Rhodium(II)-Catalyzed Alkynyl Carbene Insertion into N-H Bonds

The first insertion of an alkynyl carbene into N-H bonds under Rh-catalysis is developed. Alkynyl hydrazone carboxylates are used as donor-acceptor carbene precursors and are exquisitely inserted into the N-H bonds of various amines, amides, and 1,2-diamines. A wide variety of 3-alkynyl 3,4-dihydroquinoxalin-2(1H)-ones and densely functionalized α-alkynyl α-amino esters are obtained in good to excellent yields. Further, chemoselective N-H insertion reactions, mechanistic studies, and various synthetic transformations for obtaining valuable heterocycles are demonstrated.

P(NMe2)3-Mediated Regioselective N-Alkylation of 2-Pyridones via Direct Deoxygenation of α-Keto Esters

A practical and regioselective direct N-alkylation of 2-pyridones is enabled by use of α-keto esters in the P(NMe2)3-mediated deoxygenation process. The reaction proceeds under mild conditions to produce N-alkylated 2-pyridones with high selectivity and generality, and the protocol is shown to be applicable for the scale-up synthesis, which makes it promising for practical applications.

O-H Insertion of Hydrogenphosphate Derivatives and α-Diazo Compounds

An efficient O-H insertion of hydrogenphosphate derivatives and α-diazo compounds has been developed to construct α-phosphoryloxy scaffolds. Diverse α-phosphoryloxy skeletons could be obtained under mild and catalyst-free conditions in good yields. The control experiments suggest a protonation and nucleophilic addition process of α-diazo compounds via a diazonium ion pair for this transformation.

Regioselective O-alkylation of 2-pyridones by TfOH-catalyzed carbenoid insertion

Selective alkylation of 2-pyridone could solve a challenge in chemistry and streamline the synthesis of important molecules. Here we report the regioselective O-alkylation of 2-pyridones by TfOH-catalyzed carbenoid insertion. In the catalytic system, alkylation of 2-pyridone was achieved with unprecedented regioselectiviy (>99 : 1). This protocol is characterized by mild reaction conditions, metal-free, and simplicity. Moreover, this method provides the desired products in good yield and demonstrates a broad substrate scope in this transformation.

Catalytic Transformations of 2-Pyridones by Rhodium-Mediated Carbene Transfer

An enantioselective cyclopropanation reaction of N-substituted 2-pyridones with diazo compounds has been realized by using a chiral rhodium complex as the catalyst, and the corresponding chiral cyclopropanes could be formed in good yields with high enantioselectivities. Moreover, using acceptor-acceptor dimethyl 2-diazomalonate as the carbene precursor, a novel 1,4-rearrangement of a Boc group from N to C has also been discovered under rhodium catalysis.

Coordination effect enabled palladium-catalyzed regioselective O-alkylation of 2-pyridones

A novel tactic for the regioselective O-alkylation of 2-pyridones has been realized through palladium catalysis in moderate to high yields. The coordination effect between palladium and nitrogen on the pyridine ring plays a versatile role.

Rhodium-Catalyzed Dearomative Rearrangement of 2-Oxypyridines with Cyclopropenes: Access to N-Alkylated 2-Pyridones

A rhodium-catalyzed dearomative O-to-N rearrangement reaction of 2-oxypyridines has been developed by using cyclopropenes as the carbene precursors. This protocol features broad substrate scope and mild reaction conditions, providing a...

Chemo- and Enantioselective Insertion of Furyl Carbene into the N-H Bond of 2-Pyridones

Asymmetric carbene insertion reactions represent one of the most important protocols to construct carbon-heteroatom bonds. The use of donor-acceptor diazo compounds bearing an ester group is however a prerequisite for achieving high enantioselectivity. Herein, we report a chemo- and enantioselective formal N-H insertion of 2-pyridones that has been accomplished for the first time with enynones as the donor-donor carbene precursors. DFT calculations indicate an unprecedented enantioselective 1,4-proton transfer from O to C. The rhodium catalyst provides a chiral pocket in which the steric repulsion and the π-π interaction of the propeller ligand play a critical role in determining the selectivities.

Divergent Gold Catalysis: Unlocking Molecular Diversity through Catalyst Control

The catalyst-directed divergent synthesis, commonly termed as "divergent catalysis", has emerged as a promising technique as it allows chartering of structurally distinct products from common substrates simply by modulating the catalyst system. In this regard, gold complexes emerged as powerful catalysts as they offer unique reactivity profiles as compared to various other transition metal catalysts, primarily due to their salient electronic and geometrical features. Owing to the tunable soft π-acidic nature, gold catalysts not only evolved as superior contenders for catalyzing the reactions of alkynes, alkenes, and allenes but also, more intriguingly, have been found to provide divergent reaction pathways over other π-acid catalysts such as Ag, Pt, Pd, Rh, Cu, In, Sc, Hg, Zn, etc. The recent past has witnessed a renaissance in such examples wherein, by choosing gold catalysts over other transition metal catalysts or by fine-tuning the ligands, counteranions or oxidation states of the gold catalyst itself, a complete reactivity switch was observed. However, reviews documenting such examples are sporadic; as a result, most of the reports of this kind remained scattered in the literature, thereby hampering further development of this burgeoning field. By conceptualizing the idea of "Divergent Gold Catalysis (DGC)", this review aims to consolidate all such reports and provide a unified approach necessary to pave the way for further advancement of this exciting area. Based on the factors governing the divergence in product formation, an explicit classification of DGC has been provided. To gain a fundamental understanding of the divergence in observed reactivities and selectivities, the review is accompanied by mechanistic insights at appropriate places.

Ruthenium-Catalyzed Chemoselective N-H Bond Insertion Reactions of 2-Pyridones/7-Azaindoles with Sulfoxonium Ylides

A ruthenium-catalyzed highly chemoselective N-alkylation of 2-pyridones has been developed, affording N-alkylated 2-pyridone derivatives in good yields and excellent N-selectivity. The key to achieve this unprecedented N-H rather than O-H insertion reaction is the use of CpRu(PPh₃)₂Cl as the catalyst and sulfoxonium ylides as the alkylation reagents. Moreover, this protocol is also amenable to 7-azaindoles by slightly varying the reaction conditions. Furthermore, sulfonium ylides are also suitable alkylation reagents, providing the N-alkylated 2-pyridones in good selectivity.

Visible-light-promoted selective O-alkylation of 2-pyridones with α-aryldiazoacetates

A visible-light-promoted O-H insertion reaction between 2-pyridones and α-aryldiazoacetates has been developed. Upon visible light irradiation, the reaction proceeds smoothly under mild and catalyst-free conditions. A wide scope of 2-pyridones and α-aryldiazoacetates are well tolerated, and various O-alkylated 2-pyridones are obtained with perfect selectivity and good functional group tolerance. A photoinduced radical process is probably responsible for the excellent selectivity.

Catalyst-free synthesis of α,α-disubstituted carboxylic acid derivatives under ambient conditions via a Wolff rearrangement reaction

Herein, a hexafluoroisopropanol (HFIP)-promoted Wolff rearrangement reaction was developed, delivering various α,α-disubstituted carboxylic acid derivatives in good to excellent yields.

Highly Enantioselective O-H Bond Insertion Reaction of α-Alkyl- and α-Alkenyl-α-diazoacetates with Water

Catalytic asymmetric reactions in which water is a substrate are rare. Enantioselective transition-metal-catalyzed insertion of carbenes into the O-H bond of water can be used to incorporate water into the stereogenic center, but the reported chiral catalysts give good results only when α-aryl-α-diazoesters are used as the carbene precursors. Herein we report the first highly enantioselective O-H bond insertion reactions between water and α-alkyl- and α-alkenyl-α-diazoesters as carbene precursors, with catalysis by a combination of achiral dirhodium complexes and chiral phosphoric acids or chiral phosphoramides. Participation of the phosphoric acids or phosphoramides in the carbene transfer reaction markedly suppressed competing side reactions, such as β-H migration, carbene dimerization, and olefin isomerization, and thus ensured good yields of the desired products. Fine-tuning of the ester moiety facilitated enantiocontrol of the proton transfer reactions of the enol intermediates and resulted in excellent enantioselectivity. This protocol represents an efficient new method for preparation of multifunctionalized chiral α-alkyl and α-alkenyl hydroxyl esters, which readily undergo various transformations and can thus be used for the synthesis of bioactive compounds. Mechanistic studies revealed that the phosphoric acids and phosphoramides promoted highly enantioselective [1,2]- and [1,3]-proton transfer reactions of the enol intermediates. Maximization of molecular orbital overlap in the transition states of the proton transfer reactions was the original driving force to involve the proton shuttle catalysts in this process.

Gold-Catalyzed Oxidation of Thioalkynes To Form Phenylthio Ketene Derivatives via a Noncarbene Route

Gold-catalyzed oxidations of thioalkynes with 8-methylquinoline oxides afford 2-phenylthioketenes that can be trapped efficiently with alcohols. The synthetic utility is manifested by terminal and internal thioalkynes over a wide scope, bearing esters, ketones, alkyl, and oxime substituents. Our density functional theory calculations suggest that gold-catalyzed oxidations of terminal and internal thioalkynes with 8-methylquinoline oxides generate gold-bound ketene intermediates without the intermediacy of α-oxo gold carbene.

Gold-catalyzed C5-alkylation of indolines and sequential oxidative aromatization: access to C5-functionalized indoles

A novel protocol towards the rapid assembly of C5-alkylated indole derivatives has been developed via gold-catalyzed regioselective functionalization and subsequent oxidative aromatization.