Selective hydrogenation for fine chemicals: Recent trends and new developments

In this overview, recent trends and developments for the selective hydrogenation of multifunctional molecules are discussed and assessed from the point of view of fine chemicals synthesis. In a first part, the design and preparation of catalysts and ligands with interesting properties are summarized, particularly meant for the catalysis specialist. ne following topics are described in some detail: How enantioselective homogeneous catalysts are designed and tested; new effective chiral monodentate phosphines; successful bidentate phosphines ligand families (with axially chiral biaryl- and ferrocenyl-based backbones, new phospholanes and with stereogenic phosphorus); novel bidentate ligand families with P-O and P-N bonds; and oxazoline-based ligands. A short overview on immobilized chiral complexes and of the toolbox of heterogeneous catalysis (bimetallic, colloidal and modified catalysts) concludes this chapter. In a second part, progress for selected catalytic transformations and generic selectivity problems is described, intended mainly for the organic chemist who has to solve specific synthetic problems. Emphasis is on the following topics: The enantioselective hydrogenation of olefins with various substitution patterns; the chemo- and enantioselective hydrogenation of ketones; the diastereo- and enantioselective hydrogenation of C=N functions; the stereoselective hydrogenation of aromatic rings; chemoselectivity and hydroxylamine accumulation in the reduction of functionalized nitroarenes; chemoselectivity and new protecting groups for catalytic debenzylation; the mild hydrogenation of carboxylic acid derivatives; and the chemoselective hydrogenation of nitriles. In the last parts of the review, transfer hydrogenation and mechanistic issues are discussed, followed by a short conclusions and outlook paragraph. 1 Introduction 2 Design, Preparation and Homogeneous Catalysts 2.1 Designing Homogeneous 2.2 Catalytic Test Results 2.3 Chiral Monodentate Ligands 2.4 Bidentate Phosphine Ligand Families 2.5 Bidentate Ligand Families with P-O and P-N Bonds 2.6 Bidentate Oxazoline-Phosphorus and Related Ligands 2.7 Miscellaneous Chiral Ligands 2.8 Immobilized Chiral Complexes 3 Design and Preparation of Heterogeneous Catalysts 3.1 Toolbox of Heterogeneous Catalysts 3.2 Bimetallic Catalysts 3.3 Colloidal Catalysts 3.4 Modified Catalysts 4 Catalytic Transformations; Selectivity Problems 4.1 Enantibselective Hydrogenation of Alkenes 4.2 Chemo- and Enantioselective Hydrogenation of Ketones 4.3 Diastereo- and Enantioselective Hydrogenation of C=N Functions 4.4 Stereoselective Hydrogenation of Aromatic Rings and the Application of Novel Catalysts 4.5 Aromatic Nitro Groups: Chemoselectivity and Hydroxylamine Accumulation 4.6 Catalytic Debenzylation: Chemoselectivity and New Protecting Groups 4.7 Mild Hydrogenation of Carboxylic Acid Derivatives 4.8 Chemoselective Hydrogenation of Nitriles 5 Transfer Hydrogenation 5.1 Ru, Rh and Ir Complexes for Enantioselective Transfer Hydrogenations 5.2 Catalytic Results 6 Mechanistic Considerations 6.1 Activation of Hydrogen and Addition to Unsaturated Substrates 6.2 Enantioselective Hydrogenation 6.3 Hydrogenation without Substrate Coordination 7 Conclusions and Outlook.