Rhodium-Catalyzed Asymmetric Conjugate Alkynylation/Aldol Cyclization Cascade for the Formation of α-Propargyl-β-hydroxyketones

Rhodium(I)-Catalyzed Asymmetric Cascade Reactions

Rhodium(I)-catalyzed asymmetric cascade reactions have emerged as powerful tools in contemporary organic synthesis, enabling efficient construction of complex molecular architectures. These transformations proceed through organorhodium intermediates, which undergo additions to reactive π-bonds, subsequently triggering cascade reactions with neighboring functional groups to effectively forge multiple carbon-carbon bonds and stereogenic centers in a single step under mild conditions. This article reviews the pioneering developments and recent breakthroughs from 2002 to 2024, highlighting the attractive advantages of rhodium(I)-catalyzed asymmetric cascade reactions and their profound impacts on synthetic organic chemistry.

Palladium-catalyzed carbon-carbon bond cleavage of primary alcohols: decarbonylative coupling of acetylenic aldehydes with haloarenes

In the current work, a palladium-catalyzed C-C bond cleavage reaction of primary alcohols has been developed. This transformation was characterized by a broad substrate scope, superior functional group tolerance, and high efficiency for selective C-C bond cleavage and was then followed by alkynyl-aryl cross coupling. Mechanism studies indicated that the propargyl alcohols underwent β-H elimination to form aldehydes rather than having undergone β-C elimination. The corresponding aldehyde intermediates then proceeded through a decarbonylation and coupling reaction with haloarenes to yield diarylacetylenes.

Stereodivergency in Copper-Catalyzed Borylative Difunctionalizations: The Impact of Boron Coordination

A reagent-controlled diastereodivergent copper-catalyzed borylative difunctionalization is reported. The formation of Lewis adducts that guide selectivity is commonly invoked in organic reaction mechanisms. Using density functional theory calculations, we identified BpinBdan as a sterically similar and less Lewis acidic alternative to B2pin2. Using a newly developed borylative aldol domino reaction as the proof-of-concept, we demonstrate a change in stereochemical outcome by a simple change of borylating reagent-B2pin2 affords the diastereomer associated with coordination control while BpinBdan overturns this mode of binding. We show that this strategy can be generalized to other scaffolds and, more importantly, that BpinBdan does not alter the diastereomeric outcome of the reaction when coordination is not involved. BpinBdan can be viewed as a mechanistic probe for coordination in future copper-catalyzed borylation reactions.

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.

Recent Progress in Asymmetric Domino Intramolecular Cyclization/Cascade Reactions of Substituted Olefins

The domino cyclization/coupling strategy is one of the most effective methods to produce cyclized and multi-functionalized compounds from olefins, which has attracted huge attention from chemists and biochemists especially for its considerable potential of enantiocontrol. Nowadays, more and more studies are developed to achieve difunctionalization of substituted olefins through an asymmetric domino intramolecular cyclization/cascade reaction, which is still an elegant choice to accomplish several synthetic ideas such as complex natural products and drugs. This review surveys the recent advances in this field through reaction type classification. It might serve as useful knowledge desktop for the community and accelerate their research.

Enolates ambushed - asymmetric tandem conjugate addition and subsequent enolate trapping with conventional and less traditional electrophiles

Metal enolates are useful intermediates and building blocks indispensable in many organic synthetic transformations. Chiral metal enolates obtained by asymmetric conjugate additions of organometallic reagents are structurally complex intermediates that can be employed in many transformations. In this review, we describe this burgeoning field that is reaching maturity after more than 25 years of development. The effort of our group to broaden possibilities to engage metal enolates in reactions with new electrophiles is described. The material is divided according to the organometallic reagent employed in the conjugate addition step, and thus to the particular metal enolate formed. Short information on applications in total synthesis is also given.

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.

Enantioselective zinc-mediated conjugate alkynylation of saccharin-derived 1-aza-butadienes

Diethylzinc and a bis(hydroxyl)malonamide ligand allow the first conjugate alkynylation of α,β-unsaturated imines. Excellent enatioselectivities are obtained with aliphatic alkynes.

Copper-catalyzed asymmetric tandem borylative addition and aldol cyclization

A highly enantioselective asymmetric copper-catalyzed tandem conjugate addition/aldol cyclization of electron-deficient olefins with B₂pin₂ was developed, which provided a rapid access to indanes bearing three consecutive chiral stereogenic centers.

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.