Gold-catalyzed fluorination of alkynes/allenes: mechanistic explanations and reaction scope

Since the beginning of this century, there has been a great deal of research on homogeneous gold-catalyzed alkyne fluorination due to the precious values of fluorinated scaffolds in many bioactive natural products, drugs, and agrochemicals. This area of research, which originally took advantage of gold's mild Lewis acidity and tendency to form π-complexes with alkynes, has gained new momentum after Sadighi's discovery in 2007 of Au-catalyzed hydrofluorination of internal alkynes. The methods have enabled direct access to valuable fluoroalkanes, fluoroalkenes, α-fluorocarbonyls, and fluorinated carbo- and hetero-cycles in one pot from readily available alkyne precursors. Both nucleophilic and electrophilic fluorination modes with versatile reactivity have been used to achieve several new cascade reactions. This study covers the literature reports published since 2007 and provides a comprehensive summary of the methods, applications, and mechanistic insights into gold-catalyzed alkyne fluorination using electrophilic and nucleophilic fluorinating reagents.

Gold-catalyzed hydrofluorination of terminal alkynes using potassium bifluoride (KHF2)

A gold-catalyzed hydrofluorination of terminal alkynes is reported. The salient features of this study showcase the use of potassium bifluoride, KHF2, as the fluorinating agent in conjunction with a fluorinated solvent, hexafluoroisopropanol. The reaction is mediated by a well-defined precursor, [Au(IPr)(OH)] with an acid activator. A wide range of functionalized terminal alkynes can be converted to the corresponding monofluoroalkenes in up to 97% yield.

Silver-Catalyzed (Z)-β-Fluoro-vinyl Iodonium Salts from Alkynes: Efficient and Selective Syntheses of Z-Monofluoroalkenes

Monofluoroalkenes are stable and lipophilic amide bioisosteres used in medicinal chemistry. However, efficient and stereoselective methods for synthesizing Z-monofluoroalkenes are underdeveloped. We envisage (Z)-β-fluoro-vinyl iodonium salts (Z-FVIs) as coupling partners for the diverse and stereoselective synthesis of Z-monofluoroalkenes. Disclosed herein is the development and application of a silver(I)-catalyzed process for accessing a broad scope of (Z)-FVIs with exclusive Z-stereoselectivity and regioselectivity from alkynes in a single step. Experimental and computational studies provide insight into the mechanism of the catalytic cycle and the role of the silver(I) catalyst, and the reactivity of (Z)-FVIs is explored through several stereospecific derivatizations.

Catalytic HF Shuttling between Fluoroalkanes and Alkynes

In this paper, we report BF3  ⋅ OEt2 as a catalyst to shuttle equivalents of HF from a fluoroalkane to an alkyne. Reactions of terminal and internal aliphatic alkynes led to formation of difluoroalkane products, while diarylalkynes can be selectively converted into fluoroalkenes. The method tolerates numerous sensitive functional groups including halogen, protected amine, ester and thiophene substituents. Mechanistic studies (DFT, probe experiments) suggest the catalyst is involved in both the defluorination and fluorination steps, with BF3 acting as a Lewis acid and OEt2 a weak Lewis base that mediates proton transfer. In certain cases, the interconversion of fluoroalkene and difluoroalkane products was found to be reversible. The new catalytic system was applied to demonstrate proof-of-concept recycling of poly(vinylidene difluoride).

Hydrofluorination of Alkynes: From (E) to (Z)

The hydrofluorination of alkynes is an efficient synthetic route to monofluoroalkenes or difluoroalkanes. Both fluorinated motifs have found applications in medicinal chemistry and beyond. This review explores the recent advances in the hydrofluorination of diverse alkynes through various activation methods, from classical coinage metal catalysis to metal-free conditions. The range of alkynes goes from the simplest unactivated alkynes to activated ones (ynones and derivatives, ynamides, alkynyl sulfides and sulfones as much as haloalkynes). Regio- and stereoselective methods exists, but there is still room for improvement depending on the type of alkyne.

Au(I)-Catalyzed Regioselective Hydrofluorination of Propargylamines Using Aqueous HF

Gold-catalyzed regioselective hydrofluorination of alkynes using aqueous HF has been achieved by employing an amide directing group. For both aryl- and alkyl-substituted propargylamines, the fluorination occurred at the site distal to the amino group.

Gold catalyzed hydrofluorination of propargyl alcohols promoted by fluorine-hydrogen bonding

The hydroxyl group was discovered to promote gold catalyzed hydrofluorination of alkynes via hydrogen bonding interaction. Based on this strategy, propargyl alcohols could be hydrofluorinated smoothly using Et₃N·3HF under acidic additive-free conditions, which provided a straightforward alternative protocol for the synthesis of 3-fluoroallyl alcohols.

Au(I) Catalyzed HF Transfer: Tandem Alkyne Hydrofluorination and Perfluoroarene Functionalization

HF transfer reactions between organic substrates are potentially useful transformations. Such reactions require the development of catalytic systems that can promote both defluorination and fluorination steps in a single reaction sequence. Herein, we report a catalytic protocol in which an equivalent of HF is generated from a perfluoroarene | nucleophile pair and transferred directly to an alkyne. The reaction is catalyzed by [Au(IPr)NⁱPr₂] (IPr = N,N′-1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). HF transfer generates two useful products in the form of functionalized fluoroarenes and fluoroalkenes. Mechanistic studies (rate laws, KIEs, density functional theory (DFT) calculations, competition experiments) are consistent with the Au(I) catalyst facilitating a catalytic network involving both concerted S_NAr and hydrofluorination steps. The nature of the nucleophile impacts the turnover-limiting step. The cS_NAr step is turnover-limiting for phenol-based nucleophiles, while protodeuaration likely becomes turnover-limiting for aniline-based nucleophiles. The approach removes the need for direct handling of HF reagents in hydrofluorination and offers possibilities to manipulate the fluorine content of organic molecules through catalysis.

Synthesis of Gold(I)-Trifluoromethyl Complexes and their Role in Generating Spectroscopic Evidence for a Gold(I)-Difluorocarbene Species

Readily prepared and bench-stable [Au(CF₃ )(NHC)] compounds were synthesized by using new methods, starting from [Au(OH)(NHC)], [Au(Cl)(NHC)] or [Au(L)(NHC)]HF₂ precursors (NHC=N-heterocyclic carbene). The mechanism of formation of these species was investigated. Consequently, a new and straightforward strategy for the mild and selective cleavage of a single carbon/fluorine bond from [Au(CF₃ )(NHC)] complexes was attempted and found to be reversible in the presence of an additional nucleophilic fluoride source. This straightforward technique has led to the unprecedented spectroscopic observation of a gold(I)-NHC difluorocarbene species.

Assembly of Thiosubstituted Benzoxazoles via Copper-Catalyzed Coupling of Thiols with 5-Iodotriazoles Serving as Diazo Surrogates

An efficient cascade approach to thiosubstituted benzoxazoles has been developed. The transformation starts with in situ generation of a diazo compound via annulation-triggered electrocyclic opening of the 1,2,3-triazole ring. The subsequent Cu-catalyzed trapping of diazo intermediates by various thiols affords the desired heterocycles in generally good yields of up to 91%. The protocol features very good functional group tolerance and is applicable to substrates with different electronic properties.

Stereodivergent Alkyne Hydrofluorination Using Protic Tetrafluoroborates as Tunable Reagents

The discovery of safe, general, and practical procedures to prepare vinyl fluorides from readily available precursors remains a synthetic challenge. The metal-free hydrofluorination of alkynes constitutes an attractive though elusive strategy for their preparation. Introduced here is an inexpensive and easily handled reagent that enables the development of simple and scalable protocols for the regioselective hydrofluorination of alkynes to access both the E and Z isomers of vinyl fluorides. These reaction conditions were suitable for a diverse collection of alkynes, including several highly functionalized pharmaceutical derivatives. Computational and experimental mechanistic studies support C-F bond formation through vinyl cation intermediates, with the E- and Z-hydrofluorination products forming under kinetic and thermodynamic control, respectively.

A HF Loaded Lewis-Acidic Aluminium Chlorofluoride for Hydrofluorination Reactions

The very strong Lewis acid aluminium chlorofluoride (ACF) was loaded with anhydrous HF. The interaction between the surface of the catalyst and HF was investigated using a variety of characterization methods, which revealed the formation of polyfluorides. Moreover, the reactivity of the HF-loaded ACF towards the hydrofluorination of alkynes was studied.

Efficient synthesis of 1-iodoalkynes via Al2O3 mediated reaction of terminal alkynes and N-iodosuccinimide

Iodination of terminal alkynes using N-iodosuccinimide (NIS) in the presence of γ-Al2O3 was developed to afford 1-iodoalkynes with good to excellent yields (up to 99%). This described approach featured excellent chemoselectivity, good functional group tolerance, and utilization of an inexpensive catalyst.

Silver-Mediated Decarboxylative Fluorodiiodination of Alkynoic Acids: Synthesis of Regio- and Stereoselective Fluoroalkenes

A variety of arylalkynoic acids reacted with 1,3-diiodo-5,5-dimethylhydantoin and HF·pyridine in the presence of AgOAc to provide the corresponding 1-fluoro-2,2-diiodovinylarenes in good yields and high regioselectivity. In addition, Pd-catalyzed cross-coupling reaction of 1-fluoro-2,2-diiodovinylarenes afforded diaryl coupling products in the Suzuki reaction and monoaryl coupling products with high stereoselectivity in the Hiyama reaction. It was found that C-F-activated borylation of fluoroalkenes using Pd catalyst afforded the vinylboranes with good yields.

Quantifying electronic similarities between NHC–gold(i) complexes and their isolobal imidazolium precursors

The σ and π donor/acceptor properties of the carbon–gold bond are manifested by changes in ¹⁹⁷Au spectroscopic data.

Chemoselective and stereospecific iodination of alkynes using sulfonium iodate(i) salt

The sulfonium iodate(i) electrophilc reagent efficiently promoted the stereodivergent iodination of alkynes. The chemoselective iodination was effectively realized by switching the solvent system to access the 1-iodoalkyne and (E)-vicinal-diiodoalkenes.

Unveiling the mechanisms and secrets of chemoselectivities in Au(i)-catalyzed diazo-based couplings with aryl unsaturated aliphatic alcohols

The mechanisms and origins of the unsaturated-aliphatic-alcohols-modulated chemoselectivity of Au-catalyzed carbene transfers from diazo compounds are disclosed by DFT studies.

Hypervalent Iodine Mediated Chemoselective Iodination of Alkynes

Reported herein are practical approaches for the chemoselective mono-, di-, and tri-iodination of alkynes based on efficient oxidative iodinations catalyzed by hypervalent iodine reagents. The reaction conditions were systematically optimized by altering the iodine source and/or the hypervalent iodine reagent system. The tetrabutylammonium iodide (TBAI)/(diacetoxyiodo)benzene (PIDA) system is specific for monoiodination, while the KI/PIDA system results in di-iodination. Combining the TBAI/PIDA and KI/PIDA systems in one pot provided the corresponding tri-iodination products efficiently. These reaction conditions can be applied to the synthetically important iodination of aromatic and aliphatic alkynes, which was accomplished in good yield (up to 99%) and excellent chemoselectivity. These synthetic methods can also be applied to the efficient chemoselective synthesis of iodoalkyne derivatives, intermediates, and related biologically active compounds.