Fine-Tuning of Molybdenum Imido Alkylidene Complexes for the Cyclopolymerization of 1,6-Heptadiynes To Give Polyenes Containing Exclusively Five-Membered Rings

Abstract

Diethyl dipropargylmalonate (1) was cyclopolymerized by various well-defined molybdenum-based Schrock initiators to produce polymers exclusively based on 1,2-cyclopent-1-enylenvinylene units. For this purpose, variations in both the imido and alkoxy ligand in molybdenum alkylidene complexes of type Mo(NAr‘)(CHCMe2Ph)(OR)2 (Ar‘ = 2,6-i-Pr2C6H3; 2,6-Me2C6H3; OR = OCH(CF3)2, OCH(CH3)2, OC(CH3)3) were carried out. Polymers containing >95% 1,2-cyclopent-1-enylenvinylene units were obtained by low-temperature-initiated cyclopolymerization of 1 by Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OCH(CH3)2)2 (2). In the presence of quinuclidine, >95% five-membered ring structures were realized at room temperature using Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OC(CH3)3)2 (3) and Mo(N-2,6-i-Pr2C6H3)(CHCMe2Ph)(OC(CH3)3)2 (4). In contrast, molybdenum initiators containing electron-withdrawing alkoxides such as Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OCH(CF3)2)2(quinuclidine) (5) or the nonfluorinated analogue Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OCH(CH3)2)2(quinuclidin...