1-tert-Butyl-1H-tetrazol-5-yl fluoromethyl sulfone (TBTSO2CH2F): a versatile fluoromethylidene synthon and its use in the synthesis of monofluorinated alkenes via Julia–Kocienski olefination

Streamlining Fluoroalkenyl Arene Synthesis Illuminated with Mechanistic Insights

Fluorinated compounds are a staple of modern-day chemical innovation, and efficient strategies for their synthesis are highly valuable. In this chemical space, fluoroalkenes continue to be the object of much interest across diverse fields, including drug development and pharmaceuticals with active roles as bioisosteres. Herein, in expanding chemists' synthetic toolbox for constructing valuable organofluorine compounds, we report a "pipeline" strategy for the synthesis of fluorinated olefins, viz., gem-difluoroalkenyl and monofluoroalkenyl arenes with high E-isomeric selectivity. The advantages of this streamlined synthetic protocol include mild reaction conditions, operational simplicity, broad substrate scope, and good to excellent yields, even at gram scales. Critical to this robust procedure is the use of widely available and inexpensive "scavenger" solid-support Merrifield peptide resin for removing phosphine impurities. Further computational investigations offering clarity into this reactivity are disclosed.

Tunable Synthesis of Monofluoroalkenes and Gem-Difluoroalkenes via Solvent-Controlled Rhodium-Catalyzed Arylation of 1-Bromo-2,2-difluoroethylene

Divergent synthesis of fluorine-containing scaffolds starting from a suite of raw materials is an intriguing topic. Herein, we report the solvent-controlled rhodium-catalyzed tunable arylation of 1-bromo-2,2-difluoroethylene. The selection of the reaction solvents provides switchable defluorinated or debrominated arylation from readily available feedstock resources (both arylboronic acids/esters and 1-bromo-2,2-difluoroethylene are commercially available). This switch is feasible because of the difference in coordination ability between the solvent (CH2 Cl2 or CH3 CN) and the rhodium center, resulting in different olefin insertion. This protocol allows the convenient synthesis of monofluoroalkenes and gem-difluoroalkenes, both of which are important scaffolds in the fields of medicine and materials. Moreover, this newly developed solvent-regulated reaction system can be applied to the site-selective dechlorinated arylation of trichloroethylene. Overall, this study provides a useful strategy for the divergent synthesis of fluorine-containing scaffolds and provides insight into the importance of solvent selection in catalytic reactions.

Cu-catalyzed cascade difluoroalkylation/5-endo cyclization/β-fluorine cleavage of ynones

A copper-catalyzed, redox-neutral cascade difluoroalkylation/5-endo annulation/β-fluorine cleavage of ynones is developed, providing a direct and stereoselective method to access synthetically important α-monofluoroalkenyl cyclopentanones. Mechanistic studies suggest an unprecedented Cu^II-assisted β-fluorine fragmentation, which may be valuable for the challenging but important C-F bond activation. Moreover, the in situ generated difluorocarbene was found to serve as an effective reductant for the regeneration of copper(I) catalyst, thus avoiding the addition of external reductants.

Synthesis of gem-Difluorinated 1,3-Dienes via Synergistic Cu/Pd-Catalyzed Borodifluorovinylation of Alkynes

gem-Difluoroalkenes (=CF₂), which normally act as metabolically stable bioisosteres for carbonyl groups (C═O), are widely applied in agrochemicals and pharmaceuticals and are also used as building blocks in organic synthesis. Herein, an example of Cu/Pd-catalyzed borodifluorovinylation was achieved using alkynes, difluoroethylene bromide, and B₂pin₂ as chemical feedstocks, providing the corresponding conjugated gem-difluoroalkene scaffold with good functional group compatibility. Moreover, an array of fluorinated synthons can be obtained through further transformations.

Defluorinative C-C Bond-Forming Reaction of Trifluoromethyl Alkenes with gem-(Diborylalkyl)lithiums

We report the transition-metal-free defluorinative C-C bond-forming reaction of trifluoromethyl alkenes with gem-(diborylalkyl)lithiums. This synthetic strategy provides access to a variety of 4,4-difluoro homoallylic diboronate esters, which serve as versatile intermediates in the efficient preparation of valuable gem-difluoroalkene derivatives. Further synthetic modifications are conducted to demonstrate the synthetic utility of the obtained 4,4-difluoro homoallylic diboronate esters.

Electrochemical Fluorination of Vinyl Boronates through Donor-Stabilized Vinyl Carbocation Intermediates

The electrochemical generation of vinyl carbocations from alkenyl boronic esters and boronates is reported. Using easy-to-handle nucleophilic fluoride reagents, these intermediates are trapped to form fully substituted vinyl fluorides. Mechanistic studies support the formation of dicoordinated carbocations through sequential single-electron oxidation events. Notably, this electrochemical fluorination features fast reaction times and Lewis acid-free conditions. This transformation provides a complementary method to access vinyl fluorides with simple fluoride salts such as TBAF.

Chemoselective catalytic hydrodefluorination of trifluoromethylalkenes towards mono-/gem-di-fluoroalkenes under metal-free conditions

Fluorine-containing moieties show significant effects in improving the properties of functional molecules. Consequently, efficient methods for installing them into target compounds are in great demand, especially those enabled by metal-free catalysis. Here we show a diazaphospholene-catalyzed hydrodefluorination of trifluoromethylalkenes to chemoselectively construct gem-difluoroalkenes and terminal monofluoroalkenes by simple adjustment of the reactant stoichiometry. This metal-free hydrodefluorination features mild reaction conditions, good group compatibility, and almost quantitative yields for both product types. Stoichiometric experiments indicated a stepwise mechanism: hydridic addition to fluoroalkenes and subsequent β-F elimination from hydrophosphination intermediates. Density functional theory calculations disclosed the origin of chemoselectivity, regioselectivity and stereoselectivity, suggesting an electron-donating effect of the alkene-terminal fluorine atom.

Highly E-Selective Synthesis of α-Fluoro-β-arylalkenyl Sulfones from gem-Difluoroalkenes with Sodium Sulfinates

The straightforward synthesis of α-fluoro-β-arylalkenyl sulfones under transition-metal- and base-free conditions has been described, which displays broad functional group compatibility and high stereoselectivity. In particular, the strategy is also applied to the late-stage modification of complex natural products and drugs.

Monofluorinated 5-membered rings via fluoromethylene transfer: synthesis of monofluorinated isoxazoline N-oxides

The synthesis of five-membered rings using fluoromethylene transfer chemistry is an attractive method for building fluorinated products of high value. This work demonstrates for the first time that one-fluorine-one-carbon modification of a substrate could be a viable strategy to access monofluorinated five-membered rings. The synthetic methodology was developed to access monofluorinated isoxazoline-N-oxides in one step starting from substituted 2-nitroacrylates using fluoromethylsulfonium reagents.

Stereoselective Synthesis of Trisubstituted (Z)-Alkenes from Ketones via the Julia-Kocienski Olefination Using 1-Methyl- and 1-tert-Butyl-1H-tetrazol-5-yl Alkyl Sulfones

1-Methyl-1H-tetrazol-5-yl (MT) alkyl sulfones 1b, 3b, and 7b react with various unsymmetrical ketones in the presence of LiHMDS in THF at low temperature to give trisubstituted (Z)-alkenes in good yields stereoselectively (Z/E = 91:9 to 99:1). For sterically less demanding ketones, olefination using t-Bu reagents 1d and 3d generated (Z)-alkenes with higher stereoselectivity (93:7-99:1).

Stereoselective Synthesis of Terminal Monofluoroalkenes from Trifluoromethylated Alkenes

Herein we report the hydrodefluorination reaction of trifluoromethylated alkenes to access terminal monofluoroalkenes. The use of LiAlH₄ allowed the stereoselective synthesis of the terminal monofluoroalkenes in good to excellent yields with good to excellent diastereoselectivities. Mechanistic studies suggested a hydroalumination reaction followed by a stereoselective fluoride elimination.

Modular radical cross-coupling with sulfones enables access to sp3-rich (fluoro)alkylated scaffolds

Cross-coupling chemistry is widely applied to carbon-carbon bond formation in the synthesis of medicines, agrochemicals, and other functional materials. Recently, single-electron-induced variants of this reaction class have proven particularly useful in the formation of C(sp²)-C(sp³) linkages, although certain compound classes have remained a challenge. Here, we report the use of sulfones to activate the alkyl coupling partner in nickel-catalyzed radical cross-coupling with aryl zinc reagents. This method's tolerance of fluoroalkyl substituents proved particularly advantageous for the streamlined preparation of pharmaceutically oriented fluorinated scaffolds that previously required multiple steps, toxic reagents, and nonmodular retrosynthetic blueprints. Five specific sulfone reagents facilitate the rapid assembly of a vast set of compounds, many of which contain challenging fluorination patterns.

Manganese-catalyzed synthesis of monofluoroalkenes via C–H activation and C–F cleavage

A redox-neutral manganese-catalyzed α-fluoroalkenylation of arenes via C–H activation and C–F cleavage has been described.

Enantioselective synthesis of Iboga alkaloids and vinblastine via rearrangements of quaternary ammoniums

An efficient and novel strategy for the enantioselective syntheses of various iboga alkaloids has been developed. The salient features include a gold-catalyzed oxidation of a terminal alkyne followed by cyclization, a Stevens rearrangement and a tandem sequence that combines the gold-catalyzed oxidation, cyclization and [1,2]-shift. The catharanthine analogs provided by our approach were further converted to the vinca alkaloid vinblastine and its analogs, which confirmed the remarkable sensitivity of the cytotoxicity to the C20' substituent of vinblastine.

Kinetically controlled E-selective catalytic olefin metathesis

A major shortcoming in olefin metathesis, a chemical process that is central to research in several branches of chemistry, is the lack of efficient methods that kinetically favor E isomers in the product distribution. Here we show that kinetically E-selective cross-metathesis reactions may be designed to generate thermodynamically disfavored alkenyl chlorides and fluorides in high yield and with exceptional stereoselectivity. With 1.0 to 5.0 mole % of a molybdenum-based catalyst, which may be delivered in the form of air- and moisture-stable paraffin pellets, reactions typically proceed to completion within 4 hours at ambient temperature. Many isomerically pure E-alkenyl chlorides, applicable to catalytic cross-coupling transformations and found in biologically active entities, thus become easily and directly accessible. Similarly, E-alkenyl fluorides can be synthesized from simpler compounds or more complex molecules.

Regioselective Fluorination of 1-(2,2-Dibromovinyl)benzene Derivatives with Wet Tetra-n-butylammonium Fluoride: One-Pot Synthesis of (Z)-1-(2-Bromo-1-fluorovinyl)benzenes

A direct fluorination of 1-(2,2-dibromovinyl)benzene derivatives using wet tetra-n-butylammonium fluoride (TBAF·3H2O) as either a base or a fluorine source in toluene was accomplished, which provided (Z)-1-(2-bromo-1-fluorovinyl)benzene compounds in up to 81% yields with high regioselectivities. This reaction results strongly depend upon the reaction conditions. The mechanism of this reaction was investigated as well.

Direct fluorination of styrenes

We have developed a practical method to synthesize fluorostyrene compounds. A mild and regioselective mono-fluorination reaction occurred smoothly for various di- and trisubstituted styrenes in the presence of RuCl3 and N-fluorobenzenesulfonimide (NFSI). A tandem alkyne hydroarylation-olefin fluorination reaction was also developed using an Au catalyst.

Difluoromethyl 2-pyridyl sulfone: a versatile carbonyl gem-difluoroolefination reagent

The gem-difluoroolefination of carbonyl compounds with difluoromethyl 2-pyridyl sulfone (2-PySO₂CF₂H) has been summarized in this tutorial account.

Designer HF-based fluorination reagent: highly regioselective synthesis of fluoroalkenes and gem-difluoromethylene compounds from alkynes

Hydrogen fluoride (HF) and selected nonbasic and weakly coordinating (toward cationic metal) hydrogen-bond acceptors (e.g., DMPU) can form stable complexes through hydrogen bonding. The DMPU/HF complex is a new nucleophilic fluorination reagent that has high acidity and is compatible with cationic metal catalysts. The gold-catalyzed mono- and dihydrofluorination of alkynes using the DMPU/HF complex yields synthetically important fluoroalkenes and gem-difluoromethlylene compounds regioselectively.

A practical route for the highly stereoselective synthesis of tetrasubstituted fluoroalkenes

A series of tetrasubstituted fluoroalkene derivatives were synthesized by the reaction of α-fluoro-β-carbonyl benzothiazol-2-yl sulfones with various nucleophiles in good yields with high stereoselectivities. The predominant cis configuration of fluorine and alkynyl groups was observed. A single isomer was obtained when a ketone, acetate or amide was used as the substrate in the presence of a base.

Introduction of fluorine and fluorine-containing functional groups

Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo- and transition-metal catalysis. The most challenging transformation remains the formation of the parent C-F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal-fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost-efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.

Stereoselective synthesis of conjugated fluoro enynes

Metalation-electrophilic fluorination of TMS- and TIPS-protected 1,3-benzothiazol-2-yl (BT) propargyl sulfones gave corresponding BT fluoropropargyl sulfones, Julia-Kocienski reagents for the synthesis of fluoro enynes. Both reagents reacted with aldehydes under mild DBU- or LHMDS-mediated conditions, giving high yields of conjugated fluoro enynes with E-stereoselectivity. In comparison to DBU-mediated reactions, stereoselectivity was higher in low-temperature LHMDS-mediated reactions. Two ketones were shown to react as well, using LHMDS as base. In situ removal of the TMS group gave terminal conjugated 2-fluoro 1,3-enynes. Synthetic utility of the fluoro enynes was demonstrated by conversion to internal alkynes and to stereoisomeric fluoro dienes via Sonogashira and Heck couplings.

Expedient synthesis of α-substituted fluoroethenes

A mild and efficient synthesis of 1-aryl-1-fluoroethenes from benzothiazolyl (aryl)fluoromethyl sulfones and paraformaldehyde, under DBU- or Cs(2)CO(3)-mediated conditions at room temperature, is described. A comparable diethyl fluoro(naphthalen-2-yl)methylphosphonate reagent does not react with paraformaldehyde under these mild conditions. The utility of the methodology for synthesis of terminal α-fluoroalkenes bearing electron-withdrawing functionalities is also shown.

Asymmetric organocatalytic monofluorovinylations

The development of highly enantio- and diastereoselective organocatalytic monofluorovinylations is presented. Based on the application of α-fluoro-β-keto-benzothiazolesulfones, the formal addition of a monofluorovinylic anion synthon to a range of acyclic and cyclic enones, as well as imines, is shown. These procedures give selective access to both E- and Z-isomers of the monofluorovinylated products, which are isolated as the pure diastereoisomers in good to excellent yields with up to 99% ee. Furthermore, the application of this concept for the formation of highly enantioenriched bicylic compounds containing a monofluorovinyl moiety is also described. In addition, a mechanistic rationale for the observed E:Z-selectivities is presented.