Hydrogen Bonding Phase-Transfer Catalysis with Ionic Reactants: Enantioselective Synthesis of γ-Fluoroamines

Activation of fluoride anion as nucleophile in water with data-guided surfactant selection

A principal component surfactant_map was developed for 91 commonly accessible surfactants for use in surfactant-enabled organic reactions in water, an important approach for sustainable chemical processes. This map was built using 22 experimental and theoretical descriptors relevant to the physicochemical nature of these surfactant-enabled reactions, and advanced principal component analysis algorithms. It is comprised of all classes of surfactants, i.e. cationic, anionic, zwitterionic and neutral surfactants, including designer surfactants. The value of this surfactant_map was demonstrated in activating simple inorganic fluoride salts as effective nucleophiles in water, with the right surfactant. This led to the rapid development (screening 13-15 surfactants) of two fluorination reactions for β-bromosulfides and sulfonyl chlorides in water. The latter was demonstrated in generating a sulfonyl fluoride with sufficient purity for direct use in labelling of chymotrypsin, under physiological conditions.

Integrating experimental and computational approaches for deep eutectic solvent-catalyzed glycolysis of post-consumer polyethylene terephthalate

To achieve a sustainable and circular economy, developing effective plastic recycling methods is essential. Despite advances in the chemical recycling of plastic waste, modern industries require highly efficient and sustainable solutions to address environmental problems. In this study, we propose an efficient glycolysis strategy for post-consumer polyethylene terephthalate (PET) using deep eutectic solvents (DESs) to produce bis(2-hydroxyethyl) terephthalate (BHET) with high selectivity. Choline chloride (ChCl)- and urea-based DESs were synthesized using various metal salts and were tested for the glycolysis of PET waste; ChCl-Zn(OAc)2 exhibited the best performance. The DES-containing solvent system afforded a complete PET conversion, producing BHET at a high yield (91.6%) under optimal reaction conditions. The degradation mechanism of PET and its interaction with DESs were systematically investigated using density functional theory-based calculations. Furthermore, an intuitive machine learning model was developed to predict the PET conversion and BHET selectivity for different DES compositions. Our findings demonstrate that the DES-catalyzed glycolysis of post-consumer PET could enable the development of a sustainable chemical recycling process, providing insights to identify the new design of DESs for plastic decomposition.

Enhancing polyethylene terephthalate conversion through efficient microwave-assisted deep eutectic solvent-catalyzed glycolysis

Chemical recycling of plastics is a promising approach for effectively depolymerizing plastic waste into its constituent monomers, thereby contributing to the realization of a sustainable circular economy. Glycolysis, which converts polyethylene terephthalate (PET) into the monomer bis(2-hydroxyethyl) terephthalate (BHET), has emerged as a cost-effective and commercially viable chemical recycling process. However, glycolysis requires long reaction times and high energy consumption, limiting its industrialization. In this study, we develop an energy-efficient microwave-assisted deep eutectic solvent-catalyzed glycolysis method to degrade PET effectively and rapidly, resulting in a high BHET yield. This combined approach enables the quantitative degradation of PET within 9 min, achieving a high BHET yield of approximately 99% under optimal reaction conditions. Furthermore, the proposed approach has a low specific energy consumption (45 kJ/g) and minimizes waste generation. The thermal behavior of PET and its degradation mechanism are systematically investigated using scanning electron microscopy and density functional theory-based calculations. The results obtained suggest that the proposed straightforward, swift, and energy-efficient strategy has the potential to offer a sustainable solution to plastic waste management challenges and expedite the industrialization of chemical recycling.

Chiral N,N'-Dioxide Ligands Tune Diastereoselectivity in Mg(II)-Catalyzed Asymmetric Ring-Opening Desymmetrization of Azetidiniums

A diastereodivergent asymmetric desymmetrization of azetidinium salts with benzothiazoleamides as carbon nucleophiles through a chiral N,N'-dioxide/Mg(II) complex-promoted ring-opening reaction is realized by tuning ligands. Both syn- and anti-chiral δ-amino acid derivatives bearing benzothiazole structure were obtained in moderate to good yields and dr and ee values. DFT calculations indicated that the diastereodivergency stems from the different size of the chiral pocket formed by variable substructures of the ligands, leading to the opposite attack direction of the nucleophiles.

Strategies for Nucleophilic C(sp3)-(Radio)Fluorination

This Perspective surveys the progress and current limitations of nucleophilic fluorination methodologies. Despite the long and rich history of C(sp³)-F bond construction in chemical research, the inherent challenges associated with this transformation have largely constrained nucleophilic fluorination to a privileged reaction platform. In recent years, the Doyle group─along with many others─has pursued the study and development of this transformation with the intent of generating deeper mechanistic understanding, developing user-friendly fluorination reagents, and contributing to the invention of synthetic methods capable of enabling radiofluorination. Studies from our laboratory are discussed along with recent developments from others in this field. Fluoride reagent development and the mechanistic implications of reagent identity are highlighted. We also outline the chemical space inaccessible by current synthetic technologies and a series of future directions in the field that can potentially fill the existing dark spaces.

Regiodivergent Nucleophilic Fluorination under Hydrogen Bonding Catalysis: A Computational and Experimental Study

The controlled programming of regiochemical outcomes in nucleophilic fluorination reactions with alkali metal fluoride is a problem yet to be solved. Herein, two synergistic approaches exploiting hydrogen bonding catalysis are presented. First, we demonstrate that modulating the charge density of fluoride with a hydrogen-bond donor urea catalyst directly influences the kinetic regioselectivity in the fluorination of dissymmetric aziridinium salts with aryl and ester substituents. Moreover, we report a urea-catalyzed formal dyotropic rearrangement, a thermodynamically controlled regiochemical editing process consisting of C-F bond scission followed by fluoride rebound. These findings offer a route to access enantioenriched fluoroamine regioisomers from a single chloroamine precursor, and more generally, new opportunities in regiodivergent asymmetric (bis)urea-based organocatalysis.

Gold-Catalyzed Nucleophilic Ring-Opening Reaction of 2-Alkynylazetidines with Alcohols

The reaction of 2-alkynylazetidines and alcohols with a gold catalyst is described. A variety of substituted δ-amino-α,β-unsaturated ketones were synthesized via gold-promoted nucleophilic attack of alcohols followed by ring-opening of azetidine ring.

Deconstructive isomerization of azetidinols via C-C bond cleavage enabled by N-heterocyclic carbene (NHC) catalysis

Herein, we describe an N-heterocyclic carbene (NHC)-catalyzed deconstructive isomerization of azetidinols via an inert C-C bond cleavage. It provides a direct and supplementary pathway to access α-amino ketone and oxazol-2-one derivatives in moderate to good yields. DFT calculation supports the proposed mechanism in which NHC undergoes a concerted proton transfer and ring-opening process. This reaction features non-metal catalysis, simple reaction operation, excellent regioselectivity and gram-scale synthesis.

Strained Ammonium Precursors for Radiofluorinations

The increasing application of positron emission tomography (PET) in nuclear medicine has stimulated the extensive development of a multitude of novel and versatile techniques to introduce fluorine‐18, especially for the radiolabelling of biologically or pharmacologically active molecules. Taking into consideration that the introduction of fluorine‐18 (t_1/2=109.8 min) mostly proceeds under harsh conditions, radiolabelling of such molecules represents a challenge and is of enormous interest. Ideally, it should proceed in a regioselective manner under mild physiological conditions, in an acceptable time span, with high yields and high specific activities. Special attention has been drawn to 2‐fluoroethyl and 3‐fluoropropyl groups, which are often the active sites of radiofluorinated compounds. Precursors containing an ammonium leaving group – such as a strained azetidinium or aziridinium moiety – can help to overcome these obstacles leading to a convenient and mild introduction of [¹⁸F]fluoride with high radiochemical yields.

Hydrogen Bonding Phase-Transfer Catalysis with Alkali Metal Fluorides and Beyond

Phase-transfer catalysis (PTC) is one of the most powerful catalytic manifolds for asymmetric synthesis. Chiral cationic or anionic PTC strategies have enabled a variety of transformations, yet studies on the use of insoluble inorganic salts as nucleophiles for the synthesis of enantioenriched molecules have remained elusive. A long-standing challenge is the development of methods for asymmetric carbon-fluorine bond formation from readily available and cost-effective alkali metal fluorides. In this Perspective, we describe how H-bond donors can provide a solution through fluoride binding. We use examples, primarily from our own research, to discuss how hydrogen bonding interactions impact fluoride reactivity and the role of H-bond donors as phase-transfer catalysts to bring solid-phase alkali metal fluorides in solution. These studies led to hydrogen bonding phase-transfer catalysis (HB-PTC), a new concept in PTC, originally crafted for alkali metal fluorides but offering opportunities beyond enantioselective fluorination. Looking ahead, the unlimited options that one can consider to diversify the H-bond donor, the inorganic salt, and the electrophile, herald a new era in phase-transfer catalysis. Whether abundant inorganic salts of lattice energy significantly higher than those studied to date could be considered as nucleophiles, e.g., CaF₂, remains an open question, with solutions that may be found through synergistic PTC catalysis or beyond PTC.

Synthesis of Chiral Tetrahydro-3-benzazepine Motifs by Iridium-Catalyzed Asymmetric Hydrogenation of Cyclic Ene-carbamates

A highly efficient N,P-ligated iridium complex is presented for the simple preparation of chiral tetrahydro-3-benzazepine motifs by catalytic asymmetric hydrogenation. Substrates bearing both 1-aryl and 1-alkyl substituents were smoothly converted to the corresponding hydrogenated product with excellent enantioselectivity (91-99% ee) and in isolated yield (92-99%). The synthetic value of this transformation was demonstrated by a gram-scale hydrogenation and application in the syntheses of trepipam and fenoldopam.

Asymmetric Azidation under Hydrogen Bonding Phase-Transfer Catalysis: A Combined Experimental and Computational Study

Asymmetric catalytic azidation has increased in importance to access enantioenriched nitrogen containing molecules, but methods that employ inexpensive sodium azide remain scarce. This encouraged us to undertake a detailed study on the application of hydrogen bonding phase-transfer catalysis (HB-PTC) to enantioselective azidation with sodium azide. So far, this phase-transfer manifold has been applied exclusively to insoluble metal alkali fluorides for carbon–fluorine bond formation. Herein, we disclose the asymmetric ring opening of meso aziridinium electrophiles derived from β-chloroamines with sodium azide in the presence of a chiral bisurea catalyst. The structure of novel hydrogen bonded azide complexes was analyzed computationally, in the solid state by X-ray diffraction, and in solution phase by ¹H and ¹⁴N/¹⁵N NMR spectroscopy. With N-isopropylated BINAM-derived bisurea, end-on binding of azide in a tripodal fashion to all three NH bonds is energetically favorable, an arrangement reminiscent of the corresponding dynamically more rigid trifurcated hydrogen-bonded fluoride complex. Computational analysis informs that the most stable transition state leading to the major enantiomer displays attack from the hydrogen-bonded end of the azide anion. All three H-bonds are retained in the transition state; however, as seen in asymmetric HB-PTC fluorination, the H-bond between the nucleophile and the monodentate urea lengthens most noticeably along the reaction coordinate. Kinetic studies corroborate with the turnover rate limiting event resulting in a chiral ion pair containing an aziridinium cation and a catalyst-bound azide anion, along with catalyst inhibition incurred by accumulation of NaCl. This study demonstrates that HB-PTC can serve as an activation mode for inorganic salts other than metal alkali fluorides for applications in asymmetric synthesis.

Ring-opening fluorination of bicyclic azaarenes

We have discovered a ring-opening fluorination of bicyclic azaarenes. Upon treatment of bicyclic azaarenes such as pyrazolo[1,5-a]pyridines with electrophilic fluorinating agents, fluorination of the aromatic ring is followed by a ring-opening reaction. Although this overall transformation can be classified as an electrophilic fluorination of an aromatic ring, it is a novel type of fluorination that results in construction of tertiary carbon-fluorine bonds. The present protocol can be applied to a range of bicyclic azaarenes, tolerating azines and a variety of functional groups. Additionally, mechanistic studies and enantioselective fluorination have been examined.

Synthesis of tertiary alkyl fluorides and chlorides by site-selective nucleophilic ring-opening reaction of α-aryl azetidinium salts

Site-selective nucleophilic ring-opening reactions of 2-arylazetidine-2-carboxylic acid ester-derived tetraalkyl ammonium salts 2 with tetrabutylammonium halides (Bu₄NX) to give tertiary alkyl halides are successfully demonstrated. For example, a nucleophilic ring-opening reaction of 2-(o-tolyl) derivative 2a with 1.2 equivalents of tetrabutylammonium fluoride (Bu₄NF) in THF at 60 °C preferentially proceeded at a more substituted carbon atom (2-position) compared to a less-substituted carbon atom (4-position) and afforded tert-butyl 4-(dimethylamino)-2-fluoro-2-(o-tolyl)butanoate 3aa in 71% yield as the corresponding tertiary alkyl fluoride. This result was applied to synthesize optically active organofluorine compounds starting from commercially available (R)-1-phenylethylamine.

Site-selective nucleophilic ring-opening of 2 with Bu₄NX proceeded at a much-substituted 2-position preferentially and produced tertiary alkyl halides 3.

Multigram synthesis of N-alkyl bis-ureas for asymmetric hydrogen bonding phase-transfer catalysis

Fluorine is a key element present in ~35% of agrochemicals and 25% of marketed pharmaceutical drugs. The availability of reliable synthetic protocols to prepare catalysts that allow the efficient incorporation of fluorine in organic molecules is therefore essential for broad applicability. Herein, we report a protocol for the multigram synthesis of two representative enantiopure N-alkyl bis-urea organocatalysts derived from (S)-(-)-1,1'-binaphthyl-2,2'-diamine ((S)-BINAM). These tridentate hydrogen bond donors are highly effective phase-transfer catalysts for solubilizing safe and inexpensive metal alkali fluorides (KF and CsF) in organic solvents for enantioselective nucleophilic fluorinations. The first catalyst, characterized by N-isopropyl substitution, was obtained by using a two-step sequence consisting of reductive amination followed by urea coupling from commercially available starting materials (14 g, 48% yield and 5-d total synthesis time). The second catalyst, featuring N-ethyl alkylation and meta-terphenyl substituents, was accessed via a novel, scalable, convergent route that concluded with the coupling between N-ethylated (S)-BINAM and a preformed isocyanate (52 g and 52% overall yield). On this scale, the synthesis requires ~10 d. This can be reduced to 5 d by performing some steps in parallel. Compared to the previous synthetic route, this protocol avoids the final chromatographic purification and produces the desired catalysts in very high purity and improved yield.

Closing the gap between 19F and 18F chemistry

Positron emission tomography (PET) has become an invaluable tool for drug discovery and diagnosis. The positron-emitting radionuclide fluorine-18 is frequently used in PET radiopharmaceuticals due to its advantageous characteristics; hence, methods streamlining access to 18F-labelled radiotracers can make a direct impact in medicine. For many years, access to 18F-labelled radiotracers was limited by the paucity of methodologies available, and the poor diversity of precursors amenable to 18F-incorporation. During the last two decades, 18F-radiochemistry has progressed at a fast pace with the appearance of numerous methodologies for late-stage 18F-incorporation onto complex molecules from a range of readily available precursors including those that do not require pre-functionalisation. Key to these advances is the inclusion of new activation modes to facilitate 18F-incorporation. Specifically, new advances in late-stage 19F-fluorination under transition metal catalysis, photoredox catalysis, and organocatalysis combined with the availability of novel 18F-labelled fluorination reagents have enabled the invention of novel processes for 18F-incorporation onto complex (bio)molecules. This review describes these major breakthroughs with a focus on methodologies for C-18F bond formation. This reinvigorated interest in 18F-radiochemistry that we have witnessed in recent years has made a direct impact on 19F-chemistry with many laboratories refocusing their efforts on the development of methods using nucleophilic fluoride instead of fluorination reagents derived from molecular fluorine gas.

Cobalt-Catalyzed Divergent Aminofluorination and Diamination of Styrenes with N-Fluorosulfonamides

A cobalt-catalyzed aminofluorination reaction of styrenes with N-fluorosulfonamides serving as both the amination and fluorination agents has been developed. The switch of selectivity in this catalytic reaction from aminofluorination to diamination could be easily achieved by the addition of 1.0 equiv of PPh₃. Both transformations tolerated a wide array of substrates under mild reaction conditions.

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.

Reactivities of electrophilic N-F fluorinating reagents

Electrophilic fluorination represents one of the most direct and useful methods available for the selective introduction of fluorine into organic compounds. Electrophilic fluorinating reagents of the N-F class have revolutionised the incorporation of fluorine atoms into both pharmaceutically- and agrochemically-important substrates. Since the earliest N-F reagents were commercialised in the 1990s, their reactivities have been investigated using qualitative and, more recently, quantitative methods. This review discusses the different experimental approaches employed to determine reactivities of N-F reagents, focussing on the kinetics studies reported in recent years. We make critical evaluations of the experimental approaches against each other, theoretical approaches, and their applicability towards practical problems. The opportunities for achieving more efficient synthetic electrophilic fluorination processes through kinetic understanding are highlighted.

Synthesis of optically active 2-substituted azetidine-2-carbonitriles from chiral 1-arylethylamine via α-alkylation of N-borane complexes

The base-promoted α-alkylation of N-((S)-1-arylethyl)azetidine-2-carbonitriles 3via formation of their N-borane complexes 4 was investigated. For example, treatment of diastereomerically pure borane N-((S)-1′-(4′′-methoxyphenyl)ethyl)azetidine-2-carbonitrile complex (1S,2S,1′S)-4b with 1.2 equivalents of LDA at −78 °C followed by 1.3 equivalents of benzyl bromide at −78 °C and warming to room temperature produced α-benzylated (2S,1′S)-5ba in 72% yield and (2R,1′S)-5ba in 2% yield. A mechanism for this diastereoselective α-alkylation was proposed. Our method enables the production of optically active 2-substituted azetidine-2-carbonitriles, such as α-benzylated (S)-10a and (R)-10a, starting from commercially available (S)-(1-(4-methoxyphenyl)ethyl)amine.

Synthesis of optically active 2-substituted azetidine-2-carbonitriles by LDA-promoted diastereoselective α-alkylation of borane N-((S)-1-arylethyl)azetidine-2-carbonitrile complexes was demonstrated.

Impact of Multiple Hydrogen Bonds with Fluoride on Catalysis: Insight from NMR Spectroscopy

Hydrogen-bonding interactions have been explored in catalysis, enabling complex chemical reactions. Recently, enantioselective nucleophilic fluorination with metal alkali fluoride has been accomplished with BINAM-derived bisurea catalysts, presenting up to four NH hydrogen-bond donors (HBDs) for fluoride. These catalysts bring insoluble CsF and KF into solution, control fluoride nucleophilicity, and provide a chiral microenvironment for enantioselective fluoride delivery to the electrophile. These attributes encouraged a ¹H/¹⁹F NMR study to gain information on hydrogen-bonding networks with fluoride in solution, as well as how these arrangements impact the efficiency of catalytic nucleophilic fluorination. Herein, NMR experiments enabled the determination of the number and magnitude of HB contacts to fluoride for thirteen bisurea catalysts. These data supplemented by diagnostic coupling constants ^1hJ_{NH···F^–} give insight into how multiple H bonds to fluoride influence reaction performance. In dichloromethane (DCM-d₂), nonalkylated BINAM-derived bisurea catalyst engages two of its four NH groups in hydrogen bonding with fluoride, an arrangement that allows effective phase-transfer capability but low control over enantioselectivity for fluoride delivery. The more efficient N-alkylated BINAM-derived bisurea catalysts undergo urea isomerization upon fluoride binding and form dynamically rigid trifurcated hydrogen-bonded fluoride complexes that are structurally similar to their conformation in the solid state. Insight into how the countercation influences fluoride complexation is provided based on NMR data characterizing the species formed in DCM-d₂ when reacting a bisurea catalyst with tetra-n-butylammonium fluoride (TBAF) or CsF. Structure–activity analysis reveals that the three hydrogen-bond contacts with fluoride are not equal in terms of their contribution to catalyst efficacy, suggesting that tuning individual electronic environment is a viable approach to control phase-transfer ability and enantioselectivity.