Fluorine substituents have become a widespread and important drug component, their introduction facilitated by the development of safe and selective fluorinating agents. Organofluorine affects nearly all physical and adsorption, distribution, metabolism, and excretion properties of a lead compound. Its inductive effects are relatively well understood, enhancing bioavailability, for example, by reducing the basicity of neighboring amines. In contrast, exploration of the specific influence of carbon-fluorine single bonds on docking interactions, whether through direct contact with the protein or through stereoelectronic effects on molecular conformation of the drug, has only recently begun. Here, we review experimental progress in this vein and add complementary analysis based on comprehensive searches in the Cambridge Structural Database and the Protein Data Bank.

Fluorine in Pharmaceuticals: Looking Beyond Intuition.

The Many Roles for Fluorine in Medicinal Chemistry

Over the past decade, the most significant, conceptual advances in the field of fluorination were enabled most prominently by organo- and transition-metal catalysis. The most challenging transformation remains the formation of the parent C-F bond, primarily as a consequence of the high hydration energy of fluoride, strong metal-fluorine bonds, and highly polarized bonds to fluorine. Most fluorination reactions still lack generality, predictability, and cost-efficiency. Despite all current limitations, modern fluorination methods have made fluorinated molecules more readily available than ever before and have begun to have an impact on research areas that do not require large amounts of material, such as drug discovery and positron emission tomography. This Review gives a brief summary of conventional fluorination reactions, including those reactions that introduce fluorinated functional groups, and focuses on modern developments in the field.

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Introduction of Fluorine and Fluorine-Containing Functional Groups

Recent advances in catalysis have made the incorporation of fluorine into complex organic molecules easier than ever before, but selective, general and practical fluorination reactions remain sought after. Fluorination of molecules often imparts desirable properties, such as metabolic and thermal stability, and fluorinated molecules are therefore frequently used as pharmaceuticals or materials. But the formation of carbon-fluorine bonds in complex molecules is a significant challenge. Here we discuss reactions to make organofluorides that have emerged within the past few years and which exemplify how to overcome some of the intricate challenges associated with fluorination.

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Catalysis for fluorination and trifluoromethylation

Hydrogen bonding is responsible for the structure of much of the world around us. The unusual and complex properties of bulk water, the ability of proteins to fold into stable three-dimensional structures, the fidelity of DNA base pairing, and the binding of ligands to receptors are among the manifestations of this ubiquitous noncovalent interaction. In addition to its primacy as a structural determinant, hydrogen bonding plays a crucial functional role in catalysis. Hydrogen bonding to an electrophile serves to decrease the electron density of this species, activating it toward nucleophilic attack. This principle is employed frequently by Nature's catalysts, enzymes, for the acceleration of a wide range of chemical processes. Recently, organic chemists have begun to appreciate the tremendous potential offered by hydrogen bonding as a mechanism for electrophile activation in small-molecule, synthetic catalyst systems. In particular, chiral hydrogen-bond donors have emerged as a broadly applicable class of catalysts for enantioselective synthesis. This review documents these advances, emphasizing the structural and mechanistic features that contribute to high enantioselectivity in hydrogen-bond-mediated catalytic processes.

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Asymmetric catalysis by chiral hydrogen-bond donors.

Asymmetric fluorination, trifluoromethylation, and perfluoroalkylation reactions.

The design and development of new high-performance catalysts for applications in asymmetric catalytic reactions is of ongoing interest in organic chemistry. The combination of a Lewis acid and a Lewis base working in concert is now considered state of the art in stereoselective syntheses. The synergistic activation by two or more reactive centers allows high reaction rates and excellent transfer of stereochemical information. Despite the self-quenching reaction between Lewis acids and Lewis bases that might lead to an inactive catalyst, considerable effort has been directed towards the development of the dual-activation concept. The ultimate goal is to mimic nature by the discovery of catalytic systems analogous to enzymatic processes that involve metal-ion cocatalysts. With this aim, the dual activation concept greatly broadens the range of artificial catalysts. The most efficient catalytic systems are reviewed, and the mechanisms of action are discussed.

Towards perfect catalytic asymmetric synthesis: dual activation of the electrophile and the nucleophile.

A new efficient catalytic enantioselective electrophilic fluorination of cyclic and acyclic β-ketoesters is developed. As low as 1 mol % of chiral bis(oxazoline)-copper triflate complexes catalyses the fluorination by means of N -fluorobenzenesulfonimide. The use of 1,1,1,3,3,3-hexafluoroisopropanol (HFIP) is crucial for achieving high enantioselectivity.

Copper(II) triflate-bis(oxazoline)-catalysed enantioselective electrophilic fluorination of β-ketoesters

Cyclic and acyclic β-ketoesters were efficiently trifluoromethylated with 5-trifluoromethyldibenzothiophenium tetrafluoroborate in the presence of a phase-transfer catalyst to afford the corresponding α-substituted α-trifluoromethyl β-ketoesters in good to excellent yields. In a second approach, 5-trifluoromethyldibenzothiophenium tetrafluoroborate and tetrabutylammonium difluorotriphenylstannate were used for efficient electrophilic trifluoromethylation of various silyl enol ethers leading to the corresponding α-trifluoromethyl ketones in good to high yields.

Mild electrophilic trifluoromethylation of beta-ketoesters and silyl enol ethers with 5-trifluoro methyldibenzothiophenium tetrafluoroborate.

All domains of society are impacted by fluorine chemistry. In particular, fluorine plays a key role in medicinal, pharmaceutical and agrochemical sciences in which the synthesis and evaluation of molecules featuring one or more fluorine atoms is nowadays routine in every new discovery and development program. Since the beginning of this century, enantiopure fluorinated molecules are at the forefront of innovation of the tremendous achievements in fluorine chemistry. This review introduces the reader to the recent progress in asymmetric installation of one fluorine and a trifluoromethyl group via nucleophilic, electrophilic and radical diastereo- and enantioselective reactions.

Jun-An Ma

Papers

Asymmetric fluorination, trifluoromethylation, and perfluoroalkylation reactions.

Towards perfect catalytic asymmetric synthesis: dual activation of the electrophile and the nucleophile.

Copper(II) triflate-bis(oxazoline)-catalysed enantioselective electrophilic fluorination of β-ketoesters

Mild electrophilic trifluoromethylation of beta-ketoesters and silyl enol ethers with 5-trifluoro methyldibenzothiophenium tetrafluoroborate.

Recent progress in asymmetric fluorination and trifluoromethylation reactions.