A Palladium-Catalyzed Cyclization−Oxidation Sequence: Synthesis of Bicyclo[3.1.0]hexanes and Evidence for SN2 C−O Bond Formation

Transition metal catalysed cascade C-C and C-O bond forming events of alkynes

The past few decades have witnessed the emergence of domino reactions as a powerful tool for the multi-functionalization of alkynes for the rapid and smooth construction of complex molecular architectures. In this context, employing transition metal catalysis, vicinal/geminal cascade functionalization of alkynes involving C-C and C-O bond-formation reactions, has become a preferred strategy for the synthesis of oxygenated motifs. Despite this significant progress, reviews documenting such strategies are either metal/functional group-centric or target-oriented, thus hampering further developments. Therefore, in this review, different conceptual approaches based on C-C and C-O bond-forming events of alkynes such as carboxygenation (C-C and CO bond formation), carboalkoxylation (C-C and C-OR bond formation), and carboacetoxylation (C-C and C-OAc bond formations) are discussed, and examples from the literature from the last two decades are presented. Further, we have presented detailed insights into the mechanism of different transformations.

Synthesis of 3-Azabicyclo[3.1.0]hexane Derivates

3-Azabicyclo[3.1.0]hexanes are an important class of nitrogen-containing heterocycles that have been found to be key structural features in a wide range of biologically active natural products, drugs, and agrochemicals. As a cutting-edge area, the synthesis of these derivatives has made spectacular progress in recent decades, with various transition-metal-catalyzed and transition-metal-free catalytic systems being developed. In this review, we provide an overview of recent advances in the efficient methods for the synthesis of 3-azabicyclo[3.1.0]hexane derivatives since 2010, emphasizing the scope of substrates and synthesis' applications, as well as the mechanisms of these reactions.

Pd-Catalyzed Asymmetric 5-exo-trig Cyclization/Cyclopropanation/Carbonylation of 1,6-Enynes for the Construction of Chiral 3-Azabicyclo[3.1.0]hexanes

We herein disclose a mild and efficient access to chiral 3-azabicyclo[3.1.0]hexanes via a Pd-catalyzed asymmetric 5-exo-trig cyclization/cyclopropanation/carbonylation of 1,6-enynes. Various nucleophiles, such as alcohols, phenols, amines and water, are well compatible with the reaction system. This reaction forms three C-C bonds, two rings, two adjacent quaternary carbon stereocenters as well as one C-O/C-N bond with excellent regio- and enantioselectivities. The products could be further functionalized to generate a library of 3-azabicyclo[3.1.0]hexane frameworks.

Palladium-Catalyzed Organic Reactions Involving Hypervalent Iodine Reagents

The chemistry of polyvalent iodine compounds has piqued the interest of researchers due to their role as important and flexible reagents in synthetic organic chemistry, resulting in a broad variety of useful organic molecules. These chemicals have potential uses in various functionalization procedures due to their non-toxic and environmentally friendly properties. As they are also strong electrophiles and potent oxidizing agents, the use of hypervalent iodine reagents in palladium-catalyzed transformations has received a lot of attention in recent years. Extensive research has been conducted on the subject of C—H bond functionalization by Pd catalysis with hypervalent iodine reagents as oxidants. Furthermore, the iodine(III) reagent is now often used as an arylating agent in Pd-catalyzed C—H arylation or Heck-type cross-coupling processes. In this article, the recent advances in palladium-catalyzed oxidative cross-coupling reactions employing hypervalent iodine reagents are reviewed in detail.

Pd-Catalyzed and ligand-enabled alkene difunctionalization via unactivated C-H bond functionalization

Palladium-catalyzed and ligand-enabled C-H functionalization methods have emerged as a powerful approach for the preparation of therapeutically important motifs and complex natural products. Olefins, owing to their natural abundance, have been extensively employed for the formation of C-C and C-X bonds and the generation of various heterocycles. Traditionally, activated as well as starting materials with preinstalled functional groups, and also halide substrates under transition metal catalysis, have been employed for olefin difunctionalization. However, strategies for employing unactivated C-H bond functionalization to achieve alkene difunctionalization have rarely been explored. A possible solution to this challenge is the application of bulky ligands which enhances the reductive elimination pathway and inhibits β-hydride elimination to selectively yield difunctionalized alkene products. This feature article summarizes the utilization of unreactive C-H bonds in the Pd-catalyzed and ligand-enabled difunctionalization of alkenes.

Oxidation of Pd(II) with disilane in a palladium-catalyzed disilylation of aryl halides: a theoretical view

Density functional theory (DFT) calculation has been used to reveal the mechanism of the Pd-catalyzed disilylation reaction of aryl halides. The DFT calculations indicate that the reaction starts with the oxidative addition of the C-I bond to the Pd(0) catalyst. Concerted metalation-deprotonation (CMD) can then generate a five-membered palladacycle. Insertion of Pd(ii) into the Si-Si bond in disilane followed by two sequential steps of reductive eliminations yields the disilylation product and regenerates the Pd(0) catalyst. According to the NPA charge analysis along the reaction coordinates, the formal oxidative addition of the Si-Si bond to palladium could be considered as the insertion of palladium into the Si-Si bond. However, the conventional oxidative addition of the C-I bond to palladium is exactly an oxidation process with the electron transfer from the palladium atom to the C-I bond. Therefore, electron rich Pd(0) is beneficial for the oxidation process, and Pd(ii) prone to acquire electrons is beneficial for the insertion process.

Synthesis of Highly Conjugated Functionalized 2-Pyridones by Palladium-Catalyzed Aerobic Oxidative Dicarbonation Reactions of N-(Furan-2-ylmethyl) Alkyne Amides and Alkenes as Coupling Partners

A mild, step-economical method for the synthesis of highly conjugated functionalized 2-pyridones from N-(furan-2-ylmethyl) alkyne amides is reported. This method involves Pd-catalyzed aerobic oxidative dicarbonation reactions of alkynes with carbon nucleophiles of a furan ring and an acrylate or styrene as coupling partners. The UV-vis absorption spectra of some of the 2-pyridones indicated that they absorbed shortwave radiation, suggesting their potential utility for filtration of such radiation.

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.

Hypervalent Iodine Reagents in Palladium-Catalyzed Oxidative Cross-Coupling Reactions

Hypervalent iodine compounds are valuable and versatile reagents in synthetic organic chemistry, generating a diverse array of useful organic molecules. Owing to their non-toxic and environmentally friendly features, these reagents find potential applications in various oxidative functionalization reactions. In recent years, the use of hypervalent iodine reagents in palladium-catalyzed transformations has been widely studied as they are strong electrophiles and powerful oxidizing agents. For instance, extensive work has been carried out in the field of C–H bond functionalization via Pd-catalysis using hypervalent iodine reagents as oxidants. In addition, nowadays, iodine(III) reagents have been frequently employed as arylating agents in Pd-catalyzed C–H arylation or Heck-type cross-coupling reactions. In this review, recent advancements in the area of palladium-catalyzed oxidative cross-coupling reactions using hypervalent iodine reagents are summarized in detail.

Copper(II)-Mediated Intramolecular Cyclopropanation of Ketene N,X-Acetals (X = S, O, N) under Mild Conditions

CuBr₂-mediated intramolecular oxidative cyclopropanation of α-oxo ketene N,S-acetals was efficiently achieved to afford 2-thioalkyl-3-azabicyclo-[3.1.0]hexenes under mild conditions. 2-Oxyalkyl- and 2-aminoallyl-3-azabicyclo-[3.1.0]hexenes were also obtained from the corresponding α-oxo ketene N,O- and N,N-acetals. Further C-S bond transformations led to diverse 2-aryl-substituted 3-azabicyclo[3.1.0]hexene derivatives in good to high yields.

Pd(II)-Catalyzed Intramolecular Acetoxylative (3 + 2) Annulation of Propargylic Alcohol and Alkene: Polycyclic Oxa-heterocycle Synthesis and Mechanistic Insight

A Pd(II)-catalyzed intramolecular and highly selective acetxoylative (3 + 2) annulation of propargylic alcohol and alkene is reported. Mechanistically, a hydroxy-guided regioselective alkyne acetoxypalladation is followed by 6-exo-trig alkene insertion to form an alkyl-Pd(II) intermediate. After oxidation, the resulting cyclometalated alkoxy-Pd(IV)-alkyl undergoes direct reductive elimination to afford a polycyclic oxa-heterocycle. When an additional coordinating site or ligand accessible by palladium is present, an S_N2-type C-C reductive elimination of alkyl-Pd(IV) instead occurs along with hydroxy acetylation, affording 3-bicyclo[4.1.0]heptan-5-one products.

Recent advances in the synthesis of cyclopropanes

Cyclopropanes, one of the most important strained rings, have gained much attention for more than a century because of their interesting and unique reactivity. They not only exist in many natural products, but have also been widely used in the fields of organic synthesis, medicinal chemistry and materials science as versatile building blocks. Based on the sustainable development in this area, this review mainly focuses on the recent advances in the synthesis of cyclopropanes classified by the type of catalytic system, including regio-, diastereo-, and enantio-selective reactions.

Photocatalytic decarboxylative [2 + 2 + 1] annulation of 1,6-enynes with N-hydroxyphthalimide esters for the synthesis of indene-containing polycyclic compounds

An efficient photoredox-mediated [2 + 2 + 1] annulation of 1,6-enynes with N-hydroxyphthalimide esters was reported for the synthesis of spiro and non-spiro indene-containing polycyclic frameworks.

Oxidative tandem annulation of 1-(2-ethynylaryl)prop-2-en-1-ones catalyzed by cooperative iodine and TBHP

Metal-free I₂-catalyzed tandem annulation of 1,6-enynes to access 1H-cyclopropa-[b]naphthalene-2,7-diones using TBHP is presented.

Efficient assembly of ynones via palladium-catalyzed sequential carbonylation/alkynylation

A convenient and efficient strategy to construct functionalized ynones by a palladium-catalyzed cascade annulation reaction has been developed.

Recent advances in the synthesis of cyclopropanes

Cyclopropanes have gained much attention by virtue of their interesting structure and unique reactivity. This review discusses the recent advances in the synthesis of cyclopropanes, and some of the related applications will be discussed.

Palladium-Catalyzed Direct C-H Trifluoroethylation of Aromatic Amides

A simple and direct C-H trifluoroethylation of aromatic amides has been developed. The protocol is applicable to a variety of aromatic amides, including ones derived from amino acids. The developed method can be used for further modifications of peptides. Preliminary mechanistic studies have been done by isolating the reaction intermediate.

Palladium(II)-Catalyzed Transformation of 3-Alkylbenzofurans to [2,3'-Bibenzofuran]-2'(3'H)-ones: Oxidative Dimerization of 3-Alkylbenzofurans

An unprecedented oxidative dimerization by palladium catalysis has been developed using PhI(OPiv)₂ as a by-standing oxidant. This provides a facile method for the synthesis of quaternary 2,3'-bibenzofuran-2'(3')-ones from readily accessible substrates. A plausible mechanism involving a Pd(II)-Pd(IV) catalytic cycle is proposed; a trace amount of water is required for subsequent oxidation.

Intramolecular cyclization of N-allyl propiolamides: a facile synthetic route to highly substituted γ-lactams (a review)

The development of simple and efficient methods for construction of substituted γ-lactams is an important synthetic goal because such ring skeletons are present in numerous natural compounds that display diverse biological activities.

Palladium-catalyzed decarboxylative alkoxycarbonylation of potassium aryltrifluoroborates with potassium oxalate monoesters

An efficient method for the preparation of aryl esters has been developed via Pd-catalyzed decarboxylative coupling of potassium aryltrifluoroborates and potassium oxalate monoesters.

Palladium-catalyzed Heck-type reaction of oximes with allylic alcohols: synthesis of pyridines and azafluorenones

An efficient palladium-catalyzed Heck-type reaction of oximes with allylic alcohol has been developed for the synthesis of pyridines and azafluorenones.

Pd-Catalyzed oxidative isomerization of propargylic acetates: highly efficient access to α-acetoxyenones via alkenyl Csp2–O bond-forming reductive elimination from PdIV

A Pd(ii)/(iv)-catalyzed oxidative isomerization of propargylic acetates developed for the synthesis of polysubstituted alkenyl acetates is described.

Asymmetric intramolecular α-cyclopropanation of aldehydes using a donor/acceptor carbene mimetic

Enantioselective α-alkylation of carbonyl is considered as one of the most important processes for asymmetric synthesis. Common alkylation agents, that is, alkyl halides, are notorious substrates for both Lewis acids and organocatalysts. Recently, olefins emerged as a benign alkylating species via photo/radical mechanisms. However, examples of enantioselective alkylation of aldehydes/ketones are scarce and direct asymmetric dialkylation remains elusive. Here we report an intramolecular α-cyclopropanation reaction of olefinic aldehydes to form chiral cyclopropane aldehydes. We demonstrate that an α-iodo aldehyde can function as a donor/acceptor carbene equivalent, which engages in a formal [2+1] annulation with a tethered double bond. Privileged bicyclo[3.1.0]hexane-type scaffolds are prepared in good optical purity using a chiral amine. The synthetic utility of the products is demonstrated by versatile transformations of the bridgehead formyl functionality. We expect the concept of using α-iodo iminium as a donor/acceptor carbene surrogate will find wide applications in chemical reaction development.

The formation of chiral centres adjacent to carbonyl groups is a highly challenging task in asymmetric catalysis. Here, the authors report the asymmetric diakylation of carbonyls via an intramolceular α-cyclopropanation procedure.

Palladium-catalyzed oxidative 6-exo-trig cyclization of 1,6-enynes: facile synthesis of bicyclo[4.1.0]heptan-5-ones

We here describe a new palladium-catalyzed oxidative 6-exo-trig cyclization of 1,6-enynes at room temperature using tBuONO as an oxidant for the synthesis of 3-bicyclo[4.1.0]heptan-5-ones.