Palladium‐Catalyzed Oxidative Carbocyclization–Borylation of Enallenes to Cyclobutenes

A highly efficient palladium‐catalyzed oxidative borylation of enallenes was developed for the selective formation of cyclobutene derivatives and fully‐substituted alkenylboron compounds. Cyclobutenes are formed as the exclusive products in MeOH in the presence of H₂O and Et₃N, whereas the use of AcOH leads to alkenylboron compounds. Both reactions showed a broad substrate scope and good tolerance for various functional groups, including carboxylic acid ester, free hydroxy, imide, and alkyl groups. Furthermore, transformations of the borylated products were conducted to show their potential applications.     

Palladium(II)/Brønsted Acid‐Catalyzed Enantioselective Oxidative Carbocyclization–Borylation of Enallenes

An enantioselective oxidative carbocyclization–borylation of enallenes that is catalyzed by palladium(II) and a Brønsted acid was developed. Biphenol‐type chiral phosphoric acids were superior co‐catalysts for inducing the enantioselective cyclization. A number of chiral borylated carbocycles were synthesized in high enantiomeric excess.     

Palladium‐Catalyzed Carbonylation Reactions of Aryl Halides and Related Compounds

A CO group richer : (Hetero)arenes are vital intermediates in the manufacture of agrochemicals, dyes, pharmaceuticals, and other industrial products. In the past decades transition‐metal‐catalyzed coupling reactions of aryl halides with all types of nucleophiles have been developed. This Review summarizes recent work in the area of palladium‐catalyzed carbonylation reactions of aryl halides and related compounds (see scheme).

     

Synthesis of 2‐Alkynoates by Palladium(II)‐Catalyzed Oxidative Carbonylation of Terminal Alkynes and Alcohols

A homogeneous Pd^II catalyst, utilizing a simple and inexpensive amine ligand (TMEDA), allows 2‐alkynoates to be prepared in high yields by an oxidative carbonylation of terminal alkynes and alcohols. The catalyst system overcomes many of the limitations of previous palladium carbonylation catalysts. It has an increased substrate scope, avoids large excesses of alcohol substrate and uses a desirable solvent. The catalyst employs oxygen as the terminal oxidant and can be operated under safer gas mixtures.     

Palladium‐Catalyzed Oxidative Carbocyclizations

Palladium‐catalyzed oxidative carbon–carbon bond‐forming annulations, that is, carbocyclization reactions, have recently emerged as efficient and atom‐economical routes to carbo‐ and heterocycles, whereby less functionalized substrates and fewer synthetic steps are needed to obtain a target molecule compared with traditional non‐oxidative carbon–carbon bond‐forming reactions. In this review, the synthetic efforts in palladium‐catalyzed oxidative carbocyclization reactions are summarized.     

Highly Selective Construction of Seven‐Membered Carbocycles by Olefin‐Assisted Palladium‐Catalyzed Oxidative Carbocyclization–Alkoxycarbonylation of Bisallenes

An olefin‐assisted palladium‐catalyzed oxidative carbocyclization–alkoxycarbonylation of bisallenes to afford seven‐membered carbocycles has been established. This dehydrogenative coupling reaction showed excellent substrate scope and functional group compatibility. The reaction exhibited high chemo‐ and regioselectivity, and ester 3 was the only product obtained. The olefin unit has been proven to be indispensable during the reaction. Moreover, intramolecular oxidative coupling suggests that the reaction proceeds via a (π‐allyl)palladium intermediate.     

Palladium‐Catalyzed Aminocarbonylation of Allylic Alcohols

A benign and efficient palladium‐catalyzed aminocarbonylation reaction of allylic alcohols is presented. The generality of this novel process is demonstrated by the synthesis of β,γ‐unsaturated amides including aliphatic, cinnamyl, and terpene derivatives. The choice of ligand is crucial for optimal carbonylation processes: Whereas in most cases the combination of PdCl₂ with Xantphos ( L6 ) gave best results, sterically hindered substrates performed better in the presence of simple triphenylphosphine ( L10 ), and primary anilines gave the best results using cataCXium® PCy ( L8 ). The reactivity of the respective catalyst system is significantly enhanced by addition of small amounts of water. Mechanistic studies and control experiments revealed a tandem allylic alcohol amination/C?N bond carbonylation reaction sequence.     

Ligand‐Free Palladium‐Catalyzed Aerobic Oxidative Coupling of Carboxylic Anhydrides with Arylboronic Acids

We report a new, effective and environmentally friendly protocol for selective aerobic oxidative coupling of arylboronic acids with carboxylic anhydrides in the presence of ligand‐free palladium catalyst. The aryl benzoates are obtained in good to excellent yields.     

From Anilines to Isatins: Oxidative Palladium‐Catalyzed Double Carbonylation of CH Bonds

A novel palladium‐catalyzed C? H double carbonylation introduces two adjacent carbonyl groups for the synthesis of isatins from readily available anilines. The reaction proceeds under atmospheric pressure of CO with high regioselectivity and without any additives. Density functional theory investigations indicate that the palladium‐catalyzed double carbonylation catalytic cycle is plausible.     

Investigating the Mechanism of Palladium‐Catalyzed Radical Oxidative C(sp3)−H Carbonylation: A DFT Study

Palladium (Pd)‐catalyzed radical oxidative C?H carbonylation of alkanes is a useful method for functionalizing hydrocarbons, but there is still a lack of understanding of the mechanism, which restricts the application of this reaction. In this work, density functional theory (DFT) calculations were carried out to study the mechanism for a Pd‐catalyzed radical esterification reaction. Two plausible reaction pathways have been proposed and validated by DFT calculations. The computational results reveal that the generated alkyl radical prefers to add to the carbon monoxide (CO) molecule to form a carbonyl radical before bonding with the Pd species. Radical addition onto Pd followed by CO migratory insertion was unfavorable owing to the high energy barrier of the migratory insertion step. The regioselectivity of the C(sp³)?H carbonylation was also investigated by DFT. The results show that the regioselectivity is controlled by both the bond dissociation energy of the reacting C?H bond and the stability of the corresponding generated carbon radical. Competitive side reactions also affected the yield and regioselectivity owing to the rapid consumption of the stable radical intermediate.