The halogen-metal exchange reaction between perfluoro-n-heptyl iodide and n-butyllithium

Abstract

Perfluoro-n-heptyl iodide and 1–1.5 molar equivalents of n-butyllithium in diethyl ether or halide-free diethyl ether/pentane undergo 40–50% halogen-metal exchange below −90° and form a stable heterogeneous system. The stability of the perfluoro-n-heptyllithium present is attributed to its association with perfluoro-n-heptyl iodide. Such a mixture reacts only slowly with acetaldehyde below −90° to give a low yield of 1-(perfluoro-n-heptyl)ethanol. Halogen-metal exchange is complete in the presence of a sufficient excess of the fluoroalkyliodide, in accord with the relative carbanion stabilities. In the presence of a large excess of n-butyllithium the system is considerably less stable and trans-1-n-butyltridecafluoro-1-heptene is a major product. The stable heterogeneous mixture becomes homogeneous above ca −90°, and in the presence of acetaldehyde the fluoroalkyllithium then undergoes a fast reaction to give a quantitative yield of the fluoroalcohol. Perfluoro-n-heptyllithium decomposes very rapidly even at −75°, but it can be utilised at this and higher temperatures by producing it in the presence of an appropriate substrate. Perfluoro-n-propyl iodide and perfluoroisopropyl iodide also undergo 40–50% halogen-metal exchange when treated with an excess of n-butyllithium in ether below ca. −90°, indicating that the ability of perfluoroalkyllithiums to form stable heterogeneous systems is general. Carboxylation of perfluoro-n-propyllithium, perfluoroisopropyllithium, and perfluoro-n-heptyllithium was effected.