Heterogenised iridium complexes for the asymmetric hydrogenation of imines

Recent advances in the asymmetric reduction of imines by recycled catalyst systems

Recent advances relating to the asymmetric reduction of imines to afford optically active amines via recyclable catalyst systems are reviewed.

Catalytic self-assembled monolayers on Au nanoparticles: the source of catalysis of a transphosphorylation reaction

The catalytic activity of a series of Au monolayer protected colloids (Au MPCs) containing different ratios of the catalytic unit triazacyclononane⋅Zn(II) (TACN⋅Zn(II) ) and an inert triethyleneglycol (TEG) unit was measured. The catalytic self-assembled monolayers (SAMs) are highly efficient in the transphosphorylation of 2-hydroxy propyl 4-nitrophenyl phosphate (HPNPP), an RNA model substrate, exhibiting maximum values for the Michaelis-Menten parameters k(cat) and K(M) of 6.7×10(-3) s(-1) and 3.1×10(-4) M, respectively, normalized per catalytic unit. Despite the structural simplicity of the catalytic units, this renders these nanoparticles among the most active catalysts known for this substrate. Both k(cat) and K(M) parameters were determined as a function of the mole fraction of catalytic unit (x(1)) in the SAM. Within this nanoparticle (NP) series, k(cat) increases up till x(1) ≈0.4, after which it remains constant and K(M) decreases exponentially over the range studied. A theoretical analysis demonstrated that these trends are an intrinsic property of catalytic SAMs, in which catalysis originates from the cooperative effect between two neighboring catalytic units. The multivalency of the system causes an increase of the number of potential dimeric catalytic sites composed of two catalytic units as a function of the x(1) , which causes an apparent increase in binding affinity (decrease in K(M)). Simultaneously, the k(cat) value is determined by the number of substrate molecules bound at saturation. For values of x(1) >0.4, isolated catalytic units are no longer present and all catalytic units are involved in catalysis at saturation. Importantly, the observed trends are indicative of a random distribution of the thiols in the SAM. As indicated by the theoretical analysis, and confirmed by a control experiment, in case of clustering both k(cat) and K(M) values remain constant over the entire range of x(1) .

Enantioselective hydrogenation of acyclic aromatic N-aryl imines catalyzed by an iridium complex of (S,S)-1,2-bis(tert-butylmethylphosphino)ethane

[reaction: see text] An iridium(I) complex of (S,S)-1,2-bis(tert-butylmethylphosphino)ethane with tetrakis(3,5-bis(trifluoromethyl)phenyl)borate as the counterion catalyzes the hydrogenation of acyclic aromatic N-aryl imines under 1 atm of hydrogen pressure at room temperature to give the corresponding optically active secondary amines with up to 99% ee.

Recycling Asymmetric Hydrogenation Catalysts by Their Immobilization onto Ion-Exchange Resins

Ion-exchange resins can be used as supports for the preparation of single-site, heterogenised asymmetric hydrogenation catalysts. The immobilised catalysts obtained can be efficiently and conveniently recovered and recycled. This article reviews the significant contributions in the field including the main concepts behind the design and the applications of this type of catalyst.

In Quest of Factors That Control the Enantioselective Catalytic Markovnikov Hydroboration/Oxidation of Vinylarenes

This study attempts to rationalise the unpredictable performance of transition metal catalysed asymmetric hydroboration of vinylarenes on varying the precursor of the catalyst from cationic to neutral species, [M(cod)(L-L)]BF4, [M(mu-Cl)(cod)]2/(L-L), the metal (M=Rh and Ir), and the hydroborating reagent (catecholborane, pinacolborane). The approaches are based on the agreement between experimental data provided by (R)-Binap and (R)-Quinap modified catalytic systems and computational data evidenced by DFT calculations and QM/MM strategies. Unprecedentedly high enantiomeric excesses in the hydroboration/oxidation of vinylarenes with both electron-withdrawing substituents ((R)-(+)-1-p-F-phenylethanol, ee up to 92 %) and electron-releasing substituents ((R)-(+)-1-p-MeO-phenylethanol, ee up to 98 %), can be attributed to a rhodium halide key intermediate.

Enantioselective hydrogenation of 3-alkylidenelactams: high-throughput screening provides a surprising solution

High-throughput screening of 256 potential catalysts (8 metal precursors x 32 phosphine ligands) has identified [(BDPP)Ir(COD)]BF4 as a catalyst for the enantioselective hydrogenation of 3-alkylidenelactams. This result is surprising given the highly flexible backbone of the BDPP ligand and the ineffectiveness of this catalyst in other applications. The asymmetric hydrogenation appears fairly general for five- and six-membered lactams and in one case has been scaled up to a 20 kg level.