Stereoisomerization of the bornyl radical during aliphatic nucleophilic substitutions

The mechanism of the reaction between the anion of 4-methoxycarbonyl-1-methyl-1,4-dihydropyridine (1(-)) and endo-2-bromo-1,7,7-trimethylbicyclo[2.2.1]heptane (bornyl bromide) and between 1(-) and exo-2-bromo-1,7,7-trimethylbicyclo[2.2.1]heptane (isobornyl bromide) has been investigated. In both cases the endo and exo substitution products were produced with a small dominance of the inversion product. While the product distribution was independent of temperature for the reaction with bornyl bromide, the yield of the endo-product increased at lower temperatures in the reaction with isobornyl bromide. Clear-cut evidence of a mechanism change in the latter case was not observable in an Arrhenius plot. Different mechanistic interpretations as regards to S(N)2 and electron transfer reactions are discussed.