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 <i>N</i>-heterocyclic carbene (NHC) ligand of the general formulas Mo-(<i>N</i>-Ar)-(CHCMe₂R)-(NHC)-(X)₂ and [Mo-(<i>N</i>-Ar)-(CHCMe₂R)-(NHC or CAAC)-(X)]⁺ [(BAr^F)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 α-selectivity was found to be proportional to the steric demand of the monomer. The influence of the coordinated carbene ligand on the regioselectivity of insertion was investigated by combining experimental data with the results obtained from DFT-optimized structures. The study revealed that the degree of α-selectivity depends on the steric size of the NHC or CAAC ligand and on the partial charge on the oxygen atom of the ester group of the coordinated monomer. The correlation is explained by two additional molybdenum-oxygen (ester) coordinations, which stabilize relevant intermediates. A correlation fit with a satisfactory <i>R</i> ² of 0.854 confirms these effects quantitatively and the model was verified by repeated k-fold cross-validation.