Beyond the Balz-Schiemann reaction: the utility of tetrafluoroborates and boron trifluoride as nucleophilic fluoride sources

Regiodivergent hydrosilylation, hydrogenation, [2π + 2π]-cycloaddition and C-H borylation using counterion activated earth-abundant metal catalysis

The widespread adoption of earth-abundant metal catalysis lags behind that of the second- and third-row transition metals due to the often challenging practical requirements needed to generate the active low oxidation-state catalysts. Here we report the development of a single endogenous activation protocol across five reaction classes using both iron- and cobalt pre-catalysts. This simple catalytic manifold uses commercially available, bench-stable iron- or cobalt tetrafluoroborate salts to perform regiodivergent alkene and alkyne hydrosilylation, 1,3-diene hydrosilylation, hydrogenation, [2π + 2π]-cycloaddition and C-H borylation. The activation protocol proceeds by fluoride dissociation from the counterion, in situ formation of a hydridic activator and generation of a low oxidation-state catalyst.

Earth-Abundant Metal Catalysis Enabled by Counterion Activation

A precatalyst activation strategy has been developed for earth-abundant metal catalysis enabled by counterion dissociation and demonstrated through alkene hydroboration. Commercially available iron and cobalt tetrafluoroborate salts were found to catalyze the hydroboration of aryl and alkyl alkenes with good functional group tolerance (Fe, 12 substrates; Co, 13 substrates) with three structurally distinct ligands. Key to this endogenous activation was counterion dissociation to generate fluoride which indirectly activates the precatalyst by reaction with pinacol borane.

Difluorinative ring expansions of benzo-fused carbocycles and heterocycles are achieved withp-(difluoroiodo)toluene

Fluorination of exocyclic alkenes and allenes withp-TolIF₂gives ring-expanded β,β-difluoridesviaa 1,2-phenyl shift.

Expanding the Balz-Schiemann Reaction: Organotrifluoroborates Serve as Competent Sources of Fluoride Ion for Fluoro-Dediazoniation

The Balz-Schiemann reaction endures as a method for the preparation of (hetero)aryl fluorides yet is eschewed due to the need for harsh conditions or high temperatures along with the need to isolate potentially explosive diazonium salts. In a departure from these conditions, we show that various organotrifluoroborates (RBF₃ ^- s) may serve as fluoride ion sources for solution-phase fluoro-dediazoniation in organic solvents under mild conditions. This methodology was successfully extended to a one-pot process obviating aryl diazonium salt isolation. Sterically hindered (hetero)anilines are fluorinated under unprecedentedly mild conditions in good-to-excellent yields. Taken together, this work expands the repertoire of RBF₃ ^- s to act as fluorine ion sources in an update to the classic Balz-Schiemann reaction.

An aryne triggered ring-opening fluorination of cyclic thioethers with potassium fluoride

Herein, we report an aryne triggered ring-opening fluorination protocol of a great variety of saturated sulfur heterocycles. A key factor for the success is the identification of a suitable mediator. Compared to previous methods, this transition-metal free protocol employs low-cost potassium fluoride as the fluorine source. The operational simplicity and mild reaction conditions allow for the rapid synthesis of a wide range of aliphatic fluoride compounds in good yields.

Radical Chlorodifluoromethylation: Providing a Motif for (Hetero)arene Diversification

A method for the radical chlorodifluoromethylation of (hetero)arenes using chlorodifluoroacetic anhydride is reported. This operationally simple protocol proceeds under mild photochemical conditions with high functional group compatibility and complements the large body of literature for the trifluoromethylation of (hetero)arenes. Introduction of the chlorodifluoromethyl motif enables rapid diversification to a wide array of aromatic scaffolds. This work showcases the chlorodifluoromethyl group as an attractive entryway to otherwise synthetically challenging electron-rich difluoromethyl(hetero)arenes. Furthermore, facile conversion of the CF2Cl moiety into the corresponding aryl esters, gem-difluoroenones, and β-keto-esters is demonstrated.

Modern Approaches for Asymmetric Construction of Carbon-Fluorine Quaternary Stereogenic Centers: Synthetic Challenges and Pharmaceutical Needs

New methods for preparation of tailor-made fluorine-containing compounds are in extremely high demand in nearly every sector of chemical industry. The asymmetric construction of quaternary C-F stereogenic centers is the most synthetically challenging and, consequently, the least developed area of research. As a reflection of this apparent methodological deficit, pharmaceutical drugs featuring C-F stereogenic centers constitute less than 1% of all fluorine-containing medicines currently on the market or in clinical development. Here we provide a comprehensive review of current research activity in this area, including such general directions as asymmetric electrophilic fluorination via organocatalytic and transition-metal catalyzed reactions, asymmetric elaboration of fluorine-containing substrates via alkylations, Mannich, Michael, and aldol additions, cross-coupling reactions, and biocatalytic approaches.

cis-Specific cyanofluorination of vinyl azides enabled by electron-donor-acceptor complexes: synthesis of α-azido-β-fluoronitriles

Here we report a novel electron-donor-acceptor (EDA) complex-enabled three-component cyanofluorination of vinyl azides under metal-free conditions in a cis-specific manner. This reaction protocol is operationally simple without exclusion of either moisture or oxygen, allowing access to a wide range of highly-functionalized α-azido-β-fluoronitriles bearing quaternary carbons that are difficult to obtain by existing methods.

A Cross-Dehydrogenative Annulation Strategy towards Synthesis of Polyfluorinated Phenanthridinones with Copper

The first cross-dehydrogenative annulation of (hetero)aromatic amides with polyfluoro(hetero)arenes is presented. This operationally simple oxidative annulation process is mediated by inexpensive copper salt, accommodates a wide range of substrates with exquisite chemo- and regioselectivity profile, and produces demanding polyfluorinated phenanthridinones in high yields (up to 92 %). Using alkenyl amides under identical conditions, the synthesis of polyfluorinated 2-quinolones has also been accomplished. Given the importance of fluorinated heterocycles in the pharmaceutical industry and drug discovery, this work is highly significant.

Metal-Free Synthesis of (E)-Monofluoroenamine from 1-Sulfonyl-1,2,3-triazole and Et2O·BF3 via Stereospecific Fluorination of α-Diazoimine

A general, stereospecific, and straightforward method for the rapid preparation of functionalized (E)-monofluoroenamines is reported. Rather than transition metals (Rh, Ni, Pd, Cu, Ag, etc.), Et₂O·BF₃ was employed to promote the formation of α-diazoimine through the Dimroth equilibrium of common 1-sulfonyl-1,2,3-triazole for the first time. An overall migration of fluoride from boron to the diazo-linked carbon of α-diazoimine was achieved. Derivations and late-stage modification of bioactive molecule were conducted. A plausible mechanism was also proposed.

Recent advances in the synthesis of fluorinated hydrazones

Fluorinated compounds are widely used in pharmaceuticals, agrochemicals, and materials.

p-Toluenesulfonic acid catalysed fluorination of α-branched ketones for the construction of fluorinated quaternary carbon centres

A catalytic and concise fluorination of α-branched ketones for the construction of the challenging quaternary C–F bond could be achieved.

Fluorination of some highly functionalized cycloalkanes: chemoselectivity and substrate dependence

A study exploring the chemical behavior of some dihydroxylated β-amino ester stereo- and regioisomers, derived from unsaturated cyclic β-amino acids is described. The nucleophilic fluorinations involving hydroxy-fluorine exchange of some highly functionalized alicyclic diol derivatives have been carried out in view of selective fluorination, investigating substrate dependence, neighboring group assistance and chemodifferentiation.

China's flourishing synthetic organofluorine chemistry: innovations in the new millennium

The new millennium has witnessed the rapid development of synthetic organofluorine chemistry all over the world, and chemists in China have made significant contributions in this field. This review aims to provide a brief introduction to China's primary innovations from 2000 to early 2017, covering fluorination, fluoroalkylation, fluoromethylthiolation, fluoroolefination and polyfluoroarylation, as well as synthesis with fluorinated building blocks. Recent advances in the chemistry of difluorocarbene and the chemistry of carbon–fluorine bond activation are also discussed. As a conclusion, the review ends with some personal perspectives on the future development of China's synthetic organofluorine chemistry.

Triflic Acid Promoted Decarboxylation of Adamantane-oxazolidine-2-one: Access to Chiral Amines and Heterocycles

We have developed a one-step procedure to a variety of chiral lipophilic and conformationally rigid amines and heterocycles by decarboxylation of adamantane-oxazolidine-2-one. Triflic acid or aluminum triflate promote the addition of diverse nucleophiles to the oxazolidine-2-one moiety accompanied by the release of carbon dioxide. The resulting amine or heterocycle is then protonated/metalated by the catalyst (promotor). Additionally, the starting racemic material, adamantane-oxazolidine-2-one, was resolved into single enantiomers using a chiral auxiliary to access enantio-enriched products and to study the racemization pathway of chiral 1,2-disubstituted adamantane derivatives.

Fluorinative ring-opening of cyclopropanes by hypervalent iodine reagents. An efficient method for 1,3-oxyfluorination and 1,3-difluorination

A new method is presented for 1,3-difluorination and 1,3-oxyfluorination reactions. The process is based on iodonium mediated opening of 1,1-disubstituted cyclopropanes. The reaction proceeds with high chemo- and regioselectivity under mild reaction conditions typically at room temperature in a couple of hours. The reaction probably occurs via electrophilic ring-opening of cyclopropanes.

Silver-catalyzed radical carbofluorination of unactivated alkenes with acetic acid in aqueous solution

The AgNO₃-catalyzed carbofluorination of unactivated alkenes with acetic acid provides an efficient and general route to γ-fluorinated carboxylic acids.

Silver-mediated fluorination of alkyl iodides with TMSCF3 as the fluorinating agent

The first example of a silver-mediated fluorination of alkyl iodides with the Ruppert–Prakash reagent (TMSCF₃) as the fluorinating agent is reported.

Divergent synthesis from reactions of 2-trifluoromethyl-1,3-conjugated enynes with N-acetylated 2-aminomalonates

Divergent synthesis of fluorinated heterocycles from the reactions of 2-trifluoromethyl-1,3-conjugated enynes with N-acetylated 2-aminomalonates and subsequent gold-catalysed cyclization were developed.

Silver-induced self-immolative Cl–F exchange fluorination of arylsulfur chlorotetrafluorides: synthesis of arylsulfur pentafluorides

Cl–F exchange fluorination of (Het)ArSF₄Cl without fluorinating reagents is disclosed.

Mechanism of Silver-Mediated Geminal Difluorination of Styrenes with a Fluoroiodane Reagent: Insights into Lewis-Acid-Activation Model

Fluorination mediated by the cyclic hypervalent fluoroiodane reagent (1) often requires an exogenous Lewis acid. The widely accepted Lewis-acid-activation model is that a given Lewis acid binds to the oxygen atom of 1 (O-coordination) to polarize the I-O bond. Computational studies of silver-mediated geminal difluorination of styrenes with 1 reveal a new "F-coordination" model that is energetically much preferred over the commonly accepted "O-coordination" model. The calculations rationalize the regioselective formation of the geminal difluorination product.

Siladifluoromethylation and Difluoromethylation onto C(sp(3)), C(sp(2)), and C(sp) Centers Using Ruppert-Prakash Reagent and Fluoroform

Siladifluoromethylations and difluoromethylations on sp(3), sp(2), and sp carbons of lithiated carbamates, arenes, and terminal alkynes, respectively, have been attained by employing the Ruppert-Prakash reagent (CF3TMS) and fluoroform (CF3H) as the CF2 sources. The advantage of this reaction is that the (sila)difluoromethylated compounds can be obtained by simple treatment of easily accessible substrates, lithium bases, and CF3TMS or CF3H. Furthermore, the products bearing the TMS group can be transformed into the valuable compounds with the CF2 fragment via the carbon-carbon bond forming reactions.