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

Article abstract of DOI:10.1016/S0040-4020(01)90451-X

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.