Alkylation of aromatics with diols in superacidic media

Friedel–Crafts alkylation of simple aromatics (benzene, toluene, o-, m- and p-xylene) with various types of diols in the presence of superacidic trifluoromethanesulfonic acid was studied. 1,2- and 1,3-diols were found to bring about only monoalkylation yielding unusual monoalkylated products: bis(1-methyl-2-phenylpropyl) ether was formed in the reaction of 2,3-butanediol, and 2,4-dimethyl-2,4-pentanediol produced mono- and di-tert-butyl-substituted benzenes. Higher diol homologues, in turn, undergo electrophilic cyclialkylative ring closure (cyclialkylation) to form tetralin derivatives with high selectivities and in good yields. Additional primary alkylation processes are monoalkylation to yield alkylbenzenes and the formation of diphenylalkanes. The products thus formed may participate in secondary transformations, which are a second cyclialkylation (formation of octahydroanthracene and -phenanthrene derivatives), isomerization and dehydrogenation. Mechanistic studies with stereoisomeric 2,5-hexanediols point to the involvement of open dicationic intermediates in the alkylation process.