Erling Grovenstein

Aryl Migrations in Organometallic Compounds of the Alkali Metals

Publisher Summary The most common and thoroughly studied rearrangements of organoalkali compounds are sigmatropic shifts of aryl groups. That the reaction of 2-chloro-1,1,1-triphenylethane with sodium in dioxane or isooctane-diethyl ether gives 1,1,2-triphenyIethylsodium. The dependence of the extent of rearrangement on the alkali metal used constitutes part of the general evidence in favor of these rearrangements, being rearrangements of organometallic compounds rather than of free radicals. In all the above examples 2, 2, 2-triphenylethyl alkali metal compounds have been generated by reactions of alkali metals on halides or quaternary ammonium salts. Similar results have been obtained by metal–metal exchange reactions not involving elementary metals. The organoalkali compounds normally exist as associated compounds— ethyllithium and n-butyllithium are hexamers, and tertbutyllithium is a tetramer in benzene solution. Because free ions and loose ion pairs have similar degrees of solvation, the studies on competitive reactions are also in accord with benzyl cleavage occurring in a free anion because the concentration of such anions must be very small, it is reasonable to assume that it is predominantly the loose ion pair that undergoes benzyl cleavage.

Carbanions 27. Rearrangements of (9-alkyl-9-fluorenyl)-methyllithium (or cesium) and 2,2-diphenyl-3,3-dimethyl-butyllithium ☆

Abstract A study has been made upon the products from warming various (9-alkyl-9-fluorenyl)methyllithium (or cesium) compounds in THF to near 0 °C followed by carbonation. When the 9-alkyl group is ethyl, the result is chiefly the protonated product (9-alkyl-9- fluorenyl)methane; a similar product evidently is formed when the 9-alkyl group is 1-norbornyl. When the 9-alkyl group is tert-butyl, the minor product is 9-neopentylfluorene-9-carboxylic acid from a [1,2]-migration of the tert-butyl group while the major product is 9-methylfluorene-9-carboxylic acid from an intramolecular elimination as shown by deuterium labeling. When the 9-alkyl is neopentyl, the major product is 9-neopentyl-9,10-dihydro-phenanthrene-9-carboxylic acid along with some 9-neopentylphenanthrene which becomes the major product in diethyl ether solution at 35 °C. 2,2-Diphenyl-3,3-dimethylbutyllithium undergoes predominantly [1,2]-phenyl migration in THF at 0 °C. From an x-ray crystal study upon 9-tert-butyl-9-(chloromethyl)fluorene and 9-neopentyl-9-(chloromethyl)fluorene, it is concluded that steric acceleration is responsible for the unusual reactions of (9-alkyl-9-fluorenyl)methyllithiums when the 9-alkyl groups are tert-butyl and neopentyl.

Cation Effects in Organoalkali Metal Chemistry

Organoalkali metal compounds can be regarded as substances in which the organic anion is joined to the counter alkali metal cation primarily by electrostatic forces between cations and anions. This generalization [1] is believed to hold even for organolithium compounds with localized negative charge, although here some small carbon-lithium covalency may enhance bonding [2]. From the simplified structure R-M+, it is tempting to think that the nature of M+ is unimportant, in other words, that one is concerned only with the chemistry of “carbanions” in discussing the reaction of R-M+.