Homologation of Electron-Rich Benzyl Bromide Derivatives via Diazo C-C Bond Insertion

Nitrogen-Based Organofluorine Functional Molecules: Synthesis and Applications

Fluorine and nitrogen form a successful partnership in organic synthesis, medicinal chemistry, and material sciences. Although fluorine-nitrogen chemistry has a long and rich history, this field has received increasing interest and made remarkable progress over the past two decades, driven by recent advancements in transition metal and organocatalysis and photochemistry. This review, emphasizing contributions from 2015 to 2023, aims to update the state of the art of the synthesis and applications of nitrogen-based organofluorine functional molecules in organic synthesis and medicinal chemistry. In dedicated sections, we first focus on fluorine-containing reagents organized according to the type of fluorine-containing groups attached to nitrogen, including N-F, N-RF, N-SRF, and N-ORF. This review also covers nitrogen-linked fluorine-containing building blocks, catalysts, pharmaceuticals, and agrochemicals, underlining these components' broad applicability and growing importance in modern chemistry.

Triflic Acid-Catalyzed Dehydrative Amination of 2-Arylethanols with Weak N-Nucleophiles in Hexafluoroisopropanol

The catalytic deoxyamination of readily available 2-arylethanols offers an appealing, simple, and straightforward means of accessing β-(hetero)arylethylamines of biological interest. Yet, it currently represents a great challenge to synthetic chemistry. In most cases, the alcohol has to be either pre-activated in situ or converted into a reactive carbonyl intermediate, limiting the substrate scope for some methods. Examples of direct dehydrative amination of 2-arylethanols are thus still scarce. Here, we describe a catalytic protocol based on the synergy of triflic acid and hexafluoroisopropanol, which enables the direct and stereospecific amination of a broad array of 2-arylethanols, and does not require any pre-activation of the alcohol. This approach yields high value-added products incorporating sulfonamide, amide, urea, and aniline functionalities. In addition, this approach was applied to the sulfidation of 2-arylethanols. Mechanistic experiments and DFT computations indicate the formation of phenonium ions as key intermediates in the reaction.

Aryl versus Alkyl Redox-Active Diazoacetates ─ Light-Induced C-H Insertion or 1,2-Rearrangement

Diazo compounds with redox-active leaving groups are versatile reagents for orthogonal functionalizations, previously utilized in the Rh-catalyzed synthesis of highly substituted cyclopropanes. Photochemical activation of aryl-substituted diazoacetates generates carbenes, whereas redox-active esters can furnish C-radicals via the photoexcitation of EDA complexes. However, the photochemical behavior of these two functionalities, while present in one molecule, remains to be defined. We demonstrate that under light irradiation, reactions occur only on the diazo moiety, leaving the NHPI functionality intact. Not only aryl- but also alkyl-substituted NHPI diazoacetates are activated by blue light; either C-H insertion or the hydrogen/carbon 1,2-rearrangement occurs depending on the aryl/alkyl group.

Cobalt-Catalyzed Wagner-Meerwein Rearrangements with Concomitant Nucleophilic Hydrofluorination

We report a cobalt-catalyzed Wagner-Meerwein rearrangement of gem-disubstituted allylarenes that generates fluoroalkane products with isolated yields up to 84 %. Modification of the counteranion of the N-fluoropyridinium oxidant suggests the substrates undergo nucleophilic fluorination during the reaction. Subjecting the substrates to other known metal-mediated hydrofluorination procedures did not lead to observable 1,2-aryl migration. Thus, indicating the unique ability of these cobalt-catalyzed conditions to generate a sufficiently reactive electrophilic intermediate capable of promoting this Wagner-Meerwein rearrangement.

Metal-Free Arylation of Benzothiophenes at C4 by Activation as their Benzothiophene S-Oxides

Benzothiophenes, activated by oxidation to the corresponding S-oxides, undergo C-H/C-H-type coupling with phenols to give C4 arylation products. While an electron-withdrawing group at C3 of the benzothiophene is important, the process operates without a directing group and a metal catalyst, thus rendering it compatible with sensitive functionalities-e.g. halides and formyl groups. Quantum chemical calculations suggest a formal stepwise mechanism involving heterolytic cleavage of an aryloxysulfur species to give a π-complex of the corresponding benzothiophene and a phenoxonium cation. Subsequent addition of the phenoxonium cation to the C4 position of the benzothiophene is favored over the addition to C3; Fukui functions predict that the major regioisomer is formed at the more electron-rich position between C3 and C4. Varied selective manipulation of the benzothiophene products showcase the synthetic utility of the metal-free arylation process.

Reductive Opening of Oxetanes Catalyzed by Frustrated Lewis Pairs: Unexpected Aryl Migration via Neighboring Group Participation

B(C₆F₅)₃ was found to catalyze an unusual double reduction of oxetanes by hydrosilane with aryl migration via neighboring group participation. Control experiments suggested that the phenonium ion serves as the key intermediate. Minor modification of this protocol also led to simple hydrosilylative opening of oxetanes.

Indium-Catalyzed Formal Carbon-Halogen Bond Insertion: Synthesis of α-Halo-α,α-disubstituted Esters from Benzylic Halides and Diazo Esters

One-carbon-unit insertion into carbon-halogen (C-X) bonds accompanied by the formation of a new C-X bond and carbon-chain elongation is a powerful synthetic method of complex organohalides. Herein, we developed an indium trihalide catalyzed formal insertion of diazo esters into a C-X (X = Cl, Br, I) bond. In the present system, the reactions of α-aryl diazo esters with benzylic chlorides, bromides, and iodides yielded α-chloro, α-bromo, and α-iodo esters, respectively.

Catalyst-dependent chemoselective insertion of diazoalkanes into N-H\C-H\O-H\C-O bonds of 2-hydroxybenzothiazoles

The control of chemoselective insertions of diazoalkanes with 2-hydroxybenzothiazoles is challenging. Herein, the chemoselective N-H, O-H, C-O or C-H bond insertions of diazoalkanes with 2-hydroxybenzothiazoles are achieved using B(C6F5)3, Rh2(OAc)4...