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.

Regio‐ and Stereoselective Synthesis of Enynyl Boronates via Ruthenium‐Catalyzed Hydroboration of 1,4‐Diaryl‐Substituted 1,3‐Diynes

“…The addition of the BÀ H bond was carried out at 100°C, because we previously determined this temperature as the most suitable for the hydroboration of terminal and internal alkynes in the presence of Ru catalysts. [33] To exclude non-catalytic hydroboration of 1,4-diphenylbuta-1,3-diyne, the reaction of 1 a with 2 was performed, but, after 24 hours, no products were observed ( Table 1, entry 1). The low acidity of the BÀ H bond in alkoxyboranes (if compared to alkylboranes e. g. disiamylborane [27] ) often requires the application of elevated temperatures, catalysts, and an excess of borane.…”

“…Hence, searching for a novel and selective method for boryl-substituted enynes is still of great importance. Our group has broad experience in the synthesis of organoboron and organosilicon compounds via hydrosilylation [32] and hydroboration [33] reactions of alkynes in the presence of TM-catalysts in conventional and green (scCO 2 , PEG, IL) solvents.…”

Application of Green Solvents: PEG and scCO2 in the Mono- or Biphasic Catalytic Systems for the Repetitive Batch Coupling of Vinylsilanes with Vinyl Boronates toward 1-Boryl-1-silylethenes

“…These systems allowed the catalyst to be reused several times without any significant changes in its activity and stability. 6 , 13 Moreover, no significant Ru leaching was observed. Ammonium, pyridinium, or imidazolium ILs with various anions were also used for Ru–H catalyst immobilization in the homocoupling of vinylsilanes.…”

“…These can have selective functionalization based on the different reactivities of these metalloids. They can be synthesized by various catalytic methods, such as the hydroboration of silyl-substituted alkynes, 6 metathesis, 7 the silaborylation of alkynes, 8 or silylative coupling (trans-silylation) reactions. 9 Most of these transformations require the application of a molecular catalyst and proceed as homogeneous processes.…”

Ionic liquids as an enabling tool to integrate reaction and separation processes

“…58, 59 An illustrative example is the repetitive batch hydroboration of terminal and internal alkynes catalysed by Ru(CO)Cl(H)(PPh3)3 in biphasic ILs/scCO2. 60 Using 1 mol % of the catalyst in [EMPyrr][OTf]/scCO2 the hydroboration of phenylacetylene with pinacolborane takes place with high efficiency and the product is directly extracted in the scCO2 phase without the use of any organic solvent. The catalyst was active for 8 cycles cycles at a much lower reaction temperature than in standard protocols (i.e.…”