mechanism involving bimetallic cooperativity between the two rhodium centers in the form of an intramolecular hydride transfer is proposed. Mono- and bimetallic model complexes in which the possibility for bimetallic cooperativity has been reduced or eliminated are very poor catalysts.

https://science.sciencemag.org/content/sci/260/5115/1784.full.pdf

A bimetallic hydroformylation catalyst: high regioselectivity and reactivity through homobimetallic cooperativity.

Bimetallic Hydroformylation Catalysis: In Situ Characterization of a Dinuclear Rhodium(II) Dihydrido Complex with the Largest Rh–HNMR Coupling Constant

Bimetallic hydroformylation: a zwitterionic Rh–IRhI tetraphosphine ligand-based bimetallic complex exhibiting facile CO addition and phosphine ligand rearrangement equilibrium

Hydroformylierungskatalyse: In‐situ‐Charakterisierung eines zweikernigen Dihydridorhodium(II)‐Komplexes mit der bisher größten gemessenen Rh‐H‐NMR‐Kopplungskonstante

Spencer G. Train

Papers

A bimetallic hydroformylation catalyst: high regioselectivity and reactivity through homobimetallic cooperativity.

Bimetallic Hydroformylation Catalysis: In Situ Characterization of a Dinuclear Rhodium(II) Dihydrido Complex with the Largest Rh–HNMR Coupling Constant

Bimetallic hydroformylation: a zwitterionic Rh–IRhI tetraphosphine ligand-based bimetallic complex exhibiting facile CO addition and phosphine ligand rearrangement equilibrium

Hydroformylierungskatalyse: In‐situ‐Charakterisierung eines zweikernigen Dihydridorhodium(II)‐Komplexes mit der bisher größten gemessenen Rh‐H‐NMR‐Kopplungskonstante