Equatorial Preference in the C−H Activation of Cycloalkanes: GaCl3-Catalyzed Aromatic Alkylation Reaction

Publisher Website
Google Scholar
Abstract
GaCl3 catalyzes the aromatic alkylation of naphthalene or phenanthrene using cycloalkanes. The C−C bond formation predominantly takes place at the least hindered positions of the substrates, and equatorial isomers regarding the cycloalkane moiety are generally obtained. The reaction of bicyclo[4.4.0]decane and naphthalene occurs at the 2-position of naphthalene and at the 2- or 3-carbons of the cycloalkane, and the products possess a trans configuration at the junctures and an equatorial configuration at the naphthyl groups. Notably, cis-bicyclo[4.4.0]decane turns out to be much more reactive than the trans isomer, and a turnover number “TON” up to 20 based on GaCl3 is attained. 1,2-Dimethylcyclohexane reacts similarly, and the cis isomer is more reactive than the trans isomer. Monoalkylcycloalkanes react at the secondary carbons provided that the alkyl group is smaller than tert-butyl. Adamantanes react at the tertiary 1-position. The alkylation reaction is considered to involve the C−H activation of cycloalkanes with GaCl3 at the tertiary center followed by the migration of carbocations and electrophilic aromatic substitution yielding thermodynamically stable products. The stereochemistry of the reaction reveals that GaCl3 activates the equatorial tertiary C−H bond rather than the axial tertiary C−H bond.