Kinetics of Pentadienyl Cation Cyclizations

Abstract

A series of methyl-substituted pentadienyl cations has been prepared and characterized. A careful study was then made of the rate at which these cations cyclize to give cyclopentenyl cation products. Rate constants and activation parameters have been determined for this rearrangement in several different solvents and at several different cation concentrations. There is an unexpectedly large factor of 108 difference in rate between the extremes of the cations studied and a rationalization of this is presented.The cyclopentenyl cation products themselves undergo numerous rearrangements involving methyl or hydride shifts and a detailed discussion of the tetramethyl case is given. It is shown that a 3-cyclopenten-1-yl cation intermediate is involved in these rearrangements.