Origin of Stereoselectivity in Ring Opening Metathesis Polymerization with Cationic Molybdenum Imido Alkylidene CAAC Complexes

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Stereoselective ring opening metathesis polymerization (ROMP) of enantiomerically pure 2,3-dicarbomethoxynorborn-5-ene ((+)-DCMNBE) was accomplished by the action of cationic tetra- and pentacoordinated molybdenum imido alkylidene cyclic alkyl amino carbene (CAAC) complexes that are chiral at molybdenum. The same catalysts were also utilized to perform the ROMP of 2,3-dimethoxymethylnorborn-5-ene ((+)-DMMNBE). All complexes were moderately to highly active and showed high trans -isoselectivity, offering up to 97% trans -isotactic (it) repeat units. In all cases, tetracoordinated complexes were the active species, resulting in pentacoordinated transition states. A theoretical model was elaborated using the buried volume (% V bur ) values of all ligands from single-crystal X-ray analysis together with the structures of the density functional theory (DFT) generated molybdacyclobutane intermediates. The model demonstrates the steric effects of all ligands at molybdenum on the trans -isoselectivity of the reaction, as predicted by the turnstile mechanism, and includes a positive correlation between the bulky CAAC ligand with high values of % V bur of the other ligands and a high trans -isoselectivity. It was also successfully extended to molybdenum imido alkylidene N -heterocyclic carbene (NHC) complexes, proved to be of sufficient accuracy with a root mean squared error (RMSE) of 6.19% and was verified by Monte Carlo cross-validation (MCCV).

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