Studies on Grignard reagents. Part I. Fluorine nuclear magnetic resonance spectra of fluoroaryl Grignard reagents

Abstract

The fluorine resonance spectra of fluoroaryl Grignard reagents, the corresponding bisarylmagnesiums, and mixtures of the two, have been measured over a range of temperature. It has thus been possible to slow the fluoroaryl exchange and distinguish unambiguously between “RMgX” and “R₂Mg” species. Values for the ratio 2[“RMgX”]/[“R₂Mg”] in ether solutions of the pure Grignard reagents are: C₆F₅MgBr, 1·0; C₆F₅MgCl, 2·0; C₆F₅Mgl, 1·4; 3-F,5,4Cl₂C₆H₂·MgBr, ca. 2·5; C₁₀H₆FMgl, 20; p-F·C₆H₄·MgBr, 20; p-F·C₆H₄·Mgl 20. Limited additional information concerning the nature of these “RMgX” and “R₂Mg” species has been obtained from the fluorine chemical shifts. The rates of fluoroaryl exchange qualitatively follow the sequence C₆F₅Mgl < C₆F₅MgBr < C₆F₅MgCl C₁₀H₆FMgl < C₆H₂Cl₂FMgBr < p-F·C₆H₄·Mgl < p-F·C₆H₄·MgBr. For the p-fluorophenyl and pentafluorophenyl systems, mixing the bisarylmagnesium and magnesium iodide rapidly produces a conventional Grignard reagent.