A Ring-Fusion/Ring-Fission Mechanism for the Metathesis Reaction of Macrocyclic Formaldehyde Acetals

Important insight has been obtained into the mechanism of the reversible acid-catalysed transacetalation of cyclophane formaldehyde acetals (formals) Ci in CDCl3, at 25 °C. The order of appearance of the lowest oligomers in the early stages of the equilibration reaction is fully consistent with ring-fusion/ring-fission processes in which oxonium ion intermediates undergo SN2 reactions, according to an acid-catalysed bimolecular (A2) mechanism. The alternative acid-catalysed monomolecular (A1) reaction path, based on "back-biting" processes of carbenium ions generated by SN1- type cleavage of oxonium ion intermediates, predicts sequences that are in marked contrast with experimental findings.