Hydrosilylation of Alkynes under Continuous Flow Using Polyurethane‐Based Monolithic Supports with Tailored Mesoporosity

Non-porous polyurethane-based monoliths are prepared under solvent-induced phase separation conditions. They possess low specific surface areas of 0.15 m(2) g(-1), pore volumes of 1 mu L g(-1), and a non-permanent, solvent-induced microporosity with pore dimensions <= 1 nm. Mesoporosity can be introduced by varying the monomers and solvents. A tuning of the average solubility parameter of the solvent mixture by increasing the macroporogen content results in a decrease in the volume fraction of micropores from 70% to 40% and an increase in the volume fraction of pores in the range of 1.7-9.6 nm from 22% to 41% with only minor changes in the volume fraction of larger mesopores in the range of 9.6-50 nm. The polymeric monoliths are functionalized with quaternary ammonium groups, which allowed for the immobilization of an ionic liquid that contained the ionic Rh-catalyst [1-(pyrid-2-yl)-3-mesityl)-imidazol-2-ylidene))(eta(4)-1,5-cyclooctadiene)Rh(I) tetrafluoroborate]. The supported catalyst is used in the hydrosilylation of 1-alkynes with dimethylphenylsilane under continuous flow using methyl-tert-butyl ether as second liquid transport phase. E/Z-selectivity in hydrosilylation is compared to the one of the analogous biphasic reactions. The strong increase in Z-selectivity is attributed to a confinement effect provided by the small mesopores.