Borane-catalyzed arylation of aryldiazoacetates with N,N-dialkylanilines

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·H2O-Catalyzed N-H and C-H Functionalization of Aromatic Amines with Sulfoxonium Ylides under Metal-Free Conditions

Hydrogen-bonded aggregates of B(C6F5)3 and water are proven as strong Brønsted acid catalysts, which have the advantages of low toxicity and broad compatibility. Sulfoxonium ylides are stable surrogates of diazo compounds, which participate in various reactions due to their versatile reactivity. Based on these characteristics, a strategy for C-N bond or C-C bond construction of aromatic amines with sulfoxonium ylides under metal-free catalytic conditions was established. This method has advantages of mild conditions and excellent yield, which is suitable for the N-H or C-H functionalization of various aromatic amine compounds.

Borane-Catalyzed Coupling of Diazooxindoles and Difluoroenoxysilanes to Tetrasubstituted Monofluoroalkenes

A highly stereoselective coupling reaction of diazooxindoles with difluoroenoxysilanes catalyzed by Lewis acidic boranes has been developed. The reaction proceeded at ambient temperature under transition metal-free conditions with wide functional group tolerance. By using this simple procedure, a series of tetrasubstituted monofluoroalkenes can be accessed in good yield with high selectivity.

Facile Friedel-Crafts alkylation of arenes under solvent-free conditions

The Friedel-Crafts alkylation of arenes is an important part of electrophilic aromatic substitution reactions. However, the reactivity of arenes is weakened by electron-withdrawing substituents, leading to limited substrate scopes and applications. Herein, we developed an efficient HOTf-promoted Friedel-Crafts alkylation reaction of broad arenes with α-aryl-α-diazoesters under metal-free and solvent-free conditions.

Dearomative Insertion of Fluoroalkyl Carbenes into Azoles Leading to Fluoroalkyl Heterocycles with a Quaternary Center

The skeletal ring expansion of heteroarenes through carbene insertion is gaining popularity in synthetic chemistry. Efficient strategies for heterocyclic ring expansion to access heterocycles containing a fluoroalkyl quaternary carbon center through fluoroalkyl carbene insertion are highly desirable because of their broad applications in medicinal chemistry. Herein, we report a general strategy for the dearomative one-carbon insertion of azoles using fluoroalkyl N-triftosylhydrazones as fluoroalkyl carbene precursors, resulting in ring-expanded heterocycles in excellent yields with good functional-group compatibility. The broad generality of this methodology in the late-stage diversification of pharmaceutically interesting bioactive molecules and versatile transformations of the products has been demonstrated.

Pathway to Construct α-Acyloxy Esters by B(C6F5)3-Catalyzed O-H Insertion of Carboxylic Acids with Sulfoxonium Ylides

We report the first example of B(C6F5)3-catalyzed O-H insertion reaction of sulfoxonium ylides and carboxylic acids, achieving efficient construction of diester moieties under metal-free condition. This protocol is characterized by broad substrate tolerance, particularly for various phenylacetic acids, and good compatibility with water/air condition, which is superior to most other methods.