Anhydrous Phosphazenium Fluorides as Sources for Extremely Reactive Fluoride Ions in Solution

Strong Bases Design: Key Techniques and Stability Issues

Theoretical design of molecular superbases has been attracting researchers for more than twenty years. General approaches were developed to make the bases potentially stronger, but less attention was paid to the stability of the predicted structures. Hence, only a small fraction of the theoretical research has led to positive experimental results. Possible stability issues of extremely strong bases are extensively studied in this work using quantum chemical calculations on a high level of theory. Several step-by-step design examples are discussed in detail, and general recommendations are given to avoid the most common stability problems. New potentially stable structures are theoretically studied to demonstrate the future prospects of molecular superbases design.

pH Indicators for Strong Molecular Bases: A Theoretical Approach

The transition points in hexamethylphosphoramide are theoretically studied for a series of acid-base indicators. Three new indicators with multiple transition points and deeply colored low-nucleophilic anions are designed. A general basicity scale is established for highly basic hexamethylphosphoramide solutions, expanding beyond the basicity of the strongest currently known molecular bases.

Speciation and kinetics of fluoride transfer from tetra-n-butylammonium difluorotriphenylsilicate ('TBAT')

Tetra-n-butylammonium difluorotriphenylsilicate (TBAT) is a conveniently handled anhydrous fluoride source, commonly used as a surrogate for tetra-n-butylammonium fluoride (TBAF). While prior studies indicate that TBAT reacts rapidly with fluoride acceptors, little is known about the mechanism(s) of fluoride transfer. We report on the interrogation of the kinetics of three processes in which fluoride is transferred from TBAT, in THF and in MeCN, using a variety of NMR methods, including chemical exchange saturation transfer, magnetisation transfer, diffusion analysis, and 1D NOESY. These studies reveal ion-pairing between the tetra-n-butylammonium and difluorotriphenylsilicate moieties, and a very low but detectable degree of fluoride dissociation, which then undergoes further equilibria and/or induces decomposition, depending on the conditions. Degenerate exchange between TBAT and fluorotriphenylsilane (FTPS) is very rapid in THF, inherently increases in rate over time, and is profoundly sensitive to the presence of water. Addition of 2,6-di-tert-butylpyridine and 3 Å molecular sieves stabilises the exchange rate, and both dissociative and direct fluoride transfer are shown to proceed in parallel under these conditions. Degenerate exchange between TBAT and 2-naphthalenyl fluorosulfate (ARSF) is not detected at the NMR timescale in THF, and is slow in MeCN. For the latter, the exchange is near-fully inhibited by exogenous FTPS, indicating a predominantly dissociative character to this exchange process. Fluorination of benzyl bromide (BzBr) with TBAT in MeCN-d3 exhibits moderate progressive autoinhibition, and the initial rate of the reaction is supressed by the presence of exogenous FTPS. Overall, TBAT can act as a genuine surrogate for TBAF, as well as a reservoir for rapidly-reversible release of traces of it, with the relative contribution of the pathways depending, inter alia, on the identity of the fluoride acceptor, the solvent, and the concentration of endogenous or exogenous FTPS.

Reactions Using Freons and Halothane as Halofluoroalkyl/Halofluoroalkenyl Building Blocks

In recent years, hydrofluorocarbon compounds such as chlorofluorocarbons, hydrochlorofluorocarbons, and 2-bromo-2-chloro-1,1,1-trifluoroethane (halothane) have been used as fluorine-containing building blocks to construct functional fluorine-containing compounds, e. g., polymers, liquid crystals, and medicines. Hydrofluorocarbons promote the formation of reactive fluoroalkyl or fluoroalkenyl species via anionic or radical processes, and these species can act as nucleophiles or electrophiles depending on the reaction conditions. Progress in fluorine chemistry using hydrofluorocarbons in the last 30 years is described in this review and diverse reactions are discussed, including the fluoroalkyl/alkenyl products and proposed mechanisms involved.

SN2 versus E2 reactions in a complex microsolvated environment: theoretical analysis of the equilibrium and activation steps of a nucleophilic fluorination

The reactivity of the fluoride ion towards alkyl halides is highly dependent on the solvating environment. In polar aprotic solvents with large counter-ions is highly reactive and produces substantial E2 product, whereas in polar protic solvents leads to slow kinetics and high selectivity for S_N2 reactions. The use of a more complex environment with stoichiometric addition of tert-butanol to acetonitrile solvent is able to module the reactivity and selectivity of tetrabutylammonium fluoride (TBAF). In the present work, we have performed a detailed theoretical analysis of this complex reaction system by density functional theory, continuum solvation model, and including explicit tert-butanol molecules. A kinetic model based on the free energy profile was also used to predict the reactivity and selectivity. The results indicated that the TBAF(tert-butanol) complex plays the key role to increase the S_N2 selectivity, whereas higher aggregates are not relevant. The E2 product is formed exclusively via free TBAF, because the solvating tert-butanol in the TBAF(tert-butanol) complex inhibits the E2 pathway. Our analysis suggests that diols or tetraols could produce an improved selectivity.

A cyclic divalent N(I) species isoelectronic to carbodiphosphoranes

The formation of a rare type of diphosphazenium cation is described. Its synthesis features a unique oxidative dealkylation of an iminophosphorane-phosphole by a silver(I) salt. DFT study of this compound reveals the low valent character of the N(I) center.

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.

Heavy silylchalcogenido lanthanates synthesis Ph4P[Cp3La-ESiMe3] (E = S, Se, and Te) via fluoride-induced demethylation of dimethylcarbonate to Ph4P[OCO2Me] key intermediate

We report a new high-yield synthesis of so far not accessible tetraphenylphosphonium methylcarbonate Ph₄P[OCO₂Me] via solvothermal fluoride-induced demethylation and MeF elimination at Me₂CO₃ (DMC) by Ph₄P-F, structurally characterized as λ⁵-fluorophosphoran by XRD. The synthetic value of Ph₄P[OCO₂Me] key compound for preparing nearly all kinds of other Ph₄P[anion] salts with perfectly crystallizing (not symmetry frustrated) cation is demonstrated by examples beyond ionic liquid research: a complete set of silylchalcogenide salts Ph₄P[ESiMe₃] (E = S, Se, and Te) including the first example of a structurally characterized non-coordinating, naked [Te-SiMe₃]^- anion is presented. With this set of soft Lewis bases and metal organic Lewis acids [Cp₃La] at hand, a comprehensive series of crystalline 1 : 1 lanthanate complexes Ph₄P[Cp₃La-ESiMe₃] has been prepared. Their structural features and trends such as complexation induced Si-E bond elongation and a pronounced trend in La-E-Si bond angle contraction with E = S < Se < Te are discussed. Heteronuclear ¹H, ¹³C, ²⁹Si, and ¹³⁹La NMR studies provide a set of ¹³⁹La NMR shifts for homologs of heavy chalcogen-lanthanum complexes.

Phosphorus-Containing Superbases: Recent Progress in the Chemistry of Electron-Abundant Phosphines and Phosphazenes

The renaissance of Brønsted superbases is primarily based on their pronounced capacity for a large variety of chemical transformations under mild reaction conditions. Four major set screws are available for the selective tuning of the basicity: the nature of the basic center (N, P, …), the degree of electron donation by substituents to the central atom, the possibility of charge delocalization, and the energy gain by hydrogen bonding. Within the past decades, a plethora of neutral electron-rich phosphine and phosphazene bases have appeared in the literature. Their outstanding properties and advantages over inorganic or charged bases have now made them indispensable as auxiliary bases in deprotonation processes. Herein, an update of the chemistry of basic phosphines and phosphazenes is given. In addition, due to widespread interest, their use in catalysis or as ligands in coordination chemistry is highlighted.

The role of intermolecular forces in ionic reactions: the solvent effect, ion-pairing, aggregates and structured environment

The environment enclosing an ionic species has a critical effect on its reactivity. In a more general sense, medium effects are not limited to the solvent, but involve the counter ion effect (ion pairing), formation of larger aggregates and structured environment as provided by the host in the case of host-guest complexes. In this review, a general view of the medium effect on anion-molecule reactions is presented. Nucleophilic substitution reactions of aliphatic (SN2) and aromatic (SNAr) systems, as well as elimination reactions (E2), are the focus of the discussion. In particular, nucleophilic fluorination with KF, CsF and tetraalkylammonium fluoride was used as the main model, because of the importance of this kind of reaction and the recent advances in the study of these systems. The solvent effect, ion pairing, formation of aggregates and formation of complexes with crown ethers, cryptands and calixarenes are discussed. For a deeper insight into the medium effect, many results of reliable theoretical calculations in close agreement with experiments were chosen as examples.

The Influence of Weakly Coordinating Cations on the O-H⋅⋅⋅O- Hydrogen Bond of Silanol-Silanolate Anions

The reaction of a saline phosphazenium hydroxide hydrate with siloxanes led to a novel kind of silanol-silanolate anions. The weakly coordinating behavior of the cation renders the formation of silanol-silanolate hydrogen bonds possible, which otherwise suffer from detrimental silanolate-oxygen cation interactions. We investigated the influence of various weakly coordinating cations on silanol-silanolate motifs, particularly with regard to different cation sizes. While large cations favor the formation of intramolecular hydrogen bonds resulting in cyclic structures, the less bulky tetramethyl ammonium cation encourages the formation of polyanionic silanol-silanolate chains in the solid state.

Insights into the Stabilization of Fluoride Ions in Ionic Liquids: Pointers to Better Fluorinating Agents

The fluorination efficiency of a fluorinating agent depends on the free availability of the fluoride ions, which in turn depends on its interaction with its solvation shell. A stable fluoride-based poor solvate ionic liquid (SIL) comprising 1-ethyl-3-methylimidazolium (EMIM) cation and ethylene glycol (EG) was recently reported and demonstrated as a fluorinating agent. Herein, we performed ab initio calculations and ab initio molecular dynamics simulations to gain a microscopic understanding of the intermolecular interactions in this SIL in gas, liquid, and crystalline phases. Ethylene glycol (EG), being capable of forming hydrogen bond(s) with the fluoride ion, prevents the latter from reacting with the EMIM cation. Fluoride forms hydrogen bonds with both the cation and the EG molecule, but it was found to have more affinity toward EG, forming a stronger hydrogen bond with its hydroxyl proton than with the acidic proton of the cation. An optimal concentration of EG in the SIL balances its contribution to stabilizing the fluoride ion and yet making fluoride available for fluorination.

Hydrogen-bond promoted nucleophilic fluorination: concept, mechanism and applications in positron emission tomography

Due to the tremendous interest in carbon-fluorine bond-forming reactions, research efforts in this area have been dedicated to the development of facile processes to synthesize small fluorine-containing organic molecules. Among others, PET (Positron Emission Tomography) is one of the most important applications of fluorine chemistry. Recognizing the specific requirements of PET processes, some groups have focused on fluorination reactions using alkali metal fluorides, particularly through SN2-type reactions. However, a common "misconception" about the role of protic solvents and hydrogen bonding interactions in this class of reactions has hampered the employment of these excellent promoters. Herein, we would like to review recent discoveries in this context, showing straightforward nucleophilic fluorination reactions using alkali metal fluorides promoted by protic solvents. Simultaneous dual activation of reacting partners by intermolecular hydrogen bonding and the enhancement of the "effective fluoride nucleophilicity", which is Nature's biocatalytic approach with the fluorinase enzyme, are the key to this unprecedentedly successful nucleophilic fluorination.

Potassium fluoride activation for the nucleophilic fluorination reaction using 18-crown-6, [2.2.2]-cryptand, pentaethylene glycol and comparison with the new hydro-crown scaffold: a theoretical analysis

Ion-pair binding in the hydro-crown ether leads to more effective catalytic cycle for nucleophilic fluorination.

Hydrogen Bonding: Regulator for Nucleophilic Fluorination

The recent advances in nucleophilic fluorination, regulated through hydrogen bonding interactions are summarized. Two main categories of fluorine nucleophiles are discussed. Alkali-metal fluorides are widely used in various fluorination transformations because they are inexpensive and safe nucleophilic fluorine sources. But the non-controllable nucleophilicity and strong basicity of some of them cause undesired side reactions, which led to the introduction of hydrogen bonding to fine tune their nucleophilicity and basicity. In contrast, an HF-based fluorine nucleophile, HF/DMPU, is in some aspects superior to the conventional HF/pyridine (Olah's reagent) or HF/Et3 N because of the higher hydrogen bond basicity of DMPU. It has been used in several nucleophilic fluorinations such as fluorination of alkynes, fluoro-Prins reaction and fluorination of aziridines.

Synthesis, Characterization, and Reactivity of Thermally Stable Anhydrous Quaternary Ammonium Fluorides

The synthesis and properties of a new class of anhydrous quaternary ammonium fluorides, based on the rigid skeleton azabicyclo[2.2.2]octane, is described. Compounds 2a-d were easily prepared by passing the corresponding ammonium iodides over fluoride-based resin followed by drying their hydrated form at 100 or 140 °C under reduced pressure. The stability (experimental and theoretical study), solubility, reactivity, and characterization by solution and solid-state MAS NMR are discussed.

Silver-mediated fluorination of alkyl iodides with TMSCF3 as the fluorinating agent

The first example of a silver-mediated fluorination of alkyl iodides with the Ruppert–Prakash reagent (TMSCF₃) as the fluorinating agent is reported.

Divalent N(I) compounds with two lone pairs on nitrogen

Carbon with the C(0) state has been reported recently, examples of which were known for the past decades. Silicon in the Si(0) state and phosphorus in the P(I) state are also known experimentally. This prompted us to search for divalent N(I) compounds, which resulted in the identification of ::N(←L)(2)(⊕) systems with bicoordinated nitrogen in the N(I) formal oxidation state. It was found that several biguanide derivatives (especially in their protonated state) belong to this class. Quantum chemical analysis provided the structural details, molecular orbitals, charge localization (vs delocalization) trends, etc. This class of compounds has been found to be characterized by two lone pairs on the central nitrogen, very similar to the central carbon in divalent C(0) compounds (::C(←L)(2)). The new bonding environment for nitrogen reported in this article, divalent nitrogen N(I), is clearly different from the nitrenium ions NR(2)(⊕). The electronic structure and reactivity of representative examples of this novel class of divalent nitrogen N(I) systems (::N(←L)(2)(⊕)) have been analyzed in detail, in terms of molecular orbitals, atomic charges, protonation energies, complexation energies with Lewis acids like BH(3), AlCl(3), and AuCl and compared with those of divalent C(0) systems.

Nucleophilic fluorination reactions starting from aqueous fluoride ion solutions

The sulfonium borane 2(+) reacts with fluoride anions in MeOH/H(2)O mixtures to afford the zwitterionic fluoroborate 2-F as an easily isolable nonhygroscopic solid. In dry acetonitrile, 2-F reacts with PhS(-) to afford the anionic fluoroborate 1-F(-). The latter is very labile and acts as a nucleophilic fluorination reagent toward a variety of substrates including alkylhalides and electron-deficient aromatic compounds. This approach may become broadly applicable to nucleophilic fluorination procedures that involve wet fluoride sources.

C-F Bond Formation for the Synthesis of Aryl Fluorides

A selection of carbon-fluorine bond-forming reactions is presented with particular focus on transition metal-mediated fluorination. A brief summary of conventional fluorination reactions is followed by a discussion of fluorination reactions mediated by palladium and silver. Investigations into the mechanism as well as the conceptual difficulty associated with transition metal-mediated carbon-fluorine bond formation are presented.

The organometallic fluorine chemistry of palladium and rhodium: studies toward aromatic fluorination

Although springing from two established fields, organometallic chemistry and fluorine chemistry, organometallic fluorine chemistry is still in its early stages. However, developments in this area are expected to provide new tools for the synthesis of selectively fluorinated organic compounds that have been in high demand. Selectively fluorinated organic molecules currently account for up to 40% of all agrochemicals and 20% of all pharmaceuticals on the market. Our research efforts have been focused on the development of new organometallic and catalytic methods for the selective introduction of fluorine and the CF(3) group into the aromatic ring. Monofluorinated and trifluoromethylated aromatic compounds are still made by the old technologies that employ stoichiometric quantities of hazardous and costly materials. In this Account, we describe our studies toward the development of safe, catalytic alternatives to these methods. We have synthesized, characterized, and studied the reactivity of the first aryl palladium(II) fluoride complexes. We have demonstrated for the first time that a Pd-F bond can be formed in a soluble and isolable molecular complex: this bond is more stable than previously thought. Toward the goal of fluoroarene formation via Ar-F reductive elimination, we have studied a number of sigma-aryl Pd(II) fluorides stabilized by various P, N, and S ligands. It has been established that numerous conventional tertiary phosphine ligands, most popular in Pd catalysis, are unlikely to be useful for the desired C-F bond formation at the metal center because of the competing, kinetically preferred P-F bond-forming reaction. A metallophosphorane mechanism has been demonstrated for the P-F bond-forming processes at Rh(I) and Pd(II), which rules out the possibility of controlling these reactions by varying the amount of phosphine in the system, a most common and often highly efficient technique in homogeneous catalysis. The novel F/Ph rearrangement of the fluoro analogue of Wilkinson's catalyst [(Ph(3)P)(3)RhF] and P-F bond-forming reactions at Pd(II) are insensitive to phosphine concentration and, because of the small size of fluorine, occur even with bulky phosphine ligands. These observations may guide further efforts toward metal-catalyzed nucleophilic fluorination of haloarenes. We have also developed aryne-mediated and CuF(2)/TMEDA-promoted aromatic fluorination reactions. The formation of fluoroarenes from the corresponding iodo- and bromoarenes in the presence of the CuF(2)/TMEDA system is the first example of a transition metal-mediated fluorination of nonactivated aryl halides in the liquid phase. Progress has also been made toward the development of aromatic trifluoromethylation. We have found unexpectedly facile and clean benzotrifluoride formation as a result of Ph-CF(3) reductive elimination from [(Xantphos)Pd(Ph)CF(3)]. This observation demonstrates for the first time that the notoriously strong and inert metal-CF(3) bond can be easily cleaved (at 50-80 degrees C) as a result of reductive elimination to produce the desired aryl-trifluoromethyl bond, the only previously missing link of the catalytic loop. Our study of the novel complex [(Ph(3)P)(3)RhCF(3)] has led to a rationale for the long-puzzling strong trans influence (electron donation) of the CF(3) group which, in complete contrast, is known to be an electron acceptor in organic chemistry.

First solvation shell effects on ionic chemical reactions: new insights for supramolecular catalysis

This article presents a detailed discussion on medium effect on ionic S(N)2 reactions. Ion pairing and the first solvation shell play the main role on reactivity. Ab initio calculations are used to show that a combination of apolar molecular cavity with polar hydroxyl groups can have catalytic effects on these reactions. The present results open an avenue of development for supramolecular catalysts able to control bimolecular ionic reactions. In particular, this new catalytic concept could be useful in nucleophilic fluorinations.

Synthesis of N6 ,N6-Dialkyl Adenine Nucleosides With In Situ Formed Hexaalkylphosphorus Triamides

Reactions between secondary amines and phosphorus trichloride (PCl₃) leads to the formation of the corresponding tris(dialkylamino)phosphines or hexaalkylphosphorus triamides [HAPT: (R₂N)₃P]. Reaction of silyl-protected 2'-deoxyinosine and acetyl-protected inosine with the in situ formed HAPT and iodine (I₂) leads to the formation of N⁶,N⁶-dialkyl adenosine and 2'-deoxyadenosine. In some cases the stoichiometry of the amine is important as also the use of a tertiary amine base. The effect of amine stoichiometry on the reaction of HAPT with I₂ has been studied by ³¹P{¹H} NMR.