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

Abstract

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.

Ligand Control of the Metal Coordination Sphere: Structures, Reactivity and Catalysis

“…It was found by 1 H and 31 P{ 1 H} variabletemperature NMR spectroscopy that a dynamic behaviour was taking place in the Rh(III) complex, but not in the Ir(III) complex, that led the two ligands to exchange their role above coalescence temperature. This dynamic behaviour, which involves opening and closing of the metal-oxygen bond of the P,O chelate, became shortly after known as hemilabile behaviour [22,23]. Related observations were made with Ru(II) complexes and applied to the reversible binding of carbon monoxide under ambient conditions (Scheme 2) [24].…”

Nucleophilic and free-radical additions of phosphines and phosphine chalcogenides to alkenes and alkynes

“…The chemo-and regioselective addition of secondary phosphine sulfides to alkyl vinyl ethers contributes to better understanding the reactivity of the both classes of compounds and offers a facile straightforward route to potent cocatalysts and "hemilabile" ligands 137 (e.g., triphenylphosphine sulfide is more effective ligand for palladium-catalyzed bisalkoxycarbonylation of olefins than triphenylphosphine 138 ).…”

Expedient routes to 4,4-dialkyl- 5-methylene-1,3-oxazolines

“…Recent investigations [1][2][3][4][5][6][7][8][9][10][11] have shown that 1,3-oxazolines (4,5-dihydro-oxazoles or 2-oxazolines)…”

“…The asymmetric catalysis employing enantiomerically pure oxazoline ligands has been developed intensively during the last fifteen years. 1 Additionally, 1,3-oxazolines are well known as fuel and oil additives, antistatic agents for polyolefins and textiles, corrosion inhibitors, polyvinyl chloride stabilizers, rubber plasticizers, photosensitizers, antipyrenes, curing agents, etc. 12 Attractive representatives of the oxazoline family are the 5-methylene derivatives, owing to the presence of the double bond in their structure, which is capable of further modification.…”