Some observations on isotopic scramblings accompanying acetolysis in the 2-norbornyl system

Abstract

Evidence is presented to show that in the acetolysis of isotopically labeled exo- or endo-2-norbornyl brosylate (exo- or endo-1-OBs) in NaOAc buffered HOAc, the initially formed product, exo-1-OAc, could undergo further isotopic scrambling during its subsequent contact with the reaction medium. These scramblings in the acetate apparently involved relatively large amounts of rearrangement of the label from C-2 to -3. After partial acetolysis of exo- or endo-1-OBs-2-d, n.m.r. analysis showed that the recovered endo-brosylate was unchanged, while the D-label in the recovered exo-brosylate has definitely rearranged from C-2 to -1 and probably also to C-6. Thus, in the acetolysis of labeled exo-1-OBs as commonly carried out in HOAc–NaOAc, isotopic scramblings could occur not only in the solvolysis reaction itself, but also prior to the reaction due to internal return giving rise to scrambled brosylate and subsequent to the reaction due to further scrambling in the initial product. Other mechanistic implications of the results are also discussed.