Dynamic Covalent Metathesis in the CC/CN Exchange between Knoevenagel Compounds and Imines

Fast and reversible
dynamic covalent CC/CN exchange
takes place without catalyst in nonpolar solvents between barbiturate-derived
Knoevenagel (Kn) compounds and imines. A detailed study of the reaction
indicates that it proceeds by an associative organo-metathesis mechanism
involving the formation of a four-membered ring azetidine intermediate
by addition of the imine CN group to the CC bond of
the Kn compound. This intermediate could be generated cleanly and
stabilized at low temperature by condensation of the o,p-dinitrophenyl Kn derivative with the cyclic imine 1-azacyclohexene.
It was characterized by extensive NMR and mass spectrometric studies.
The process described represents a genuine dynamic covalent organo-metathesis
through a four-membered ring adduct as intermediate. It paves the
way for the exploration of a wide set of dynamic systems involving
(strongly) polarized CC bonds and various imines, extending
also into covalent dynamic polymers and polymolecular assemblies.