N-Heterocyclic carbenes have become universal ligands in organometallic and inorganic coordination chemistry. They not only bind to any transition metal, be it in low or high oxidation states, but also to main group elements such as beryllium, sulfur, and iodine. Because of their specific coordination chemistry, N-heterocyclic carbenes both stabilize and activate metal centers in quite different key catalytic steps of organic syntheses, for example, C-H activation, C-C, C-H, C-O, and C-N bond formation. There is now ample evidence that in the new generation of organometallic catalysts the established ligand class of organophosphanes will be supplemented and, in part, replaced by N-heterocyclic carbenes. Over the past few years, this chemistry has been the field of vivid scientific competition, and yielded previously unexpected successes in key areas of homogeneous catalysis. From the work in numerous academic laboratories and in industry, a revolutionary turning point in oraganometallic catalysis is emerging.

http://aether.cmi.ua.ac.be/artikels/Artikels%20Gitte/Artikels/Reviews/NHC-%20liganden%20Herrmann.pdf

N-heterocyclic carbenes: a new concept in organometallic catalysis.

Dehydrogenation as a substrate-activating strategy in homogeneous transition-metal catalysis.

The Golden Age of Transfer Hydrogenation

Hydrogen autotransfer in the N-alkylation of amines and related compounds using alcohols and amines as electrophiles.

Hydrogen transfer reduction processes are attracting increasing interest from synthetic chemists in view of their operational simplicity and high selectivity. In this tutorial review the most significant advances recently achieved in the stereoselective reduction of unsaturated organic compounds catalyzed by homogeneous transition metal complexes are critically reviewed. A sharp growth of the synthetic applications of this technique in the synthesis of fine chemicals is predictable as the use of transition metal catalyzed reactions will become more familiar to synthetic chemists.

Asymmetric transfer hydrogenation: chiral ligands and applications

Geringe Katalysatormengen und kurze Reaktionsdauer genugen zur quantitativen Reduktion von Ketonen durch 2-Propanol und den Komplex [RuX(CNN)(dppb)] (siehe Struktur; X=H, Cl), dessen dreizahniger Ligand sich von 6-(4-Methylphenyl)-2-pyridylmethylamin ableitet. Die Reduktion scheint uber eine reversible Insertion des Substrats in die Ru-H-Bindung zu verlaufen, bei der ein RutheniumII)-alkoxid entsteht. dppb=Ph2P(CH2)4PPh2.

Ruthenium(II) terdentate CNN complexes: superlative catalysts for the hydrogen-transfer reduction of ketones by reversible insertion of a carbonyl group into the Ru-H bond.

2-(Aminomethyl)pyridine−Phosphine Ruthenium(II) Complexes: Novel Highly Active Transfer Hydrogenation Catalysts

The ruthenium and osmium complexes [MCl(2)(diphosphane)(L)] (M=Ru, Os; L=bidentate amino ligand) and [MCl(CNN)(dppb)] (CNN=pincer ligand; dppb=1,4-bis-(diphenylphosphino)butane), containing the N−H moiety, have been found to catalyze the acceptorless dehydrogenation of alcohols in tBuOH and in the presence of KOtBu. The compounds trans-[MCl(2)(dppf)(en)] (M=Ru 7, Os 13; dppf=1,1'-bisdiphenylphosphino)ferrocene; en=ethylenediamine) display very high activity and different substrates, including cyclic and linear alcohols, are efficiently oxidized to ketones by using 0.8-0.04 mol % of catalyst. The effect of the base and the comparison of the catalytic activity of the Ru versus Os complexes are reported. The ruthenium complex 7 generally leads to a faster conversion into ketones with respect to the osmium complex 13, which displays better activity in the dehydrogenation of 5-en-3β-hydroxy steroids. The synthesis of new Ru and Os complexes [MCl(2)(PP)(L)] (PP=dppb, dppf; L=(±)-trans-1,2-diaminocyclohexane,2-(aminomethyl)pyridine, and 2-aminoethanol) of trans and cis configuration is also reported.

Pincer and Diamine Ru and Os Diphosphane Complexes as Efficient Catalysts for the Dehydrogenation of Alcohols to Ketones

We describe the isolation of the first
CNN pincer osmium complexes [OsX(CNN)P 2 ] to display
high catalytic activity and productivity in the reduction of
ketones by either TH or HY (TOFand TON up to 10 6 h 1 and
10 5 , respectively). A high enantioselectivity (up to 98% ee ) is
possible in both reactions with a remarkably low catalyst
loading (0.005–0.002 mol%). To the best of our knowledge,
this is the first example of an osmium complex that catalyzes
the asymmetric HY of ketones. Evidence for an Os-OR vs.
Os-H equilibrium suggests that both these species are
involved in the catalytic pathways. Mechanistic studies as
well as the preparation of new CNN pincer osmium catalysts
are currently underway.

Pierluigi Rigo

Papers

Ruthenium(II) terdentate CNN complexes: superlative catalysts for the hydrogen-transfer reduction of ketones by reversible insertion of a carbonyl group into the Ru-H bond.

2-(Aminomethyl)pyridine−Phosphine Ruthenium(II) Complexes: Novel Highly Active Transfer Hydrogenation Catalysts

Pincer and Diamine Ru and Os Diphosphane Complexes as Efficient Catalysts for the Dehydrogenation of Alcohols to Ketones

Osmium(II) CNN pincer complexes as efficient catalysts for both asymmetric transfer and H2 hydrogenation of ketones.

Cyclometalated ruthenium(II) complexes as highly active transfer hydrogenation catalysts.