Rational and Then Serendipitous Formation of Aza Analogues of Hoveyda-Type Catalysts Containing a Chelating Ester Group Leading to a Polymerization Catalyst Family

Analogues of the well-known Hoveyda-Grubbs catalyst bearing both a chelating ester function and a chelating nitrogen atom were obtained. These complexes behave differently depending on the character of the chelating amine. Complexes containing a secondary amine underwent unexpected spontaneous oxidation of the amine group, leading to the Schiff base analogues. In contrast, complexes containing a tertiary amine were prone to intramolecular cyclization in the presence of a base (Et3N). Probing the activity of such (pre)catalysts in ring-closing metathesis reactions (RCMs) revealed their dormant character and excellent thermo-switchability. In particular, complexes bearing an iminium nitrogen fragment were found to be very useful in a delayed ring-opening metathesis polymerization (ROMP) and were therefore commercialized.