Influence of the supported ionic-liquid layer thickness on Z-selectivity in 1-alkyne hydrosilylation under continuous flow

Abstract

1-Butyl-3-methylimidazolium tetrafluoroborate containing different rhodium(I) N-heterocyclic carbene (NHC) complexes was immobilized as a supported ionic-liquid phase (SILP) inside the mesopores of a silica monolith to study the impact of SILP thickness (d_SILP) from the thin-SILP-limit (d_SILP ≈ 1 nm) to complete mesopore filling (d_SILP ≈ 15 nm) on Z/E-selectivity in the rhodium-catalyzed hydrosilylation of phenylacetylene with dimethylphenylsilane. A coupled analytical platform allowed monitoring of both yield and selectivity of the produced isomer pattern online in continuous-flow experiments of 600 minutes using methyl tert-butyl ether as mobile phase. The approach provided new insights into the mechanistic aspects of the reaction under liquid confinement conditions created by the varied SILP thickness. With decreasing d_SILP, the selectivity of a Rh-catalyst based on a chelating NHC is shifted towards the β(Z)-isomer, climaxing in a boost of the Z/E-ratio for d_SILP = 1 nm by a factor of >30, while the selectivity is mostly unaffected for catalysts based on nonchelating NHCs. The spatial dimension of 1 nm reflects the rigid part of the SILP characterized by a quasi-frozen morphology of the ionic liquid. It shapes a local, spatially as well as molecularly confined catalytic environment, which, together with a tailored catalyst, facilitates the predominant formation of the β(Z)-isomer under kinetic control. Contrariwise, the random, mobile part of the adjoining bulk SILP, emerging with increasing d_SILP, generally favors the formation of the β(E)-isomer under thermodynamic control.