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The N-methyl bis-imidazolium salts 3 were converted to the silver carbenes 7, then reacted to give palladium(II) complexes 4 with trans-chelating, bidentate bis-imidazolylidine ligands. The similar salts 8, that do not have N-methyl substituents, gave the tetradentate complexes 9 in direct reactions with palladium chloride. The potential of these complexes for asymmetric catalysis is discussed.

The carbene complex 1 can mediate hydrogenations of dienes with up to 20∶1.0 diastereoselectivity and 99% ee; the scope and limitations of these reactions were investigated.

Hydrogenation of 2,3-diphenylbutadiene mediated by a chiral iridium carbene oxazoline complex was studied. The kinetics of the reaction showed that it occurred in two distinct stages. The first corresponded to slow consumption of the diene to form half-hydrogenated products, i.e., monoenes. After all the diene was consumed, however, the reaction was much faster and more stereoselective. These data were interpreted in terms of possible catalytic intermediates.

This work explores the potential of iridium complexes of the N-heterocyclic carbene oxazoline ligands 1 in asymmetric hydrogenations of arylalkenes. The accessible carbene precursors, imidazolium salts 2, and robust iridium complexes 5 facilitated a discovery/optimization approach that featured preparation of a small library of iridium complexes, parallel hydrogenation reactions, and automated analysis. Three of the complexes (5ab, 5ad, and 5dp) and a similar rhodium complex (6ap) were studied by single-crystal X-ray diffraction techniques. This revealed molecular features of 6ap, and presumably the corresponding iridium complex 5ap, that the others do not have. In enantioselective hydrogenations of arylalkenes complex 5ap was the best for many, but not all, substrates. The enantioselectivities and conversions observed were sensitive to minor changes to the catalyst and substrate structure. Ligands with aliphatic N-heterocyclic carbene substituents gave complexes that are inactive, and do not lose the 1,5-cyclooctadiene ligands under the hydrogenation conditions. Experiments to investigate this unexpected observation imply that it is of a steric, rather than an electronic, origin. Temperature and pressure effects on the conversions and enantioselectivities of these reactions had minimal effects for some alkenes, but profound effects for others. In one case, the enantioselectivities obtained at high-pressure/low-temperature conditions were opposite to those obtained under high-temperature/low-pressure conditions (-64% enantiomeric excess versus +89% enantiomeric excess); a transformation from one prevalent mechanism to another is inferred from this. The studies of pressure dependence revealed that many reactions proceeded with high conversions, and optimal enantioselectivities in approximately 2 h when only 1 bar of hydrogen was used. Deuterium-labeling experiments provide evidence for other types of competing mechanisms that lead to D-incorporation at positions that do not correspond to direct addition to the double bond.

Density functional theory (PBE and B3LYP) was used to study asymmetric hydrogenations of alkenes catalyzed by an iridium imidazolylidine oxazoline complex. The calculation predicts that the alkene preferentially coordinates to the site trans to the carbene. The coordinated alkene then reacts first with the H2 ligand, then with the hydride to form alkane. Finally, the alkane is released by equilibrating with extrinsic H2 and alkene. Enantioface selectivities for hydrogenations of trisubstituted alkenes seem to be driven primarily by steric interactions with the adamantyl part of the ligand; only the smallest substituents can adopt a site close to it. Application of this theoretical model leads to correct predictions regarding the experimentally observed sense and magnitude of the enantioselectivities.