BRIDGEHEAD CARBOCATIONS - SOLVOLYSIS OF A SERIES OF 5-SUBSTITUTED BICYCLE [3.1.1]HEPTYL BROMIDES - NUCLEOPHILIC SOLVENT ASSISTANCE TO IONIZATION OF 1-BROMOBICYCLO[3.1.1]HEPTANE

The synthesis of a range of 5-substituted bicyclo[3.1.1]heptyl bromide s for solvolytic studies is described. It is found that the substituen t has a profound effect on the rate of solvolysis of the system and ac ts principally in accordance with the magnitude of its inductive/field constant sigma(I). The most spectacular example of the effect of the substituent is provided by the COOMe group which leads to a retardatio n in the rate of methanolysis by a factor of 6.5 x 10(5). While a line ar relationship in the plot of log k and sigma(I) is generally obeyed, as expected for a mechanism mediated by the bicyclo[3.1.1]heptyl brid gehead cation, two of the bromides, 1-bromobicyclo[3.1.1]heptane and i ts 5-methoxy derivative, show deviant behaviour and react more rapidly than predicted on the basis of the Hammett plot. Evidence is presente d to show that the enhanced rate of the parent is the result of nucleo philic assistance by the solvent. Anchimeric assistance in the solvoly sis of 5-methoxybicyclo[3.1.1]heptyl bromide is attributed to the powe rful p-donor property of the methoxy substituent which stabilizes the transition state in a unique concerted ring-opening and ionization ste p.