Novel Metathesis Catalysts Based on Ruthenium 1,3‐Dimesityl‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidenes: Synthesis, Structure, Immobilization, and Catalytic Activity

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Abstract
The synthesis of novel ruthenium-based metathesis catalysts containing the saturated 1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene ligand, that is, [RuCl2(NHC){CH-2-(2-PrO)-5-NO2-C6H3}] (1) and [Ru(CF3COO)2(NHC){CH-2-(2-PrO)-5-NO2-C6H3}] (2) (NHC=1,3-bis(2,4,6-trimethylphenyl)-3,4,5,6-tetrahydropyrimidin-2-ylidene) is described. Both catalysts are highly active in ring-closing metathesis (RCM) and ring-opening cross-metathesis (ROCM). Compound 1 shows moderate activity in enyne metathesis. Compound 2 is not applicable to enyne metathesis since it shows high activity in the cyclopolymerization of diethyl dipropargylmalonate (DEDPM). Poly(DEDPM) prepared by the action of 2 consists of 95 % five-membered rings, that is, poly(cyclopent-1-enevinylene)s, and 5 % of six-membered rings, that is, poly(cyclohex-1-ene-3-methylidene)s. The polymerization proceeds in a nonliving manner and results in polyenes with broad polydispersities (1.9≦PDI≦2.3). Supported analogues of 2 were prepared by immobilization on hydroxymethyl-Merrifield resin and a monolithic support derived from ring-opening-metathesis polymerization (ROMP). Catalyst loadings of 1 and 2.5 %, respectively, were obtained. Both supported versions of 2 showed excellent reactivity. With 0.24–2 % of the supported catalysts, yields in RCM and ROCM were in the range of 76–100 %. Leaching of ruthenium was low and resulted in Ru contaminations of the products of less than 0.000014 % (0.14 ppm).