Monolith- and Silica-Supported Carboxylate-Based Grubbs–Herrmann-Type Metathesis Catalysts

The synthesis of silica- and monolith-supported Grubbs–Herrmann-type catalysts is described. Two polymerizable, carboxylate-containing ligands, exo, exo-7-oxanorborn-2-ene-5,6-dicarboxylic anhydride and 7-oxanorborn-2-ene-5-carboxylic acid were surface-immobilized onto silica- and ring-opening metathesis (ROMP-) derived monolithic supports using “grafting-from” techniques. The “1st generation Grubbs catalyst”, RuCl2(CHPh)(PCy3)2, was used for these purposes. In addition, a poly(norborn-2-ene-b-exo, exo-norborn-2-ene-5,6-dicarboxylic anhydride)-coated silica 60 was prepared. The polymer supported anhydride and carboxylate groups were converted into the corresponding mono- and disilver salts, respectively, and reacted with the Grubbs–Herrmann catalyst RuCl2(CHPh)(IMesH2)(PCy3) [IMesH2=1,3-bis(2,4,6-trimethylphenyl)-4,5-dihydroimidazol-2-ylidene]. Heterogenization was accomplished by exchange of one chlorine ligand with the polymeric, immobilized silver carboxylates to yield monolith-supported catalysts 4, 5, and 6 as well as silica-supported systems 7, 8 and 9. The actual composition of these heterogenized catalysts was proven by the synthesis of a homogeneous analogue, RuCl[7-oxanorbornan-2-(COOAg)-3-COO](CHPh)(IMesH2)(PCy3) (3). All homogeneous and heterogeneous catalysts were used in ring-closing metathesis (RCM) of diethyl diallylmalonate, 1,7-octadiene, diallyldiphenylsilane, methyl trans-3-pentenoate, diallyl ether, N,N-diallyltrifluoracetamide and t-butyl N,N-diallylcarbamate allowing turnover numbers (TON's) close to 1000. In a flow-through set-up, an auxiliary effect of pendant silver carboxylates was observed with catalyst 5, where the silver moiety functions as a (reversible) phosphine scavenger that both accelerates initiation and stabilizes the catalyst by preventing phosphine elution. Detailed catalytic studies were carried out with the monolith-supported systems 4 and 6 in order to investigate the effects of temperature and chain-transfer agents (CTA's) such as cis-1,4-diacetoxybut-2-ene. In all RCM experiments Ru-leaching was low, resulting in a Ru-content of the RCM products ≤3.5 μg/g (3.5 ppm).