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

TL;DR

This paper shows how changing the size of carbene ligands in molybdenum catalysts affects the regioselectivity of cyclopolymerization reactions.

Contribution

The study introduces a quantitative model linking carbene ligand size and ester oxygen charge to polymerization selectivity.

Findings

Catalysts with larger NHC/CAAC ligands increase α-insertion selectivity in cyclopolymerization.

DFT analysis shows two molybdenum–oxygen coordinations stabilize intermediates, influencing selectivity.

A model with R² of 0.854 confirms the relationship between ligand size and regioselectivity.

Abstract

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 formulas Mo­(N-Ar)­(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 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 R
2 of 0.854 confirms
these effects quantitatively and the model was verified by repeated
k-fold cross-validation.

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