Rhodium-Catalyzed Asymmetric Conjugate Alkynylation of β,γ-Unsaturated α-Ketoesters

Recent advances in catalytic enantioselective carbometallation of cyclopropenes and cyclobutenes

Enantioenriched small carbocycles are key structures in numerous natural products and pharmaceutically important molecules as well as vital intermediates in organic synthesis. Although various catalytic approaches for the construction of such molecules from acyclic precursors have been developed, direct enantioselective functionalization of preformed three-membered and four-membered rings represents the most straightforward and modular strategy, enabling rapid and diversified synthesis of enantioenriched cyclopropanes and cyclobutanes from a single set of starting materials without the need for the incorporation of specific functional groups. In this Feature Article, we have summarized the recent advances in catalytic enantioselective functionalization of cyclopropenes and cyclobutenes through carbometallation. The plausible mechanisms of such reactions and future of this field are also discussed.

Direct observation of β-alkynyl eliminations from unstrained propargylic alkoxide Cu(i) complexes by C-C bond cleavage

β-Carbon eliminations of aryl, allylic, and propargylic alkoxides of Rh(i), Pd(ii), and Cu(i) are key elementary reactions in the proposed mechanisms of homogeneously catalysed cross-coupling, group transfer, and annulation. Besides the handful of studies with isolable Rh(i)-alkoxides, β-carbon eliminations of Pd(ii)- and Cu(i)-alkoxides are less definitive. Herein, we provide a comprehensive synthetic, structural, and mechanistic study on the β-alkynyl eliminations of isolable secondary and tertiary propargylic alkoxide Cu(i) complexes, LCuOC(H)(Ph)C[triple bond, length as m-dash]CPh and LCuOC(ArF)2C[triple bond, length as m-dash]CPh (L = N-heterocyclic carbene (NHC), dppf, S-BINAP), to produce monomeric (NHC)CuC[triple bond, length as m-dash]CPh, dimeric [(diphosphine)CuC[triple bond, length as m-dash]CPh]2, and the corresponding carbonyl. Selective β-alkynyl over β-hydrogen elimination was observed for NHC- and diphosphine-supported secondary propargylic alkoxide complexes. The mechanism for the first-order reaction of β-carbon elimination of (IPr*Me)CuOC(ArF)2C[triple bond, length as m-dash]CPh is proposed to occur through an organized four-centred transition state via a Cu-alkyne π complex based on Eyring analysis of variable-temperature reaction rates by UV-vis kinetic analysis to provide ΔH ‡ = 24(1) kcal mol-1, ΔS ‡ = -8(3) e.u., and ΔG ‡ (25 °C) = 27 kcal mol-1 over a temperature range of 60-100 °C. Additional quantitative UV-vis kinetic studies conclude that the electronic and steric properties of the NHC ligands engendered a marginal effect on the elimination rate, requiring 2-3 h at 100 °C for completion, whereas complete β-alkynyl eliminations of diphosphine-supported propargylic alkoxides were observed in 1-2 h at 25 °C.

Switchable Enantioselectivity in Conjugate Alkyne Addition of β,γ-Unsaturated α-Keto Esters by Asymmetric Binary Acid Catalysis

Switchable enantioselectivity was uncovered in the enantioselective catalytic conjugate addition of β,γ-unsaturated α-keto esters with terminal alkynes to the chiral Lewis acid complex of In(BF4)3 and chiral phosphoric acid.

Cobalt-Catalyzed Diastereo- and Enantioselective Carbon-Carbon Bond Forming Reactions of Cyclobutenes

Catalytic enantioselective functionalization of cyclobutenes constitutes a general and modular strategy for construction of enantioenriched complex cyclobutanes bearing multiple stereogenic centers, as chiral four-membered rings are common motifs in biologically active molecules and versatile intermediates in organic synthesis. However, enantioselective synthesis of cyclobutanes through such a strategy remained significantly limited. Herein, we report a series of unprecedented cobalt-catalyzed carbon-carbon bond forming reactions of cyclobutenes that are initiated through enantioselective carbometalation. The protocols feature diastereo- and enantioselective introduction of allyl, alkynyl, and functionalized alkyl groups. Mechanistic studies indicated an unusual 1,3-cobalt migration and subsequent β-carbon elimination cascade process occurred in the allyl addition. These new discoveries established a new elementary process for cobalt catalysis and an extension of diversity of nucleophiles for enantioselective transformations of cyclobutenes.

Alkynyl Borrowing: Silver-Catalyzed Amination of Secondary Propargylic Alcohols via C(sp3)-C(sp) Bond Cleavage

The silver-catalyzed alkynyl borrowing amination of secondary propargyl alcohols via C(sp³)-C(sp) bond cleavage has been developed. This new strategy was based on the β-alkynyl elimination of propargyl alcohols and alkynyl as the borrowing subject. This alkynyl borrowing amination featured high atom economy, wide functional group tolerance, and high efficiency.

Palladium-catalyzed intermolecular alkynylcarbonylation of unactivated alkenes: easy access to β-alkynylcarboxylic esters

A palladium-catalyzed intermolecular alkynylcarbonylation of unactivated alkenes has been established with ethynyl benziodoxolones (EBXs) as alkynylation reagents, providing β-alkynylcarboxylic esters efficiently from simple alkenes. The reaction features moderate to excellent regioselectivity and excellent functional group compatibility under mild reaction conditions.

Base-promoted [4 + 2] annulation of pyrrole-2-carbaldehyde derivatives with β,γ-unsaturated α-ketoesters: syntheses of 5,6-dihydroindolizines

A base-promoted [4 + 2] annulation of pyrrole-2-carbaldehyde derivatives with β,γ-unsaturated α-ketoesters for the syntheses of multisubstituted 5,6-dihydroindolizines was developed.

Iridium-Catalyzed Regioselective Hydroalkynylation of Internal Alkenes Directed by an Oxime

We report here an iridium-catalyzed hydroalkynylation of allylic alcohols protected by an oxime group. Catalytic alkynylation occurs exclusively at the distal position of the alkene. This method generates γ-alkynyl alcohol oximes directly from internal alkenes and terminal alkynes. The oxime group can be readily removed to afford a free alcohol, thus providing an indirect route for the catalytic hydroalkynylation of allylic alcohols.

Organocatalyzed Enantioselective Conjugate Addition of Boronic Acids to β,γ-Unsaturated α-Ketoesters

We report herein the (R)-3,3'-Br₂-BINOL-catalyzed enantioselective conjugate addition of organic boronic acids to β,γ-unsaturated α-ketoesters to generate the corresponding Michael addition products in moderate to high yields and with moderate to excellent enantioselectivities (up to 99% ee). This catalytic system features characteristics of mild reaction conditions, high efficiency, and tolerance to alkenylboronic acids and heteroarylboronic acids.

Enantioselective Synthesis of Spiro-oxiranes: An Asymmetric Addition/Aldol/Spirocyclization Domino Cascade

Enantioenriched spiro-oxiranes bearing three contiguous stereocenters were synthesized using a rhodium-catalyzed asymmetric addition/aldol/spirocyclization sequence. Starting from a linear substrate, the cascade enabled the formation of a spirocyclic framework in a single step. sp² - and sp-hybridized carbon nucleophiles were found to be competent initiators for this cascade, giving arylated or alkynylated products, respectively. Derivatization studies demonstrated the synthetic versatility of both the epoxide and the alkyne moieties of the products. DFT calculations were used to reconcile spectroscopic discrepancies observed between the solution- and solid-state structures of the products.

Rhodium-catalyzed annulative coupling of N-aryl-2-aminopyridine and propargylic amine via selective C–C and C–H bond activation

A Rh(iii)-catalyzed/Cu(ii)-mediated cascade reaction between N-aryl-2-aminopyridine and propargylic amine has been developed.

Rhodium-Catalyzed Pyridine N-Oxide Assisted Suzuki-Miyaura Coupling Reaction via C(O)-C Bond Activation

A rhodium-catalyzed Suzuki-Miyaura coupling reaction via C(O)-C bond activation to form 2-benzoylpyridine N-oxide derivatives is reported. Both the C(O)-C(sp²) and C(O)-C(sp³) bond could be activated during the reaction with yields up to 92%. The N-oxide moiety could be employed as a traceless directing group, leading to free pyridine ketones.

Hydroalkynylative cyclization of 1,6-enynes with terminal alkynes

A rhodium-catalyzed highly regio- and enantioselective hydroalkynylation generating cis-hydrobenzofuranone-tethered enynes has been developed. The reaction proceeds with a dynamic kinetic head-to-head insertion and symmetry breaking Michael addition cascade.

A rhodium-catalyzed highly regio- and enantioselective hydroalkynylation, generating cis-hydrobenzofuranone-tethered enynes has been developed. The reaction proceeds with a selective head-to-head insertion and symmetry breaking Michael addition cascade. One product was produced from tens of possible isomers through precise control of chemo-, regio-, and stereoselectivities using a single rhodium catalyst. Notable features of this method include 100% atom-economy, mild reaction conditions and a very broad substrate scope.

Construction of Acyclic Quaternary Carbon Stereocenters by Catalytic Asymmetric Hydroalkynylation of Unactivated Alkenes

Quaternary carbon stereocenters are common structural motifs in organic synthesis. The construction of these stereocenters in a catalytic and enantioselective manner remains a prominent synthetic challenge. In particular, methods for the synthesis of alkyne-substituted quaternary carbon stereocenters are very rare. Previous catalytic systems for hydroalkynylation of alkenes create tertiary stereocenters. We describe here an iridium catalyzed asymmetric hydroalkynylation of nonactivated trisubstituted alkene. The hydroalkynylation of β,γ-unsaturated amides occurs with high regio- and enantioselectivities to afford alkyne-substituted acyclic quaternary carbon stereocenters. Computational and experimental data suggest that the enantioselectivity is not only determined by the facial selectivity of the alkene but also by an alkene isomerization process. This strategy provides an efficient method to access alkyne-substituted acyclic quaternary carbon stereocenters with minimally functionalized starting materials.

Recent Advances in Metal-Catalyzed Asymmetric 1,4-Conjugate Addition (ACA) of Nonorganometallic Nucleophiles

The metal-catalyzed asymmetric conjugate addition (ACA) reaction has emerged as a general and powerful approach for the construction of optically active compounds and is among the most significant and useful reactions in synthetic organic chemistry. In recent years, great progress has been made in this area with the use of various chiral metal complexes based on different chiral ligands. This review provides comprehensive and critical information on the enantioselective 1,4-conjugate addition of nonorganometallic (soft) nucleophiles and their importance in synthetic applications. The literature is covered from the last 10 years, and a number of examples from before 2007 are included as background information. The review is divided into multiple parts according to the type of nucleophile involved in the reaction (such as C-, B-, O-, N-, S-, P-, and Si-centered nucleophiles) and metal catalyst systems used.

Cu-Catalyzed Oxyalkynylation and Aminoalkynylation of Unactivated Alkenes: Synthesis of Alkynyl-Featured Isoxazolines and Cyclic Nitrones

A convenient and efficient vicinal oxyalkynylation/aminoalkynylation of internal unactivated alkenes is achieved by means of a Cu-catalyzed radical cascade reaction of unsaturated ketoximes with ethynylbenziodoxolone (EBX) reagents. This protocol enables the synthesis of structurally valuable isoxazolines or cyclic nitrones and the introduction of an important alkyne group in a single operation. The reaction is characterized by a broad substrate scope for both unsaturated ketoximes and alkynylation reagents and a low catalyst loading.

Iridium-catalysed conjugated alkynylation of α,β-unsaturated amide through alkene isomerization

We have developed an iridium-catalysed conjugated alkynylation of α,β-unsaturated amide.

Catalytic activations of unstrained C–C bond involving organometallic intermediates

In this review, the transition-metal-catalyzed C–C bond activation of relatively stable and unstrained molecules involving organometallic intermediates is discussed in detail.