Asymmetric Activation/Deactivation of Racemic Ru Catalysts for Highly Enantioselective Hydrogenation of Ketonic Substrates We are grateful to Drs. H. Kumobayashi and N. Sayo of the Takasago International Corp. for providing the BINAP ligands. We also thank Dr. Kenji Yoza of the Nippon Bruker Co. for X-ray analysis of the

Nonlinear Effects in Asymmetric Catalysis by Design: Concept, Synthesis, and Applications

Asymmetric synthesis constitutes a key technology for the preparation of enantiomerically pure compounds as well as for the selective control of individual stereocenters in the synthesis of complex compounds. It is thus of extraordinary importance for the synthesis of chiral drugs, dietary supplements, flavors, and fragrances, as well as novel materials with tunable and reconfigurable chiroptical properties or the assembly of complex natural products. Typically, enantiomerically pure catalysts are used for this purpose. To prepare enantiomerically pure ligands or organocatalysts, one can make use of the natural chiral pool. Ligands and organocatalysts with an atropisomeric biphenyl and binaphthyl system have become popular, as they are configurationally stable and contain a C2-symmetric skeleton, which has been found to be particularly privileged. For catalysts with opposite configurations, both product enantiomers can be obtained. Configurationally flexible biphenyl systems initially appeared to be unsuitable for this purpose, as they racemize after successful enantiomer separation and thus are neither storable nor afford a reproducible enantioselectivity. However, there are strategies that exploit the dynamics of such ligands to stereoconvergently enrich one of the catalyst enantiomers. This can be achieved, for example, by coordinating an enantiomerically pure additive to a ligand-metal complex, which results in deracemization of the configurationally flexible biphenyl system, thereby enriching the thermodynamically preferred diastereomer. In this Account, we present our strategy to design stereochemically flexible catalysts that combine the properties of supramolecular recognition, stereoconvergent alignment, and catalysis. Such systems are capable to recognize the chirality of the target product, leading to an increase in enantioselectivity during asymmetric catalysis. We have systematically developed and investigated these smart catalyst systems and have found ways to specifically design and synthesize them for various applications. In addition to (i) reaction product-induced chiral amplification, we have developed systems with (ii) intermolecular and (iii) intramolecular recognition, and successfully applied them in asymmetric catalysis. Our results pave the way for new applications such as temperature-controlled enantioselectivity, controlled inversion of enantioselectivity with the same chirality of the recognition unit, generation of positive nonlinear effects, and targeted design of autocatalytic systems through dynamic formation of transient catalysts. Understanding such systems is of enormous importance for catalytic processes leading to symmetry breaking and amplification of small imbalances of enantiomers and offer a possible explanation of homochirality of biological systems. In addition, we are learning how to target supramolecular interactions to enhance enantioselectivities in asymmetric catalysis through secondary double stereocontrol. Configurationally flexible catalysts will enable future resource-efficient development of asymmetric syntheses, as enantioselectivities can be fully switched by stereoselective alignment of the stereochemically flexible ligand core on demand.

Efficient Amplification in Soai's Asymmetric Autocatalysis by a Transient Stereodynamic Catalyst

Mechanisms leading to a molecular evolution and the formation of homochirality in nature are interconnected and a key to the underlying principles that led to the emergence of life. So far proposed mechanisms leading to a non-linear reaction behavior are based mainly on the formation of homochiral and heterochiral dimers. Since homochiral and heterochiral dimers are diastereomers of each other, the minor enantiomer is shifted out of equilibrium with the major enantiomer by dimer formation and thus a reaction or catalysis can be dominated by the remaining molecules of the major enantiomer. In this article a mechanism is shown that leads to homochirality by the formation of a highly catalytically active transient intermediate in a stereodynamically controlled reaction. This is demonstrated by Soai's asymmetric autocatalysis, in which aldehydes are transformed into the corresponding alcohols by addition of dialkylzinc reagents. The mechanism of chirogenesis proposed here shows that an apparently inefficient reaction is the best prerequisite for a selection mechanism. In addition, stereodynamic control offers the advantage that the minor diastereomeric intermediate can be interconverted into the major diastereomer and thus be stereoeconomically efficient. This is supported by computer simulation of reaction kinetics.

The enantioselectivity in asymmetric ketone hydrogenation catalyzed by RuH2(diphosphine)(diamine) complexes: insights from a 3D-QSSR and DFT study

The 3D-QSSR method was carried out to investigate the enantioselectivity of the asymmetric ketone hydrogenation (AKH) catalyzed by RuH₂(diphosphine)(diamine) complexes integrating with DFT method, which could provide a way to design homogeneous transition-metal catalysts.

Enhanced bioconversion rate and released substrate inhibition in (R)-phenylephrine whole-cell bioconversion via partial acetone treatment

An approach was developed to enhance the efficiency for the bioconversion of 1-(3-hydroxyphenyl)-2-(methyamino)-ethanone to (R)-phenylephrine. The strain Serratia marcescens N10612, giving the benefit of 99% enantiomeric excess in (R)-PE conversion, was used. The fermentation was devised to harvest cells with high hydrophobic prodigiosin content inside the cells. Then, the partial acetone extraction was applied to remove prodigiosin from the cells. The treatment was found to increase the cells conversion rate without loss of the cells NADPH redox system. When using 50% (v/v) acetone for 5min, the processed cells can give a specific conversion rate of 16.03μmol/h/g-cells. As compared the treated cells with cells under the basal medium, the maximum reaction rate (Vmax) increased from 6.69 to 10.27 (μmol/h/g-cells), the dissociation constant (Km) decreased from 0.236 to 0.167mM and the substrate inhibition constant (KSi) increased from 0.073 to 1.521mM. The 20-fold increase in substrate inhibition constant referred to a great release from the substrate inhibition for the use of S. marcescens N10612 in the bioconversion, which would greatly benefit the bioconversion to be industrialized.

The N-H functional group in organometallic catalysis

The organometallic approach is one of the most active topics in catalysis. The application of NH functionality in organometallic catalysis has become an important and attractive concept in catalyst design. NH moieties in the modifiers of organometallic catalysts have been shown to have various beneficial functions in catalysis by molecular recognition through hydrogen bonding to give catalyst-substrate, ligand-ligand, ligand-catalyst, and catalyst-catalyst interactions. This Review summarizes recent progress in the development of the organometallic catalysts based on the concept of cooperative catalysis by focusing on the NH moiety.

Asymmetric catalysis of homo-coupling of 3-substituted naphthylamine and hetero-coupling with 3-substituted naphthol leading to 3,3'-dimethyl-2,2'-diaminobinaphthyl and -2-amino-2'-hydroxybinaphthyl

Optically active 3,3'-dimethyl-2,2'-diamino-1,1'-binaphthyl (DM-DABN) and 3,3'-dimethyl-2-amino-2'-hydroxybinaphthyl (DM-NOBIN) derivatives were synthesized by Cu-(-)-sparteine complex-catalyzed enantioselective homo- and hetero-coupling of 2-naphthylamine, respectively. The difference in enantioselectivity was observed by changing the concentration of oxygen.

Solution structures and behavior of trans-RuH(eta(1)-BH(4)) (binap)(1,2-diamine) complexes

The solution structures of a number of trans-RuH(eta(1)-BH(4))[(S)-tolbinap](1,2-diamine) precatalysts [TolBINAP = 2,2'-bis(di-4-tolylphosphino)-1,1'-binaphthyl; 1,2-diamine==(S,S)- or (R,R)-1,2-diphenylethylenediamine (DPEN), ethylenediamine (EN), and (S)-1,1-di(4-anisyl)-2-isopropylethylenediamine (DAIPEN)] have been determined using 2D NMR ((1)H--(1)H DQF-COSY, (1)H--(13)C HMQC, (1)H--(31)P HSQC, and (1)H--(15)N HSQC), and a double-pulsed field-gradient spin-echo (DPFGSE) NOE technique. All the octahedral Ru complexes adopt a trans configuration with respect to the BH(4) and hydride ligands. Amine protons of trans-RuH(eta(1)-BH(4))[(S)-tolbinap](1,2-diamine) complexes undergo H/D exchange in (CD(3))(2)CDOD. This inherent high acidity, coupled with the lability and chemical properties of the BH(4) ligand, allows for precatalyst activation without the need for an added base, in contrast to trans-RuCl(2)[(S)-tolbinap](1,2-diamine) precatalysts, which require a strong base for generation of a catalytic species. The H/BH(4) complex in a 2-propanol solution is converted to catalytically active [trans-RuH{(S)-tolbinap}{(S,S)-dpen}(ROH)](+) [(RO)(ROH)(n)](-) (R = (CH(3))(2)CH), a loosely associated ion pair of the discrete (solvated) cationic fragment and anionic species.

Asymmetric Activation

While nonracemic catalysts can generate nonracemic products with or without the nonlinear relationship in enantiomeric excesses between catalysts and products, racemic catalysts inherently give only a racemic mixture of chiral products. Asymmetric catalysts, either in nonracemic or racemic form, can be further evolved into highly activated catalysts with association of chiral activators. This asymmetric activation process is particularly useful in racemic catalysis through selective activation of one enantiomer of the racemic catalyst. Recently, a strategy whereby a racemic catalyst is selectively deactivated by a chiral additive has been reported to yield nonracemic products. However, reported herein is an alternative and conceptually opposite strategy in which a chiral activator selectively activates, rather than deactivates, one enantiomer of a racemic chiral catalyst. The advantage of this activation strategy over the deactivation counterpart is that the activated catalyst can produce a greater enantiomeric excess in the products-even with the use of a catalytic amount of activator relative to chiral catalyst-than that attained by the enantiomerically pure catalyst on its own. Therefore, asymmetric activation could provide a general and powerful strategy for not only the use of atropisomeric, racemic ligands but also chirally flexible and proatropisomeric ligands without enantiomeric resolution!