Reactions of an allylic carbocation with nucleophiles in aqueous solvents

Abstract

The allylic carbocation formed from 3-(2-X-propan-2-yl)indene (1-X)(X = Cl, OAc, or OH₂⁺), or from 2-acetoxy-1-isopropylideneindan (2-OAc) in aqueous solvents containing ca. 75 vol % water reacts rapidly with nucleophiles. The selectivity is very low: β_nucca. 0.07 with alcohols as nucleophiles and β_nucca. 0.03 with substituted acetate anions. The nucleophile attacks at both ends of the allylic system but preferentially at the exocyclic carbon atom, giving the thermodynamically more stable product. Azide anion reacts with the carbocation about 50 times as fast as does water. The solvolysis of the allylic isomers (1-OAc) and (2-OAc) is accompanied by intramolecular rearrangement of the acetates as well as hydron abstraction by the leaving acetate anion, yielding the 1,2- and 1,4-elimination products. The elimination product compositions are quite different, which indicates two discrete ion-pair intermediates. It is concluded also that the isomerization proceeds by an ion-pair route, since in less polar solvents it has only been observed along with solvolysis and elimination. The experimental results for the acetates accommodate well a mechanism in which solvolysis, rearrangement, and elimination are connected via two contact ion-pair intermediates.