Origin of Stereoselectivity\nin Ring Opening Metathesis\nPolymerization with Cationic Molybdenum Imido Alkylidene CAAC Complexes

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