Transition metal catalysis and nucleophilic fluorination

Transition metal catalyzed transformations using fluorinating reagents have been developed extensively for the preparation of synthetically valuable fluorinated targets. This is a topic of critical importance to facilitate laboratory and industrial chemical synthesis of fluorine containing pharmaceuticals and agrochemicals. Translation to (18)F-radiochemistry is also emerging as a vibrant research field because functional imaging based on Positron Emission Tomography (PET) is increasingly used for both diagnosis and pharmaceutical development. This review summarizes how fluoride sources have been used for the catalytic nucleophilic fluorination of various substrates inclusive of aryl triflates, alkynes, allylic halides, allylic esters, allylic trichloroacetimidates, benzylic halides, tertiary alkyl halides and epoxides. Until recently, progress in this field of research has been slow in part because of the challenges associated with the dual reactivity profile of fluoride (nucleophile or base). Despite these difficulties, some remarkable breakthroughs have emerged. This includes the demonstration that Pd(0)/Pd(II)-catalyzed nucleophilic fluorination to access fluoroarenes from aryl triflates is feasible, and the first examples of Tsuji-Trost allylic alkylation with fluoride using either allyl chlorides or allyl precursors bearing O-leaving groups. More recently, allylic fluorides were also made accessible under iridium catalysis. Another reaction, which has been greatly improved based on careful mechanistic work, is the catalytic asymmetric hydrofluorination of meso epoxides. Notably, each individual transition metal catalyzed nucleophilic fluorination reported to date employs a different F-reagent, an observation indicating that this area of research will benefit from a larger pool of nucleophilic fluoride sources. In this context, a striking recent development is the successful design, synthesis and applications of a fluoride-derived electrophilic late stage fluorination reagent. This new class of reagents could greatly benefit preclinical and clinical PET imaging.     

Organomediated Enantioselective 18F Fluorination for PET Applications

The first organomediated asymmetric ¹⁸F fluorination has been accomplished using a chiral imidazolidinone and [¹⁸F]N‐fluorobenzenesulfonimide. The method provides access to enantioenriched ¹⁸F‐labeled α‐fluoroaldehydes (>90 % ee), which are versatile chiral ¹⁸F synthons for the synthesis of radiotracers. The utility of this process is demonstrated with the synthesis of the PET (positron emission tomography) tracer (2S,4S)‐4‐[¹⁸F]fluoroglutamic acid.     

Synthesis of 18F‐Difluoromethylarenes from Aryl (Pseudo) Halides

A general method for the synthesis of [¹⁸F]difluoromethylarenes from [¹⁸F]fluoride for radiopharmaceutical discovery is reported. The method is practical, operationally simple, tolerates a wide scope of functional groups, and enables the labeling of a variety of arenes and heteroarenes with radiochemical yields (RCYs, not decay‐corrected) from 10 to 60 %. The ¹⁸F‐fluorination precursors are readily prepared from aryl chlorides, bromides, iodides, and triflates. Seven ¹⁸F‐difluoromethylarene drug analogues and radiopharmaceuticals including Claritin, fluoxetine (Prozac), and [¹⁸F]DAA1106 were synthesized to show the potential of the method for applications in PET radiopharmaceutical design.     

Synthesis and Structure Elucidation of Benzoylated Deoxyfluoropyranosides

Benzoylated deoxyfluoropyranosides have been synthesized, starting with protected, unprotected, or fluorinated precursors. Fluorination of eight derivatives was compared using DAST and Deoxo-Fluor as reagents. Deoxo-Fluor was found to be especially useful in the fluorination of methyl 2,3,4-O-tribenzoyl α-D-mannopyranoside and β-D-glucopyranoside, resulting in better yields and avoiding the 1,6-methoxy migration experienced with DAST for one derivative. The two reagents gave comparable yields in the fluorination of other methyl pyranosides, confirming Deoxo-Fluor as a safer alternative to DAST. Methyl α-D-mannopyranoside underwent fluorination to yield the 4,6-difluorotalopyranoside and the corresponding cyclic sulfite. The NMR spectroscopic properties of 11 benzoyl deoxy-fluoropyranosides are reported.     

Synthesis of substituted 5-bromomethyl-4-nitroimidazoles and use for the preparation of the hypoxia-selective multikinase inhibitor SN29966

5-Bromomethyl-4-nitroimidazoles have utility as bioreductive trigger precursors for the preparation of hypoxia-selective prodrugs. Here we describe an efficient two-step synthesis of 5-(bromomethyl)-1-methyl-4-nitro-1H-imidazole, a preferred precursor, employing an N-bromosuccinimide mediated radical bromination. Use of this precursor to prepare SN29966, a promising hypoxia-selective irreversible pan-ErbB inhibitor is reported along with the preparation of four other prodrug candidates. 5-Bromomethyl-4-nitroimidazole analogues bearing electron-donating and electron-withdrawing substituents at the N-1 and C-2 positions are also described.     

Mild Copper‐Catalyzed Fluorination of Alkyl Triflates with Potassium Fluoride

A chemoselective catalytic fluorination of alkyl triflates is described using potassium fluoride as a fluoride source. Excellent yields of the desired alkyl fluorides are obtained after one hour at 45 °C using 2 mol % of the copper catalyst. With 10 mol % of the catalyst, full conversion can be achieved in less than 10 minutes at 45 °C, and thus makes this procedure potentially suited for the preparation of ¹⁸F‐labeled PET probes. As a result of the mild reaction conditions, only the substitution products are observed with no evidence of common side reactions, such as elimination. Reported is a preliminary study of the reaction scope, which demonstrates that the fluorination can be performed in the presence of a wide range of functional groups. Evidence suggests an unusual role of the [IPrCuOTf] catalyst as a phase‐transfer catalyst and points to [IPrCuF] as the active fluorinating reagent (IPr=1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene).     

Site‐Selective,Late‐Stage C−H 18F‐Fluorination on Unprotected Peptides for Positron Emission Tomography Imaging

Peptides are often ideal ligands for diagnostic molecular imaging due to their ease of synthesis and tuneable targeting properties. However, labelling unmodified peptides with ¹⁸F for positron emission tomography (PET) imaging presents a number of challenges. Here we show the combination of photoactivated sodium decatungstate and [¹⁸F]‐N‐fluorobenzenesulfonimide effects site‐selective ¹⁸F‐fluorination at the branched position in leucine residues in unprotected and unaltered peptides. This streamlined process provides a means to directly convert native peptides into PET imaging agents under mild aqueous conditions, enabling rapid discovery and development of peptide‐based molecular imaging tools.     

Site‐Selective,Late‐Stage C−H 18F‐Fluorination on Unprotected Peptides for Positron Emission Tomography Imaging

Peptides are often ideal ligands for diagnostic molecular imaging due to their ease of synthesis and tuneable targeting properties. However, labelling unmodified peptides with ¹⁸F for positron emission tomography (PET) imaging presents a number of challenges. Here we show the combination of photoactivated sodium decatungstate and [¹⁸F]‐N‐fluorobenzenesulfonimide effects site‐selective ¹⁸F‐fluorination at the branched position in leucine residues in unprotected and unaltered peptides. This streamlined process provides a means to directly convert native peptides into PET imaging agents under mild aqueous conditions, enabling rapid discovery and development of peptide‐based molecular imaging tools.     

Stereoselective Synthesis of Tetrahydrofuran Spiro‐β‐Lactams by Ru‐Catalyzed Metathesis of 7‐Oxabicyclo[2.2.1]heptenes

A new method for the synthesis of spiro‐β‐lactams tethered to tetrahydrofuran rings is described. The procedure is based on Ru‐catalyzed metathesis sequences with oxanorbornene precursors easily obtained by the Staudinger [2+2] cycloaddition of related imines.     

One-pot α-nucleophilic fluorination of acetophenones in a deep eutectic solvent

Two methods of nucleophilic fluorination to prepare α-fluoroacetophenones from α-bromoacetophenones by using KF with PEG-400 or TBAF with ZnF₂ are described. On the fundamental of nucleophilic fluorination, a novel method of one-pot fluorination to prepare α-fluoroacetophenones directly from acetophenones in DES was developed.     

An efficient preparation of new sulfonyl fluorides and lithium sulfonates

An efficient preparation of several polyfluoroalkanesulfonyl fluorides is reported. This method, based on the synthesis of polyfluoroalkyl trimethyl silanes (precursors of polyfluoroalkylsulfinates) as intermediates, allows the successive transformations to be carried out in one pot. Moreover, these sulfonyl fluorides can be obtained from the corresponding sulfinates by electrophilic fluorination. This original approach avoids isolation and purification of some thermally or hydrolytically unstable intermediates. A series of new sulfonyl fluorides have been thus prepared from halogenodifluoromethylated precursors RCF2X (X = F, Br; R = ArC(O), ArS(O)n(CF2)m; n = 0, 1, 2; m = 1, 2) and have been transformed into the corresponding lithium sulfonates, which have potential applications as electrolytes for lithium batteries.     

An Effective Synthetic Route to Ef5

EF5 (a 2-nitroimidazole containing an N-(pentafluoropropyl) acetamide substituent) is a very sensitive probe for quantifying the amount of hypoxia within cells; a much improved, short step, synthetic procedure is described for EF5, whose X-ray structure is also presented.     

1-[18F]Fluoroethyleneglycol-2-nitroimidazoles, a new class of potential hypoxia PET markers

Summary 1-[2-(2-Fluoroethoxy)ethyl]-2-1H-nitroimidazole (3a), 1-{2-[2-(2-fluoroethoxy)ethoxy]ethyl}-2-1H-nitroimidazole (3b) and 1-(2-{2-[2-(2-fluoroethoxy)ethoxy]ethoxy}ethyl)-2-1H-nitroimidazole (3c) were synthesized in a two step sequence.Coupling the ditosylate of di-, tri- or tetraethylene glycol with 2-nitroimidazole followed by fluoride substitution afforded the reference compounds in high yield and¹⁸F labeling gave the corresponding markers in 70-82% radiochemical yield.     

A General Copper‐Mediated Nucleophilic 18F Fluorination of Arenes

Molecules labeled with fluorine‐18 are used as radiotracers for positron emission tomography. An important challenge is the labeling of arenes not amenable to aromatic nucleophilic substitution (S_NAr) with [¹⁸F]F^?. In the ideal case, the ¹⁸F fluorination of these substrates would be performed through reaction of [¹⁸F]KF with shelf‐stable readily available precursors using a broadly applicable method suitable for automation. Herein, we describe the realization of these requirements with the production of ¹⁸F arenes from pinacol‐derived aryl boronic esters (arylBPin) upon treatment with [¹⁸F]KF/K₂₂₂ and [Cu(OTf)₂(py)₄] (OTf=trifluoromethanesulfonate, py=pyridine). This method tolerates electron‐poor and electron‐rich arenes and various functional groups, and allows access to 6‐[¹⁸F]fluoro‐L ‐DOPA, 6‐[¹⁸F]fluoro‐m‐tyrosine, and the translocator protein (TSPO) PET ligand [¹⁸F]DAA1106.     

Innovative Syntheses of Cyano(fluoro)borates: Catalytic Cyanation,Electrochemical and Electrophilic Fluorination

Different types of high-yield, easily scalable syntheses for cyano(fluoro)borates Kt[BF_n(CN)_4−n] (n=0–2) (Kt=cation), which are versatile building blocks for materials applications and chemical synthesis, have been developed. Tetrafluoroborates react with trimethylsilyl cyanide in the presence of metal-free Brønsted or Lewis acid catalysts under unprecedentedly mild conditions to give tricyanofluoroborates or tetracyanoborates. Analogously, pentafluoroethyltrifluoroborates are converted into pentafluoroethyltricyanoborates. Boron trifluoride etherate, alkali metal salts, and trimethylsilyl cyanide selectively yield dicyanodifluoroborates or tricyanofluoroborates. Fluorination of cyanohydridoborates is the third reaction type that includes direct fluorination with, for example, elemental fluorine, stepwise halogenation/fluorination reactions, and electrochemical fluorination (ECF) according to the Simons process. In addition, fluorination of [BH(CN)₂{OC(O)Et}]⁻ to result in [BF(CN)₂{OC(O)Et}]⁻ is described.     

Synthesis and biological evaluation of 5-substituted-4-nitroimidazole ribonucleosides

The imidazole nucleosides, 4(5)-bromo-5(4)-nitro-1-β-D-ribofuranosylimidazoles, have been prepared via glycosylation of the trimethylsilylated aglycone, 4(5)-bromo-5(4)-nitroimidazole, with tetra-O-acetyl-β-D-ribo-furanose followed by removal of the acetyl protecting groups. The 5-bromo-4-nitro-1-β-D-ribofuranosylimidazole nucleoside was acetonated to produce 5-bromo-4-nitro-1-(2,3-O-isopropylidene-β-D-ribofuranosyl)-imidazole which was cyclized to provide the corresponding anhydronucleoside 5,5′-anhydro-4-nitro-5-oxo-1-(2,3-O-isopropylidene-β-D-ribofuranosyl)imidazole. Sodium hydrosulfide treatment of 5-bromo-4-nitroimidazole nucleoside provided 5-mercapto-4-nitro-1-β-D-ribofuranosylimidazole 5-sodium salt which was alkylated with E-1,5-diiodopent-1-ene to yield 5-(E-1-iodo-1-penten-5-yl)thio-4-nitro-1-β-D-ribofuranosylimidazole. The corresponding iodine-125-labeled compound was prepared similarly using radiolabeled diiodopentene. The 5-bromo-4-nitroimidazole, 5-mercapto-4-nitroimidazole, and 5-iodopentenylthio-4-nitroimidazole nucleosides were cytotoxic to Molt-3 cells in vitro at concentrations higher than 10 μg/mL. The radiolabeled 5-iodopentenylthio-4-nitroimidazole nucleoside showed 2-fold higher uptake in a rapidly growing tumor as compared to uptake in a relatively slower growing tumor in mice.     

18F‐Labeling of Aryl‐SCF3, ‐OCF3 and ‐OCHF2 with [18F]Fluoride

We report that halogenophilic silver(I) triflate permits halogen exchange (halex) nucleophilic ¹⁸F‐fluorination of aryl‐OCHFCl, ‐OCF₂Br and ‐SCF₂Br precursors under mild conditions. This Ag^I‐mediated process allows for the first time access to a range of ¹⁸F‐labeled aryl‐OCHF₂, ‐OCF₃ and ‐SCF₃ derivatives, inclusive of [¹⁸F]riluzole. The ¹⁸F‐labeling of these medicinally important motifs expands the radiochemical space available for PET applications.     

New synthesis methods for fluorinated carbon nanofibres and applications

Several new synthesis methods of fluorinated carbon nanofibres, such as controlled fluorination using fluorinating agent (TbF₄ or XeF₂), or assisted fluorination under UV and gamma irradiation, are reviewed and compared with the direct fluorination using undiluted fluorine gas. The results highlight the different fluorination mechanisms for the direct fluorination and the new methods. The other advantage of those alternative fluorination routes is the possibility to provide fine tuning of the fluorination level, i.e. from F/C atomic ratio close to zero, as a functionalization, to the unity (CF₁) according to the required application, electrochemical or tribological. Two applications are described in this paper as a function of the fluorine content: protection against ozonation and use as solid lubricants.     

Targeted Fluorination with the Fluoride Ion by Manganese‐Catalyzed Decarboxylation

We describe the first catalytic decarboxylative fluorination reaction based on the nucleophilic fluoride ion. The reported method allows the facile replacement of various aliphatic carboxylic acid groups with fluorine. Moreover, the potential of this method for PET imaging has been demonstrated by the successful ¹⁸F labeling of a variety of carboxylic acids with radiochemical conversions up to 50 %, representing a targeted decarboxylative ¹⁸F labeling method with no‐carrier‐added [¹⁸F]fluoride. Mechanistic probes suggest that the reaction proceeds through the interaction of the manganese catalyst with iodine(III) carboxylates formed in situ from iodosylbenzene and the carboxylic acid substrates.     

Nitroimidazoles. V . Synthesis of 1-methyl-2-(2-methyl-4-thiazolyl)nitroimidazoles

Reaction of readily available 2-methyl-4-formylthiazole ( 1 ) with glyoxal and ammonia gave 2-(2-methyl-4-thiazolyl)imidazole ( 2 ). Nitration of 2 with a mixture of nitric acid-sulfuric acid at 100° yielded 2-(2-methyl-4-thiazolyl)-4,5-dinitroimidazole ( 3 ) as the sole reaction product, while nitration at 65° afforded 2-(2-methyl-4-thiazolyl)-4-(or 5)-nitroimidazole ( 4 ). N-Methylation of compound 4 in the presence of base gave 1-methyl-2-(2-methyl-4-thiazolyl)4-nitroimidazole ( 6 ), whereas N-methylation with diazomethane afforded 1-methyl-2-(2-methyl-4-thiazolyl)-5-nitroimidazole ( 5 ). N-Methylation of compound 3 yielded 1-methyl-2-(2-methyl-4-thiazolyl)-3,5-dinitroimidazole ( 7 ) in high yield.