Long-Range Kinetic Effects on the Alternating Ring Opening Metathesis of Bicyclo[4.2.0]oct-6-ene-7-carboxamides and Cyclohexene

Abstract

We report an investigation of rates of ruthenium-catalyzed alternating ring opening metathesis (AROM) of cyclohexene with two different Ru-cyclohexylidene carbenes derived from bicyclo[4.2.0]oct-6-ene-7-carboxamides (A monomer) that bear different side chains. These monomers are propylbicyclo[4.2.0]oct-6-ene-7-carboxamide and N-(2-(2-ethoxyethoxy)ethanylbicyclo[4.2.0]oct-6-ene-7-carboxamide. The amide substitution of these monomers directly affects both the rate of the bicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening and the rate of reaction of the resulting carbene with cyclohexene (B monomer). The resulting Ru-cyclohexylidenes underwent reversible ring opening metathesis with cyclohexene. However, the thermodynamic equilibrium disfavored cyclohexene ring opening. Utilization of triphenylphosphine forms a more stable PPh3 ligated complex, which suppresses the reverse ring closing reaction and allowed direct measurements of the forward rate constants for formation of various A-B and A-B-A′ complexes through carbene-catalyzed ring-opening metathesis and thus gradient polymer structure-determining steps. The relative rate of the propylbicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening is 3-fold faster than that of the N-(2-(2-ethoxyethoxy)ethanylbicyclo[4.2.0]oct-6-ene-7-carboxamide. In addition, the rate of cyclohexene ring-opening catalyzed by the propyl bicyclooctene is 1.4 times faster than when catalyzed by the ethoxyethoxy bicyclooctene. Also, the subsequent rates of bicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening by propyl-based Ru-hexylidene are 1.6-fold faster than ethoxyethoxy-based Ru-hexylidene. Incorporation of the rate constants into reactivity ratios of bicyclo[4.2.0]amide-cyclohexene provides prediction of copolymerization kinetics and gradient copolymer structures.

Glycopolymers Prepared by Alternating Ring-Opening Metathesis Polymerization Provide Access to Distinct, Multivalent Structures for the Probing of Biological Activity

“…Despite previous accomplishments in synthesizing long, alternating AB copolymers with large molecular weights, , the highest degree of polymerization that could be achieved for the [4.2.0] glycopolymers was 50 AB units. The presence of sugar moieties on bicyclo[4.2.0]oct-6-ene-7-carboxamide seemed to significantly lower the reactivity of the reaction, most likely due to steric hindrance as suggested by previous studies. , Despite allowing the reaction to run for longer than 20 h, copolymers significantly longer than 50 AB units could not be achieved. We have previously demonstrated that polynorbornene 100-mers displaying mannose and fucose efficiently induce AE in mouse sperm − and polynorbornene 50-mers display the same number of ligands as the [4.2.0] glycopolymers.…”