Formation of Peralkylcyclohexadienyllithium Compounds

Article abstract of DOI:10.1021/jo00316a012

1,1,2,3,5,6-Hexamethyl-4-methylene-2,5-cyclohexadiene (1) reacts with alkyllithium compounds, RLi (R= n-Bu, sec-Bu, and t-Bu), at 2O deg C in hydrocarbon solution or in the presence of ethers or tertiary amines, which act as ligands, to give stable, soluble 1,1,2,3,5,6-hexamethyl-4-alkylcyclohexadienyllithium compounds, 3a-c.On heating, the latter aromatize to pentamethylalkylbenzenes, 4a-c, with extrusion of methyllithium, while on hydrolysis of 3a-c isomeric substituted cyclohexadienes are obtained.The effects have been investigated of varying RLi structure, ligand, and temperature on the rates of addition and aromatization reactions.Ligand metalation by RLi is a significant side reaction.Addition is faster with ligands known to reduce the association of alkyllithium compounds.Aromatization is faster in the presence of THF which favors formation of loose ion pairs and is slower with heavy substitution on the ring.It is proposed that large substituents (neopentyl) destabilize the transition states for aromatization due to steric interactions.