Recyclable Hydroboration of Alkynes Using RuH@IL and RuH@IL/scCO₂ Catalytic Systems

Abstract

This paper reports on the first detailed studies on green and sustainable methods for the repetitive batch hydroboration of terminal and internal alkynes by the effective immobilization of Ru(CO)Cl(H)(PPh3)3 in various ionic liquids (RuH@IL) or in biphasic ionic liquid/supercritical CO2 (RuH@IL/scCO2) systems. The systems RuH(1 mol %)@[EMPyrr][OTf](IL8) and RuH(1 mol %)@[EMPyrr][NTf2](IL9) were found to be the most effective immobilization approaches, with both allowing the completion of over 10 complete catalytic cycles in the hydroboration of a series of different alkynes. An increase in the catalyst content to 2 mol % (RuH(2 mol %)@[EMPyrr][NTf2](IL9)) allowed for the completion of 25 repetitive batches, which proved the high utility of the developed system. In each case, high yields of products were obtained, and their purity was confirmed through NMR, MS, and ICP techniques. On the other hand, the use of RuH(1 mol %)@/[EMPyrr][OTf](IL8)/scCO2 in the hydroboration of phenylacetylene with pinacolborane, permitted effective completion of 8 cycles, at a much lower temperature than that at which RuH@IL was used (40 vs 100 °C). Moreover, the application of such a system has a positive impact on the environment and process sustainability via the exchanging of organic solvent for scCO2 used for product extraction and separation when working with RuH@IL. The strategies presented in this work are, so far, the most effective and recyclable systems based on ILs for the hydroboration of alkynes with high TONs. In the future, they will be applied in continuous flow regime.