Monolithic Disk‐Supported Metathesis Catalysts for Use in Combinatorial Chemistry

Abstract

Two metathesis catalysts, RuCl₂(PCy₃)(NHC)(CHPh) (1) [NHC=1‐(2,4,5‐trimethylphenyl)‐3‐(6‐hydroxyhexyl)‐imidazol‐2‐ylidene] and Mo(N‐2,6‐i‐Pr₂‐C₆H₃)(CHCMe₂Ph)(BIPHEN) (2) [BIPHEN=(R)‐3,3′‐di‐t‐butyl‐5,5′,6,6′‐tetramethyl‐2,2′‐biphenolate) have been immobilized on polymeric, monolithic discs using a “grafting from” protocol. Monolithic discs were prepared via ring‐opening metathesis polymerization (ROMP) from norborn‐2‐ene (NBE), tris(norborn‐5‐ene‐2‐ylmethyleneoxy)methylsilane [(NBE‐CH₂O)₃‐SiCH₃], 2‐propanol, toluene and RuCl₂(PCy₃)₂(CHPh). Catalyst loadings of 0.55 and 0.7 wt %, respectively, were obtained. Monolithic disc‐immobilized 1 was used in various metathesis‐based reactions including ring‐closing metathesis (RCM), ring‐opening cross metathesis and enyne metathesis. Using 0.23–0.59 mol % of supported 1, turnover numbers (TONs) up to 330 were achieved. Monolithic disc‐immobilized 2 was used in various enantioselective RCM and desymmetrization reactions. Using 9–13 mol % of supported catalyst, excellent yields up to 100% and high enantiomeric excess (ee≤88%) were observed. In both cases, metal leaching was low (≤3 and ≤2%, respectively). In addition, 1 catalyzed the cyclopolymerization of diethyl dipropargylmalonate (DEDPM) to yield poly(ene)s consisting of 5‐membered rings, i.e., cyclopent‐1‐ene‐1‐vinylene units. The polymerization proceeded via non‐stoichiometric initiation yielding polymers with unimodal molecular weight distribution. Using a catalyst to monomer ratio of 1 : 170, molecular weights of M_w=16,400 and M_n=11,700 g/mol, PDI=1.40 were obtained.