BRIDGEHEAD CARBOCATIONS - A SOLVOLYTIC STUDY OF 1-BROMOBICYCLO[1.1.1.]PENTANE AND ITS BRIDGEHEAD-SUBSTITUTED DERIVATIVES

The rate constants (k) for solvolysis of a range of 3-X-substituted bi cyclo[1.1.1]pentyl bromides in 80% aqueous ethanol have been measured and a linear relationship found to exist between log k and the inducti ve/field constant sigma(I). These observations, together with the resu lts of isotope and solvent effect studies described below, have been i nterpreted as evidence for the formation of 1-bicyclo[1.1.1]pentyl cat ions in the ionization step in preference to mechanisms in which eithe r ring-opening is concerted with ionization to give 3-methylenecyclobu tyl cations or ionization is accompanied by ring-contraction to give b icyclo[1.1.0]butyl-1-carbinyl cations. The latter mechanisms would hav e been expected to show the effects of strong pi-donor substituent par ticipation, a phenomenon which was not observed. A comparison of the r ates of solvolysis of the parent bromide and its 3-deuterated analog i s characterized by a large deuterium-isotope effect (k(H)/k(D) = 1.30) which, combined with the observation that 1-bromobicyclo[1.1.1]pentan e solvolyses with enhanced rates in solvents which are strong hydrogen bonding accepters, lends support to the view that the 1-bicyclo[1.1.1 ]pentyl cation derives substantial stabilization from sigma-hyperconju gative interactions.