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

We report an investigation of rates of ruthenium-catalyzedalternatingring opening metathesis (AROM) of cyclohexene with two different Ru-cyclohexylidenecarbenes 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-carboxamideand N-(2-(2-ethoxyethoxy)ethanylbicyclo[4.2.0]oct-6-ene-7-carboxamide.The amide substitution of these monomers directly affects both therate of the bicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening andthe rate of reaction of the resulting carbene with cyclohexene (Bmonomer). The resulting Ru-cyclohexylidenes underwent reversible ringopening metathesis with cyclohexene. However, the thermodynamic equilibriumdisfavored cyclohexene ring opening. Utilization of triphenylphosphineforms a more stable PPh3 ligated complex, which suppressesthe reverse ring closing reaction and allowed direct measurementsof the forward rate constants for formation of various A-B and A-B-A ' complexes through carbene-catalyzed ring-opening metathesis and thusgradient polymer structure-determining steps. The relative rate ofthe propylbicyclo[4.2.0]oct-6-ene-7-carboxamide ring opening is 3-foldfaster 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 thepropyl bicyclooctene is 1.4 times faster than when catalyzed by theethoxyethoxy bicyclooctene. Also, the subsequent rates of bicyclo[4.2.0]oct-6-ene-7-carboxamidering opening by propyl-based Ru-hexylidene are 1.6-fold faster thanethoxyethoxy-based Ru-hexylidene. Incorporation of the rate constantsinto reactivity ratios of bicyclo[4.2.0]amide-cyclohexene providesprediction of copolymerization kinetics and gradient copolymer structures.