α-Quaternary Chiral Aldehydes from Styrenes, Allylic Alcohols, and Syngas via Multi-catalyst Relay Catalysis

Palladium/Amine Dual-Catalyzed Tsuji-Trost Fluoroallylation of Aldehydes with gem-Difluorinated Cyclopropanes

We herein disclose the Pd/amine dual-catalyzed ring-opening cross-coupling reaction between gem-difluorinated cyclopropanes (gem-F2CPs) with aldehydes, which enables the diversity-oriented synthesis (DOS) of 2-fluoroallylic aldehydes bearing all-carbon quaternary centers with features of broad scope and excellent functional group tolerance. The synthetic value of this Tsuji-Trost system was further demonstrated by late-stage functionalization of natural product-derived gem-F2CPs and the diverse synthesis of various fluoroallylic aldehyde derivatives, including alcohol, alkyne, alkene, and amine.

Visible-Light-Mediated [2+2] Photocycloadditions of Alkynes

Visible-light-mediated [2+2] photocycloaddition reaction can be considered an ideal solution due to its green and sustainable properties, and is one of the most efficient methods to synthesize four-membered ring motifs. Although research on the [2+2] photocycloaddition of alkynes is challenging because of the diminished reactivity of alkynes, and the more significant ring strain of the products, remarkable achievements have been made in this field. In this article, we highlight the recent advances in visible-light-mediated [2+2] photocycloaddition reactions of alkynes, with focus on the reaction mechanism and the late-stage synthetic applications. Advances in obtaining cyclobutenes, azetines, and oxetene active intermediates continue to be breakthroughs in this fascinating field of research.

Biomimetic Catalytic Retro-Aldol Reaction Using a Cation-Binding Catalyst: A Promising Route to Axially Chiral Biaryl Aldehydes

Here we describe a biomimetic catalytic retro-aldol reaction of racemic α-substituted β-hydroxy ketones utilizing a chiral oligoEG cation-binding catalyst as a type-II aldolase mimic. Our investigation of various aldol substrates has demonstrated that our biomimetic retro-aldol protocol enables rapid access to highly enantiomerically enriched aldols with a selectivity factor (s) of up to 70. Additionally, we have demonstrated the synthetic strategy's feasibility for accessing diverse and valuable axially chiral aldehydes.

Palladium-Catalyzed Defluorinative Coupling of Difluoroalkenes and Aryl Boronic Acids for Ketone Synthesis

The transition-metal-catalyzed carbonylation reaction is a useful approach for ketone synthesis. However, it is often problematic to use exogenous carbonyl reagents, such as gaseous carbon monoxide. In this manuscript, we report a novel palladium-catalyzed coupling reaction of gem-difluoroalkenes and aryl boronic acids that yields bioactive indane-type ketones with an all-carbon α-quaternary center. Characterization and stoichiometric reactions of the key intermediates RCF₂ Pd^II support a water-induced defluorination and cross-coupling cascade mechanism. The vinyl difluoromethylene motif serves as an in situ carbonyl precursor which is unprecedented in transition-metal-catalyzed coupling reactions. It is expected to raise broad research interest from the perspectives of ketone synthesis, fluoroalkene functionalization, and rational design of new synthetic protocols based on the unique reactivity of difluoroalkyl palladium(II) species.

Multicomponent Assembly of Complex Oxindoles by Enantioselective Cooperative Catalysis

Chiral oxindoles are important chemical scaffolds found in many natural products, and their enantioselective synthesis thus attracts considerable attention. Highly diastereo- and enantioselective synthetic methods for constructing C3 quaternary oxindoles have been well-developed. However, the efficient synthesis of chiral 3-substituted tertiary oxindoles has been rarely reported due to the ease of racemization of the tertiary stereocenter via enolization. Therefore, we herein report on the multicomponent assembly (from N-aryl diazoamides, aldehydes, and enamines/indoles) of complex oxindoles by enantioselective cooperative catalysis. These reactions proceed under mild conditions and show broad substrate scope, affording the desired coupling products (>90 examples) with good to excellent stereocontrol. Additionally, this research also demonstrates the synthetic potential of this annulation by constructing the 6,6,5-tricyclic lactone core structure of Speradine A.

Desymmetric Partial Reduction of Malonic Esters

Desymmetrization of easily available disubstituted malonic esters is a rewarding strategy to access structurally diverse quaternary stereocenters. Particularly, asymmetric reduction of malonic esters would generate a functional group with a lower oxidation state than the remaining ester, thus allowing for more chemoselective derivatization. Here, we report a new set of conditions for the zinc-catalyzed desymmetric hydrosilylation of malonic esters that afford aldehydes as the major product. Compared with alcohol-selective desymmetrization, the partial reduction uses a higher concentration of silanes and new pipecolinol-derived tetradentate ligands, proposedly to switch the pathway of zinc hemiacetal intermediates from elimination to silylation. As a result, high aldehyde-to-alcohol ratios and enantioselectivity of aldehydes are obtained from malonic esters with a large collection of substituents. Together with the abundant reactivity of aldehydes, the partial reduction has enabled an expeditious synthesis of bioactive compounds and natural metabolites containing a quaternary stereocenter.

Employing Visible-Light Photoredox Catalysis in Multicomponent-Multicatalyst Reactions: One-Pot Synthesis of Spiroquinazolin-2-(thi)ones

The multicomponent-multicatalyst reaction ((MC)²R) and visible-light catalysis have emerged as green and powerful strategies for achieving ideal syntheses. Here, we report the first example of a visible-light-induced approach toward spiroquinazolin(thi)ones. This (MC)²R features an eco-friendly energy source and solvent, metal-free catalysts, step- and atom-economy, a relay catalysis strategy, air as green oxidant, mild conditions, and easily accessible starting materials.

Enantioselective Ni/N-Heterocyclic Carbene-Catalyzed Redox-Economical Coupling of Aldehydes, Alkynes, and Enones for Rapid Construction of Acyclic All-Carbon Quaternary Stereocenters

Acyclic quaternary carbon stereocenters exist widely in natural products and bioactive molecules, but their enantioselective construction remains a prominent challenge. In particular, multicomponent enantioselective couplings of simple precursors to acyclic all-carbon quaternary stereocenters are very rare. We describe herein an N-heterocyclic carbene (NHC)-Ni catalyzed redox-economical three-component reaction of aldehydes, alkynes, and enones that proceeds in a highly chemo-, regio-, and enantioselective manner. A wide variety of valuable acyclic α-quaternary chiral ketones were synthesized in a single step with 100% atom economy. This reaction proceeds through the formation of a transient cyclic enolate followed by an aldol reaction/ring-opening sequence. The strategy is expected to inspire new and efficient approaches to generate other acyclic quaternary stereocenters.

Combining Palladium and Chiral Organocatalysis for the Enantioselective Deconjugative Allylation of Enals via Dienamine Intermediates

A catalytic enantioselective deconjugative allylation of enals is reported. A variety of enals underwent this transformation in high yield and ee, and products can be readily transformed into γ-allyl enals via a Cope rearrangement without erosion of ee. This transformation was used to install the quaternary stereocenter in (S)-bakuchiol, enabling completion of a concise formal synthesis.

Cobalt-Catalyzed Desymmetric Isomerization of Exocyclic Olefins

Chiral cyclic olefins, 1-methylcyclohexenes, are versatile building blocks for the synthesis of pharmaceuticals and natural products. Despite the prevalence of these structural motifs, the development of efficient synthetic methods remains an unmet challenge. Herein we report a novel desymmetric isomerization of exocyclic olefins using a series of newly designed chiral cobalt catalysts, which enables a straightforward construction of chiral 1-methylcyclohexenes with diversified functionalities. The synthetic utility of this methodology is highlighted by a concise and enantioselective synthesis of a natural product, β-bisabolene. The versatility of the reaction products is further demonstrated by multifarious derivatizations.

Ternary Catalysis Enabled Three-Component Asymmetric Allylic Alkylation as a Concise Track to Chiral α,α-Disubstituted Ketones

Multicomponent reactions that involve interception of onium ylides through Aldol, Mannich, and Michael addition with corresponding bench-stable acceptors have demonstrated broad applications in synthetic chemistry. However, because of the high reactivity and transient survival of these in situ generated intermediates, the substitution-type interception process, especially the asymmetric catalytic version, remains hitherto unknown. Herein, a three-component asymmetric allylation of α-diazo carbonyl compounds with alcohols and allyl carbonates is disclosed by employing a ternary cooperative catalysis of achiral Pd-complex, Rh₂(OAc)₄, and chiral phosphoric acid CPA. This method represents the first example of three-component asymmetric allylic alkylation through an S_N1-type trapping process, which involves a convergent assembly of two active intermediates, Pd-allyl species, and enol derived from onium ylides, providing an expeditious access to chiral α,α-disubstituted ketones in good to high yields with high to excellent enantioselectivity. Combined experimental and computational studies have shed light on the mechanism of this novel three-component reaction, including the critical role of Xantphos ligand and the origin of enantioselectivity.

Asymmetric construction of acyclic quaternary stereocenters via direct enantioselective additions of α-alkynyl ketones to allenamides

Acyclic quaternary stereocenters are widely present in a series of biologically active natural products and pharmaceuticals. However, development of highly efficient asymmetric catalytic methods for the construction of these privileged motifs represents a longstanding challenge in organic synthesis. Herein, an efficient chiral phosphoric acid catalyzed direct asymmetric addition of α-alkynyl acyclic ketones with allenamides has been developed, furnishing the acyclic all-carbon quaternary stereocenters with excellent regioselectivities and high enantioselectivities. Extensive and detailed experimental mechanistic studies were performed to investigate the mechanism of this reaction. Despite a novel covalent allyl phosphate intermediate was found in these reactions, further studies indicated that a SN2-type mechanism with the ketone nucleophiles is not very possible. Instead, a more plausible mechanism involving the elimination of the allyl phosphate to give the α,β-unsaturated iminium intermediate, which underwent the asymmetric conjugate addition with the enol form of ketone nucleophiles under chiral anion catalysis, was proposed. In virtue of the fruitful functional groups bearing in the chiral products, the diverse derivatizations of the chiral products provided access to a wide array of chiral scaffolds with quaternary stereocenters.

Practical Synthesis of (Z)-α,β-Unsaturated Nitriles via a One-Pot Sequential Hydroformylation/Knoevenagel Reaction

Herein, the synthesis of (Z)-α,β-unsaturated nitriles by a sequential hydroformylation/Knoevenagel reaction has been first developed. A variety of crude α-olefins from Fischer-Tropsch synthesis, internal and special olefins, as well as alkynes could be transformed into value-added alkenyl nitriles (39 examples) up to 90% yield. Remarkably, compared with commonly used tedious multistep reactions, the one-pot protocol features cheap and easily available raw materials, excellent chemo-, regio-, and stereoselectivity, very mild reaction conditions, and easy scale-up production.

Synergistic Pd/Amine-Catalyzed Stereodivergent Hydroalkylation of 1,3-Dienes with Aldehydes: Reaction Development, Mechanism, and Stereochemical Origins

Metal-hydride-catalyzed hydroalkylation of 1,3-dienes with enolizable carbonyl compounds is an atom- and step-economical method for preparing chiral molecules with allylic stereocenters. Although high diastereo- and enantioselectivities have been achieved for many coupling partners, aldehydes have not yet been used for this purpose because they are less stable than other carbonyl compounds under basic conditions and they have the potential to rapidly epimerize at the α-position. Moreover, stereodivergent hydroalkylation reactions of 1,3-dienes to access complementary diastereomers with vicinal stereocenters is challenging. Herein, we describe a synergistic palladium/amine catalyst system that allowed us to achieve the first stereodivergent hydroalkylation reactions of 1,3-dienes with aldehydes. By choosing an appropriate combination of chiral palladium and amine catalysts, we could obtain either syn or anti coupling products, and this method therefore provides highly diastereo- and enantioselective access to complementary diastereomers of chiral aldehydes with α,β-vicinal stereocenters. Density functional theory calculations revealed a mechanism involving PdH formation and migratory insertion into the alkene, followed by C-C bond formation. The origin of the stereoselectivities was investigated by means of distortion/interaction analysis.

Pd/Cu Dual-Catalyzed Asymmetric Synthesis of Highly Functional All-Carbon Quaternary Stereocenters from Vinyl Carbonates

The challenging metal-catalyzed asymmetric synthesis of highly functional quaternary carbon centers using decarboxylative C(sp³ )-C(sp³ ) bond formation reactions is reported. The key substrate, a vinyl cyclic carbonate, is activated to provide concomitantly both the requisite nucleophile (by formal umpolung) and electrophile reaction partner preceding the asymmetric cross-coupling process. A wide screening of reaction conditions, additives and catalyst precursors afforded a protocol that gave access to a series of compounds featuring densely functionalized, elusive quaternary carbon stereocenters in appreciable yield and with enantiomeric ratios (er's) of up to 90 : 10.

Catalytic Asymmetric Amino Acid and Its Derivatives by Chiral Aldehyde Catalysis

Amine acid transformation is an important chemical process in biological systems. As a well-developed and acknowledged tool, chiral aldehyde catalysis provides good catalytic activation and stereoselective control abilities in the asymmetric reaction of N-unprotected amino acid esters and amino acid esters analogs, in which the key to success is the design of the catalysts derived from chiral BINOL aldehyde, which is based on the face control of enolate intermediates. In this review, one of the co-catalytic systems that combined with a transition metal to form a multiplex catalytic system and the well-established multiplex stereocenters of chiral aldehyde catalysis have been reviewed. Finally, a novel organocatalysis is prospected.

Visible-Light-Induced Trifluoromethylation of Allylic Alcohols

An organic photoredox-catalyzed dehydroxylative trifluoromethylation of allylic alcohols was developed in an environmentally benign manner. In this reaction, the readily available CF₃SO₂Na was selected as the trifluoromethylation reagent. The in situ generated byproduct SO₂ was reutilized to activate C-OH bond, which enabled this dehydroxylative trifluoromethylation to be performed conveniently. A variety of multifunctionalized CF₃-allylic compounds were obtained in high yields and excellent stereoselectivity.

Modular Synthesis of α-Quaternary Chiral β-Lactams by a Synergistic Copper/Palladium-Catalyzed Multicomponent Reaction

An asymmetric multicomponent, interrupted Kinugasa allylic alkylation (IKAA) reaction has been developed with a synergistic Cu-catalyzed Kinugasa system and a Pd-catalyzed allylic alkylation reaction. This unprecedented reaction provides in high yields and with high stereoselectivity a synthesis of α-quaternary chiral β-lactams, which cannot be produced with existing synthetic methods. Stereoselective coupling of two catalytic amounts of transient organometallic intermediates formed in situ is an important feature of this reaction.

Dehydrative Allylation of Alkenyl sp2 C-H Bonds

We designed a cooperative catalytic system by combining commercially available Ca(NTf₂)PF₆ and Pd(PPh₃)₄ to address the dehydrative allylation of alkenyl sp² C-H bonds in an environmentally benign manner. A novel C-OH bond cleavage method was found to be crucial for this practical protocol. A variety of alkenes and allylic alcohols equipped with wide-spectrum functional groups can be successfully incorporated into the desired cross-coupling, affording 1,4-dienes with moderate to excellent yields and high stereo- and regioselectivity.

Palladium-Catalyzed Markovnikov Hydroaminocarbonylation of 1,1-Disubstituted and 1,1,2-Trisubstituted Alkenes for Formation of Amides with Quaternary Carbon

Hydroaminocarbonylation of alkenes is one of the most promising yet challenging methods for the synthesis of amides. Herein, we reported the development of a novel and effective Pd-catalyzed Markovnikov hydroaminocarbonylation of 1,1-disubstituted or 1,1,2-trisubstituted alkenes with aniline hydrochloride salts to afford amides bearing an α quaternary carbon. The reaction makes use of readily available starting materials, tolerates a wide range of functional groups, and provides a facile and straightforward approach to a diverse array of amides bearing an α quaternary carbon. Mechanistic investigations suggested that the reaction proceeded through a palladium hydride pathway. The hydropalladation and CO insertion are reversible, and the aminolysis is probably the rate-limiting step.

Asymmetric α-Pentadienylation of Aldehydes with Cyclopropylacetylenes

By employing readily available cyclopropylacetylene and its derivatives as the pentadienylation reagent, an asymmetric regioselective asymmetric α-pentadienylation reaction of aldehydes is developed by cooperative catalysis of a chiral Pd(0) catalyst and a chiral Brønsted acid in the presence of a subschoichmetric amount of an achiral amine. α-Pentadienylated aldehydes are afforded with high yields and enantioselectivities as well as excellent E/Z ratios.

Auto-tandem palladium/phosphine cooperative catalysis: synthesis of bicyclo[3.1.0]hexenes by selective activation of Morita–Baylis–Hillman carbonates

A new palladium/phosphine cooperatively catalytic sequential annulation strategy has been developed for the synthesis of bicyclo[3.1.0]hexene derivatives.

Silver/palladium relay catalyzed 1,3-dipole annulation/allylation reactions to access fully substituted allyl imidazolidines

A silver/palladium relay catalyzed 1,3-dipole annulation/allylation reaction of iminoesters and Baylis-Hillman acetates for the construction of fully substituted allyl imidazolidines is reported. The reaction of both iminoesters and Baylis-Hillman acetates affords the fully substituted allyl imidazolidines in high yields and regioselectivities. The three component reaction is triggered by silver-catalyzed 1,3-dipole annulation, followed by the sequential palladium catalyzed allylation. Mechanistic studies reveal that the dual catalytic system plays a key role in the reaction. The developed methodology provides straightforward access to allyl imidazolidines under simple reaction conditions.

Recent applications of chiral phosphoric acids in palladium catalysis

Through the combined action of palladium catalysts and chiral phosphoric acids (CPAs) a variety of catalytic asymmetric reactions have been realized during the past decade, including allylation, alkene functionalization, and C-H activation. This review surveys key examples across these various reaction types and examines the different mechanisms by which CPAs can affect stereoinduction in these reaction systems.