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Latent Thermo-Switchable Olefin Metathesis Initiators Bearing a Pyridyl-Functionalized Chelating Carbene: Influence of the Leaving Group’s Rigidity on the Catalyst’s Performance

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posted on 2010-01-11, 00:00 authored by Anna Szadkowska, Xaver Gstrein, Daniel Burtscher, Katarzyna Jarzembska, Krzysztof Woźniak, Christian Slugovc, Karol Grela
The synthesis and characterization of two ruthenium complexes bearing chelating carbene ligands is described. Carbene precursors, 2-(2-vinylphenyl)pyridine and 10-vinylbenzo[h]quinoline, are applied to prepare (SPY-5-31)-dichloro-(κ2(C,N)-N-2-(2-vinylbenzylidene)pyridine(1,3-bis(2,4,6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene)ruthenium (VIII) and (SPY-5-31)-dichloro-(κ2(C,N)-2-(benzo[h]quinolin-10-yl)methylidene)(1,3-bis(2,4,6-trimethylphenyl)4,5-dihydroimidazol-2-ylidene)ruthenium (IX). Both catalysts/initiators are used to perform ring-closing metathesis (RCM) and ring-opening metathesis polymerizations (ROMP). RCM experiments reveal significant thermal stability of the catalysts under forcing reaction conditions such as boiling toluene for 48 h. Even challenging substrates such as diethylallyl(2-methylallyl)malonate are completely transformed with low catalyst loadings (0.1 mol % at 110 °C). The high thermal stability, i.e., latency, might be explained by a slow generation of the catalytically active methylidene species. This feature leads to high molecular weight polymers and a thermal switchability in ROMP. Initiation of polymerizations of several norbornene derivatives occurs at about 48 ± 5 °C in the case of initiator VIII and at 110 ± 9 °C in the case of IX. A substantial increase of the switching temperature in the following cases could be supported with higher rigidity of the chelating carbene moiety in IX when compared to VIII.