TBAF Fluorination for Preparing Alkyl Fluorides
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TASF Fluorination for Preparing Alkyl Fluorides Wenchao Qu1 and Ximin Li2 1 Citigroup of Biomedical Imaging Center, Weill Cornell Medicine, New York, NY, USA 2 Department of Discovery, Avid Radiopharmaceuticals, Philadelphia, PA, USA
Brief Introduction TASF, a hypervalent silicon reagent [tris (dimethylamino)sulfonium difluorotrimethylsilicate], was first reported by William J. Middleton in 1976 as a catalyst for polymerization and an anhydrous fluoride ion source [11]. The acronym TASF was originally used for both tris(dimethylamino)sulfonium salts and tris(diethylamino)sulfonium salts. Today, TASF refers more often to tris(dimethylamino)sulfonium salt, due to the commercial availability, higher purity, and the advantages of easy formation of crystalline solid, although the ethyl version has better solubility in organic solvents [18–20]. TASF can be prepared directly by reacting sulfur tetrafluoride (SF4) and dimethylaminotrimethylsilane at 70 C in ether. After the addition of SF4, the reaction is stirred at room temperature for 3 days. The desired product is obtained by filtration as a white crystalline solid (Fig. 1) [11, 12]. Various applications have been found since this reagent was invented, including the generation of enolates from silyl enol ethers, and the
enolates can be used for new C-C bond formation reactions and polymer formation (group transfer polymerization), cross-coupling reactions, cycloadditions, and generation of unusual carbanions, as well as mild and efficient fluoride sources for deprotection of sensitive silyl protecting groups [18–20]. This chapter will focus on the application of TASF as an anhydrous fluoride ion source. TASF has proved to be an efficient nucleophilic fluorination agent due to its anhydrous status and good solubility in organic solvents. The syntheses of varied fluorinated sugar analogs were reported during the early stages of utilizing this fluorination reagent [18–20], while the syntheses of fluorinated amino acids have only been reported recently [17, 23].
Preparation of Alkylfluorides Using TASF as a Nucleophilic Fluorination Reagent Fluorinated Sugar Analogs The first use of TASF as a fluorination reagent was in the preparation of fluorinated sugar analogs reported by Card and Hitz in 1984 [1]. In their report, the key intermediate 1-deoxy-1-fluoro-Dfructose was synthesized from a partially protected D-fructopyranose via standard triflation, followed by TASF fluorination and acidic deprotection. Their preliminary study showed that both (diethylamino)sulfur trifluoride (DAST) and tetrabutylammonium fluoride
© Springer Nature Singapore Pte Ltd. 2020 J. Hu, T. Umemoto (eds.), Fluorination, Synthetic Organofluorine Chemistry 1, https://doi.org/10.1007/978-981-10-3896-9
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TASF Fluorination for Preparing Alkyl Fluorides
TASF Fluorination for Preparing Alkyl Fluorides, Fig. 1 The preparation method for TASF (tris(dimethylamino) sulfonium difluorotrimethylsilicate)
TASF Fluorination for Preparing Alkyl Fluorides, Fig. 2 First report of using TASF in the synthesis of fluorinated suga
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