Asymmetric Intermolecular Heck Reaction of Propargylic Acetates and Cycloalkenes to Access Fused Cyclobutenes

Recent advances in Pd-catalyzed asymmetric cyclization reactions

Over the past few decades, there have been major developments in transition metal-catalyzed asymmetric cyclization reactions, enabling the convenient access to a wide spectrum of structurally diverse chiral carbo- and hetero-cycles, common skeletons found in fine chemicals, natural products, pharmaceuticals, agrochemicals, and materials. In particular, a plethora of enantioselective cyclization reactions have been promoted by chiral palladium catalysts owing to their outstanding features. This review aims to collect the latest advancements in enantioselective palladium-catalyzed cyclization reactions over the past eleven years, and it is organized into thirteen sections depending on the different types of transformations involved.

Enantioselective Synthesis of Chiral Cyclobutenes Enabled by Brønsted Acid-Catalyzed Isomerization of BCBs

Chiral cyclobutene units are commonly found in natural products and biologically active molecules. Transition-metal-catalysis has been extensively used in asymmetric synthesis of such structures, while organocatalytic approaches remain elusive. In this study, bicyclo[1.1.0]butanes are involved in enantioselective transformation for the first time to offer a highly efficient route toward cyclobutenes with good regio- and enantiocontrol. The utilization of N-triflyl phosphoramide as a chiral Brønsted acid promoter enables this isomerization process to proceed under mild conditions with low catalyst loading as well as good functional group compatibility. The resulting chiral cyclobutenes could serve as platform molecules for downstream manipulations with excellent reservation of stereochemical integrity, demonstrating the synthetic practicality of the developed method. Control experiments have also been performed to verify the formation of a key carbocation intermediate at the benzylic position.

Concise Synthesis of (±)-Myrioneurinol Enabled by Sequential [2+2] Cycloaddition/Retro-Mannich Fragmentation/Mannich Reaction

A concise total synthesis of (±)-myrioneurinol has been achieved in 14 steps. An efficient AgSbF₆ /t-BuCl-catalyzed intramolecular [2+2] cycloaddition reaction of the alkynone-tethered enamine was developed to prepare the highly strained cyclobutene. It was used in combination with a subsequent retro-Mannich fragmentation/Mannich reaction to efficiently construct the tricyclic core of myrioneurinol.

Tunable Lewis Basicity and Nucleophilicity of Water against α,α-Dihalo-β-acetoxyketones for the Selective Synthesis of α-Haloenones and 1,2-Diketones

The discovery and systematic study of tunable yet competitive nucleophilicity and Lewis basicity of water against novel building blocks α,α-dihalo-β-acetoxyketones (possessing a tertiary acetate) have been reported. This distinct reactivity resulted in the formation of two competitive and different products 1,2-diketones and α-haloenones from a common intermediate α,α-dihalo-β-acetoxyketones through the nucleophilicity and Lewis basicity of water, respectively. A systematic study to understand the effect of temperature and amount of water on the product distribution revealed that a lower temperature in combination with a higher amount of water shows a high preference for 1,2-diketones over α-haloenones. Measuring the dielectric constant (permittivity, ε) of various reaction media at various temperatures and a correlation with the experimental observations suggested that the reaction media with a higher dielectric constant exhibit the nucleophilic character and hence show a preference for 1,2-diketones over α-haloenones.

Palladium-catalyzed alkynylative [5 + 1] carboannulation of 1,3-diarylprop-2-yn-1-yl acetates with terminal alkynes enabled by C–H functionalization

Using 1,3-diarylprop-2-yn-1-yl acetates as the five-carbon components enables alkynylative[5 + 1] carboannulation involving C–H functionalization toward 3-ethynyl-1-methylene-1,2-dihydronaphthalenes.

Asymmetric [3 + 1]-Cycloaddition Reaction via Diazo Discrimination

The enantioselective [3 + 1]-cycloaddition of two structurally different diazo compounds has been achieved using chiral bisoxazoline copper(I) complexes as a catalyst, providing a novel route for the synthesis of cyclobutenes containing a quaternary stereocenter. Typically, this reaction represents the first example of asymmetric cross-electrophile coupling of two diazo substrates via carbene discrimination.

Dual Palladium/Copper-Catalyzed anti-Selective Intermolecular Allenylsilylation of Terminal Alkynes: Entry to (E)-Silyl Enallenes

A palladium-/copper-cocatalyzed three-component trans-allenylsilylation of terminal alkynes with propargyl acetates and PhMe₂SiBpin is described, which is driven by the regioselective allenylation of the alkyne with propargyl acetates and then silylation. This method allows the simultaneous incorporation of an allene and silicon across the C≡C bond and provides a highly chemo-, regio-, and stereoselective alkyne difunctionalization route to the synthesis of valuable (E)-silyl enallenes. The utility of this method is highlighted by late-stage derivatization of bioactive compounds.

Bifunctional Borane Catalysis of a Hydride Transfer/Enantioselective [2+2] Cycloaddition Cascade

Herein, we present a mild and efficient method for synthesizing enantioenriched tetrahydroquinoline-fused cyclobutenes through a cascade reaction between 1,2-dihydroquinolines and alkynones with catalysis by chiral spiro-bicyclic bisboranes. The bisboranes served two functions: first they catalyzed a hydride transfer to convert the 1,2-dihydroquinoline substrate to a 1,4-dihydroquinoline, and then they activated the alkynone substrate for an enantioselective [2+2] cycloaddition reaction with the 1,4-dihydroquinoline generated in situ.

Nickel-catalyzed Heck reaction of cycloalkenes using aryl sulfonates and pivalates

Nickel-catalyzed Heck reaction of cycloalkenes delivers unusual conjugated arylated isomers. Nickel(0) catalysts ligated by chelating dialkylphosphines effectively activate not only aryl triflates as electrophiles, but also less reactive aryl mesylates, tosylates and pivalates. The omission of bases allows nickel hydride species to exist long enough to perform in situ olefin isomerization of initial Heck adducts.

Enantioselective Synthesis of Indole-Fused Bicyclo[3.2.1]octanes via Palladium(II)-Catalyzed Cascade Reaction

Indole-fused bicyclo[3.2.1]octanes are highly important structural units in natural products and biologically active compounds. However, there has been limited success in the enantioselective synthesis of these skeletons due to the complexity of the structure and the control of the enantioselectivity. Herein an enantioselective construction of indole-fused bicyclo[3.2.1]octanes bearing an all-carbon quaternary bridgehead stereocenter was developed via an aminopalladition-triggered Heck-type reaction. The protocol features mild conditions and good tolerance for a wide range of functional groups. The transformation can also be scaled up to demonstrate its practicability. The mechanistic studies reveal that the formation of an intermediate indol-3-yl palladium species via C-H activation should be ruled out.

Recent developments in asymmetric Heck type cyclization reactions for constructions of complex molecules

Intramolecular carbometallation-initiated asymmetric transformations are a general and powerful approach for the construction of carbo- and heterocyclic systems with one and more stereocenters. In addition, the newly developed multiple cascade reactions are an attractive strategy for increasing the molecular complexity in one step. In recent years, great progress has been made in this area with the use of various palladium and nickel complexes with P- and N-donor chiral ligands. This review highlights recent developments in intramolecular asymmetric Heck reactions, reductive Heck reactions and various types of cascade transformations (intramolecular Heck/Heck, Heck/nucleophilic trapping, Heck/Tsuji-Trost, Heck/Suzuki-Miyaura, Heck/Sonogashira, and Heck/carbonylation) in the synthesis of complex molecules over the past 5 years. A number of examples from before 2016 are included as background information. Particular attention is paid to the use of inexpensive nickel complexes as highly efficient catalysts for a number of asymmetric reactions considered here. A perspective on current challenges and potential future developments in the field of asymmetric Heck type cyclizations is also provided.

Gold-Catalyzed Synthesis of Small Rings

Three- and four-membered rings, widespread motifs in nature and medicinal chemistry, have fascinated chemists ever since their discovery. However, due to energetic considerations, small rings are often difficult to assemble. In this regard, homogeneous gold catalysis has emerged as a powerful tool to construct these highly strained carbocycles. This review aims to provide a comprehensive summary of all the major advances and discoveries made in the gold-catalyzed synthesis of cyclopropanes, cyclopropenes, cyclobutanes, cyclobutenes, and their corresponding heterocyclic or heterosubstituted analogs.

Palladium-catalyzed enol/enolate directed oxidative annulation: functionalized naphthofuroquinone synthesis and bioactivity evaluation

A palladium-promoted oxidative annulation reaction for the synthesis of structurally diverse naphthoquinone-containing heterocycles has been developed, providing switchable access to 1,2-naphthofuroquinones and densely functionalized cyclobutene-fused 1,4-naphthofuroquinones by selective enol/enolate-directed processes. The synthetic application was extended by late-stage functionalization of an anti-HIV drug. The practical value of 1,2-naphthofuroquinone synthesis was highlighted in endothelial protective lead compound development.

Palladium(II)/Lewis Acid Cocatalyzed Oxidative Annulation of 2-Alkenylanilines and Propargylic Esters: An Access to Benzo[b]azepines

An attractive approach to valuable yet synthetically challenging benzo[b]azepines was established via palladium(II)/Lewis acid cocatalyzed oxidative [5 + 2] annulation of readily available 2-alkenylanilines and propargylic esters. The protocol features mild reaction conditions and good functional group tolerance, constituting an array of benzo[b]azepines in yields of 30-75%.

Pd-Catalyzed Heck-Type Reaction: Synthesizing Highly Diastereoselective and Multiple Aryl-Substituted P-Ligands

In this work, an efficient palladium-catalyzed Heck-type reaction was successfully used to synthesize a wide range of monoaryl- or diaryl-substituted P-ligands with excellent yields and diastereoselectivity (up to 98% yield, dr >20:1). The preliminary mechanistic studies demonstrated that it possibly underwent a cationic Heck reaction that was assisted by silver salt, and it revealed the significant role of P(O)Ar₂ for excellent yields and diastereoselectivity.

Cascade cyclization and acyl migration of propargylic esters with isocyanides: rapid access to substituted furans

A novel rearrangement of ester-activated propargylic acetate with isocyanide has been disclosed. This protocol enables a quick synthesis of polysubstituted furan derivatives.

Dual C(sp3 )-H Bond Functionalization of N-Heterocycles through Sequential Visible-Light Photocatalyzed Dehydrogenation/[2+2] Cycloaddition Reactions

Herein we describe a mild method for the dual C(sp³ )-H bond functionalization of saturated nitrogen-containing heterocycles through a sequential visible-light photocatalyzed dehydrogenation/[2+2] cycloaddition procedure. As a complementary approach to the well-established use of iminium ion and α-amino radical intermediates, the elusive cyclic enamine intermediates were effectively generated by photoredox catalysis under mild conditions and efficiently captured by acetylene esters to form a wide array of bicyclic amino acid derivatives, thus enabling the simultaneous functionalization of two vicinal C(sp³ )-H bonds.