Polymer-Supported, Carbon Dioxide-Protected N-Heterocyclic Carbenes: Synthesis and Application in Organo- and Organometallic Catalysis

The synthesis of a resin-supported, carbon dioxide-protected N-heterocyclic carbene (NHC) and its use in organocatalysis and organometallic catalysis are described. The resin-bound carbon dioxide-protected NHC-based catalyst was prepared via ring-opening metathesis copolymerization of 1,4,4a,5,8,8a-hexahydro-1,4,5,8-exo,endo-dimethano-naphthalene (DMNH6) with 3-(bicyclo[2.2.1]hept-5-en-2-ylmethyl)-1-(2-propy1)-3,4,5,6-tetrahydropyrimi-din-1-ium-2-carboxylate (M1), using the well-defined Schrock catalyst Mo[N-2,6-(2-Pr)(2)-C(6)H(3)] (CHCMe(2)Ph)(OCMe(3))(2) and was used for a series of organocatalytic reactions, i.e., for the trimerization reaction of isocyanates, as well as for the cyanosilylation of carbonyl compounds. In the latter reaction, turn-over numbers (TON) up to 5000 were achieved. In addition, the polymer-supported, carbon dioxide-protected N-heterocyclic carbene served as an excellent progenitor for various polymer-supported metal complexes. It was loaded with a series of rhodium(I), iridium(I), and palladium(IT) precursors and the resuiting Rh-, Ir-, and Pd-loaded resins were successfully used in the polymerization of phenylacetylene, in the hydrogen transfer reaction to benzaldehyde, as well as in Heck-type coupling reactions. In the latter reaction. TONs up to 100,000 were achieved. M1, as a non-supported analogue of poly-M1-b-DMNH6, as well as the complexes PdCl(2)[1,3-bis(2-Pr)tetrahydropyrimidin-2-ylidene](2) (Pd-1) and IrBr[1-(norborn-5-ene-2-ylmethyl)-3-(2-Pr)-3,4,5,6-tetrahydropyrimidin-2-ylidine](COD) (Ir-1) were used as homogeneous analogues and their reactivity in the above-mentioned reactions was compared with that of the supported catalytic systems. In all reactions investigated, the TONs achieved with the supported systems were very similar to the ones obtained with the unsupported, homogeneous ones, the turn-over frequencies (TOFs), however, were lower by up to a factor of three.