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Catalysts 2020, 10, 762 2 of 15 and separation but also influences toxicological and environmental aspects. As a consequence of these drawbacks, new more sustainable methods for the synthesis of advanced organic and organometallic molecules by the application of environmentally benign solvents and/or catalyst immobilization and recycling are continuously being developed in academia and industry [3–8]. Ionic liquids (ILs) constitute an interesting green alternative to typical organic solvents because of their unique properties, such as incombustibility, negligible vapor pressure and abilities for the dissolution of many inorganic, organic and organometallic compounds. As a result of their physicochemical properties, ILs have immense structural variability which results in a wide range of their applications, for example, lubricants [9,10], solar cells [11,12], biomass processing [13,14] and solvents in chemical syntheses [15–17]. Their affinity to TM-complexes, as well as their high polarity, mean that ILs are frequently used in catalysis as a typical solvent, ligand source or even the catalyst itself. Moreover, ILs can also be used for catalyst immobilization, very often without any interference in its structure, and therefore without affecting its initial activity. An efficient immobilization of the catalyst in IL and a properly designed product separation strategy significantly reduce metal leaching into the final product and allow for the recycling of the expensive TM-complex. This can be achieved by the extraction of products from the ionic phase through organic solvents or more preferably, supercritical CO2 (scCO2) [18]. Combining ILs and scCO2 permits the development of an approach benefitting from the advantages of both homogeneous and heterogeneous catalysis, i.e., (i) high catalyst activity, selectivity, and stability, (ii) solvent (IL) and catalyst recyclability (iii) enhanced productivity and (iv) product separation simplicity. Moreover, the presence of scCO2 significantly reduces the viscosity of ILs facilitating mass transfer, increasing the reaction rate and allows for effective extraction of the products by compressed CO2 from ILs [19]. There are a lot of examples of ILs and scCO2 being used as benign, eco-friendly alternative solvents in many catalytic processes such as hydrogenations [20,21], oxidations [22], hydroformylations [23], Pd-catalyzed C–C bond-forming couplings [24], hydrometallations [25–27] or silylative coupling [28]. These solvents permit catalyst immobilization in liquid or solid-state, and processes to be carried out under repetitive batch and continuous flow methods. Our group also has experience in the application of green solvents (ILs, scCO2, PEGs) in designing sustainable protocols for the synthesis of organoboron and organosilicon compounds by the immobilization of the molecular catalyst and their recycling [29–35]. Due to their low toxicity, high reactivity under specified reaction conditions, and ease of handling, these organometallic compounds are valuable synthons in many chemical transformations, for example, Suzuki or Hiyama couplings [36,37] or halodemetalations [34,38]. Most of the organosilicon and organoboron compounds are produced under homogeneous conditions, with the use of TM-complexes as catalysts, thus searching for efficient and more sustainable protocols for their preparation is of great importance. The utilization of ILs and/or scCO2 for the synthesis of unsaturated organoboron compounds is poorly explored. Up to date, only three reports focusing on the catalytic hydroboration of alkynes in these solvents have been described [30,31,39]. Because of the challenging regio- and stereoselectivity control of the catalytic hydroboration of alkynes, the exclusive formation of one isomer was difficult to achieve, despite the good or very good recyclability and stability of catalytic systems. In contrast to the catalytic hydroboration of alkynes, borylative coupling (trans-borylation) of vinyl boronates with olefins, especially styrene derivatives, in the presence of ruthenium hydride catalysts, smoothly leads to the exclusive formation of (E)-alkenyl boronates [40]. In continuation of our studies on the application of green solvents in the synthesis of unsaturated organoboron compounds, herein, we would like to report the first application of ILs and the Ru-H catalyst [Ru(CO)Cl(H)(PCy3)2] in the monophasic ([Ru(CO)Cl(H)(PCy3)2]@ILs) or biphasic ([Ru(CO)Cl(H)(PCy3)2]@ILs/scCO2) recyclable borylative coupling of vinylboronic acid pinacol ester with olefins (Scheme 1). The presented method is an interesting variant of traditional protocols throughPDF Image | Ru-Catalyzed Repetitive Batch Borylative Coupling of Olefins
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