Gold-catalyzed Fluorination of Alkynyl Esters and Ketones: Efficient Access to Fluorinated 1,3-Dicarbonyl Compounds

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.

DFT rationalization of the mechanism and selectivity in a gold-catalyzed oxidative cyclization of diynones with alcohols

The mechanism, regioselectivity, and chemoselectivity in a gold-catalyzed oxidative cyclization of diynones with alcohols to give furan-3-carboxylate derivatives were explored by density functional theory (DFT). The obtained results revealed that the first step of the global reaction involves a nucleophilic attack of a pyridine-N-oxide derivative on the catalyst-ligated diynone, forming a vinyl intermediate that can isomerize to an α,α'-dioxo gold carbene upon the cleavage of the N-O bond. In the second step, a nucleophilic addition is also completed via pyridine-N-oxide instead of an alcohol proposed in the experiment. In the following steps, the selective nucleophilic addition of alcohol, 1,2-alkynyl migration, five-membered cyclization, and protodeauration lead to the furan-based products with the regeneration of the gold catalyst. The unique features of regio- and chemoselectivity were investigated in detail by the global reactivity index (GRI) and distortion/interaction analyses. Apart from fully rationalizing the experimental data, the DFT results provide an important contribution to understanding, optimizing, and further developing the related types of organic transformations.

Determination of the Hammett Acidity of HF/Base Reagents

Harnessing the acidity of HF/base reagents is of paramount importance to improve the efficiency and selectivity of fluorination reactions. Yet, no general method has been reported to evaluate their acidic properties, and experimental designs are still relying on a trial-and-error approach. We report a new method based on 19F NMR spectroscopy which allows highly sensitive measures and short-time analyses. Advantageously, the basic properties of the indicators can be determined upstream by DFT calculations, affording a simple yet robust semiempirical approach. In particular, the indicators used in this study were rationally designed to fit on the conceptually appealing and commonly used Hammett scale. This method has been applied to commercially available and recently developed HF/base reagents.

Chemo- and regio-divergent access to fluorinated 1-alkyl and 1-acyl triazenes from alkynyl triazenes

The 1,1,2,2-tetrafluoroethylene unit is a prevalent pattern in bioactive molecules and functional materials. Despite being in principle a straightforward strategy to access this motif, the direct tetrafluorination of alkynes involves...

Fluorohydration of alkynes via I(I)/I(III) catalysis

Substrate specificity is ubiquitous in biological catalysis, but less pervasive in the realm of small-molecule catalysis. Herein, we disclose an intriguing example of substrate specificity that was observed whilst exploring catalysis-based routes to generate α-fluoroketones from terminal and internal alkynes under the auspices of I(I)/I(III) catalysis. Utilising p-TolI as an inexpensive organocatalyst with Selectfluor^® and amine/HF mixtures, the formation of protected α-fluoroketones from simple alkynes was realised. Whilst the transient p-TolIF₂ species generated in situ productively engaged with pentynyl benzoate scaffolds to generate the desired α-fluoroketone motif, augmentation or contraction of the linker suppressed catalysis. The prerequisite for this substructure was established by molecular editing and was complemented with a physical organic investigation of possible determinants.

Synthesis of α-Fluorinated Imides via Direct Fluorohydroxylation of Ynamides

A practical synthesis of α-fluorinated imides via the catalyst-free fluorohydroxylation of ynamides is developed. The reaction employs commercially available Selectfluor (F-TEDA-BF₄) and H₂O as the fluorine and hydroxyl sources, respectively. A broad range of aryl- or alkyl-substituted ynamides were well applicable to the reaction with good functional group tolerance under simple and mild reaction conditions. The synthetic utility of the α-fluoroimide products was demonstrated by several value-added transformations. Preliminary mechanistic studies were conducted.

Transition metal-free functionalized hydration of alkynes: one-pot synthesis of fluorinated β-keto-imidates using Selectfluor

A transition metal-free, four-component one-pot synthesis of polyfunctionalized fluorinated β-keto-imidates via the functionalized hydration of alkynes has been described. The intermediate 1,3-ketoamino moiety was obtained from easily accessible arylpropioladehyde and arlyhydroxylamine and was treated with Selectfluor delivering fluorinated β-keto-imidates. A wide variety of functional groups are tolerated under mild reaction conditions and the product applicability is highlighted.

Metal-free, Regio-, and Stereo-Controlled Hydrochlorination and Hydrobromination of Ynones and Ynamides

We developed an atom-economical and metal-free method for the regio- and stereo-selective hydrohalogenation of ynones and ynamides using easy to handle DMPU/HX (X = Br or Cl) reagents. The reaction operates under mild conditions and a range of functional groups is well tolerated. We propose that the hydrohalogenation of ynones gives the anti-addition products via a concerted multimolecular Ad_E3 mechanism and that the hydrohalogenation of ynamides produces the syn-addition products via a cationic keteniminium intermediate.