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

Observation of unusual outer-sphere mechanism using simple alkenes as nucleophiles in allylation chemistry

Transition-metal catalyzed allylic substitution reactions of alkenes are among the most efficient methods for synthesizing diene compounds, driven by the inherent preference for an inner-sphere mechanism. Here, we present a demonstration of an outer-sphere mechanism in Rh-catalyzed allylic substitution reaction of simple alkenes using gem-difluorinated cyclopropanes as allyl surrogates. This unconventional mechanism offers an opportunity for the fluorine recycling of gem-difluorinated cyclopropanes via C - F bond cleavage/reformation, ultimately delivering allylic carbofluorination products. The developed method tolerates a wide range of simple alkenes, providing access to secondary, tertiary fluorides and gem-difluorides with 100% atom economy. DFT calculations reveal that the C - C bond formation goes through an unusual outer-sphere nucleophilic substitution of the alkenes to the allyl-Rh species instead of migration insertion, and the generated carbon cation then forms the C - F bond with tetrafluoroborate as a fluoride shuttle.

Cobalt(III) Halide Metal-Organic Frameworks Drive Catalytic Halogen Exchange

The selective halogenation of complex (hetero)aromatic systems is a critical yet challenging transformation that is relevant to medicinal chemistry, agriculture, and biomedical imaging. However, current methods are limited by toxic reagents, expensive homogeneous second- and third-row transition metal catalysts, or poor substrate tolerance. Herein, we demonstrate that porous metal-organic frameworks (MOFs) containing terminal Co(III) halide sites represent a rare and general class of heterogeneous catalysts for the controlled installation of chlorine and fluorine centers into electron-deficient (hetero)aryl bromides using simple metal halide salts. Mechanistic studies support that these halogen exchange (halex) reactions proceed via redox-neutral nucleophilic aromatic substitution (SNAr) at the Co(III) sites. The MOF-based halex catalysts are recyclable, enable green halogenation with minimal waste generation, and facilitate halex in a continuous flow. Our findings represent the first example of SNAr catalysis using MOFs, expanding the lexicon of synthetic transformations enabled by these materials.

C-F Bond Insertion: An Emerging Strategy for Constructing Fluorinated Molecules

C-F Insertion reactions, where an organic fragment formally inserts into a carbon-fluorine bond in a substrate, are highly attractive, yet largely unexplored, methods to prepare valuable fluorinated molecules. The inherent strength of C-F bonds and the resulting need for a large thermodynamic driving force to initiate C-F cleavage often leads to sequestering of the released fluoride in an unreactive by-product. Recently, however, several groups have succeeded in overcoming this challenge, opening up the study of C-F insertion as an efficient and highly atom-economical approach to prepare fluorinated compounds. In this article, the recent breakthroughs are discussed focusing on the key conceptual advances that allowed for both C-F bond cleavage and subsequent incorporation of the released fluoride into the product.

Stereodivergent 1,3-difunctionalization of alkenes by charge relocation

Alkenes are indispensable feedstocks in chemistry. Functionalization at both carbons of the alkene-1,2-difunctionalization-is part of chemistry curricula worldwide1. Although difunctionalization at distal positions has been reported2-4, it typically relies on designer substrates featuring directing groups and/or stabilizing features, all of which determine the ultimate site of bond formation5-7. Here we introduce a method for the direct 1,3-difunctionalization of alkenes, based on a concept termed 'charge relocation', which enables stereodivergent access to 1,3-difunctionalized products of either syn- or anti-configuration from unactivated alkenes, without the need for directing groups or stabilizing features. The usefulness of the approach is demonstrated in the synthesis of the pulmonary toxin 4-ipomeanol and its derivatives.

Deoxyfluorination of 1°, 2°, and 3° Alcohols by Nonbasic O-H Activation and Lewis Acid-Catalyzed Fluoride Shuttling

A method for deoxyfluorination of aliphatic primary, secondary, and tertiary alcohols is reported, employing a nontrigonal phosphorus triamide for base-free alcohol activation in conjunction with an organic soluble fluoride donor and a triarylborane fluoride shuttling catalyst. Mechanistic experiments are consistent with a reaction that proceeds by the collapse of an oxyphosphonium fluoroborate ion pair with fluoride transfer. The substrate scope complements existing deoxyfluorination methods and enables the preparation of homochiral secondary and tertiary alkylfluorides by stereoinversion of the substrate alcohol.

Electrooxidative Fluorofunctionalization of Arylcyclopropanes

The work herein demonstrates the viability of an electrochemical oxidative protocol for the expedient realization of 1,3-fluorofunctionalization of arylcyclopropanes under catalyst- and oxidant-free conditions. Given the relatively low nucleophilicity of fluoride ion, the counterintuitive outcome that the ring-opening is initiated by nucleophilic fluorination is rationalized by invoking tight ion pair between aryl radical cation and BF4- counterion. By integrating alcohols, acids, and N-heterocycles as the terminating nucleophiles, straightforward 1,3-fluorooxygenation and 1,3-fluoroamination are smoothly achieved.

Photocatalytic C-F alkylation of trifluoromethyls using o-phosphinophenolate: mechanistic insights and substrate prediction

Density functional theory computations reveal the radical mechanism of photocatalytic defluoroalkylation and hydrodefluorination of N-phenyl-2,2,2-trifluoromethylacetamide with o-phosphinophenolate (PO) cooperative catalysis. The energy gaps between the singlet substrate LUMOs and triplet photocatalyst SOMOs can be used as an effective "chemical descriptor" for predicting catalyst activity. Cesium formate assisted C-F bond activation is the most favorable path. A series of available organic structures are computationally predicted as potential substrates.

Cobalt-Catalyzed Wagner-Meerwein Rearrangements with Concomitant Nucleophilic Hydrofluorination

We report a cobalt-catalyzed Wagner-Meerwein rearrangement of gem-disubstituted allylarenes that generates fluoroalkane products with isolated yields up to 84 %. Modification of the counteranion of the N-fluoropyridinium oxidant suggests the substrates undergo nucleophilic fluorination during the reaction. Subjecting the substrates to other known metal-mediated hydrofluorination procedures did not lead to observable 1,2-aryl migration. Thus, indicating the unique ability of these cobalt-catalyzed conditions to generate a sufficiently reactive electrophilic intermediate capable of promoting this Wagner-Meerwein rearrangement.

BF3-Catalyzed Intramolecular Fluorocarbamoylation of Alkynes via Halide Recycling

A BF₃-catalyzed atom-economical fluorocarbamoylation reaction of alkyne-tethered carbamoyl fluorides is reported. The catalyst acts as both a fluoride source and Lewis acid activator, thereby enabling the formal insertion of alkynes into strong C-F bonds through a halide recycling mechanism. The developed method provides access to 3-(fluoromethylene) oxindoles and γ-lactams with excellent stereoselectivity, including fluorinated derivatives of known protein kinase inhibitors. Experimental and computational studies support a stepwise mechanism for the fluorocarbamoylation reaction involving a turnover-limiting cyclization step, followed by internal fluoride transfer from a BF₃-coordinated carbamoyl adduct. For methylene oxindoles, a thermodynamically driven Z-E isomerization is facilitated by a transition state with aromatic character. In contrast, this aromatic stabilization is not relevant for γ-lactams, which results in a higher barrier for isomerization and the exclusive formation of the Z-isomer.

eFluorination Using Cheap and Readily Available Tetrafluoroborate Salts

A practical electrochemical method for the rapid, safer, and mild synthesis of tertiary hindered alkyl fluorides from carboxylic acids has been developed without the need for hydrofluoric acid salts or non-glass reactors. In this anodic fluorination, collidinium tetrafluoroborate acts as both the supporting electrolyte and fluoride donor. A wide range of functional groups has been shown to be compatible, and the possibility of scale-up using flow electrochemistry has also been demonstrated.

1-Fluoro-1-sulfonyloxylation of Alkenes by Sterically and Electronically Tuned Hypervalent Iodine: Regression Analysis toward 1,1-Heterodifunctionalization

In the heterodifunctionalization of alkenes, 1,1-regioselectivity remains elusive in sharp contrast to 1,2-regioselectivity. Herein, the 1-fluoro-1-sulfonyloxylation of styrenes with Bu₄NBF₄ and sulfonic acids using a hypervalent iodine ArI(OAc)₂ is reported. Regression analysis of substituents on ArI(OAc)₂ suggested that their electron-withdrawing ability and steric factor influence the 1,1-heterodifunctionalization. We designed o-{2,4-(CF₃)₂C₆H₃}- and p-NO₂-substituted ArI(OAc)₂ by the regression analysis to achieve high selectivity.

Salt Metathesis: Tetrafluoroborate Anion Rapidly Fluoridates Organoboronic Acids to give Organotrifluoroborates

Tetrafluoroborate (BF₄ ^- ) has long been used as a spectator counter anion. Herein, we report an unprecedented salt metathesis between a variety of BF₄ ^- salts and a series of organoboronic acids yielding the corresponding organotrifluoroborates. We identified conditions for fast and efficient fluoridation (<1 h) with minimal workup. Fundamentally, this work discloses the proclivity of BF₄ ^- to exchange fluoride atoms with organoboronates, highlighting the lability of BF₄ ^- .

Ionic Liquids Effect on the Stability of 17-Electron Cation Product of the Electrochemical Oxidation of Cymantrene

The oxidative electrochemistry of cymantrene, CpMn(CO)3 (1; Cp = [η5-C5H5]–), was examined in ionic liquids (ILs) composed of anions of varying Lewis base properties. It was observed that the cyclic voltammetric responses strongly depended on the nucleophilic properties of the IL anion. Still, all observations are consistent with the initial formation of 1+ followed by an attack from the IL anion. In bis(trifluoromethylsulfonyl)amide [NTf2]-based ILs, the process shows close to ideal electrochemical reversibility as the reaction between 1+ and [NTf2] anion is very slow. On the other hand, in tetrafluoroborate and trifluoromethanesulfonate-based IL, the oxidation of 1 shows different levels of electrochemical reversibility with a marked sign of anion attack to 1+. In contrast, 1 exhibits an irreversible oxidation process in hexafluorophosphate-based IL. The reaction rate constants for the interaction of 1+ with the different IL anions were estimated by fitting the experimental data to digital simulations of the proposed mechanism. Besides, the use of [NTf2]-based ILs as a supporting electrolyte in CH2Cl2 was also examined. The oxidation process of 1 shows a close to ideal electrochemical reversibility but low to non-chemical reversibility. This study illustrates the wide range of electrochemical environments available with ILs and demonstrates their limited utility for investigating the redox properties of metal carbonyl compounds. It also intends to warn the reader on how the IL media may influence an electrochemical study if care is not exercised.

Mechanism of the Aryl-F Bond-Forming Step from Bi(V) Fluorides

In this article, we describe a combined experimental and theoretical mechanistic investigation of the C(sp²)–F bond formation from neutral and cationic high-valent organobismuth(V) fluorides, featuring a dianionic bis-aryl sulfoximine ligand. An exhaustive assessment of the substitution pattern in the ligand, the sulfoximine, and the reactive aryl on neutral triarylbismuth(V) difluorides revealed that formation of dimeric structures in solution promotes facile Ar–F bond formation. Noteworthy, theoretical modeling of reductive elimination from neutral bismuth(V) difluorides agrees with the experimentally determined kinetic and thermodynamic parameters. Moreover, the addition of external fluoride sources leads to inactive octahedral anionic Bi(V) trifluoride salts, which decelerate reductive elimination. On the other hand, a parallel analysis for cationic bismuthonium fluorides revealed the crucial role of tetrafluoroborate anion as fluoride source. Both experimental and theoretical analyses conclude that C–F bond formation occurs through a low-energy five-membered transition-state pathway, where the F anion is delivered to a C(sp²) center, from a BF₄ anion, reminiscent of the Balz–Schiemann reaction. The knowledge gathered throughout the investigation permitted a rational assessment of the key parameters of several ligands, identifying the simple sulfone-based ligand family as an improved system for the stoichiometric and catalytic fluorination of arylboronic acid derivatives.

Tetramethylammonium Fluoride: Fundamental Properties and Applications in C-F Bond-Forming Reactions and as a Base

Nucleophilic ionic sources of fluoride are essential reagents in the synthetic toolbox to access high added-value fluorinated building blocks unattainable by other means. In this review, we provide a concise description and rationale of the outstanding features of one of these reagents, tetramethylammonium fluoride (TMAF), as well as disclosing the different methods for its preparation, and how its physicochemical properties and solvation effects in different solvents are intimately associated with its reactivity. Furthermore, herein we also comprehensively describe its historic and recent utilization, up to December 2021, in C-F bond-forming reactions with special emphasis on nucleophilic aromatic substitution fluorinations with a potential sustainable application in industrial settings, as well as its use as a base capable of rendering unprecedented transformations.

Access to enantioenriched molecules with diverse fluorinated tetrasubstituted stereocenters using hydroxy as a kinetic resolution auxiliary group

We describe in this paper that using secondary OH as the kinetic resolution auxiliary group, a series of previously unavailable fluorinated fully-substituted carbon molecules can be obtained with excellent level of enantioselectivities.

Developing organoboranes as phase transfer catalysts for nucleophilic fluorination using CsF

Despite the general high fluorophilicity of boron, organoboranes such as BEt₃ and 3,5-(CF₃)₂C₆H₃–BPin are shown herein for the first time, to our knowledge, to be effective (solid to solution) phase-transfer catalysts for the fluorination of certain organohalides with CsF. Significant (up to 30% e.e.) chiral induction during nucleophilic fluorination to form β-fluoroamines using oxazaborolidine (pre)catalysts and CsF also can be achieved. Screening different boranes revealed a correlation between calculated fluoride affinity of the borane and nucleophilic fluorination reactivity, with sufficient fluoride affinity required for boranes to react with CsF and form Cs[fluoroborate] salts, but too high a fluoride affinity leading to fluoroborates that are poor at transferring fluoride to an electrophile. Fluoride affinity is only one component controlling reactivity in this context; effective fluorination also is dependent on the ligation of Cs⁺ which effects both the phase transfer of CsF and the magnitude of the [Cs⋯F-BR₃] interaction and thus the B–F bond strength. Effective ligation of Cs⁺ (e.g. by [2.2.2]-cryptand) facilitates phase transfer of CsF by the borane but also weakens the Cs⋯F–B interaction which in turn strengthens the B–F bond – thus disfavouring fluoride transfer to an electrophile. Combined, these findings indicate that optimal borane mediated fluorination occurs using robust (to the fluorination conditions) boranes with fluoride affinity of ca. 105 kJ mol⁻¹ (relative to Me₃Si⁺) under conditions where a signficant Cs⋯F–B interaction persists.

Simple boranes with the optimal fluoride ion affinity are effective as catalysts for phase transfer nucleophilic fluorination with CsF.

Revisiting the Balz-Schiemann Reaction of Aryldiazonium Tetrafluoroborate in Different Solvents under Catalyst- and Additive-Free Conditions

The thermal and photochemical Balz-Schiemann reaction in commonly used solvents was revisited under catalyst- and additive-free conditions. The study showed that using low- or non-polar solvents could improve the pyrolysis and photolysis of aryldiazonium tetrafluoroborates, enabling effective fluorination at a low temperature or under visible-light irradiation. PhCl and hexane were exemplified as cheap and reliable solvents for both reactions, providing good to excellent yields of aryl fluorides from the corresponding diazonium tetrafluoroborates. The combination of slight heating with visible-light irradiation was beneficial for the transformation of stable aryldiazonium tetrafluoroborates. Nevertheless, the electronic and steric nature of aryldiazonium tetrafluoroborates still had a pivotal effect on both fluorinations even in these solvents.

Asymmetric Synthesis of Fluoro, Fluoromethyl, Difluoromethyl, and Trifluoromethylcyclopropanes

Fluorine-containing cyclopropanes are a subclass of cyclopropane derivatives that have generated considerable interest in medicinal chemistry for several decades. The replacement of a cyclopropane C-H or C-CH₃ bond with fluorine or a fluorinated group (such as CF₃ or CF₂H) can lead sometimes to synergistic effects in terms of biological activity and improved metabolic profile of a cyclopropane containing bioactive compound. In this context, the preparation of fluoro-, difluoromethyl-, or trifluoromethyl-cyclopropane is particularly attractive and important but quite challenging considering the unique electronic properties that result from the incorporation of a fluorine atom into a substrate or a reagent. In the past decade, we have sought to develop new routes for the stereoselective synthesis of these building blocks using the most reliable cyclopropanation methods and convenient and readily available starting materials. The challenge that had to be undertaken was how we could use the unique properties of the fluorine atom to improve upon the efficiency of a given process rather than shutting it down. This could be overcome by defining new substrate/reagent reactivity guidelines and carefully selecting whether the fluorinated group was introduced on the electrophilic or nucleophilic partner for a given reaction. In this Account, we describe our contributions in this area that take advantage of diazo-derived rhodium carbenes, zinc carbenoids, ring closure processes, and biocatalytic methods to access these important potential drug subunits. Our initial investigation relied on the development of a Michael-initiated ring closure reaction using the Reformatsky enolate derived from readily available ethyl dibromofluoroacetate and α,β-unsaturated electrophiles. The reaction proceeded extremely well but with modest to good diastereoselectivities with ester acrylates. Further extension to various fluorinated nucleophiles such as oxazolidinone based and DABCO ylides led to similar selectivities.In order to access enantioenriched fluorocyclopropanes, we then investigated the chiral dioxaborolane mediated zinc carbenoid based approaches using the fluoroiodomethylzinc carbenoid/allylic alcohol combination or the iodomethylzinc carbenoid/fluoroallylic alcohol combination. Quite surprisingly, both approaches were equally successful at providing the corresponding fluorocyclopropanes with excellent diastereo- and enantioselectivities.To broaden the scope of fluorinated cyclopropane building blocks that could be prepared with good enantiocontrol, we then investigated the rhodium-catalyzed cyclopropanation of fluoro-, difluoromethyl-, and trifluoromethyl-substituted alkenes with acceptor-acceptor and donor-acceptor diazo reagents. Depending on the substrate/reagent combination, Hashimoto's Rh₂((S)-TCPTTL)₄ or Davies' Rh₂((S)-BTPCP)₄ catalyst proved be the most efficient catalysts providing the cyclopropane derivatives with the highest enantioselectivities.More recently, a collaboration with Fasan's group led to the use of engineered myoglobins to catalyze the reaction of ethyl diazoacetate and difluoromethyl-substituted alkenes. This biocatalyzed process led to high turnover number and high enantioselectivities.Although our work has significantly increased the number of tools in the organic chemist's toolbox, continuous efforts in this area would be beneficial to the development of diastereo- and enantioselective approaches to allow the preparation of any elusive isomers of these valuable chiral building blocks.

Cs4 B4 O3 F10 : First Fluorooxoborate with [BF4 ] Involving Heteroanionic Units and Extremely Low Melting Point

Herein, a new congruently melting mixed-anion compound Cs₄ B₄ O₃ F₁₀ has been characterized as the first fluorooxoborate with [BF₄ ] involving heteroanionic units. Compound Cs₄ B₄ O₃ F₁₀ possesses two highly fluorinated anionic clusters and therefore its formula can be expressed as Cs₃ (B₃ O₃ F₆ ) ⋅ Cs(BF₄ ). The influence of [BF₄ ] units on micro-symmetry and structural evolution was discussed based on the parent compound. More importantly, Cs₄ B₄ O₃ F₁₀ shows the lowest melting point among all the available borates and thus sets a new record for such system. This work is of great significance to enrich and tailor the structure of borates using perfluorinated [BF₄ ] units.

Catalytic asymmetric nucleophilic fluorination using BF3·Et2O as fluorine source and activating reagent

Fluorination using chiral catalytic methods could result in a direct access to asymmetric fluorine chemistry. However, challenges in catalytic asymmetric fluorinations, especially the longstanding stereochemical challenges existed in BF₃·Et₂O-based fluorinations, have not yet been addressed. Here we report the catalytic asymmetric nucleophilic fluorination using BF₃·Et₂O as the fluorine reagent in the presence of chiral iodine catalyst. Various chiral fluorinated oxazine products were obtained with good to excellent enantioselectivities (up to >99% ee) and diastereoselectivities (up to >20:1 dr). Control experiments (the desired fluoro-oxazines could not be obtained when Py·HF or Et₃N·3HF were employed as the fluorine source) indicated that BF₃·Et₂O acted not only as a fluorine reagent but also as the activating reagent for activation of iodosylbenzene.

Catalytic asymmetric fluorination remains elusive, especially the longstanding stereochemical challenges which exist in BF₃Et₂O-based fluorinations. Here the authors show a catalytic asymmetric nucleophilic fluorination using BF₃·Et₂O as the fluorine reagent in the presence of chiral iodine catalyst.

Highly Stereoselective Intramolecular Carbofluorination of Internal α,β-Ynones Promoted by Selectfluor

Herein, we report a metal-free intramolecular carbofluorination protocol for the synthesis of tetrasubstituted monofluoroalkenes from internal α,β-ynones and Selectfluor with both high stereoselectivity and broad functional group tolerance. The chelation between tetrafluoroborate anion and the oxygen present in the aldehyde group rendered the reaction highly stereoselective, with the tetrafluoroborate serving as the direct fluorine source. Therefore, with addition of sodium tetrafluoroborate, Selectfluor could be reused several times without sacrificing reactivity.

Wonderful fusion of organofluorine chemistry and decarboxylation strategy

Decarboxylation strategy has been emerging as a powerful tool for the synthesis of fluorine-containing organic compounds that play important roles in various fields such as pharmaceuticals, agrochemicals, and materials science. Considerable progress in decarboxylation has been made over the past decade towards the construction of diverse valuable fluorinated fine chemicals for which the fluorinated part can be brought in two ways. The first way is described as the reaction of non-fluorinated carboxylic acids (and their derivatives) with fluorinating reagents, as well as fluorine-containing building blocks. The second way is dedicated to the exploration and the use of fluorine-containing carboxylic acids (and their derivatives) in decarboxylative transformations. This review aims to provide a comprehensive summary of the development and applications of decarboxylative radical, nucleophilic and cross-coupling strategies in organofluorine chemistry.

Electricity-Driven Post-Functionalization of Conducting Polymers

Electrochemical doping of conducting polymers (CPs) generates polarons (radical ionic species) and bipolarons (ionic species) in their backbone via multi-electron transfer between an electrode and the CP. In the electrochemical polymer reaction (ePR), these generated ionic species are regarded as reactive intermediates for further transformation of the chemical structures of CPs. This electrochemical post-functionalization can easily be used to control the degree of reactions by turning a power supply on/off, as well as tuning the applied electrode potential, which leads to fine-tuning of the various properties of the CPs, such as the HOMO/LUMO level and PL properties. This Account summarizes recent developments in the electrochemical post-functionalization of CPs. In particular, we focus on reaction design for the ePR, with respect to the preparation and structure of the precursor polymers, applicable functional groups, efficient reaction conditions, and electrolytic methodologies.

Synthesis and anticancer potential of novel 5,6-oxygenated and/or halogenated steroidal d-homo lactones

A series of 5,6-modified steroidal d-homo lactones, comprising of halogenated and/or oxygenated derivatives, was synthesized and evaluated for potential anticancer properties. Preparation of many of these compounds involved investigating alternative synthetic pathways. In silico ADME testing was performed for both novel and some previously synthesized compounds. Calculated physicochemical properties were in accordance with the Lipinski, Veber, Egan, Ghose and Muegge criteria, suggesting the potential of these molecules as orally active agents. Cytotoxicity of the synthesized steroid derivatives was tested on six tumor and one normal human cell line. None of the investigated derivatives was toxic to non-cancerous MRC-5 control cells. Most of the compounds showed significant cytotoxicity against the treated cancer cell lines. Most notably, the 3β,5α,6β-trihydroxy derivative exhibited strong cytotoxicity against multiple cell lines (MCF-7, MDA-MB-231 and HT-29), with the highest effect observed for lung adenocarcinoma (A549) cells, for which this steroid was more cytotoxic than all of the three commercial chemotherapeutic agents used as reference compounds. Molecular docking suggests the 3β,5α,6β-trihydroxy derivative could bind the EGFR tyrosine kinase domain with high affinity, providing a potential mechanism for its cytotoxicity via inhibition of EGFR signaling. The most active compounds were further studied for their potential to induce apoptosis by the double-staining fluorescence method; where the 5α,6β-dibromide, 5α,6β-dichloride and 3β,5α,6β-triol induced apoptotic changes in all three treated cell lines: MDA-MB-231, HT-29 and A549. To predict interactions with nuclear steroidal receptors, affinity for the ligand binding domains of ERα, ERβ and AR was measured using a yeast-based fluorescence assay. The 5β,6β-epoxide, dibromide and 5α-hydroxy-3,6-dioxo derivatives showed affinity for ERα, while the 5α-fluoro-6β-hydroxy and 3β-acetoxy-5α,6β-dihydroxy derivatives were identified as ERβ ligands. None of the tested compounds showed affinity for AR. Structure-activity relationships of selected compounds were also examined.

A family of rhodium(i) NHC chelates featuring O-containing tethers for catalytic tandem alkene isomerization-hydrosilylation

The rhodium complex Rh(HL)(COD)Cl, 1, L being a functionalized N-heterocyclic carbene (NHC) ligand with an oxygen-containing pendant arm, has been used as the entry point to synthesize a series of neutral and cationic Rh(i) O,C chelates. While the Rh-carbene interaction is similar in all these 16-electron complexes, structural analysis reveals that the strength of the Rh-O bond is greatly affected by the nature of the O-donor: R-O^- > R-OH > R-OBF₃. These subtle changes in the nature of the O-containing tether are found to be responsible for large differences in the alkene hydrosilylation catalytic activity of these compounds: the stronger the Rh-O interaction, the better the catalytic performances. The most active catalyst, [Rh(L)(COD)], 2, demonstrated good catalytic activity under mild reaction conditions for the hydrosilylation of a range of alkene substrates with the industrially relevant non-activated tertiary silane, 1,1,1,3,5,5,5-heptamethyltrisiloxane (MD^HM). Furthermore, this complex is an effective catalyst for the selective remote functionalization of internal olefins at room temperature via tandem alkene isomerization-hydrosilylation.

Stereodivergent Alkyne Hydrofluorination Using Protic Tetrafluoroborates as Tunable Reagents

The discovery of safe, general, and practical procedures to prepare vinyl fluorides from readily available precursors remains a synthetic challenge. The metal-free hydrofluorination of alkynes constitutes an attractive though elusive strategy for their preparation. Introduced here is an inexpensive and easily handled reagent that enables the development of simple and scalable protocols for the regioselective hydrofluorination of alkynes to access both the E and Z isomers of vinyl fluorides. These reaction conditions were suitable for a diverse collection of alkynes, including several highly functionalized pharmaceutical derivatives. Computational and experimental mechanistic studies support C-F bond formation through vinyl cation intermediates, with the E- and Z-hydrofluorination products forming under kinetic and thermodynamic control, respectively.

Hydrogen Bonds with BF4− Anion as a Proton Acceptor

The BF4− anion is characterised by weak Lewis base properties; it is usually classified as a “non-coordinating anion”. The searches through the Cambridge Structural Database (CSD) were performed and it was found that the BF4− anion often occurs in crystal structures and it is involved in numerous intermolecular interactions; hydrogen bonds are the majority of them. The hydrogen bonds involving the BF4− anion as a proton acceptor are closer to linearity with the increase of the strength of interaction that is in line with the tendency known for other hydrogen bonds. However, even for short contacts between the proton and the Lewis base centre, slight deviations from linearity occur. The MP2/aug-cc-pVTZ calculations on the BF4−…HCN complex and on the BF4−…(HCN)4 cluster were also carried out to characterise corresponding C-H…F hydrogen bonds; such interactions often occur in crystal structures.

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

Fluorine chemistry plays an increasingly important role in pharmaceutical, agricultural, and materials industries. The incorporation of fluorine-containing groups into organic molecules can improve their chemical and physical properties, which attracts continuous interest in organic synthesis. Among various reported methods, transition-metal-catalyzed fluorination/fluoroalkylation has emerged as a powerful method for the construction of these compounds. This review attempts to describe the major advances in the transition-metal-catalyzed incorporation of fluorine, trifluoromethyl, difluoromethyl, trifluoromethylthio, and trifluoromethoxy groups reported between 2011 and 2019.