Macrocyclization of Dienes under Confinement with Cationic Tungsten Imido/Oxo Alkylidene N-Heterocyclic Carbene Complexes

Macrocyclization reactions are still challenging due to competing oligomerization, which requires the use of small substrate concentrations. Here, the cationic tungsten imido and tungsten oxo alkylidene N-heterocyclic carbene complexes [[W(N-2,6-Cl-2-C6H3)(CHCMe2Ph(OC6F5)(pivalonitrile)(IMes)+ B(Ar-F)(4)(-)] (W1) and [W(O)(CHCMe2Ph(OCMe(CF3)(2))(IMes)(CH3CN)(+) B(Ar-F)(4)(-)] (W2) (IMes=1,3-dimesitylimidazol-2-ylidene; B(Ar-F)(4)(-)=tetrakis(3,5-bis(trifluoromethyl)phenyl borate) have been immobilized inside the pores of ordered mesoporous silica (OMS) with pore diameters of 3.3 and 6.8 nm, respectively, using a pore-selective immobilization protocol. X-ray absorption spectroscopy of W1@OMS showed that even though the catalyst structure is contracted due to confinement by the mesopores, both the oxidation state and structure of the catalyst stayed intact upon immobilization. Catalytic testing with four differently sized alpha,omega-dienes revealed a dramatically increased macrocyclization (MC) and Z-selectivity of the supported catalysts compared to the homogenous progenitors, allowing high substrate concentrations of 25 mM. With the supported complexes, a maximum increase in MC-selectivity from 27 to 81 % and in Z-selectivity from 17 to 34 % was achieved. In general, smaller mesopores exhibited a stronger confinement effect. A comparison of the two supported tungsten-based catalysts showed that W1@OMS possesses a higher MC-selectivity, while W2@OMS exhibits a higher Z-selectivity which can be rationalized by the structures of the catalysts.

Automated summary

The immobilization of tungsten-based catalysts inside ordered mesoporous silica results in enhanced macrocyclization and Z-selectivity, allowing high substrate concentrations. Smaller mesopores exhibit a stronger confinement effect. The two supported catalysts show different selectivities, which can be rationalized by their structures.