gem-Difluorination of carbon–carbon triple bonds using Brønsted acid/Bu₄NBF₄ or electrogenerated acid

• Mizuki Yamaguchi,
• Hiroki Shimao,
• Kengo Hamasaki,
• Keiji Nishiwaki,
• Shigenori Kashimura and
• Kouichi Matsumoto

Beilstein J. Org. Chem. 2024, 20, 2261–2269, doi:10.3762/bjoc.20.194

• University, 3-4-1 Kowakae, Higashi-osaka, Osaka, 577-8502, Japan 10.3762/bjoc.20.194 Abstract gem-Difluorination of carbon–carbon triple bonds was conducted using Brønsted acids, such as Tf2NH and TfOH, combined with Bu4NBF4 as the fluorine source. The electrochemical oxidation of a Bu4NBF4/CH2Cl2 solution

• containing alkyne substrates could also give the corresponding gem-difluorinated compounds (in-cell method). The ex-cell electrolysis method was also applicable for gem-difluorination of alkynes. Keywords: carbon–carbon triple bonds; chemical method; electrochemistry; gem-difluorination; Introduction

• blocks for further transformations. We have focused on the investigation of gem-difluorination of carbon–carbon triple bonds, because this procedure is one of the most simple but powerful and straightforward methods. In addition, there have been a few reports in the literature that seem to mainly rely on

Regio- and stereochemical stability induced by anomeric and gauche effects in difluorinated pyrrolidines

• Ana Flávia Candida Silva,
• Francisco A. Martins and
• Matheus P. Freitas

Beilstein J. Org. Chem. 2024, 20, 1572–1579, doi:10.3762/bjoc.20.140

• of the pyrrolidine ring in proline motifs has been found to induce significant conformational changes that impact the structure and biological roles of modified peptides and proteins. Vicinal difluorination of fluoroproline, for example, in (3S,4R)-3,4-difluoroproline, serves to mitigate the inherent

• the geometric arrangement necessary to most effectively accommodate anomeric and gauche effects [8][9]. It is noteworthy that vicinal difluorination has previously been demonstrated to minimize conformational bias in fluoroprolines [10], offering some insight into the potential outcomes of this study

Benzylic C(sp³)–H fluorination

• Alexander P. Atkins,
• Alice C. Dean and
• Alastair J. J. Lennox

Beilstein J. Org. Chem. 2024, 20, 1527–1547, doi:10.3762/bjoc.20.137

• FAT with Selectfluor to produce the desired benzyl fluoride. Increasing amino acid and Selectfluor loadings achieved difluorination of the benzylic substrates. This procedure was demonstrated predominately on primary benzylic substrates, but could be used to effect the fluorination of several

• the loadings of K2S2O8 and Selectfluor, selectivity for the mono- (conditions A) or difluorination (conditions B) products could be achieved. Building on their previous iron-catalysed work, Figure 11, Lectka and co-workers reported in 2014 the use of triethylborane as a radical chain initiator for C

• Et3N·3HF, provided the benzylic fluoride product. Although a majority of examples were performed with an excess of benzylic substrate (up to 6 equivalents with respect to methyl radical precursor), a broad scope with excellent functional group tolerance was demonstrated. Difluorination was possible

Synthesis of 2,2-difluoro-1,3-diketone and 2,2-difluoro-1,3-ketoester derivatives using fluorine gas

• Alexander S. Hampton,
• David R. W. Hodgson,
• Graham McDougald,
• Linhua Wang and
• Graham Sandford

Beilstein J. Org. Chem. 2024, 20, 460–469, doi:10.3762/bjoc.20.41

• generate a fluoride ion that facilitates limiting enolization processes, and an electrophilic N–F fluorinating agent that is reactive towards neutral enol species. Keywords: difluorination; difluoromethylene; direct fluorination; electrophilic fluorination; organofluorine; Introduction Fluorine is

• corresponding 2,2-difluoroketones [15]. In related kinetic studies concerning the electrophilic 2-fluorination of 1,3-diketones with Selectfluor [16][17], we demonstrated that the rate-determining step for difluorination was enolization of the intermediate 2-fluoro-1,3-diketone. Monofluorination of 1,3

• -diketones occurs rapidly because the substrates lie predominantly in their enol tautomeric forms. The resulting 2-difluoro-1,3-diketones, on the other hand, are formed in their keto-tautomeric forms. Thus, we found difluorination could only be achieved upon addition of water or a base to accelerate the

Redox-active molecules as organocatalysts for selective oxidative transformations – an unperceived organocatalysis field

• Elena R. Lopat’eva,
• Igor B. Krylov,
• Dmitry A. Lapshin and
• Alexander O. Terent’ev

Beilstein J. Org. Chem. 2022, 18, 1672–1695, doi:10.3762/bjoc.18.179

• chiral copper complex. An extraordinary example of an asymmetric difluorination of alkenes with the migration of aryl or methyl groups was shown using a chiral aryl iodide catalyst [149][150] (Scheme 35). Depending on the nature of the migrating group, two mechanisms are possible that determine the

• -organocatalyzed vinylarene diamination. Iodoarene-organocatalyzed asymmetric CH-hydroxylation of benzylic substrates. Iodoarene-organocatalyzed asymmetric difluorination of alkenes with migration of aryl or methyl groups. Examples of 1,2-diiodo-4,5-dimethoxybenzene-catalyzed electrochemical oxidative

Cathodic generation of reactive (phenylthio)difluoromethyl species and its reactions: mechanistic aspects and synthetic applications

• Sadanobu Iwase,
• Shinsuke Inagi and
• Toshio Fuchigami

Beilstein J. Org. Chem. 2022, 18, 872–880, doi:10.3762/bjoc.18.88

• from an aspect of green chemistry [7][8][9][10]. In this context, we have developed various electrochemical methodologies for efficient selective fluorination [11][12] and molecular conversion of organofluorine compounds to date [13][14][15][16][17][18]. We have also achieved the gem-difluorination of

• ; (b) o-phthalonitrile (4 mM), and (c) o-phthalonitrile (4 mM) + 1 (8 mM). Scan rate: 100 mV/s. Electrochemical gem-difluorination of sulfides bearing α-electron-withdrawing groups. Electrochemical gem-difluorodesulfurization of dithioacetals. Electrochemical gem-difluorodesulfurization of

Development of N-F fluorinating agents and their fluorinations: Historical perspective

• Teruo Umemoto,
• Yuhao Yang and
• Gerald B. Hammond

Beilstein J. Org. Chem. 2021, 17, 1752–1813, doi:10.3762/bjoc.17.123

Sustainable manganese catalysis for late-stage C–H functionalization of bioactive structural motifs

• Jongwoo Son

Beilstein J. Org. Chem. 2021, 17, 1733–1751, doi:10.3762/bjoc.17.122

• . Additionally, difluorination was observed with negligible amounts because the monofluorinated product is rendered more electronically deficient by the first fluorine atom. Based on an analysis by DFT calculations, the postulated reaction pathway of manganese-catalyzed C–H fluorination is described in Figure 1

CF₃-substituted carbocations: underexploited intermediates with great potential in modern synthetic chemistry

• Anthony J. Fernandes,
• Armen Panossian,
• Bastien Michelet,
• Agnès Martin-Mingot,
• Frédéric R. Leroux and
• Sébastien Thibaudeau

Beilstein J. Org. Chem. 2021, 17, 343–378, doi:10.3762/bjoc.17.32

Vicinal difluorination as a C=C surrogate: an analog of piperine with enhanced solubility, photostability, and acetylcholinesterase inhibitory activity

• Yuvixza Lizarme-Salas,
• Alexandra Daryl Ariawan,
• Ranjala Ratnayake,
• Hendrik Luesch,
• Angela Finch and
• Luke Hunter

Beilstein J. Org. Chem. 2020, 16, 2663–2670, doi:10.3762/bjoc.16.216

• -workers have recently described a method for the one-step, diastereoselective 1,2-difluorination of alkenes, mediated by a hypervalent iodine catalyst [24]. The substrate scope of the Jacobsen method has certain constraints but their original report did include some examples of α,β-unsaturated amides, and

• products. We were forced to conclude that the rather complex structure of 3 was incompatible with the Jacobsen catalytic system. Since the one-step difluorination method (Scheme 1) was unsuccessful, we decided to pursue a stepwise fluorination approach [26][27] (Scheme 2). Thus, the allylic alcohol 7 [28

• μM) and 2 (IC50 > 1000 μM). The attempted synthesis of 6 (a diastereoisomer of 2) via a one-step 1,2-difluorination reaction [24]. Py = pyridine; m-CPBA = m-chloroperbenzoic acid, DCM = dichloromethane. The attempted synthesis of 2 via a stepwise fluorination approach (ether series). THF

Photosensitized direct C–H fluorination and trifluoromethylation in organic synthesis

• Shahboz Yakubov and
• Joshua P. Barham

Beilstein J. Org. Chem. 2020, 16, 2151–2192, doi:10.3762/bjoc.16.183

• mono- vs difluorinated products at the benzylic position; 9-fluorenone affords benzylic monofluorination and xanthone affords benzylic difluorination (Scheme 5). Importantly, this metal-free direct C–H fluorination proceeded without the need for any specialized photochemical equipment; under visible

• -difluorination. gem-Difluorination is particularly challenging since the first introduced fluorine atom will electronically hinder the second fluorination at the same position. For difluorination, xanthone was employed as the PC and Selectfluor® II as the fluorine source (Scheme 7). The higher T1 energy may have

• been responsible for generating more Selectfluor®-derived radical cations that drove the reaction towards difluorination to afford the product 28a. However, it is important to note that only difluorinated products were detected and no monofluorinated products were observed, regardless of whether

Lipophilicity trends upon fluorination of isopropyl, cyclopropyl and 3-oxetanyl groups

• Benjamin Jeffries,
• Zhong Wang,
• Robert I. Troup,
• Anaïs Goupille,
• Jean-Yves Le Questel,
• Charlene Fallan,
• James S. Scott,
• Elisabetta Chiarparin,
• Jérôme Graton and
• Bruno Linclau

Beilstein J. Org. Chem. 2020, 16, 2141–2150, doi:10.3762/bjoc.16.182

• trifluorinated B4. Geminal difluorination caused an increase in lipophilicity compared to vicinal fluorination (compare B6 with B3 and B5, and B7 with B4 and B2). Interestingly, in analogy with the same lipophilicity of B3–B4, B6 and B7 also were equivalent in lipophilicity. Finally, the 3-fluorinated oxetanyl

• effect of the fluorine is evident from the 1H NMR chemical shift values of the CH3 groups in D1 (0.91 ppm) [29], D3 (0.98 ppm), and D2 (1.37 ppm) [30]. Compared to the monofluorinated D3, difluorination (D4) and trifluorination (D5) at the same carbon atom increases lipophilicity. There is a notable

• by fluorine. Interestingly, for γ-mono- and γ,γ-difluorination the linear butanol derivatives G3 and G4 are now less lipophilic than the branched D3 and D4, respectively. A possible explanation is that branching in D3 and D4 results in shielding of the polar C–F bonds preventing dipolar interactions

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

• Jessica Neufeld,
• Constantin G. Daniliuc and
• Ryan Gilmour

Beilstein J. Org. Chem. 2020, 16, 1627–1635, doi:10.3762/bjoc.16.135

• the substrates and the plenum of methods available to activate π-bonds [24][25]. Confidence in this approach stemmed from our recent vicinal and geminal difluorination of alkenes [26][27][28][29][30], and contemporaneous studies from the Jacobsen laboratory [31][32][33][34], under the auspices of I(I

Recent advances in transition-metal-catalyzed incorporation of fluorine-containing groups

• Xiaowei Li,
• Xiaolin Shi,
• Xiangqian Li and
• Dayong Shi

Beilstein J. Org. Chem. 2019, 15, 2213–2270, doi:10.3762/bjoc.15.218

• catalyst, AgF as fluoride source, NMO as oxidant, and DMF as solvent, they achieved the selective mono- or difluorination in high yields (Scheme 34). Notably, pyridine as an additive could prevent the decomposition of an amide substrate in a long-time reaction. Meanwhile, the group of Sanford [79

Recent advances in hypervalent iodine(III)-catalyzed functionalization of alkenes

• Xiang Li,
• Pinhong Chen and
• Guosheng Liu

Beilstein J. Org. Chem. 2018, 14, 1813–1825, doi:10.3762/bjoc.14.154

• ) reagents were applied to oxidize transition metals [21][22][23][24][25]. In an alternative way, the electrophilic hypervalent iodine(III) reagents can activate alkenes directly in a metal-free manner. Based on this strategy, dichlorination [26], 1,2-difluorination [27], gem-difluorination [28

• of substrates bearing either two or four methylene groups between the alkene and sulfonamide failed to provide the fluoroheterocycles. Difluorination of alkenes Hara and co-workers reported a vicinal difluorination of unactivated alkenes in the presence of stoichiometric amounts of difluoro

• iodotoluene [27]. Based on this seminal work, Gilmour and co-workers reported a catalytic difluorination of alkenes using an inexpensive p-iodotoluene as the catalyst and Selectfluor as the terminal oxidant [63]. Terminal olefins proved to be viable substrates for this reaction. It is worth noting that the

A survey of chiral hypervalent iodine reagents in asymmetric synthesis

• Soumen Ghosh,
• Suman Pradhan and
• Indranil Chatterjee

Beilstein J. Org. Chem. 2018, 14, 1244–1262, doi:10.3762/bjoc.14.107

• -difluorination, 1,1-difluorination and fluorolactonization protocols appeared almost simultaneously (Scheme 15). The lactate-based C2-symmetric chiral iodine precatalysts 73, 76, and 79 were used to deliver chiral fluorinated scaffolds from alkene starting materials 69 in the presence of pyr.HF as a nucleophilic

Fluorocyclisation via I(I)/I(III) catalysis: a concise route to fluorinated oxazolines

• Felix Scheidt,
• Christian Thiehoff,
• Gülay Yilmaz,
• Stephanie Meyer,
• Constantin G. Daniliuc,
• Gerald Kehr and
• Ryan Gilmour

Beilstein J. Org. Chem. 2018, 14, 1021–1027, doi:10.3762/bjoc.14.88

• transformations. To that end, it was envisaged that the protocol could be effectively translated to the fluorocyclisation of readily accessible N-allylcarboxamides (Figure 2, bottom). Whilst the initial phase of catalysis would resemble that of the catalytic difluorination, the presence of the amide would allow

• DCE (0.2 mol·L−1) with 20 mol % catalyst loading, and using Selectfluor® as the oxidant. An initial reaction screen, based on the conditions reported for our vicinal difluorination study [9], began with an exploration of the effect of amine/HF ratio. This was deemed prudent due to the perceived

• . Selected examples of bioactive compounds containing the 2-oxazoline motif. The catalytic difluorination of alkenes (top) and the proposed fluorocyclisation via the same I(I)/I(III) manifold (bottom). Substrate scope. aReaction conducted on 1 mmol scale. bReaction time increased to 40 hours. cReaction time

Addition of dithi(ol)anylium tetrafluoroborates to α,β-unsaturated ketones

• Yu-Chieh Huang,
• An Nguyen,
• Simone Gräßle,
• Sylvia Vanderheiden,
• Nicole Jung and
• Stefan Bräse

Beilstein J. Org. Chem. 2018, 14, 515–522, doi:10.3762/bjoc.14.37

• dithiolane and dithiane protecting groups which are irreplaceable intermediates for the introduction of, e.g., fluorine via gem-difluorination [1][2]. They also allow the formation of valuable building blocks that can be used for diverse transformations in organic chemistry (e.g., Umpolung) [3]. Ketene 1,3

One-pot multistep mechanochemical synthesis of fluorinated pyrazolones

• Joseph L. Howard,
• William Nicholson,
• Yerbol Sagatov and
• Duncan L. Browne

Beilstein J. Org. Chem. 2017, 13, 1950–1956, doi:10.3762/bjoc.13.189

• designed a 2-step reaction related to our recent work on liquid assisted grinding effects of the fluorination of 1,3-dicarbonyl compounds, in which the dicarbonyl will initially form a pyrazolone in the first reaction prior to undergoing difluorination in the second step (Scheme 1) [17]. Notably this

• approach will likely require a grinding auxiliary in the first step where two liquid phases react and will be catalysed by an acid to afford a solid pyrazolone material. This will then be followed by a difluorination reaction between solid–solid reactants, this reaction may perform better in the presence

Organofluorine chemistry: Difluoromethylene motifs spaced 1,3 to each other imparts facial polarity to a cyclohexane ring

• Mathew J. Jones,
• Ricardo Callejo,
• Alexandra M. Z. Slawin,
• Michael Bühl and
• David O'Hagan

Beilstein J. Org. Chem. 2016, 12, 2823–2827, doi:10.3762/bjoc.12.281

• (Scheme 3). The presence of the gem-dimethyl motif renders the carbonyl groups independent of each other and this facilitates the difluorination process relative to 5a and 5b. Nevertheless, other unidentified byproducts were also obtained, which made purification of 4c by column chromatography difficult

• compound was prepared by direct difluorination of the corresponding α,α-dimethylated 1,3-diketone with DAST. The presence of the two 1,3-diaxial fluorine atoms in structure 4c is responsible for the facial polarisation on the cyclohexane ring. We have demonstrated the intermolecular electrostatic

C–H bond halogenation catalyzed or mediated by copper: an overview

• Wenyan Hao and
• Yunyun Liu

Beilstein J. Org. Chem. 2015, 11, 2132–2144, doi:10.3762/bjoc.11.230

• functionalized 8-methylquinolinyl substrates reported by Sanford et al. [43], Daugulis and co-worker [44] established in 2013 a copper-catalyzed arene C–H o-fluorination of N-(quinolin-8-yl)benzamides 14. The mono- and/or difluorination took place in the presence of CuI, N-methylmorpholine N-oxide (NMO) and

Aminofluorination of 2-alkynylanilines: a Au-catalyzed entry to fluorinated indoles

• Antonio Arcadi,
• Emanuela Pietropaolo,
• Antonello Alvino and
• Véronique Michelet

Beilstein J. Org. Chem. 2014, 10, 449–458, doi:10.3762/bjoc.10.42

• on the terminal triple bond the desired products were isolated in moderate yields (Table 3, entries 1–3). In the presence of the 1H-pyrazolyl moiety the difluorination prevailed over the monofluorination process (Table 3, entry 4). With the 2-alkynylanilines bearing a linear alkyl group on the

Flow microreactor synthesis in organo-fluorine chemistry

• Hideki Amii,
• Aiichiro Nagaki and
• Jun-ichi Yoshida

Beilstein J. Org. Chem. 2013, 9, 2793–2802, doi:10.3762/bjoc.9.314

• inert plastic flow tubes provided flexibility, scale-up production, and improved safety of operation. Using flow chemistry, the conversion of alcohols to fluorides was achieved by Seeberger and Ley, independently (Scheme 2) [44][45][46][47]. Usually, gem-difluorination of ketones by DAST is known to be

• less efficient because it needs heating and/or long reaction time to complete the fluorination [48][49]. Nevertheless by the use of a flow microreactor device, isatin underwent gem-difluorination to give the corresponding difluoride in 73% yield (residence time in the reactor: within 1 h). Reactions