Tuning Regioselectivity in Cyclopolymerization through Carbene Ligand Size Modulation in Molybdenum Imido Alkylidene Catalysts

Herein we report on the regioselectivity in cyclopolymerization of diethyl dipropargylmalonate (DEDPM) with a set of 22 different neutral and cationic molybdenum imido alkylidene catalysts bearing either a cyclic alkyl amino carbene (CAAC) or N-heterocyclic carbene (NHC) ligand of the general formulae Mo(NAr)( CHCMe2R)(NHC)(X)2 and [Mo(N-Ar)(CHCMe2R)(NHC or CAAC)(X)]⁺ [(BArF)4]⁻ (R = C6H5, CH3; Ar = 2,6-Me2C6H3, 2,6-Cl2C6H3, 2,6-iPr2C6H3, 3,5-Me2C6H3, 2-tBu-C6H4, 2-CF3-C6H4, C6F5; X = OTf, OC6F5, OMes, OCH(CF3)2, OC(CH3)(CF3)2, 2’,4’,6’,2’’,4’’,6’’-hexamethylterphen-1-yloxy (OHMT), Br, Cl; NHC = 1,3- dimesitylimidazol-2-ylidene (IMes), 1,3-dimesityl-4,5-dihydroimidazol-2-ylidene (IMesH2), 1,3- diisopropylimidazol-2-ylidene (IiPr), 1,3-dicyclohexylimidazol-2-ylidene (ICy), 1,3-dimesityl-3,4,5,6- tetrahydropyrimidin-2-ylidene (6-Mes), 1,3,4-triphenyl-1,2,4-triazol-5-ylidene (TPT), 1-(2,6-diisopropylphenyl)- 3,3,5,5-tetramethyltetrahydropyrrol-2-ylidene (CAAC-1)). While most complexes result in predominantly α- insertion, a few also exhibit β-insertion, with α-insertion-derived triads as low as 37%. The influence of the coordinated carbene ligand on the regioselectivity of insertion has been investigated by combining experimental data with data obtained from the DFT-optimized structures. A correlation fit was obtained with a satisfactory R2 of 0.854 and the model was verified by repeated k-fold cross-validation.