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

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). Highly active metathesis catalysts [RuX2(NHC){CH‐2‐(2‐PrO)‐5‐NO2‐C6H3}] (NHC=1,3‐bis(2,4,6‐trimethylphenyl)‐3,4,5,6‐tetrahydropyrimidin‐2‐ylidene; X=Cl− (1), CF3COO− (2)) were synthesized and structurally characterized (structure of 2 depicted). Supported analogues of 2 were prepared by immobilization on a hydroxymethyl‐Merrifield resin and a monolithic support derived from ring‐opening‐metathesis polymerization. Compounds 1 and 2 and the supported versions of 2 showed high reactivity in ring‐closing metathesis and ring‐opening cross‐metathesis.