Stereoselective Ring‐Opening Metathesis Polymerization with Tungsten Sulfido Alkylidene N‐Heterocyclic Carbene Complexes

Abstract

A series of cationic tungsten sulfido alkylidene N-heterocyclic carbene (NHC) complexes (W01 -W09) of the general formula [W(S)(CHCMe 3 )(X)(NHC) (CMe 3 CN) + B(Ar F ) 4 − ] (NHC = 1,3-dimesitylimidazol-2-ylidene, IMes; 1,3-dimesityl-4,5-dichloroimidazol-2-ylidene, IMesCl 2 ; 1,3-bis(2,6xdiisopropyl)phenyl)imidazol-2-ylidene, IDipp; X = Cl, C 6 F 5 O, 2,6-Ph 2 -C 6 H 3 ; B(Ar F ) 4 − = tetrakis(3,5-bis(trifluoromethyl)phenyl)borate) are used as initiators in the stereoselective ring-opening metathesis polymerization (ROMP) of (+) 2,3-endo, exo-dicarbomethoxynorborn-5-ene ((+)DCMNBE, M1). Trans-isospecifity up to 84% is achieved along with varying percentages of cis-syndiospecifity. The different extent of trans-isospecifity is compared to the one of related benchmark cationic molybdenum and tungsten imido and tungsten oxo alkylidene NHC complexes. Mechanistic investigations suggest that the syn-isomer of a nitrile-free initiator reacts with M1 presumably in an ene anti fashion to yield a syn-first insertion product via turnstile rearrangement, which accounts for the predominant trans-isospecifity of the polymerization. The cis-syndiotactic sequences are proposed to stem from the competing ene syn addition of M1 to a nitrile-containing syn-isomer of the initiator.

Stereoselective Ring Expansion Metathesis Polymerization with Cationic Molybdenum Alkylidyne N-Heterocyclic Carbene Complexes

“…The very similar stereospecificity of the REMP with complexes 1−3 indicates a minor influence of the electronic effect of the nature of the NHC on stereoselectivity, if any, which is in line with previous reports on stereoselective ringopening metathesis polymerization (ROMP) of norbornenes. 31 In contrast, catalysts 4−6 bearing the sterically less demanding IiPr ligand and additional THF or pivalonitrile (PivCN) offer access to cis-st-rich poly((+)-DCMNBE), again with high stereospecificity up to 90% (Table 2, Figures S18 − 20).…”

Olefin Metathesis and Stereoselective Ring-Opening Metathesis Polymerization with Neutral and Cationic Molybdenum(VI) Imido and Tungsten(VI) Oxo Alkylidene Complexes Containing N-Chelating N-Heterocyclic Carbenes

“…The structure of Mo2 was determined by single-crystal X-ray analysis (Figure 3) and confirmed triflate exchange by iodide. Mo2 crystallizes in the orthorhombic space group Pca2 1 with a = 1838.01 (10) pm, b = 1032.09(5) pm, c = 2038.73 (11) pm, α = β = γ = 90°, Z = 4. The complex adopts a distorted TBP structure with τ = 0.66.…”

“…The area of stereoselective ring-opening metathesis polymerization − (ROMP) by well-defined group 5, 6 and group 8 metal alkylidenes has blossomed into a mature field over the last two decades, given the large number of publications and that all possible stereoregular structures can now be accessed, though with different ease and generality. In this regard, neutral and cationic molybdenum imido, tungsten imido, and tungsten oxo alkylidene N-heterocyclic carbene (NHC) complexes have evolved as highly active and stereoselective catalysts. ,,,− Recently, we reported on molybdenum imido alkylidene complexes containing N-, O-, and C-chelating NHCs. , In view of their unique reactivity but also stability, we extended our research to cationic molybdenum and tungsten­(VI) imido and oxo alkylidene complexes containing N-chelating NHCs.…”