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

“All new is well-forgotten old”, the proverb goes. The current “fluorine boom” is news only to a novice in the field: the exceptional importance of fluorinated organic compounds in numerous areas has been known for a long time. The sharpest increase in the number of fluorine-containing pharmaceuticals and agrochemicals is dated back to 30 years ago. Also around that time (1979), the first monograph devoted to industrial applications of organofluorine compounds was published, covering not only fluorine-containing biologically active materials but also refrigerants, propellants, surfactants, textile chemicals, polymers, and dyes. The increasingly broad realization of the key role of organofluorine compounds in numerous areas has recently attracted many new scientists to the field. The development of new methods for the selective introduction of fluorine and fluorinecontaining groups into organic molecules for biologically active and other useful materials has become a hot area. Over 100 reviews, book chapters, and highlights on this subject have appeared in the literature in the past few years. As citing all of these publications in this review article is impossible, we provide references to only the most recent, general, and comprehensive ones. Molecules bearing a trifluoromethyl group constitute one of the most important classes of selectively fluorinated compounds. As early as 1928, Lehmann reported his observations of biological activity of some trifluoromethylated organic derivatives and already in 1959 Yale published a detailed review article entitled “The Trifluoromethyl Group in Medicinal Chemistry”. Since then, numerous books and reviews have appeared in the literature covering various aspects of trifluoromethylated organic and organometallic compounds. Within this family, derivatives bearing the CF3 group on aromatic rings are particularly numerous and important. Some examples of such compounds used as active ingredients of pharmaceuticals and agrochemicals are shown in Scheme 1. Trifluoromethylated building blocks and intermediates are clearly needed to make such molecules. The simplest trifluoromethylated aromatic compound, benzotrifluoride, was originally prepared by Swarts at the end of the 19th century. In his work, Swarts treated benzotrichloride with “two thirds of its weight of antimony fluoride” to obtain a mixture of PhCF2Cl and PhCF3, from which the two were separated and isolated pure by distillation. In the early 1930s, two industrial groups, one from Kinetic Chemicals, Inc. and one from I. G. Farbenindustrie AG patented their discoveries on the successful use of HF instead of SbF3 for the Swarts reaction. These inventions were the starting point for the modern large-scale manufacturing of trifluoromethylated aromatics. Other methods have been developed for conversion of various C1 units on the ring to CF3 with a variety of fluorinating agents. While representing an outstanding discovery and a classic of organic and organofluorine chemistry, the Swarts reaction is nonetheless neither atom-economical nor environmentally benign, as it deals with stoichiometric quantities of hazardous chemicals and generates large amounts of chlorine waste. To convert a CH3 group on the ring to CF3, the methyl is first exhaustively chlorinated to produce 3 equiv of HCl as a

Aromatic Trifluoromethylation with Metal Complexes

Key limitations of the colloidal semiconductor nanorods that have been reported so far are a significant distribution of lengths and diameters as well as the presence of irregular shapes produced by the current synthetic routes and, finally, the poor ability to fabricate large areas of oriented nanorod arrays. Here, we report a seeded-growth approach to the synthesis of asymmetric core−shell CdSe/CdS nanorods with regular shapes and narrow distributions of rod diameters and lengths, the latter being easily tunable up to 150 nm. These rods are highly fluorescent and show linearly polarized emission, whereby the emission energy depends mainly on the core diameter. We demonstrate their lateral alignment as well as their vertical self-alignment on substrates up to areas of several square micrometers.

Synthesis and Micrometer-Scale Assembly of Colloidal CdSe/CdS Nanorods Prepared by a Seeded Growth Approach

Fluorine has received great attention in all fields of science. “Small atom with a big ego” was the title of the Symposium at the ACS meeting in San Francisco in 2000, where a number of the current scientific and industrial aspects of fluorine chemistry made possible by the small size and high electronegativity of the atom were discussed. This small atom has provided mankind with significant benefits in special products such as poly(tetrafluroethylene) (PTFE), freon, fluoro-liquid crystals, optical fiber, pharmaceutical and agrochemical compounds, and so on, all of which have their own unique properties that are otherwise difficult to obtain.1 For instance, at present, up to 30% of agrochemicals and 10% of pharmaceuticals currently used contain fluorine atoms. Therefore, organic fluorine compounds have received a great deal of interest and attention from the scientists involved in diverse fields of science and technology. Now, not only C-F bond formation but also selective C-F bond activation have become current subjects of active investigation from the viewpoint of effective synthesis of fluoroorganic compounds. The former is highlighted by designing a sophisticated fluorinating reagent for regioand stereocontrolled fluorination and developing versatile multifunctional and easily prepared building blocks. C-F bond formation has been treated extensively in several reviews2 and books.3 The latter is a subject that has been less explored but would be promising for selective defluorination of aliphatic fluorides, cross-coupling with aryl fluorides, and * To whom correspondence should be addressed. Phone: 81-78-803-5799. Fax: 81-78-803-5799. E-mail: amii@kobe-u.ac.jp and uneyamak@cc.okayamau.ac.jp. † Kobe University. ‡ Okayama University. Chem. Rev. 2009, 109, 2119–2183 2119

C-F bond activation in organic synthesis.

Fluoroalkylation reaction, featuring the transfer of a fluoroalkyl group to a substrate, is a straightforward and efficient method for the synthesis of organofluorine compounds. In fluoroalkylation reactions, fluorine substitution can dramatically influence the chemical outcome. On the one hand, the chemistry of alkylation with non-fluorinated reagents may not be applicable to fluoroalkylations, so it is necessary to tackle the fluorine effects to achieve efficient fluoroalkylation reactions. On the other hand, fluorine substitution may bring about new reactivities and transformations that cannot be realized in alkylation with non-fluorinated reagents; thus, fluorine substitution can be used to explore new synthetic methods. This tutorial review provides a brief overview of the unique fluorine effects in recently developed nucleophilic, electrophilic, radical, and transition metal-mediated fluoroalkylation reactions by comparing with either their non-fluorinated counterparts or fluorinated counterparts with different numbers of fluorine substituents.

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The unique fluorine effects in organic reactions: recent facts and insights into fluoroalkylations

Efficient fluoroalkylations have been proven to be a highly useful strategy for the synthesis of bioactive fluorine-containing compounds and other materials. The design and use of a single category of reagents for multiple synthetic goals are much more attractive to preparative organic chemists. In this Account, we show how we have succeeded in the nucleophilic trifluoromethylation, difluoromethylation, difluoromethylenation, (phenylsulfonyl)difluoromethylation, (phenylthio)difluoromethylation, and monofluoromethylation as well as radical (phenylsulfonyl)difluoromethylation and electrophilic difluoromethylation by using fluorinated sulfones, sulfoxides, sulfides, or fluorinated sulfonium salts. The chemistry not only provides practically powerful synthetic methods, but the molecular design concept that we have developed may also be adopted to tackle other related chemical problems.

Selective fluoroalkylations with fluorinated sulfones, sulfoxides, and sulfides.

A new and efficient method for the preparation of tri- and difluoromethylsilanes using magnesium metal-mediated reductive tri- and difluoromethylation of chlorosilanes is reported using tri- and difluoromethyl sulfides, sulfoxides, and sulfones. The byproduct of the process is diphenyl disulfide. Since phenyl trifluoromethyl sulfone, sulfoxide, and sulfide are readily prepared from trifluoromethane (CF3H) and diphenyl disulfide, the method can be considered to be catalytic in diphenyl disulfide for the preparation of (trifluoromethyl)trimethylsilane (TMS−CF3) from non-ozone-depleting trifluoromethane.

Preparation of Tri- and Difluoromethylsilanes via an Unusual Magnesium Metal-Mediated Reductive Tri- and Difluoromethylation of Chlorosilanes Using Tri- and Difluoromethyl Sulfides, Sulfoxides, and Sulfones

We have successfully accomplished the nucleophilic fluoroalkylation of α,β-enones, arynes, and activated alkynes with fluorinated sulfones. It was found that for acylic α,β-enones, although the reaction medium and the structure of the enones can all influence the regioselectivity of the nucleophilic alkylation reactions, the hard/soft nature of the carbanions played a major role. By using the 1,4- and 1,2-addition product ratio as a probe to determine the hard/soft nature of the above-mentioned four halogenated carbanions, the order of the softness of these carbanions can be given as follows: [(PhSO2)2CF−] (20) ≈ PhSO2CCl2− (32) > PhSO2CHF− (31) > PhSO2CF2− (30). In the case of fluoroalkylation of aryne (35 as the precursor) and α,β-acetylenic ketones 46 with fluorobis(phenylsulfonyl)methane (21), fluorobis(phenylsulfonyl)methylated arenes 36 and β-fluorobis(phenylsulfonyl)methylated α,β-enones 47 were obtained as the corresponding products in good yields. During the reaction between 2-fluoro-2-(phenylsul...

Nucleophilic Fluoroalkylation of α,β-Enones, Arynes, and Activated Alkynes with Fluorinated Sulfones: Probing the Hard/Soft Nature of Fluorinated Carbanions

The unprecedented nucleophilic fluoroalkylation of simple epoxides with fluorinated sulfones was achieved to give the beta-fluoroalkyl alcohols in one step. The negative "fluorine effect" in the nucleophilic fluoroalkylation of epoxides with fluorinated carbanions was probed by the reactivity comparison between carbanions PhSO2CF2- (3) and PhSO2CCl2- (4) and between carbanions PhSO2CHF- (7) and PhSO2CHCl- (13). The mediation of this fluorine effect by introducing another electron-withdrawing benzenesulfonyl group was found to be an effective way to significantly increase the nucleophilicity of the fluorinated carbanions, with the reactivity order [(PhSO2)2CF-] (16) > PhSO2CFH- (7) >> PhSO2CF2- (3).

https://pubs.acs.org/doi/pdf/10.1021/jo801797x

Jinbo Hu

Papers

The unique fluorine effects in organic reactions: recent facts and insights into fluoroalkylations

Selective fluoroalkylations with fluorinated sulfones, sulfoxides, and sulfides.

Preparation of Tri- and Difluoromethylsilanes via an Unusual Magnesium Metal-Mediated Reductive Tri- and Difluoromethylation of Chlorosilanes Using Tri- and Difluoromethyl Sulfides, Sulfoxides, and Sulfones

Nucleophilic Fluoroalkylation of α,β-Enones, Arynes, and Activated Alkynes with Fluorinated Sulfones: Probing the Hard/Soft Nature of Fluorinated Carbanions

Nucleophilic fluoroalkylation of epoxides with fluorinated sulfones.