Access to β‐Lactams by Enantioselective Palladium(0)‐Catalyzed C(sp3)H Alkylation

β‐Lactams are very important structural motifs because of their broad biological activities as well as their propensity to engage in ring‐opening reactions. Transition‐metal‐catalyzed C? H functionalizations have emerged as strategy enabling yet uncommon highly efficient disconnections. In contrast to the significant progress of Pd⁰‐catalyzed C? H functionalization for aryl–aryl couplings, related reactions involving the formation of saturated C(sp³)? C(sp³) bonds are elusive. Reported here is an asymmetric C? H functionalization approach to β‐lactams using readily accessible chloroacetamide substrates. Important aspects of this transformation are challenging C(sp³)? C(sp³) and strain‐building reductive eliminations to for the four‐membered ring. In general, the β‐lactams are formed in excellent yields and enantioselectivities using a bulky taddol phosphoramidite ligand in combination with adamantyl carboxylic acid as cocatalyst.     

Pd(II)‐Catalyzed Tandem Enantioselective Methylene C(sp3)−H Alkenylation–Aza‐Wacker Cyclization to Access β‐Stereogenic γ‐Lactams

Herein, we describe an unprecedented cascade reaction to β‐stereogenic γ‐lactams involving Pd(II)‐catalyzed enantioselective aliphatic methylene C(sp³)?H alkenylation–aza‐Wacker cyclization through syn‐aminopalladation. Readily available 3,3′‐substituted BINOLs are used as chiral ligands, providing the corresponding γ‐lactams with broad scope and high enantioselectivities (up to 98 % ee).     

Site‐Selective δ‐C(sp3)−H Alkylation of Amino Acids and Peptides with Maleimides via a Six‐Membered Palladacycle

The site‐selective functionalization of unactivated C(sp³)?H bonds remains one of the greatest challenges in organic synthesis. Herein, we report on the site‐selective δ‐C(sp³)?H alkylation of amino acids and peptides with maleimides via a kinetically less favored six‐membered palladacycle in the presence of more accessible γ‐C(sp³)?H bonds. Experimental studies revealed that C?H bond cleavage occurs reversibly and preferentially at γ‐methyl over δ‐methyl C?H bonds while the subsequent alkylation proceeds exclusively at the six‐membered palladacycle that is generated by δ‐C?H activation. The selectivity can be explained by the Curtin–Hammett principle. The exceptional compatibility of this alkylation with various oligopeptides renders this procedure valuable for late‐stage peptide modifications. Notably, this process is also the first palladium(II)‐catalyzed Michael‐type alkylation reaction that proceeds through C(sp³)?H activation.     

Palladium(II)‐Catalyzed Enantioselective Arylation of Unbiased Methylene C(sp3)−H Bonds Enabled by a 2‐Pyridinylisopropyl Auxiliary and Chiral Phosphoric Acids

Enantioselective functionalizations of unbiased methylene C(sp³)?H bonds of linear systems by metal insertion are intrinsically challenging and remain a largely unsolved problem. Herein, we report a palladium(II)‐catalyzed enantioselective arylation of unbiased methylene β‐C(sp³)?H bonds enabled by the combination of a strongly coordinating bidentate PIP auxiliary with a monodentate chiral phosphoric acid (CPA). The synergistic effect between the PIP auxiliary and the non‐C₂‐symmetric CPA is crucial for effective stereocontrol. A broad range of aliphatic carboxylic acids and aryl bromides can be used, providing β‐arylated aliphatic carboxylic acid derivatives in high yields (up to 96 %) with good enantioselectivities (up to 95:5 e.r.). Notably, this reaction also represents the first palladium(II)‐catalyzed enantioselective C?H activation with less reactive and cost‐effective aryl bromides as the arylating reagents. Mechanistic studies suggest that a single CPA is involved in the stereodetermining C?H palladation step.     

Easily Accessible Auxiliary for Palladium‐Catalyzed Intramolecular Amination of C(sp2)H and C(sp3)H Bonds at δ‐ and ε‐Positions

An easily synthesized and accessible N,O‐bidentate auxiliary has been developed for selective C? H activation under palladium catalysis. The novel auxiliary showed its first powerful application in C? H functionalization of remote positions. Both C(sp²)? H and C(sp³)? H bonds at δ‐ and ε‐positions were effectively activated, thus giving tetrahydroquinolines, benzomorpholines, pyrrolidines, and indolines in moderate to excellent yields by palladium‐catalyzed intramolecular C? H amination.     

Ligand‐Enabled Arylation of γ‐C−H Bonds

Pd^II‐catalyzed arylation of γ‐C(sp³)?H bonds of aliphatic acid‐derived amides was developed by using quinoline‐based ligands. Various γ‐aryl‐α‐amino acids were prepared from natural amino acids using this method. The influence of ligand structure on reactivity was also systematically investigated.     

Synthesis of Strained γ‐Lactams by Palladium(0)‐Catalyzed C(sp3)−H Alkenylation and Application to Alkaloid Synthesis

A variety of strained α‐alkylidene‐γ‐lactams were synthesized by palladium(0)‐catalyzed intramolecular C(sp³)?H alkenylation from easily accessible acyclic and monocyclic bromoalkene precursors. These lactams are valuable intermediates for accessing various classes of mono‐ and bicylic alkaloids containing a pyrrolidine ring, as illustrated with the synthesis of an advanced model of the marine natural product plakoridine A and of the indolizidine alkaloid δ‐coniceine.     

Palladium‐Catalyzed Intermolecular [4+1] Spiroannulation by C(sp3)−H Activation and Naphthol Dearomatization

A novel palladium‐catalyzed [4+1] spiroannulation was developed by using a C(sp³)?H activation/naphthol dearomatization approach. This bimolecular domino reaction of two aryl halides was realized through a sequence of cyclometallation‐facilitated C(sp³)?H activation, biaryl cross‐coupling, and naphthol dearomatization, thus rendering the rapid assembly of a new class of spirocyclic molecules in good yields with broad functional‐group tolerance. Preliminary mechanistic studies indicate that C?H cleavage is likely involved in the rate‐determining step, and a five‐membered palladacycle was identified as the key intermediate for the intermolecular coupling.     

Pd‐Catalyzed Enantioselective Ring Opening/Cross‐Coupling and Cyclopropanation of Cyclobutanones

A palladium‐catalyzed enantioselective sequential ring‐opening/cross‐coupling of cyclobutanones is disclosed that provides chiral indanones bearing C3‐quaternary stereocenters. The reaction process involves palladium‐catalyzed nucleophilic addition of cyclobutanones and aryl halides, enantioselective β‐carbon elimination, and intermolecular trapping of a transient σ‐alkylpalladium complex with boronic acids. Alternatively, an intramolecular cyclopropanation is realized through C?H bond functionalization in the absence of external coupling reagents, affording chiral cyclopropane‐fused‐indanones in good yields and enantioselectivity.     

Palladium‐Catalyzed γ‐C(sp3)−H Arylation of Thiols by a Detachable Protecting/Directing Group

Reported herein is a palladium‐catalyzed, directed γ‐C(sp³)?H arylation of protected thiols. The key is to utilize Michael acceptors as a dual reagent to install a protecting/directing group on thiols by a thiol‐Michael click reaction, and remove it later under basic conditions. The C?H arylation proceeds with high functional‐group tolerance and the deprotected thiols can be further transformed into other sulfur‐containing compounds. This unique mode of activation could open the door for site‐selective functionalization of thiols or other sulfur‐containing compounds at unactivated positions.     

Palladium(0)/PAr3‐Catalyzed Intermolecular Amination of C(sp3)H Bonds: Synthesis of β‐Amino Acids

An intermolecular C(sp³)? H amination using a Pd⁰/PAr₃ catalyst was developed. The reaction begins with oxidative addition of R₂N? OBz to a Pd⁰/PAr₃ catalyst and subsequent cleavage of a C(sp³)? H bond by the generated Pd? NR₂ intermediate. The catalytic cycle proceeds without the need for external oxidants in a similar manner to the extensively studied palladium(0)‐catalyzed C? H arylation reactions. The electron‐deficient triarylphosphine ligand is crucial for this C(sp³)? H amination reaction to occur.     

Ligand‐Enabled Alkynylation of C(sp3)−H Bonds with Palladium(II) Catalysts

The palladium(II)‐catalyzed β‐ and γ‐alkynylation of amide C(sp³)−H bonds is enabled by pyridine‐based ligands. This alkynylation reaction is compatible with substrates containing α‐tertiary or α‐quaternary carbon centers. The β‐methylene C(sp³)−H bonds of various carbocyclic rings were also successfully alkynylated.     

Synthesis of β‐Lactams by Palladium(0)‐Catalyzed C(sp3)−H Carbamoylation

A general and user‐friendly synthesis of β‐lactams is reported that makes use of Pd⁰‐catalyzed carbamoylation of C(sp³)−H bonds, and operates under stoichiometric carbon monoxide in a two‐chamber reactor. This reaction is compatible with a range of primary, secondary and activated tertiary C−H bonds, in contrast to previous methods based on C(sp³)−H activation. In addition, the feasibility of an enantioselective version using a chiral phosphonite ligand is demonstrated. Finally, this method can be employed to synthesize valuable enantiopure free β‐lactams and β‐amino acids.     

Carboxylate‐Assisted Oxidative Addition to Aminoalkyl PdII Complexes: C(sp3)−H Arylation of Alkylamines by Distinct PdII/PdIV Pathway

Reported is the discovery of an approach to functionalize secondary alkylamines using 2‐halobenzoic acids as aryl‐transfer reagents. These reagents promote an unusually mild carboxylate‐assisted oxidative addition to alkylamine‐derived palladacycles. In the presence of Ag^I salts, a decarboxylative C(sp³)?C(sp²) bond reductive elimination leads to γ‐aryl secondary alkylamines and renders the carboxylate motif a traceless directing group. Stoichiometric mechanistic studies were effectively translated to a Pd‐catalyzed γ‐C(sp³)?H arylation process for secondary alkylamines.     

Palladium‐Catalyzed C−H Silylation through Palladacycles Generated from Aryl Halides

A highly efficient palladium‐catalyzed disilylation reaction of aryl halides through C?H activation has been developed for the first time. The reaction has broad substrate scope. A variety of aryl halides can be disilylated by three types of C?H activation, including C(sp²)?H, C(sp³)?H, and remote C?H activation. In particular, the reactions are also unusually efficient. The yields are essentially quantitative in many cases, even in the presence of less than 1 mol % catalyst and 1 equivalent of the silylating reagent under relatively mild conditions. The disilylated biphenyls can be converted into disiloxane‐bridged biphenyls.     

Access to β‐Lactams by Enantioselective Palladium(0)‐Catalyzed C(sp3)?H Alkylation

β‐Lactams are very important structural motifs because of their broad biological activities as well as their propensity to engage in ring‐opening reactions. Transition‐metal‐catalyzed C H functionalizations have emerged as strategy enabling yet uncommon highly efficient disconnections. In contrast to the significant progress of Pd⁰‐catalyzed C H functionalization for aryl–aryl couplings, related reactions involving the formation of saturated C(sp³) C(sp³) bonds are elusive. Reported here is an asymmetric C H functionalization approach to β‐lactams using readily accessible chloroacetamide substrates. Important aspects of this transformation are challenging C(sp³) C(sp³) and strain‐building reductive eliminations to for the four‐membered ring. In general, the β‐lactams are formed in excellent yields and enantioselectivities using a bulky taddol phosphoramidite ligand in combination with adamantyl carboxylic acid as cocatalyst.     

Palladium‐Catalyzed Intramolecular Carbene Insertion into C(sp3)−H Bonds

A palladium‐catalyzed carbene insertion into C(sp³)?H bonds leading to pyrrolidines was developed. The coupling reaction can be catalyzed by both Pd⁰ and Pd^II, is regioselective, and shows a broad functional group tolerance. This reaction is the first example of palladium‐catalyzed C(sp³)?C(sp³) bond assembly starting from diazocarbonyl compounds. DFT calculations revealed that this direct C(sp³)?H bond functionalization reaction involves an unprecedented concerted metalation–deprotonation step.     

Rhodium(III)‐Catalyzed [3+2] Annulation of 5‐Aryl‐2,3‐dihydro‐1H‐pyrroles with Internal Alkynes through C(sp2)H/Alkene Functionalization

This study describes a new rhodium(III)‐catalyzed [3+2] annulation of 5‐aryl‐2,3‐dihydro‐1H‐pyrroles with internal alkynes using a Cu(OAc)₂ oxidant for building a spirocyclic ring system, which includes the functionalization of an aryl C(sp²)? H bond and addition/protonolysis of an alkene C?C bond. This method is applicable to a wide range of 5‐aryl‐2,3‐dihydro‐1H‐pyrroles and internal alkynes, and results in the assembly of the spiro[indene‐1,2′‐pyrrolidine] architectures in good yields with excellent regioselectivities.     

Copper‐Catalyzed Three‐Component Carboazidation of Alkenes with Acetonitrile and Sodium Azide

A copper‐catalyzed three‐component reaction of alkenes, acetonitrile, and sodium azide afforded γ‐azido alkyl nitriles by formation of one C(sp³)−C(sp³) bond and one C(sp³)−N bond. The transformation allows concomitant introduction of two highly versatile groups (CN and N₃) across the double bond. A sequence involving the copper‐mediated generation of a cyanomethyl radical and its subsequent addition to an alkene, and a C(sp³)−N bond formation accounted for the reaction outcome. The resulting γ‐azido alkyl nitrile can be easily converted into 1,4‐diamines, γ‐amino nitriles, γ‐azido esters, and γ‐lactams of significant synthetic value.     

Palladium‐Catalyzed Pyrazole‐Directed sp3 C−H Bond Arylation for the Synthesis of β‐Phenethylamines

We have developed a method for palladium‐catalyzed, pyrazole‐directed sp³ C−H bond arylation by aryl iodides. The reaction employs a Pd(OAc)₂ catalyst at 5–10 mol % loading and silver(I) oxide as a halide‐removal agent, and it proceeds in acetic acid or acetic acid/hexafluoroisopropanol solvent. Ozonolysis of the pyrazole moiety affords pharmaceutically important β‐phenethylamines.