Highly Chemo- and Enantioselective Hydrogenation of Linear α,β-Unsaturated Ketones

Asymmetric and Chemoselective Iridium Catalyzed Hydrogenation of Conjugated Unsaturated Oxime Ethers

Research on the chemoselective metal-catalyzed hydrogenation of conjugated π-systems has mostly been focussed on enones. Herein, we communicate the understudied asymmetric hydrogenation of enimines catalyzed by N,P-iridium complexes and chemoselective toward the alkene. A number of enoxime ethers underwent hydrogenation smoothly to yield the desired products in high yield and stereopurity (up to 99 % yield, up to 99 % ee). No hydrogenation of the C=N π-bond was observed under the applied reaction conditions (20 bar H2, rt, DCM). It was demonstrated that the chiral oxime ether could be hydrolyzed into the ketone with complete preservation of the installed stereogenity at the α-carbon. At last, a binding mode of the substrate to the active iridium catalyst and the consequence for the stereoselective outcome was proposed.

The Effect of Conformational Freedom vs Restriction on the Rate in Asymmetric Hydrogenation: Iridium-Catalyzed Regio- and Enantioselective Monohydrogenation of Dienones

Transition metal-catalyzed asymmetric hydrogenation constitutes an efficient strategy for the preparation of chiral molecules. When dienes are subjected to hydrogenation, control over regioselectivity still presents a large challenge and the fully saturated alkane is often yielded. A few successful monohydrogenations of dienes have been reported, but hitherto these are only efficient for dienes comprised of two distinctly different olefins. Herein, the reactivity of a conjugated carbonyl compound as a function of their conformational freedom is studied, based on a combined experimental and theoretical approach. It was found that alkenes in the (s)-cis conformation experience a large rate acceleration while (s)-trans restrained alkenes undergo hydrogenation slowly. Ultimately, this reactivity aspect was exploited in a novel method for the monohydrogenation of dienes based on conformational restriction ((s)-cis vs (s)-trans). This mode of discrimination conceptually differs from existing monohydrogenations and dienones constructed of two olefins similar in nature could efficiently be hydrogenated to the chiral alkene (up to 99 % ee). The extent of regioselection is even powerful enough to overcome the conventional reactivity order of substituted olefins (di>tri>tetra). This high yielding and atom-economical protocol provides an interesting opportunity to instal a stereogenic center on a carbocycle, while leaving a synthetically useful alkene untouched.

Asymmetric Hydrogenations of Acyclic α,β-Unsaturated Ketones with Chiral Frustrated Lewis Pairs (FLPs)

In this paper, we demonstrate a metal-free asymmetric hydrogenation of acyclic α,β-unsaturated ketones under the catalysis of a frustrated Lewis pair (FLP) comprising chiral oxazoline and achiral borane. A wide range of optically active α-substituted ketones were furnished in high yields with 26-85% ee's.

Enantiodivergent Hydrogenation of Exocyclic α,β-Unsaturated Lactams Enabled by Switching the N-Chirality of Iridium Catalyst

Central chirality is an important chiral element used in the design of chiral ligands and catalysts. Mostly, the attention of organic chemists is focused on developing of chiral ligands with stable stereogenic centers. However, the N-chirality in chiral ligand design has been rarely explored due to its flexibility. Here we demonstrate the design, synthesis, and application of a class of simple P,N-ligands with flexible N-chirality and their derived iridium complexes with fixed N-chiral stereocenters. Both fixed configurations of the N-stereocenter of the iridium complexes could be selectively formed from the same chiral ligand. This pair of diastereoisomeric iridium complexes showed good performance in the enantiodivergent asymmetric hydrogenation of exocyclic α,β-unsaturated lactams. The N-H group plays an impressive role in catalytic activity. Computational studies emphasized the importance of N-chirality and N-H group.

Stereoselective Iridium-N,P-Catalyzed Double Hydrogenation of Conjugated Enones to Saturated Alcohols

Asymmetric hydrogenation of prochiral substrates such as ketones and olefins constitutes an important instrument for the construction of stereogenic centers, and a multitude of catalytic systems have been developed for this purpose. However, due to the different nature of the π-system, the hydrogenation of olefins and ketones is normally catalyzed by different metal complexes. Herein, a study on the effect of additives on the Ir-N,P-catalyzed hydrogenation of enones is described. The combination of benzamide and the development of a reactive catalyst unlocked a novel reactivity mode of Crabtree-type complexes toward C=O bond hydrogenation. The role of benzamide is suggested to extend the lifetime of the dihydridic iridium intermediate, which is prone to undergo irreversible trimerization, deactivating the catalyst. This unique reactivity is then coupled with C=C bond hydrogenation for the facile installation of two contiguous stereogenic centers in high yield and stereoselectivity (up to 99% ee, 99/1 d.r.) resulting in a highly stereoselective reduction of enones.

Cobalt-Catalyzed Asymmetric 1,4-Hydroboration of Enones with HBpin

Herein, a series of new 8-OIQ cobalt complexes were synthesized and used for cobalt-catalyzed chemo- and enantioselective 1,4-hydroboration of enones with HBpin to access chiral β,β-disubstituted ketones with good to excellent chemo- and enantioselectivties. This protocol is operationally simple and shows a broad substrate scope.

Highly Enantioselective Iridium-Catalyzed Hydrogenation of Conjugated Trisubstituted Enones

Asymmetric hydrogenation of conjugated enones is one of the most efficient and straightforward methods to prepare optically active ketones. In this study, chiral bidentate Ir–N,P complexes were utilized to access these scaffolds for ketones bearing the stereogenic center at both the α- and β-positions. Excellent enantiomeric excesses, of up to 99%, were obtained, accompanied with good to high isolated yields. Challenging dialkyl substituted substrates, which are difficult to hydrogenate with satisfactory chiral induction, were hydrogenated in a highly enantioselective fashion.

Catalytic Asymmetric Transfer Hydrogenation of trans-Chalcone Derivatives Using BINOL-derived Boro-phosphates

Chiral phosphoric-acid-catalyzed asymmetric reductions of trans-chalcones have been investigated in this work. A BINOL-derived boro-phosphate-catalyzed asymmetric transfer hydrogenation of the carbon-carbon double bond of trans-chalcone derivatives employing borane as a hydride source was realized. This methodology provides a convenient procedure to access chiral dihydrochalone derivatives in high yields and with high enantioselectivities under mild conditions.

Catalytic Reductive Aldol and Mannich Reactions of Enone, Acrylate, and Vinyl Heteroaromatic Pronucleophiles

Catalytic reductive coupling of enone, acrylate, or vinyl heteroaromatic pronucleophiles with carbonyl or imine partners offers an alternative to base-mediated enolization in aldol- and Mannich-type reactions. In this review, direct catalytic reductive aldol and Mannich reactions are exhaustively catalogued on the basis of metal or organocatalyst. Stepwise processes involving enone conjugate reduction to form discrete enol or (metallo)enolate derivatives followed by introduction of carbonyl or imine electrophiles and aldol reactions initiated via enone conjugate addition are not covered.

Enantioconvergent and enantiodivergent catalytic hydrogenation of isomeric olefins

The asymmetric catalytic hydrogenation of olefins is one of the most widely studied and utilised transformations in asymmetric synthesis. This straightforward and atom-economical strategy can provide excellent enantioselectivity for a broad variety of substrates and is widely relevant for both industrial applications and academic research. In many instances the hydrogenation is stereospecific in the regard that the E-Z-geometry of the olefin governs the stereochemistry of the hydrogenation, producing an enantiodivergent outcome. Interestingly, the possibility to hydrogenate E- and Z-isomer mixtures to a single stereoisomer in an enantioconvergent manner has been reported. This avoids the need for synthesis of geometrically pure alkene starting materials and therefore constitutes a significant practical advantage. This review article aims to provide an overview of the different stereochemical outcomes in the hydrogenation of olefins. Although the field is well developed and selectivity models have been proposed for a number of catalytic systems, an organized collection of enantioconvergent results, as opposed to the more common enantiodivergent case, might promote new investigation into these phenomena.

Lewis Base-Catalyzed Enantioselective Conjugate Reduction of β,β-Disubstituted α,β-Unsaturated Ketones with Trichlorosilane: E/Z-Isomerization, Regioselectivity, and Synthetic Applications

The chiral bisphosphine dioxide-catalyzed asymmetric conjugate reduction of acyclic β,β-disubstituted α,β-unsaturated ketones with trichlorosilane affords saturated ketones having a stereogenic carbon center at the carbonyl β-position with high enantioselectivities. Because the E/Z-isomerizations of enone substrates occur concomitantly, reduction products with the same absolute configurations are obtained from either (E)- or (Z)-enones. Conjugate reduction is accelerated in the presence of an electron-rich aryl group at the β-position of the enone owing to its carbocation-stabilizing ability. Computational studies were also conducted in order to elucidate the origin of the observed enantioselectivity. The regio- and enantioselective reductions of dienones were realized and applied to the syntheses of ar-turmerone, turmeronol A, mutisianthol, and jungianol, which are optically active sesquiterpenes.

Giving a Second Chance to Ir/Sulfoximine-Based Catalysts for the Asymmetric Hydrogenation of Olefins Containing Poorly Coordinative Groups

This work identifies a family of Ir/phosphite-sulfoximine catalysts that has been successfully used in the asymmetric hydrogenation of olefins with poorly coordinative or noncoordinative groups. In comparison with analogue Ir/phosphine-sulfoximine catalysts previously reported, the presence of a phosphite group extended the range of olefins than can be efficiently hydrogenated. High enantioselectivities, comparable to the best ones reported, have been achieved for a wide range of olefins containing relevant poorly coordinative groups such as α,β-unsaturated enones, esters, lactones, and lactams as well as alkenylboronic esters.

Enantioselective conjugate hydrosilylation of α,β-unsaturated ketones

Enantioselective conjugate hydrosilylation of β,β-disubstituted α,β-unsaturated ketones was realized. In the presence of a chiral picolinamide-sulfonate Lewis base catalyst, the reactions provided various chiral ketones bearing a chiral center at the β-position in up to quantitative yields with moderate enantioselectivities.

Chemo- and Enantioselective Catalytic Hydrogenation of α,β-Unsaturated Ketones and Aldehydes as a Tool to Introduce Chiral Centers at α- or β-Positions of Ketones

Catalytic enantioselective hydrogenations of acyclic and cyclic α,β-unsaturated ketones and aldehydes are useful protocols for introduction of chiral centers at α- and/or β-positions of ketones especially in process scale reactions, which are reviewed in addition to conjugate and organocatalytic transfer reductions from the literatures since 2000.

Double Asymmetric Hydrogenation of Linear β,β-Disubstituted α,β-Unsaturated Ketones into γ-Substituted Secondary Alcohols using a Dual Catalytic System

Double asymmetric hydrogenation of linear β,β-disubstituted α,β-unsaturated ketones catalyzed by the DM-SEGPHOS/DMAPEN/Ru^II complex with t-C₄ H₉ OK afforded the γ-substituted secondary alcohols in high diastereo- and enantioselectivities. Some mechanistic experiments suggested that two different reactive species, type (I) and (II), were reversibly formed in this catalytic system: Type (I) with the diamine ligand DMAPEN enantioselectively hydrogenated the enones into the chiral allylic alcohols, and type (II) without the diamine ligand diastereoselectively hydrogenated the allylic alcohols into the γ-substituted secondary alcohols. This dual catalysis protocol was successfully applied to the reaction of a variety of aliphatic- and aromatic-substituted enone substrates.

Chiral Organosulfur Ligands/Catalysts with a Stereogenic Sulfur Atom: Applications in Asymmetric Synthesis

Asymmetric synthesis, in which chiral organocatalysts or metal complexes with chiral ligands are used, has become the most valuable methodology for the preparation of enantiomerically pure organic compounds. Among such catalysts/ligands, a growing number constitute various organosulfur compounds. This Review provides comprehensive and critical information on the plethora of sulfur-based chiral ligands and organocatalysts used in asymmetric synthesis, which have been published within the last 15 years. However, it is confined to the presentation of only those chiral catalysts/ligands that possess a stereogenic sulfur atom and includes sulfoxides, sulfinamides, N-sulfinyl ureas, sulfoximines, and some related S-chiral derivatives.

Recent advances in chiral imino-containing ligands for metal-catalyzed asymmetric transformations

In this review, the recent applications of a variety of chiral imino-containing ligands classified by different types of metal-catalyzed asymmetric reactions are summarized. The progress made in this area would encourage us to design and synthesize more novel chiral imino-containing ligands, and explore their applications in metal-catalyzed asymmetric transformations.

Highly efficient synthesis of chiral aromatic ketones via Rh-catalyzed asymmetric hydrogenation of β,β-disubstituted enones

A series of chiral aromatic ketones were achieved with excellent enantioselectivities via Rh–ZhaoPhos catalyzed asymmetric hydrogenation of β,β-disubstituted enones.

Tandem hydrogenation and condensation of fluorinated α,β-unsaturated ketones with primary amines, catalyzed by nickel

A simple homogeneous catalytic system based on nickel phosphine complexes has been developed for the transfer hydrogenation and condensation of α,β-unsaturated ketones to yield saturated ones and saturated imines using primary amines as hydrogen donors.

Improved biocatalysts from a synthetic circular permutation library of the flavin-dependent oxidoreductase old yellow enzyme

Members of the old yellow enzyme (OYE) family are widely used, effective biocatalysts for the stereoselective trans-hydrogenation of activated alkenes. To further expand their substrate scope and improve catalytic performance, we have applied a protein engineering strategy called circular permutation (CP) to enhance the function of OYE1 from Saccharomyces pastorianus. CP can influence a biocatalyst's function by altering protein backbone flexibility and active site accessibility, both critical performance features because the catalytic cycle for OYE1 is thought to involve rate-limiting conformational changes. To explore the impact of CP throughout the OYE1 protein sequence, we implemented a highly efficient approach for cell-free cpOYE library preparation by combining whole-gene synthesis with in vitro transcription/translation. The versatility of such an ex vivo system was further demonstrated by the rapid and reliable functional evaluation of library members under variable environmental conditions with three reference substrates ketoisophorone, cinnamaldehyde, and (S)-carvone. Library analysis identified over 70 functional OYE1 variants with several biocatalysts exhibiting over an order of magnitude improved catalytic activity. Although catalytic gains of individual cpOYE library members vary by substrate, the locations of new protein termini in functional variants for all tested substates fall within the same four distinct loop/lid regions near the active site. Our findings demonstrate the importance of these structural elements in enzyme function and support the hypothesis of conformational flexibility as a limiting factor for catalysis in wild type OYE.

Synthesis of chiral sulfoximine-based thioureas and their application in asymmetric organocatalysis

For the first time, chiral sulfoximine derivatives have been applied as asymmetric organocatalysts. In combination with a thiourea-type backbone the sulfonimidoyl moiety leads to organocatalysts showing good reactivity in the catalytic desymmetrization of a cyclic meso-anhydride and moderate enantioselectivity in the catalytic asymmetric Biginelli reaction. Straightforward synthetic routes provide the newly designed thiourea-sulfoximine catalysts in high overall yields without affecting the stereohomogeneity of the sulfur-containing core fragment.