Organolanthanide-mediated ring-opening ziegler polymerization (ROZP) of methylenecycloalkanes: a theoretical mechanistic investigation of alternative mechanisms for chain initiation of the samarocene-promoted ROZP of 2-phenyl-1-methylenecyclopropane.
A detailed theoretical investigation of alternative mechanisms for chain initiation of the organolanthanide-promoted ring-opening polymerization of 2-phenyl-1-methylenecyclopropane (PhMCP) with an archetypical [Cp2SmH] model catalyst is presented. Several conceivable pathways for important elementary steps, which also included ring-opening isomerization of PhMCP to phenylbutadienes, were critically scrutinized for a tentative course of the catalytic reaction. The operative mechanism starts with the first exo-methylene C=C insertion into the Sm-H bond in a 1,2 fashion and is followed by shift-based beta-alkyl eliminative cyclopropyl ring opening by cleavage of a proximal bond, while the alternative mechanism that commences with 2,1-insertion and subsequent ring opening by distal bond scission is revealed to be almost entirely precluded. The facile and irreversible insertion process is not found to occur in a regioselective fashion. The ring-opening process is analyzed as the critical step that discriminates between the two conceivable mechanisms. Opening of the cyclopropyl ring is kinetically easy and proceeds readily for the 1,2-insertion species, while a prohibitively large barrier must be overcome for ring opening of 2,1-insertion species. The isomerization of PhMCP in a ring-opened fashion, which would afford phenylbutadienes as possible products, is predicted to be a less likely process, owing to both kinetic and thermodynamic factors. The phenyl functionality has been demonstrated to distinguish between the regioisomeric ring-opening pathways, both kinetically and thermodynamically, thereby rendering this process selective with regard to the regiochemistry. Overall, chain initiation of the samarocene-mediated ring-opening polymerization of PhMCP is predicted to be a smooth, kinetically facile process.
Tobisch
Sven
S
Institut für Anorganische Chemie der Martin-Luther-Universität Halle-Wittenberg, Fachbereich Chemie, Kurt-Mothes-Strasse 2, 06210 Halle, Germany. tobisch@chemie.uni-halle.de