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) C(i) in CDCl(3), at 25 degrees 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 S(N)2 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 S(N)1-type cleavage of oxonium ion intermediates, predicts sequences that are in marked contrast with experimental findings.
Cacciapaglia
Roberta
R
Dipartimento di Chimica e IMC-CNR Sezione Meccanismi di Reazione, Università di Roma La Sapienza, Box 34--Roma 62, 00185 Roma, Italy. roberta.cacciapaglia@uniroma1.it
Di Stefano
Stefano
S
Mandolini
Luigi
L