Boron-Catalyzed Azide Insertion of α-Aryl α-Diazoesters

Late-Stage Photoredox-Catalyzed Aryl C-H Bond Diazomethylation with Atomic Carbon Reagents

Herein, we present a novel class of diazo compounds as atomic carbon reagents substituted with two orthogonal redox-active leaving groups that were exploited in the late-stage construction of chiral centers with aryl C-H bonds from aromatic feedstocks and drug molecules. Key to the strategy was the use of photoredox catalysis to enable an initial C-H diazomethylation reaction able to generate diazomethyl-substituted redox-active esters. Subsequent construction of chiral centers with readily available starting materials proceeded using a broad range of well-known diazo and redox-active ester functionalizations. Moreover, the applicability of our novel atomic carbon reagent was tested in the automated parallel synthesis of a library of Fenofibrate derivatives.

Comparison of the Efficiency of B-O and B-C Bond Formation Pathways in Borane-Catalyzed Carbene Transfer Reactions Using α-Diazocarbonyl Precursors: A Combined Density Functional Theory and Machine Learning Study

Lewis acidic boranes, especially tris(pentafluorophenyl)borane [B(C6F5)3], have emerged as metal-free catalysts for carbene transfer reactions of α-diazocarbonyl compounds in a variety of functionalization reactions. The established mechanism for how borane facilitates carbene generation for these compounds in the scientific community is based on the formation of a B-O (C=O) intermediate (path O). Herein, we report an extensive DFT study that challenges the notion of a ubiquitous path O, revealing that B-C(=N=N) bond formation (path C) for certain diazocarbonyl substrates proves to be the preferred pathway. This study elucidates, through the introduction of 22 various substituents on each side of the α-diazocarbonyl backbone, how the electron-donating and -withdrawing properties of substituents influence the competition between these B-O and B-C pathways. To elucidate the impact of the electronic features of diazo substrates on the competition between the O and C pathways in the studied dataset, we employed a machine learning approach based on the Random Forest model. This analysis revealed that substrates with higher electron density on the diazo-attached carbon, lower electron density on the carbonyl carbon, and more stable HOMO orbitals tend to proceed via path C. Furthermore, this study not only demonstrates that borane efficiency in facilitating N2 release is greatly affected by the nature of substituents on both sides of the α-diazocarbonyl functionality but also shows that for some substrates, borane is incapable of catalyzing the release of molecular nitrogen.

B(C6F5)3-Catalyzed Multicomponent Reactions of 2,3-Diketoesters, Amines, Allenes, and Nucleophiles: Synthesis of 2α-Functionalized Pyrroles

An efficient B(C6F5)3-catalyzed multicomponent reaction of 2,3-diketoesters, amines, allenes, and nucleophiles was reported, which afforded 2α-functionalized pyrroles in moderate to good yields. The reaction features low catalyst loading, usage of a small amount of solvent, high atom economy, tunable installation of diverse functional groups at 2α-position, and water being formed as the byproduct. This is the first multicomponent reaction that combines vicinal tricarbonyl compounds with allenes.

Visible-Light-Mediated Azidation of α-Diazoesters with TMSN3 via Direct Photoexcitation and SH2 Mechanism

A visible light-mediated azidation of α-diazoesters with TMSN3 to synthesize valuable α-azidoesters has been developed. Without using any catalysts and additives, the reaction proceeded smoothly under visible light irradiation at room temperature. A variety of α-diazoesters were successfully converted to the desired α-azidoesters, showing good functional group tolerance. The products could be readily transformed into triazole, α-azidoacid, and α-azidoamide. Mechanistic studies suggested that the reaction is mainly carrying out via direct photoexcitation and SH2 mechanism. This work provides a novel, mild, and practical protocol for synthesizing α-azidoesters.

Copper-Catalyzed Three-Component Tandem Reaction of Alkynes, α-Diazo Esters, and TMSN3 to Access N-Substituted 1,2,3-Triazoles

An efficient copper-catalyzed three-component tandem reaction of alkynes, α-diazo esters, and TMSN3 to construct triazoles has been developed. Through this strategy, a number of diverse N-substituted 1,2,3-triazoles were conveniently obtained in moderate to good yields from simple and readily available starting materials using K2CO3 as the base. The mechanism of the tandem Cu-catalyzed azide-alkyne cycloaddition (CuAAC) and Cu-carbenoid-participated C-N coupling reaction has been investigated.

Transition-metal-free azide insertion of N-triftosylhydrazones using a non-metallic azide source

Benzylic azides, an important class of active organic synthons, were synthesized in high yields from the easily accessible N-triftosylhydrazones with stable TMSN₃ under mild conditions. The reaction features high efficiency and excellent functional group tolerance, as illustrated by gram-scale synthesis and the synthesis of drug-like molecules. Mechanistic studies reveal that azidation occurs at the electron-deficient diazo-carbon via the elimination of N₂ by an azide ion.

B(C6F5)3-catalyzed oxidation of α-diazoesters using DMF and molecular oxygen as oxygen sources

A metal-free catalytic oxidation of α-diazoesters via a green environmental-friendly route was developed. The α-diazoesters were converted to α-ketoesters using DMF and molecular oxygen as oxygen sources and B(C₆F₅)₃ as the catalyst, without any additives. This protocol has a broad adaptability of substrates and good compatibility with a range of functional groups, and it offers new insight into reactions catalyzed by B(C₆F₅)₃.

Synthesis of functionalized S-benzyl dithiocarbamates from diazo-compounds via multi-component reactions with carbon disulfide and secondary amines

Triflic acid promoted multi-component synthesis of functionalized S-benzyl dithiocarbamates from diazo compounds, carbon disulfide and secondary amines is reported. The reactions proceeded at room temperature and gave the desired dithiocarbamates in good yields. Wide-substrate scope and easy operation are the important features of this methodology.

TBAI-Catalyzed S-H and N-H Insertion Reactions of α-Diazoesters with Thiophenols and Amines under Metal-Free Conditions

Mild, convenient, and effective TBAI-catalyzed S-H and N-H insertion reactions of α-diazoesters with thiophenols and aromatic amines under metal-free conditions have been described, furnishing a straightforward and general platform for the synthesis of various thioethers and 2-amino-2-oxoacetates in moderate to excellent yields. Moreover, this strategy features simple operation, mild conditions, broad substrate scope, and easy scale-up.

Understanding the Influence of Donor-Acceptor Diazo Compounds on the Catalyst Efficiency of B(C6 F5 )3 Towards Carbene Formation

Diazo compounds have been largely used as carbene precursors for carbene transfer reactions in a variety of functionalization reactions. However, the ease of carbene generation from the corresponding diazo compounds depends upon the electron donating/withdrawing substituents either side of the diazo functionality. These groups strongly impact the ease of N2 release. Recently, tris(pentafluorophenyl)borane [B(C6 F5 )3 ] has been shown to be an alternative transition metal-free catalyst for carbene transfer reactions. Herein, a density functional theory (DFT) study on the generation of carbene species from α-aryl α-diazocarbonyl compounds using catalytic amounts of B(C6 F5 )3 is reported. The significant finding is that the efficiency of the catalyst depends directly on the nature of the substituents on both the aryl ring and the carbonyl group of the substrate. In some cases, the boron catalyst has negligible effect on the ease of the carbene formation, while in other cases there is a dramatic reduction in the activation energy of the reaction. This direct dependence is not commonly observed in catalysis and this finding opens the way for intelligent design of this and other similar catalytic reactions.

Triarylborane Catalyzed Carbene Transfer Reactions Using Diazo Precursors

Reactive carbenes generated from diazo compounds are key intermediates for a range of organic reactions to afford synthetically useful organic compounds. The majority of these reactions have been carried out using transition metal catalysts. However, the formation of carbene intermediates using main group elements has not been widely investigated for synthetic purposes. Recent studies have demonstrated that triarylboranes can be used for the in situ generation of reactive carbene intermediates in both stoichiometric and catalytic reactions. These new reactivities of triarylboranes have gained significant attention in synthetic chemistry particularly in catalytic studies. The range of organic compounds that have been synthesized through these reactions are important as pharmaceuticals or agrochemicals. In this perspective, we highlight the recent progress and ongoing challenges of carbene transfer reactions generated from their corresponding diazo precursors using triarylboranes as catalysts. We also highlight the stoichiometric use of triarylboranes in which the boranes not only activate the diazo functionality to afford a carbene intermediate but also actively participate in the reactions as a reagent. The different mechanisms for activation and carbene transfer are described along with the mechanistic and computational studies that have aided the elucidation of these reaction pathways. Potential opportunities for the use of boranes as a catalyst toward different carbene transfer reactions and their future prospects are discussed.

Tris(pentafluorophenyl)borane-Catalyzed Carbenium Ion Generation and Autocatalytic Pyrazole Synthesis-A Computational and Experimental Study

In recent years, metal-free organic synthesis using triarylboranes as catalysts has become a prevalent research area. Herein we report a comprehensive computational and experimental study for the highly selective synthesis of N-substituted pyrazoles through the generation of carbenium species from the reaction between aryl esters and vinyl diazoacetates in the presence of catalytic tris(pentafluorophenyl)borane [B(C6 F5 )3 ]. DFT studies were undertaken to illuminate the reaction mechanism revealing that the in situ generation of a carbenium species acts as an autocatalyst to prompt the regiospecific formation of N-substituted pyrazoles in good to excellent yields (up to 81 %).

B(C6F5)3-Catalyzed Electron Donor-Acceptor Complex-Mediated Aerobic Sulfenylation of Indoles under Visible-Light Conditions

An efficient B(C₆F₅)₃-catalyzed aerobic oxidative C-S cross-coupling reaction of thiophenol with indoles was developed, affording a wide range of diaryl sulfides in good yields. An electron donor-acceptor complex between B(C₆F₅)₃ and indoles was formed, facilitating the photoinduced single-electron transfer (SET) from indole substrates to the B(C₆F₅)₃ catalyst. This protocol demonstrates a new reaction model using B(C₆F₅)₃ as a single-electron oxidant.

Borane catalysed cyclopropenation of arylacetylenes

Triarylboranes have gained substantial attention as catalysts for C-C bond forming reactions due to their remarkable catalytic activities. Herein, we report B(C₆F₅)₃ catalysed cyclopropenation of a wide variety of arylacetylenes using donor-acceptor diazoesters. A mild reaction protocol has been developed for the synthesis of functionalised cyclopropenes (33 examples) in good to excellent yields.

B(C6F5)3-catalyzed O-H insertion reactions of diazoalkanes with phosphinic acids

A highly efficient base-, metal-, and oxidant-free catalytic O-H insertion reaction of diazoalkanes and phosphinic acids in the presence of B(C6F5)3 has been developed. This powerful methodology provides a green approach towards the synthesis of a broad spectrum of α-phosphoryloxy carbonyl compounds with good to excellent yields (up to 99% yield). The protocol features the advantages of operational simplicity, high atom economy, practicality, easy scalability and environmental friendliness.

Tris(pentafluorophenyl)borane catalyzed C-C and C-heteroatom bond formation

A series of boron based Lewis acids have been reported to date, but among them, tris(pentafluorophenyl)borane (BCF) has gained the most significant attention in the synthetic chemistry community. The viability of BCF as a potential Lewis acid catalyst has been vastly explored in organic and materials chemistry due to its thermal stability and commercial availability. Most explorations of BCF chemistry in organic synthesis has occurred in the last two decades and many new catalytic reactivities are currently under investigation. This review mainly focuses on recent reports from 2018 onwards and provides a concise knowledge to the readers about the role of BCF in metal-free catalysis. The review has mainly been categorized by different types of organic transformation mediated through BCF catalysis for the C-C and C-heteroatom bond formation.

Regiospecific and site-selective C–H allylation of phenols with vinyldiazo compounds catalyzed by In(iii)

An In(OTf)3-catalyzed regiospecific, site-selective, and C–H chemoselective insertion reaction of phenols with vinyldiazoacetates was developed. The reactions of aryl or alkyl substituted vinyldiazoacetates exhibited different selective manners.

When phosphoryl azide meets mechanochemistry: clean, rapid, and efficient synthesis of phosphoryl amides under B(C6F5)3 catalysis in a ball mill

We described herein the first example associated with B(C6F5)3-catalyzed preparation of phosphoryl amides under mechanochemical conditions.

An asymmetric hydrocyanation/Michael reaction of α-diazoacetates via Cu(i)/chiral guanidine catalysis

An asymmetric one-pot hydrocyanation/Michael reaction of α-aryl diazoacetates with trimethylsilyl cyanide, tert-butanol, and N-phenylmaleimides has been realized using a chiral guanidinium salt/CuBr catalyst.