Optically Active Iridium Imidazol-2-ylidene-oxazoline Complexes:  Preparation and Use in Asymmetric Hydrogenation of Arylalkenes

Abstract

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.

Asymmetric Hydrogenation of Olefins Using Chiral Crabtree-type Catalysts: Scope and Limitations

“…For some alkene substrates, no pressure-dependence was observed, whereas for others, pressure had significantly influenced both the reaction rate and enantioselectivity. 39 Scheme 8 Hydrogenation of an alkene starting from an η 2 -H 2 complex (F) and going through an Ir(III)/Ir(V) catalytic cycle. The reaction has been suggested to go through either a concerted migratory insertion-oxidative The fact that the sense of enantioselectivity is controlled by the olefin geometry can be used to gain access to both enantiomers of the hydrogenated product with the same catalyst, provided that isomerization can be supressed.…”

Ligand Design for Catalytic Asymmetric Reduction

“…A chiral carbene-oxazoline ligand L5 is also useful for this purpose. 59 The mechanism of this reaction is to be elucidated by experimental [60][61][62][63] and theoretical 64,65 studies. Chiral titanocene catalysts also show high enantioselectivity for hydrogenation of simple olefins.…”

Polyimide-Supported Dichloro-1,3-bis(p-dimethylaminobenzyl)benzimidazolidin-2-ilidenruthenium (II) as Effective Catalyst for Hydrosilylation Reactions

“…These properties have led to a number of applications where NHC-based catalysts exhibit superior performance. Such NHC-metal complexes have been successfully utilized in cross-coupling reactions [16] and related processes, including hydrogenation [17], hydroformylation [18] hydrosilylation [19], oxidation [20], metathesis [21], cycloisomerisation of olefins [22], the synthesis of furans [23] and cyclopropanation reactions.…”