Acid-catalysed intramolecular allylic rearrangement of methyl ethers in aqueous solvent. Evidence for the intermediacy of ion–molecule pairs in solvolysis

Abstract

2-Methoxyisopropylideneindan (2-OMe) undergoes a facile acid-catalysed intramolecular allylic isomerization in 9.1 vol% acetonitrile in water producing 3-(1-methoxy-1-methyl)ethylindene (1-OMe), along with solvolysis yielding 3-(2-hydroxy-1-methylethyl)indene (1-OH) and 2-hydroxy-isopropylideneindan (2-OH). The isomerization is 11 times slower than the formation of alcohols. The ether (1-OMe) also solvolyses, but more slowly, to give (1-OH) and trace amounts of (2-OH) and (2-OMe). This seems to be the first demonstration of a carbocation-molecule pair as intermediate in a heterolytic reaction in a highly aqueous solvent. Analysis of the kinetic data, in combination with results obtained from solvolysis of the corresponding chloride 3-(1-chloro-1-methylethyl)indene (1-Cl), suggests that the intramolecular rearrangement of the ethers may in fact have two discrete ion-molecule pairs in common with the solvolysis.