Selectivity in Ring-opening Metathesis Polymerization of Z-Cyclooctenes Catalyzed by a Second-generation Grubbs Catalyst
Martinez, H.; Miró, P.; Charbonneau, P.; Hillmyer, M. A.; Cramer, C.
ACS Catal. 2012, 2, 2547 (doi:10.1021/cs300549u).
Mechanistic details of the ring-opening metathesis polymerization of 3-substituted-Z-cyclooctenes (3RCOEs) catalyzed by the second-generation Grubbs' catalyst were systematically studied at the M06-2X/SDD|6-311+G(2df,p)//M06-L/SDD|6-31G(d) level of theory to elucidate factors contributing to observed regioselectivities. All possible conformational isomers for Z-cyclooctene (4 total) and 3-methyl-Z-cyclooctene (16 total) were taken into account. The potential energy surfaces for both the initiation and propagation steps were calculated including all stereochemically distinct approaches of the cycloalkene to the active catalyst. In contrast to the situation with smaller cycloalkenes, the rate-limiting step for the polymerization of Z-cyclooctenes was determined to be the breakdown of the metallacyclobutane intermediate. This change is attributed to increased repulsive interactions between the growing polymer chain and the mesityl groups of the N-heterocyclic carbene ligand when the effective cone angle increases upon ring opening. Most of the observed regioselectivity for 3RCOEs derives from differential steric interactions, but solvation also plays a role.