Z‑Selective Ring-Opening Metathesis Polymerization of 3‑Substituted Cyclooctenes by Monoaryloxide Pyrrolide Imido Alkylidene (MAP) Catalysts of Molybdenum and Tungsten

Ring-opening metathesis polymerization of a series of 3-substituted cyclooctenes (3-MeCOE, 3-HexCOE, and 3-PhCOE) initiated by various Mo and W MAP complexes leads to cis,HT-poly­(3-RCOE) polymers. The apparent rate of polymerization of 3-HexCOE by W­(N-t-Bu)­(CH-t-Bu)­(Pyr)­(OHMT) (1c; Pyr = pyrrolide; OHMT = O-2,6-Mesityl2C6H3) is greater than the rate of polymerization by Mo­(N-t-Bu)­(CH-t-Bu)­(Pyr)­(OHMT) (1b), but both gave the same cis,HT polymer structures. Formation of HT-poly­(3-RCOE) employing 1c takes place via propagating species in which the R group (methyl, hexyl, or phenyl) is on C2 of the propagating alkylidene chain, a type of intermediate that has been modeled through the preparation of W­(N-t-Bu)­(CHCHMeEt)­(Pyr)­(OHMT). The rate of ROMP is exceedingly sensitive to steric factors: e.g., W­(N-t-Bu)­(CH-t-Bu)­(Me2Pyr)­(OHMT), the dimethylpyrrolide analogue of 1c, essentially did not polymerize 3-HexCOE at 22 °C. When a sample of W­(N-t-Bu)­(CHCHMeEt)­(Pyr)­(OHMT) and 3-methyl-1-pentene in CDCl3 is cooled to −20 °C, the alkylidene resonances for W­(N-t-Bu)­(CHCHMeEt)­(Pyr)­(OHMT) disappear and resonances that can be ascribed to protons in a synα/synα′ disubstituted trigonal bipyramidal metallacyclobutane complex appear. 3-Methyl-1-pentene is readily lost from this metallacycle on the NMR time scale at room temperature.