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3.20 Asymmetric Fluorination, Monofluoromethylation, Difluoromethylation, and Trifluoromethylation Reactions

DOI: 10.1055/sos-SD-203-00543

Gouverneur, V.; Lozano, O.Science of Synthesis: Stereoselective Synthesis, (20113851.

General Introduction

In recent years, fluorine chemistry has established itself as an important area of research, which benefits agrochemical, medicinal, and material science.[‌1‌‌3‌] The properties of the CF bond assist in interpreting the behavior of organofluorine compounds.[‌4‌,‌5‌] For instance, fluorine is the most electronegative element (χ=4) and has the smallest atomic radius of the second row of the periodic table. Its ionization energy (I) is highly endothermic (401.2 kcal·mol1) but its electron affinity (Eea) is a very favorable exothermic process (+78.3 kcal·mol1). The fluorine atom is intermediate in size between hydrogen and oxygen but closer to oxygen [van der Waals radii (Å): H (1.20); O (1.52); F (1.47)]. Fluorine forms the strongest bond to carbon in organic chemistry (109.9 kcal·mol1 for CH3F) and is highly polarized with the electron density located mainly on fluorine. The substantial ionic nature of the CF bond is reflected in its large dipole moment, which, combined with other effects (hyperconjugation, weak hydrogen-bond acceptor, charge dipole interactions CFX+), accounts for the conformational behavior of many fluorinated compounds. These unique characteristics have encouraged direct fluorination, in the context of mono-, di-, and trifluoromethylation, as a means of tailoring the physical properties of organic compounds to demand. Selected characteristics of fluoromethanes are given in Table 1.

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References