Transfer hydrogenation

Abstract: The catalytic cycles for the H2-hydrogenation (H) and transfer hydrogenation (T) of CO and CN bonds catalyzed by over 100 ruthenium hydride complexes in organic and aqueous media can be classified into two main classes: the hydride transfer step is inner sphere (I) or outer sphere (O). Important subclasses of these mechanisms are cases where an ancillary ligand may assist in the hydride transfer step (IL or OL, respectively). The types of hydride complexes and their reactivity toward CO (ketones, aldehydes, CO2) and CN (imines) bonds is examined. Features of the different types of catalytic cycles are described. It is clear that the ligand assisted cases lead to very efficient catalysts for the selective hydrogenation of these polar bonds (e.g. Noyori’s metal–ligand bifunctional catalysis).

Abstract: 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. The 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 CN 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.

Abstract: During the last decade, the use of N-heterocyclic carbene ligands (NHCs) based on imidazolium ions and related heterocycles has emerged as an alternative to phosphines in the design of new organometallic catalysts. We review catalysts with chelate and pincer NHC ligands, including complexes of palladium, ruthenium, rhodium and iridium. Transfer hydrogenation and Heck chemistry are given special attention. Also discussed are Suzuki and Sonogashira coupling and immobilization on clay supports. Synthetic aspects are covered as well as a discussion of structural features, catalytic properties and catalyst recovery and recycling.