Mechanism of the Regio- and Stereoselective Cyclopolymerization of 1,6-Hepta- and 1,7-Octadiynes by High Oxidation State Molybdenum–Imidoalkylidene N-Heterocyclic Carbene Initiators

The regio- and stereoselective cyclopolymerization of a series of 1,6-heptadiynes and 1,7-octadiynes, i.e., of 4,4-bis(ethoxycarbonyl)-1,7-heptadiyne (M1), 4-ethoxycarbonyl-4-(1R,2S,5R)-(−)-menthyloxycarbonyl-1,6-heptadiyne (M2), (R,S)-4,5-bis((1R,2S,5R)-(−)-menthyloxymethylcarboxymethyl)-1,7-octadiyne (M3), and (S,S/R,R)-4,5-bis((1R,2S,5R)-(−)-menthyloxymethylcarboxymethyl)-1,7-octadiyne (M4), by the action of the molybdenum–imidoalkylidene N-heterocyclic carbene (NHC) complexes (Mo(N-2,6-Me2C6H3)(CHCMe2Ph)(OTf)2(IMesH2) (I1), Mo(N-2,6-Me2C6H3)(CHCMe3)(OTf)2(IMesH2) (I2), (Mo(N-2,6-Me2-C6H3)(CHCMe2Ph)(OTf)(OCH(CF3)2)(IMesH2) (I3), Mo(N-2,6-Cl2C6H3)(CHCMe3)(OTf)2(IMes) (I4, IMesH2 = 1,3-dimesitylimidazolidin-2-ylidene, IMes = 1,3-dimesitylimidazolin-2-ylidene) and by the molybdenum–imidoalkylidene monoaryloxide monopyrrolide (MAP) complexes Mo(N-2,6-iPr2-C6H3)(CHCMe2Ph)(2,5-Me2-pyrrolide)(O-2,6-Me2-C6H3) (MAP1), Mo(N-2,6-iPr2-C6H3)(CHCMe2Ph)(2,5-Me2-pyrrolide)(O-2,6-Mes2-C6H3) (MAP2), and Mo(N-2-tBu-C6H4)...