Palladium-Catalyzed Asymmetric Allylic C–H Alkylation of 1,4-Dienes with Cyclic β-Keto Esters

Metal-Catalyzed Enantioconvergent Transformations

Enantioconvergent catalysis has expanded asymmetric synthesis to new methodologies able to convert racemic compounds into a single enantiomer. This review covers recent advances in transition-metal-catalyzed transformations, such as radical-based cross-coupling of racemic alkyl electrophiles with nucleophiles or racemic alkylmetals with electrophiles and reductive cross-coupling of two electrophiles mainly under Ni/bis(oxazoline) catalysis. C-H functionalization of racemic electrophiles or nucleophiles can be performed in an enantioconvergent manner. Hydroalkylation of alkenes, allenes, and acetylenes is an alternative to cross-coupling reactions. Hydrogen autotransfer has been applied to amination of racemic alcohols and C-C bond forming reactions (Guerbet reaction). Other metal-catalyzed reactions involve addition of racemic allylic systems to carbonyl compounds, propargylation of alcohols and phenols, amination of racemic 3-bromooxindoles, allenylation of carbonyl compounds with racemic allenolates or propargyl bromides, and hydroxylation of racemic 1,3-dicarbonyl compounds.

Bimetallic Palladium/Cobalt Catalysis for Enantioselective Allylic C-H Alkylation via a Transient Chiral Nucleophile Strategy

An asymmetric allylic C-H functionalization has been developed by making use of transient chiral nucleophiles, as well as bimetallic synergistic catalysis with an achiral Pd⁰ catalyst and a chiral N,N'-dioxide-Co^II complex. A variety of β-ketoesters and N-Boc oxindoles coupled with allylbenzenes and aliphatic terminal alkenes were well tolerated, furnishing the desired allylic alkylation products in high yields (up to 99 %) with excellent regioselectivities and enantioselectivities (up to 99 % ee).

Aminothiaindanone as an Accessible Scaffold for a Three-Point Chemical Diversity

Aminothiaindanone heterocycle appears to be a scaffold of interest in medicinal chemistry. To increase the chemical diversity in this series, the introduction of three-point chemical diversity on the cyclopenta[b]thiophen-4-one scaffold was explored. About thirty newly functionalized thiophene-containing bicycles were obtained using various chemical reactions, paving the way for novel possibilities in medicinal chemistry projects.

Access to chiral γ-butenolides via palladium-catalyzed asymmetric allylic C-H alkylation of 1,4-dienes

Asymmetric allylic C-H alkylation of 1,4-pentadienes with α-angelica lactones has been developed by tri-axial phosphoramidite-palladium catalysis. This reaction can tolerate a range of functional groups under mild conditions, furnishing versatile chiral γ,γ-disubstituted butenolides in high yields with good to high levels of stereoselectivity.

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.

Palladium-Catalyzed Asymmetric Allylic C-H Functionalization: Mechanism, Stereo- and Regioselectivities, and Synthetic Applications

Asymmetric functionalization of inert C-H bonds is undoubtedly a synthetically significant yet challenging bond-forming process, allowing for the preparation of densely functionalized molecules from abundantly available feedstocks. In the past decade, our group and others have found that trivalent phosphorus ligands are capable of facilitating Pd-catalyzed allylic C-H functionalization of α-alkenes upon using p-quinone as an oxidant. In these reactions, a 16-electron Pd(0) complex bearing a monodentate phosphorus ligand, a p-quinone, and an α-alkene has been identified as a key intermediate. Through a concerted proton and two-electron transfer process, electrophilic π-allylpalladium is subsequently generated and can be leveraged to forge versatile chemical bonds with a wide range of nucleophiles. This Account focuses on describing the origin, evolution, and synthetic applications of Pd-catalyzed asymmetric allylic C-H functionalization reactions, with an emphasis on the fundamental mechanism of the concerted proton and two-electron transfer process in allylic C-H activation.Enabled by the cooperative catalysis of the palladium complex of triarylphosphine, a primary amine, and a chiral phosphoric acid, an enantioselective α-allylation of aldehydes with α-alkenes is established. The combination of chiral phosphoric acid and a palladium complex of a chiral phosphoramidite ligand allows the allylic C-H alkylation of α-alkenes with pyrazol-5-ones to give excellent enantioselectivities, wherein the chiral ligand and chiral phosphoric acid synergistically control the stereoselectivity. Notably, the palladium-phosphoramidite complexes are also efficient catalysts for allylic C-H alkylation, with a wide scope of nucleophiles. In the case of 1,4-dienes, the geometry and coordination pattern of the nucleophile are able to vary the transition states of bond-forming events and thereby determine the Z/E-, regio-, and stereoselectivities.These enantioselective allylic C-H functionalization reactions are tolerant of a wide range of nucleophiles and α-alkenes, providing a large library of optically active building blocks. Based on enantioselective intramolecular allylic C-H oxidation, the formal synthesis of (+)-diversonol is accomplished, and enantioselective intramolecular allylic C-H amination can enable concise access to letermovir. In particular, the asymmetric allylic C-H alkylation of 1,4-dienes with azlactones offers highly enantioenriched α,α-disubstituted α-amino acid derivatives that are capable of serving as key building blocks for the enantioselective synthesis of lepadiformine alkaloids. In addition, a tachykinin receptor antagonist and (-)-tanikolide are also synthesized with chiral molecules generated from the corresponding allylic C-H alkylation reactions.