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Catalysts 2020, 10, 762 4 of 15 the product yield was slightly lower. Simultaneously, the application of trifluoromethanesulfonate anion ([OTf]−) resulted in the lower conversion of substrates than for bis(trifluoromethylsulfonyl)imide anion ([NTf2]−). Table 1. Borylative coupling of vinylboronic acid pinacol ester (1) with styrene (2a) in the presence of [Ru(CO)Cl(H)(PCy3)2]@IL—IL screening. IL—Ionic liquids. Entry 1 2 3 4 5 6 7 8 9 Ionic Liquid [EMPyr][OTf] [EMPyr][NTf2 ] [BMPyr][OTf] [BMPyr][NTf2 ] [BMIm][NTf2 ] [BMIm][OTf] [BMIm][PF6 ] [BMIm][BF4 ] [BMIm]Cl Conversion of 1 [%] a 95 100 91 93 97 100 88 54 trace Yield of 3a [%] b 95 100 91 93 97 100 88 54 n.d Reaction conditions: [Ru-H]:1:2a = 0.03:1:4, 130 ◦C, ionic liquid (1 g), inert atmosphere. a Determined by GC-MS. b Determined by GC-MS and 1H NMR. Borylative coupling of 1 with 2a in 1-butyl-3-methylimidazolium-based ionic liquids with [OTf]− or [NTf2]− anions occurred smoothly with very high yields with the formation of the desired product 3a and excellent selectivity (Table 1, entry 5–6). In contrast to pyrrolidinium cation, the quantitative conversion of 1 was observed for [OTf]− anion. Lower yields were observed when moisture-sensitive anions such as hexafluorophosphate ([PF6])− or tetrafluoroborate ([BF4])− were used (Table 1, entry 7–8). The presence of traces of water in those ILs probably caused partial deactivation of the moisture-sensitive catalyst. When [BMIm]Cl was used as a solvent and immobilization medium, no conversion of 1 was noticed. This can be explained by the strong coordination ability of chloride anion to the metal center of the catalyst, and inhibition of its activity. This observation is in agreement with previous reports [28,30]. [EMPyr][NTf2] and [BMIm][OTf], which ensured the complete conversion of 1 (Table 1, entry 2 and 6), were chosen for further reaction conditions screening. We optimized parameters crucial for obtaining the highest reaction yield, i.e., temperature, time, the molar ratio of substrates, as well as catalyst loading (Table 2). Initially, for [EMPyr][NTf2], the high catalyst loading of [Ru(CO)Cl(H)(PCy3)2] (3 mol%) was maintained while the other parameters were tuned. The quantitative yields of 3a were observed above 110 ◦C. The same results were noticed when the time of the reaction was reduced from 24 to 6 h. Moreover, only a 1.5-fold excess of 2a towards 1 was necessary to get a full conversion of 1. In the final stage of optimization studies in [EMPyr][NTf2], the influence of catalyst loading on reaction efficiency was determined. It was found that 1 mol% of [Ru(CO)Cl(H)(PCy3)2] is essential for achieving a very high reaction yield (Table 2, entry 12). Although the application of the equimolar ratio of substrates resulted in a complete conversion of 1, the presence of a homocoupling product of 1 was observed (Table 2, entry 14). Therefore, a small excess of olefin is essential for the elimination of the side-homocoupling reaction of vinyl boronate. Replacement of ruthenium hydride catalyst with coordinated PCy3 ligand to [Ru(CO)Cl(H)(PPh3)3] leads to a lower conversion of 1 and the formation of undefined by-products (Table 2, entry 15). Based on the most effective reaction conditions in [EMPyr][NTf2], borylative coupling of 1 with 2a in [BMIm][OTf] was determined. Similar to pyrrolidinium-based IL, the reaction in [BMIm][OTf] should be performed at least at 110 ◦C for 6 h with a slight excess of 2a towards 1 and 1 mol% of [Ru(CO)Cl(H)(PCy3)2], to achieve the best process efficiency (Table 2, entry 16). It is worth emphasizing that the application of ILs as a solvent enhances reaction rate compared to an analogous reaction performed in poly (ethylene glycol), another green solvent with similar properties to ILs [29] The same phenomenon has already been observed by us during studies on the catalytic hydroboration of alkynes in Ru(CO)Cl(H)(PPh3)3@ILs systems [30].PDF Image | Ru-Catalyzed Repetitive Batch Borylative Coupling of Olefins
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