Highly Enantioselective Hydrogenation of Simple Ketones Catalyzed by a Rh–PennPhos Complex

Even alkyl methyl ketones undergo asymmetric hydrogenation with high enantioselectivity when a rhodium complex of the conformationally rigid chiral ligand 1 (Me-PennPhos; R=CH₃) is used as the catalyst. Basic additives such as 2,6-lutidine contribute to the achievement of high enantiomeric excesses.     

Enantioselective Hydrogenation of Olefins with Iridium–Phosphanodihydrooxazole Catalysts

High turnover numbers and up to 98% ee were obtained in the catalytic hydrogenation of unfunctionalized aryl-substituted olefins with iridium–phosphanyldihydrooxazole complexes 1 (see reaction scheme). The anion of the complex—for example, hexafluorophosphate or tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF⁻)—has a remarkable effect on the reactivity and longevity of the catalyst.     

Synthesis of Optically Active α-Amino- phosphinic Acids by Catalytic Asymmetric Hydrogenation in Organic Solvents and Aqueous Micellar Media

Enantiomeric excesses of up to 99 % could be obtained in the synthesis of the biologically interesting acylated α-aminophosphinic acids 1 (R¹=Me, Ph; R²=H, Me, Et; R³=H, F, iPr) by asymmetric hydrogenation with rhodium complex catalysts and subsequent hydrolysis [Eq. (1)]. cod=1,5-cyclooctadiene.     

Highly Enantioselective Iridium‐Catalyzed Hydrogenation of Cyclic Enamides

The MaxPHOX–Ir catalyst system provided the highest selectivity ever reported for the reduction of cyclic enamides derived from α‐ and β‐tetralones. This result indicates that iridium catalysts are also proficient in reducing alkenes bearing metal‐coordinating groups. In the present system, selectivity was pressure‐dependent: In most cases, a decrease in the H₂ pressure to 3 bar resulted in an increase in enantioselectivity. Moreover, the process can be carried out in environmentally friendly solvents, such as methanol and ethyl acetate, with no loss of selectivity.     

Solution pH: A Selectivity Switch in Aqueous Organometallic Catalysis—Hydrogenation of Unsaturated Aldehydes Catalyzed by Sulfonatophenylphosphane–Ru Complexes

The equilibribrium distribution of water-soluble ruthenium hydrides [HRuCl(tppms)₃] and [H₂Ru(tppms)₄] (tppms = (3-sulfonatophenyl)diphenylphosphane) in the reaction with H₂ is governed by the pH value. As a consequence, the selectivity in the hydrogenation of cinnamaldehyde for reaction at C=C or C=O can be completely inverted by changing the pH value (see drawing below).     

Macrocyclic Cyclopropenes by Highly Enantioselective Intramolecular Addition of Metal Carbenes to Alkynes

The intramolecular addition of a diazo ester group to a triple bond in the presence of chiral dirhodium(II ) carboxamidate catalysts gives macrocyclic lactones with a fused cycloproprene ring [Eq. (a)]. This efficient reaction is characterized by high enantiocontrol (up to 98% ee) and chemoselectivity.     

Novel Catalytic Hydrogenolysis of Trimethylsilyl Enol Ethers by the Use of an Acidic Ruthenium Dihydrogen Complex

The heterolytic cleavage of H ₂ is the key to the novel catalytic hydrogenolysis of trimethylsilyl enol ethers catalyzed by [RuCl(η²-H₂)(dppe)₂]OTf (dppe = 1,2-bis(diphenylphosphanyl)ethane, OTf = trifluoromethanesulfonate), which results in the formation of a ketone and Me₃SiH (see scheme). In addition, the stoichiometric, ruthenium-assisted protonation of a prochiral lithium enolate with H₂ gave a chiral ketone with high enantioselectivity (up to 75 % ee).     

Conformationally Flexible Biphenyl‐phosphane Ligands for Ru‐Catalyzed Enantioselective Hydrogenation

Stereomutation of a BIPHEP/RuCl ₂ /diamine complex (shown schematically) is possible because of the conformational flexibilty of BIPHEP ligands. The result is an asymmetric activation in the Ru‐catalyzed hydrogenation of carbonyl compounds to optically active alcohols. Whereas a racemic BINAP/RuCl₂ complex with a chiral diamine activator gives a 1:1 mixture of two diastereomers, unequal amounts of the diastereomers can be produced from a BIPHEP/RuCl₂ complex and a chiral diamine. Ar=3,5‐dimethylphenyl, BINAP=2,2′‐bis(diphenylphosphanyl)‐1,1′‐binaphthyl, BIPHEP=2,2′‐bis(diarylphosphanyl)biphenyl.     

Solution and Solid-State Studies of a Chiral Zinc–Sulfonamide Complex Relevant to Enantioselective Cyclopropanations

A highly distorted tetrahedron formed by the four nitrogen atoms around zinc in the crystalline zinc–sulfonamide complex 1 may explain its catalytic activity in asymmetric cyclopropanations. The agent is formed by deprotonation of (R,R)-N,N′-cyclohexane-1,2-diyl)bis(n-butanesulfonamide) with diethylzinc and addition of 2,2′-bipyridyl.