Diastereo- and enantioselective hydrogenative aldol coupling of vinyl ketones: design of effective monodentate TADDOL-like phosphonite ligands

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.

NiH-Catalyzed Regio- and Enantioselective Hydroalkylation for the Synthesis of β- or γ-Branched Chiral Aromatic N-Heterocycles

A nickel hydride-catalyzed regio- and enantioselective hydroalkylation reaction was developed to give access to a library of chiral β- or γ-branched aromatic N-heterocycles. This intriguing asymmetric transformation features excellent selectivities, step- and atom-economies, and generating two kinds of chiral products through one synthetic strategy. Furthermore, the possible reaction mechanism was extensively investigated using numerous control experiments and density functional theory calculations.

Rhodium-catalyzed intramolecular reductive aldol-type cyclization: Application for the synthesis of a chiral necic acid lactone

A rhodium-catalyzed intramolecular reductive aldol-type cyclization is described to give β-hydroxylactones with high diastereoselectivities. The stereoselectivity of this cyclization is highly solvent dependent and can give syn- or anti-β-hydroxylactones with high diastereoselectivity. This methodology was also applied to the synthesis of a chiral necic acid lactone which is a structural component of the pyrrolizidine alkaloid monocrotaline.

Enantioselective Synthesis of Secondary β-Trifluoromethyl Alcohols via Catalytic Asymmetric Reductive Trifluoroalkylation and Diastereoselective Reduction

Fluorinated motifs are frequently encountered in drugs and agrochemicals. Incorporating fluorine-containing motifs in drug candidates for lead optimization in pharmaceutical research and development has emerged as a powerful tool. The construction of molecules that feature a trifluoromethyl (CF₃-) group on a stereogenic carbon has accumulated broad research efforts. Unlike its well-explored, biologically active methyl counterpart, asymmetric construction of β-trifluoromethylated alcohols bearing adjacent stereocenters still remains elusive. Through retrosynthetic analysis, we posited that followed by sequential reduction of carbonyl, the initial construction of chiral α-trifluoromethylated ketones could render the desired product in a facile, one-pot fashion. Herein, we developed the first example of nickel-catalyzed asymmtric reductive cross-coupling trifluoroalkylation of acyl chlorides for enantioselective synthesis of diverse α-trifluoromethylated ketones. The one-pot reduction of these α-trifluoromethylated ketones furnished corresponding alcohols bearing β-CF₃-substituted stereogenic carbons with excellent diastereoselectivity and complete enantioselective retention. High yields/enantioselectivity, mild conditions, and good functional group compatibility are shown in the system. Utilities of the method are also illustrated by applying asymmetric, late-stage trifluoroalkylation of biologically active complex molecules, revealing tremendous potential for development of CF₃-containing chiral drugs.

From Hydrogenation to Transfer Hydrogenation to Hydrogen Auto-Transfer in Enantioselective Metal-Catalyzed Carbonyl Reductive Coupling: Past, Present, and Future

Atom-efficient processes that occur via addition, redistribution or removal of hydrogen underlie many large volume industrial processes and pervade all segments of chemical industry. Although carbonyl addition is one of the oldest and most broadly utilized methods for C-C bond formation, the delivery of non-stabilized carbanions to carbonyl compounds has relied on premetalated reagents or metallic/organometallic reductants, which pose issues of safety and challenges vis-à-vis large volume implementation. Catalytic carbonyl reductive couplings promoted via hydrogenation, transfer hydrogenation and hydrogen auto-transfer allow abundant unsaturated hydrocarbons to serve as substitutes to organometallic reagents, enabling C-C bond formation in the absence of stoichiometric metals. This perspective (a) highlights past milestones in catalytic hydrogenation, hydrogen transfer and hydrogen auto-transfer, (b) summarizes current methods for catalytic enantioselective carbonyl reductive couplings, and (c) describes future opportunities based on the patterns of reactivity that animate transformations of this type.

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.

Feedstock Reagents in Metal-Catalyzed Carbonyl Reductive Coupling: Minimizing Preactivation for Efficiency in Target-Oriented Synthesis

Use of abundant feedstock pronucleophiles in catalytic carbonyl reductive coupling enhances efficiency in target-oriented synthesis. For such reactions, equally inexpensive reductants are desired or, ideally, corresponding hydrogen autotransfer processes may be enacted wherein alcohols serve dually as reductant and carbonyl proelectrophile. As described in this Minireview, these concepts allow reactions that traditionally require preformed organometallic reagents to be conducted catalytically in a byproduct-free manner from inexpensive π-unsaturated precursors.

Reductive C-C Coupling from α,β-Unsaturated Nitriles by Intercepting Keteniminates

We present an atom-economic strategy to catalytically generate and intercept nitrile anion equivalents using hydrogen transfer catalysis. Addition of α,β-unsaturated nitriles to a pincer-based Ru-H complex affords structurally characterized κ-N-coordinated keteniminates by selective 1,4-hydride transfer. When generated in situ under catalytic hydrogenation conditions, electrophilic addition to the keteniminate was achieved using anhydrides to provide α-cyanoacetates in high yields. This work represents a new application of hydrogen transfer catalysis using α,β-unsaturated nitriles for reductive C-C coupling reactions.

Rhodium-Catalyzed Aldehyde Arylation via Formate-Mediated Transfer Hydrogenation: Beyond Metallic Reductants in Grignard/Nozaki-Hiyami-Kishi-Type Addition

The first intermolecular carbonyl arylations via transfer hydrogenative reductive coupling are described. Using rhodium catalysts modified by tBu2PMe, sodium formate-mediated reductive coupling of aryl iodides with aldehydes occurs in a chemoselective fashion in the presence of protic functional groups and lower halides. This work expands the emerging paradigm of transfer hydrogenative coupling as an alternative to pre-formed carbanions or metallic reductants in C═X addition.

Lewis-Base-Catalyzed Reductive Aldol Reaction To Access Quaternary Carbons

A synthetic method for the efficient construction of β-hydroxylactones and lactams bearing α-quaternary carbon centers is described. This transformation relies on an electronically differentiated Lewis base catalyst, which is uniquely capable of promoting a reductive aldol reaction of α,α-disubstituted and α,α,β-trisubstituted enones. This approach provides a valuable synthetic alternative for carbon-carbon bond formation in complex molecular settings due to its orthogonal reactivity compared to that of traditional aldol reactions. Based on this method described herein, lactones, lactams, and morpholine amides bearing α-quaternary carbon centers are accessible in yields up to 85% and 50:1 dr.

Catalytic enantioselective OFF ↔ ON activation processes initiated by hydrogen transfer: concepts and challenges

Hydrogen transfer initiated processes are eco-compatible transformations allowing the reversible OFF ↔ ON activation of otherwise unreactive substrates. The minimization of stoichiometric waste as well as the unique activation modes provided by these transformations make them key players for a greener future for organic synthesis. Long limited to catalytic reactions that form racemic products, considerable progress on the development of strategies for controlling diastereo- and enantioselectivity has been made in the last decade. The aim of this review is to present the different strategies that enable enantioselective transformations of this type and to highlight how they can be used to construct key synthetic building blocks in fewer operations with less waste generation.

Catalytic enantioselective aldol reactions

Recent developments in catalytic asymmetric aldol reactions have been summarized.

Reductive C-C Coupling via Hydrogenation and Transfer Hydrogenation: Departure from Stoichiometric Metals in Carbonyl Addition

Metal catalyzed reductive couplings of π-unsaturated reagents with carbonyl compounds via hydrogenation or transfer hydrogenation has emerged as an alternative to the use of stoichiometric organometallic reagents in carbonyl addition.

Total Synthesis of Swinholide A: An Exposition in Hydrogen-Mediated C-C Bond Formation

Diverse hydrogen-mediated C-C couplings enable construction of the actin-binding marine polyketide swinholide A in only 15 steps (longest linear sequence), roughly half the steps required in two prior total syntheses. The redox-economy, chemo- and stereoselectivity embodied by this new class of C-C couplings are shown to evoke a step-change in efficiency.

Metal-catalyzed reductive coupling of olefin-derived nucleophiles: Reinventing carbonyl addition

α-Olefins are the most abundant petrochemical feedstock beyond alkanes, yet their use in commodity chemical manufacture is largely focused on polymerization and hydroformylation. The development of byproduct-free catalytic C-C bond-forming reactions that convert olefins to value-added products remains an important objective. Here, we review catalytic intermolecular reductive couplings of unactivated and activated olefin-derived nucleophiles with carbonyl partners. These processes represent an alternative to the longstanding use of stoichiometric organometallic reagents in carbonyl addition.

Broad Spectrum Enolate Equivalent for Catalytic Chemo-, Diastereo-, and Enantioselective Addition to N-Boc Imines

Alkynyl ketones are attractive but challenging nucleophiles in enolate chemistry. Their susceptibility to other reactions such as Michael additions and the difficulty of controlling the enolate geometry make them difficult substrates. Mannich-type reactions, which previously have not been reported using N-carbamoyl-imines with simple ketone enolates, became our objective. In this report, we describe the first direct catalytic Mannich-type reaction between various ynones and N-Boc imines, whose stereocontrol presumably derives from catalyst control of enolate geometry. This method produces α-substituted β-amino ynones with excellent chemo-, diastereo-, and enantioselectivity. The products can be readily transformed into a broad range of molecular scaffolds upon further one-step transformations, demonstrating the utility of ynones as masked synthetic equivalents for a variety of unsymmetrically substituted acyclic ketones. In particular, alkynyl alkyl ketones resolve the long-standing problem of the inability to use the enolates of unsymmetrical dialkyl ketones lacking α-branching for regio- and stereoselective reactions.

Reductive Cyclization of Halo-Ketones to Form 3-Hydroxy-2-Oxindoles via Palladium Catalyzed Hydrogenation: A Hydrogen-Mediated Grignard Addition

The reductive cyclization of N-oxoacyl ortho-bromoanilides to form 3-hydroxy-2-oxindoles under the conditions of palladium catalyzed hydrogenation is described. This work may be viewed as a prelude to intermolecular hydrogen-mediated Grignard-type reductive couplings of organic halides with carbonyl compounds.

Catalytic enantioselective 1,2-diboration of 1,3-dienes: versatile reagents for stereoselective allylation

More with boron: The development of catalytic enantioselective 1,2-diboration of 1,3-dienes enables a new strategy for enantioselective carbonyl allylation reactions (see scheme). These reactions occur with outstanding levels of stereoselection and can be applied to both monosubstituted and 1,1-disubstituted dienes. The carbonyl allylation reactions provide enantiomerically enriched functionalized homoallylic alcohol products.

Unlocking Hydrogenation for C-C Bond Formation: A Brief Overview of Enantioselective Methods

Hydrogenation of π-unsaturated reactants in the presence of carbonyl compounds or imines promotes reductive C-C coupling, providing a byproduct-free alternative to stoichiometric organometallic reagents in an ever-increasing range of C=X (X = O, NR) additions. Under transfer hydrogenation conditions, hydrogen exchange between alcohols and π-unsaturated reactants triggers generation of electrophile-nucleophile pairs, enabling carbonyl addition directly from the alcohol oxidation level, bypassing discrete alcohol oxidation and generation of stoichiometric byproducts.

Catalytic enantioselective diboration of cyclic dienes. A modified ligand with general utility

The enantioselective 1,4-diboration of cyclic dienes with a new taddol-derived phosphonite ligand occurs with excellent enantioselectivity. Oxidation delivers the derived 1,4-diol, whereas homologation can be used to deliver a chiral 1,6-diol.