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

Abstract

Non‐porous polyurethane‐based monoliths are prepared under solvent‐induced phase separation conditions. They possess low specific surface areas of 0.15 m2 g−1, pore volumes of 1 µL 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))(η4‐1,5‐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.

Effect of liquid confinement on regioselectivity in the hydrosilylation of alkynes with cationic Rh(i) N-heterocyclic carbene catalysts

“…Clearly, Rh@SILP ROMP-10 outrivals the performance of Rh@SILP PUR , a mesoporous polyurethane-based monolithic support with mesopores in the range between 2 and 50 nm lled with Rh-1 in [BMIM][BF 4 ] and run under continuous conditions, too, also using MTBE as the second organic transport phase. 25 Only Rh@SBA-15 with Rh-1 conned in the mesoporous SBA-15 with uniform, narrow distributed 6.2 nm pores performed better, at least with aliphatic 1-alkynes. 26…”

“…to the (b)-Z-selective hydrosilylation of alkynes using Rh(I) NHC catalysts. 25,26 Here, we propose for the rst time the use of a liquid connement for the enhancement of (b)-Z-selectivity in alkyne hydrosilylation reactions and provide a more detailed study on the inuence of the layer thickness of the supported ionic-liquid-phase (SILP) on (b)-Z-selectivity using connement that is not created by mesopores but by the SILP, the second organic transport phase, here methyl tert-butyl ether (MTBE), and the support itself. For these purposes, we used ringopening metathesis polymerization (ROMP) derived, mostly non-porous polymeric monolithic supports that were surface-functionalized with ionic groups to ensure for good adhesion of the SILP.…”

“…[27][28][29][30][31] Generally, they can be prepared via free radical polymerization; however, large-volume monoliths have so far only been prepared by electron beam-induced free radical polymerization, [32][33][34][35][36][37][38] and polyaddition. 25 By carefully varying the thickness of the SILP, a liquid connement was accomplished and conrmed by a substantial increase in (b)-Z-selectivity in the hydrosilylation of both aliphatic and aromatic 1-alkynes.…”