Palladium-Catalyzed Enantioselective C(sp2)–H Imidoylation by Desymmetrization

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.

Recent progress in metal assisted multicomponent reactions in organic synthesis

To prepare complicated organic molecules, straightforward, sustainable, and clean methodologies are urgently required. Thus, researchers are attempting to develop imaginative approaches. Metal-catalyzed multicomponent reactions (MCRs) offer optimal molecular diversity, high atomic efficiency, and energy savings in a single reaction step. These versatile protocols are often used to synthesize numerous natural compounds, heterocyclic molecules, and medications. Thus far, the majority of metal-catalyzed MCRs under investigation are based on metal catalysts such as copper and palladium; however, current research is focused on developing novel, environmentally friendly catalytic systems. In this regard, this study demonstrates the effectiveness of metal catalysts in MCRs. The aim of this study is to provide an overview of metal catalysts for safe application in MCRs.

Palladium-Catalyzed Direct C(sp2)-H Cyanomethylation of Arylamides using Chloroacetonitrile

In this study, we devised a palladium-catalyzed efficient and versatile method for C(sp2)-H ortho-cyanomethylation of arylamides with a broad substrate scope and moderate to excellent yields. An inexpensive and commercially available chloroacetonitrile was employed as a cyanomethylating source. This method is also compatible with the air atmosphere. Further, the synthetic feasibility of this technique is established by gram-scale synthesis and functional group transformation of the products.

Stereoselective Construction of Unsymmetrically Linked Heterocycles via Palladium-Catalyzed Alkyne Insertion/Cycloimidoylation Cascade

A novel strategy to access unsymmetrically linked heterocycles via palladium-catalyzed acylcycloimidoylation of alkyne-tethered carbamoyl chlorides with isocyanides has been developed. Functionalized isocyanides were successfully applied as imine-containing heterocycle precursors to capture the vinyl-Pd^II intermediate, which was generated from a syn-carbopalladation of alkyne, followed by subsequent intramolecular C-H bond activation/imidoylative Heck reactions. Methylene oxindoles within Z-tetrasubstituted olefins were obtained in high yields with excellent stereoselectivities. Broad functional groups were well tolerated under mild reaction conditions.

Diastereoselective palladium-catalyzed C(sp3)-H cyanomethylation of amino acid and carboxylic acid derivatives

In this study, we report an efficient protocol for Pd-catalyzed methylene β-C(sp³)-H cyanomethylation of 8-aminoquinoline-directed α-amino acids using inexpensive chloroacetonitrile. Iodoacetonitrile generated in situ from chloroacetonitrile reacts with methylene C(sp³)-H bonds of α-amino acids with excellent diastereoselectivity, enabling access to a wide range of important γ-cyano-α-amino acids. Our protocol works well with different amino acid and carboxylic acid derivatives with good chemical yields and high functional group tolerance.

Kinetic resolution of sulfur-stereogenic sulfoximines by Pd(ii)-MPAA catalyzed C-H arylation and olefination

A direct Pd(ii)-catalyzed kinetic resolution of heteroaryl-enabled sulfoximines through an ortho-C-H alkenylation/arylation of arenes has been developed. The coordination of the sulfoximine pyridyl-motif and the chiral amino acid MPAA ligand to the Pd(ii)-catalyst controls the enantio-discriminating C(aryl)-H activation. This method provides access to a wide range of enantiomerically enriched unreacted aryl-pyridyl-sulfoximine precursors and C(aryl)-H alkenylation/arylation products in good yields with high enantioselectivity (up to >99% ee), and selectivity factor up to >200. The coordination preference of the directing group, ligand effect, geometry constraints, and the transient six-membered concerted-metalation-deprotonation species dictate the stereoselectivity; DFT studies validate this hypothesis.

Pd-Catalyzed Asymmetric Acyl-Carbamoylation of an Alkene to Construct an α-Quaternary Chiral Cycloketone

Herein, we report the palladium-catalyzed asymmetric acyl-carbamoylation of an alkene by employing thioesters as the acyl electrophiles and t-BuNC as the carbamoyl reagent, affording an α-quaternary chiral cycloketone in synthetically useful yields with excellent enantioselectivity. The reaction proceeded via asymmetric 1,2-migratory insertions of acyl-Pd into alkenes and subsequent migratory insertion of isocyanides into C(sp³)-Pd^II. The product could be diversified to some valuable skeletons with retention of enantiopurity, demonstrating the synthetic utility of this protocol.

Catalytic Enantioselective Isocyanide-Based Reactions: Beyond Passerini and Ugi Multicomponent Reactions

The development of catalytic enantioselective isocyanide-based reactions is currently of great interest because the resulting products are valuable in organic synthesis, pharmacological chemistry, and materials science. This review assembles and comprehensively summarizes the recent achievements in this rapidly growing area according to the reaction types. Special attention is paid to the advantages, limitations, possible mechanisms, and synthetic applications of each reaction. In addition, a personal outlook on the opportunities for further exploration is given at the end.

Radical Borylative Cyclization of Isocyanoarenes with N-Heterocyclic Carbene Borane: Synthesis of Borylated Aza-arenes

Borylated aza-arenes are of great importance in the area of organic synthesis. A radical borylative cyclization of isocyanoarenes with N-heterocyclic carbene borane (NHC-BH₃) under metal-free conditions was developed. The reaction allows the efficient assembly of several types of borylated aza-arenes (phenanthridines, benzothiazoles, etc.), which are difficult to access using alternative methods. Mild reaction conditions, a good functional-group tolerance, and generally good efficiencies were observed. The utility of these products is demonstrated, and the mechanism is discussed.

Chiral Catalysts for Pd0 -Catalyzed Enantioselective C-H Activation

In the past few decades, processes that involve transition-metal catalysis have represented a major part of the synthetic chemist's toolbox. Recently, the interest has shifted from the well-established cross-coupling reactions to C-H bond functionalization, thus making it a current frontier of transition-metal-catalyzed reactions. Constant progress in this field has led to the discovery of enantioselective methods to generate and control various types of stereogenic elements, thereby demonstrating its high value to generate scalemic chiral molecules. The present review is dedicated to enantioselective Pd⁰ -catalyzed C-H activation, which may be considered as an evolution of Pd⁰ -catalyzed cross-couplings, with a focus on the different chiral ligands and catalysts that enable these transformations.

Advances in palladium-catalysed imidoylative cyclization of functionalized isocyanides for the construction of N-heterocycles

Palladium-catalysed isocyanide insertion reactions have witnessed great progress in recent years. In particular, imidoylative cyclization of functionalized isocyanides was successfully developed by taking advantage of the adjustable substituents on the isocyano group, opening a new avenue to access a variety of nitrogen-containing heterocycles. In this review article, we summarize the advances of functionalized isocyanide insertion reactions and highlight the breakthroughs of enantioselective palladium catalysed imidoylation reactions by using this strategy. Additionally, copper-catalysed cyclization reactions of functionalized isocyanides are briefly discussed.

Recent Advances in Palladium-Catalyzed Isocyanide Insertions

Isocyanides have long been known as versatile chemical reagents in organic synthesis. Their ambivalent nature also allows them to function as a CO-substitute in palladium-catalyzed cross couplings. Over the past decades, isocyanides have emerged as practical and versatile C1 building blocks, whose inherent N-substitution allows for the rapid incorporation of nitrogeneous fragments in a wide variety of products. Recent developments in palladium catalyzed isocyanide insertion reactions have significantly expanded the scope and applicability of these imidoylative cross-couplings. This review highlights the advances made in this field over the past eight years.

Palladium-Catalyzed Cycloaddition of Alkynylimines, Double Isocyanides, and H2O/KOAc

In this work, a palladium-catalyzed cyclization of alkynylimines and double isocyanides is described. This facile procedure is efficient for synthesizing various 4-amidyl-2-aminopyrroles. Mechanism investigation indicates that a four-membered ring-fused pyrrole species is a key intermediate and the reaction involves [4 + 1] cycloaddition, protonation, nucleophilic addition, 1,4-addition of isocyanide, and rearomatization. Interestingly, the linear dipyrrole derivative is found to be an appropriate fluoride ion probe with a remarkable emission change, which could serve as a potential candidate for optoelectronic conjugated materials.

Palladium-catalysed dearomative aryl/cycloimidoylation of indoles

The first example of a dearomative palladium-catalysed isocyanide insertion reaction has been developed using functionalized isocyanides as the reaction partner of N-(2-bromobenzoyl)indoles. The imidoyl-palladium intermediate generated by tandem indole double bond/isocyanide insertion reactions could be trapped by intramolecular functional groups such as the C(sp2)-H bond and alkenes, affording diversified indoline derivatives bearing C3 imine-containing heterocycles. The dearomative aryl/cycloimidoylation of indoles proceeded smoothly in good to excellent yields with a wide functional group tolerance.

Divergent oriented synthesis (DOS) of aza-heterocyclic amides through palladium-catalyzed ketenimination of 2-iodo-N-(propa-1,2-dien-1-yl)anilines

A palladium-catalyzed tandem reaction of N-(2-iodophenyl)-4-methyl-N-(propa-1,2-dien-1-yl)benzenesulfonamide with isocyanide is described to divergently produce aza-heterocyclic amides.

Access to 1-amino-3,4-dihydroisoquinolines via palladium-catalyzed C-H bond aminoimidoylation reaction from functionalized isocyanides

Efficient access to 1-amino-3,4-dihydroisoquinolines, through palladium-catalyzed intramolecular C-H bond aminoimidoylation of α-benzyl-α-isocyanoacetates, has been developed. Consecutive isocyanide insertion and C-H bond activation were realized with C-N and C-C bonds formation in one step under redox neutral conditions, employing O-benzoyl hydroxylamines as electrophilic amino sources.

Thio radical-induced denitrogenative annulation of 1-azido-2-isocyanoarenes to construct 2-thiolated benzimidazoles

A method for the synthesis of 2-thiolated benzimidazoles is described starting from thiols and 1-azido-2-isocyanoarenes. The isocyano group works as an acceptor of various thio radicals, followed by denitrogenative annulation of the resulting imidoyl radical intermediates to the azido group, with nitrogen loss as the only process involving high bond-forming efficiency. The one-pot method for the synthesis of these products with high functional group tolerance in the benzimidazole-based ring is not available in previous literature.

Pd-Catalyzed C(sp2)-H Imidoylative Annulation: A General Approach To Construct Dibenzoox(di)azepines

A general method to construct the scaffolds of dibenzooxazepine and dibenzodiazepine, through Pd-catalyzed isocyanide insertion and intramolecular C(sp²)-H activation, has been developed. This is the first example of seven-membered heterocycle formation by C-H imidoylative annulation.

Cascade double isocyanide insertion and C-N coupling of 2-iodo-2'-isocyano-1,1'-biphenyls

A palladium-catalyzed double isocyanide insertion using 2-iodo-2'-isocyano-1,1'-biphenyls followed by copper-catalyzed intramolecular C-N coupling, delivering a unique heterocyclic structure containing both phenanthridine and carbazole scaffolds, has been developed. In this cascade process, four chemical bonds, including two C-C, one C-O, and one C-N bonds are formed consecutively without isolating an intermediate. The strategy of using a functionalized isocyanide in double insertion provides a quick approach for constructing heterocyclic systems with high bond-forming efficiency.

Enantioselective Synthesis of Planar Chiral Pyridoferrocenes via Palladium-Catalyzed Imidoylative Cyclization Reactions

A highly efficient synthesis of planar chiral pyrido[3,4- b] ferrocenes by a palladium-catalyzed enantioselective isocyanide insertion/desymmetric C(sp²)-H bond activation reaction was developed. Various planar chiral pyridoferrocenes were obtained in high yields with good to excellent enantioselectivity under mild conditions (up to 99% yield, 99% ee), enabled by a unique SPINOL-derived phosphoramidite ligand.