The applications of ionic liquids into the synthesis protocols have become into the focus of interest due to their versatility and tunability. However, it was shown that the number of chemical steps could be over 20 or even 30, reducing their impact towards green chemistry (Jessop 2011). Using a biomass-originated building block for the synthesis of an IL, this number could be significantly reduced. It was shown that valerate-based ionic liquids could easily be synthesized from γ-valerolactone (Fegyverneki et al. 2010; Orha et al. 2018), which can be obtained by a two-step synthesis via valorization of lignocellulosic biomass wastes (Tukacs et al. 2014 and 2017).
To demonstrate their further applicability, they were first compared to the conventional 1-butyl-3-methylimidazolium cation-based ILs in the coupling of iodobenzene (1a) and triethoxyphenylsilane (2a) as a model reaction (Scheme 1) under typically used “Hiyama conditions” using tetrabutylammonium fluoride as a F− source (Ismalaj et al. 2014). As with Sonogashira coupling in ILs (Orha et al. 2019), negligible conversion rates of 1a were detected in butyl-methylimidazolium cation containing ILs (Table 1 entries 1–3). The reaction efficiency could be significantly increased by the introduction of tetrabutylphosphonium 4-ethoxyvalerate [TBP][4EtOV], resulting in the conversion of 1a. The product biphenyl (3a) was isolated with a yield of 58% (Table 1, entry 6), which could be assumed to be higher by subsequent optimization of the reaction parameters.
The selection of a palladium source could have a significant effect on the efficiency of a catalytic transformation (Błaszczyk et al. 2009). When different precursors were compared, it was revealed that using of bis(triphenylphosphine)palladium(II) dichloride (Pd(PPh3)2Cl2) and tris(dibenzylideneacetone)dipalladium(0) (Pd2(dba)3) gave acceptable yields of 3a (Table 2, entries 1 and 5). A similar observation was reported for the Sonogashira reaction performed in biomass-originated ILs (Orha et al. 2019).
The residual moisture content could dramatically affect a transition-metal-catalyzed reaction. In this case, however, the fluoride activator ([N(C4H9)4][F], hereafter TBAF) containing 3 eq water, the water content of the reaction mixtures was ca 10 m/m%. It exceeds the typical residual water content of GVL-based ionic liquids (Strádi et al. 2015, Orha et al. 2018). Consequently, that, the protocol is hardly sensitive to water up to 10%, can be assumed and no special handling of the reaction regarding the exclusion of air and moisture is necessary.
Generally, the organosilane activation with fluoride ion, that is, the formation of the pentavalent silicon center, is considered as a key step. In this way, a facile bond breaking of the carbon–silicon bond during transmetalation is favored. It was demonstrated that alkali metal salt such as NaF or CsF could also act as silane activators for Hiyama coupling reactions (Gurung et al. 2013; Monguchi et al. 2012). In addition, both ammonium- and phosphonium-based ionic liquid have been proved to be good extractants of Pd suggesting their complexing ability (Katsuta et al. 2011; Regel-Rosocka et al. 2015). Because of high water tolerance of the catalytic reactions, we attempt to convert 1a and 2 to 3 by the use of cheaper NaF in the presence of the different amount of water. However, no conversion was detected up to 20 wt% water. When 30 wt% of water loading was applied, moderate (39%) conversion of 1a to 3a was detected in the presence of 2 eqv NaF, even higher activator concentration at 130 °C for 24 h. To conclude, the fluoride activator cannot be eliminated from the system.
Hereafter, to facilitate C–C bond coupling involving several iodoaromatic substances (1a–o) and triethoxyphenylsilane (2), bis(triphenylphosphine)palladium(II) dichloride was selected as a catalyst precursor by the use of 1.5 eq TBAF in the absence of any additional ligands and auxiliary base in [TBP][4EtOV] at 130 °C for 24 h (Table 3). It was demonstrated that the catalytic system generally could be utilized for the conversion of various iodoaromatic compounds. The substrate reactivity was not affected by the electronic parameters of the aromatic substrates. Thus, no Hammet-sigma (σp) correlation can be established for para-substituted species. Both electron-donating groups, i.e., methyl, tert-butyl, methoxy (Table 3 entries 2–6), and electron-withdrawing groups, i.e., chloro, fluoro or trifluoromethoxy (Table 2, entries 7–12) were tolerated on the aryl iodide. No significant differences of the isolated yields were observed. Under identical conditions, iodopyridine derivatives and 2-iodothiophene were easily converted to the corresponding biphenyls (3 m–o). The conversion of 4-chloro-1-iodobenzene did not lead to the formation of 1,4-diphenylbenzene.
The GVL-based [TBP][4EtOV] ionic liquid has been proven as an excellent reaction media for transition-metal-catalyzed C–N (Ullmann type) (Orha et al. 2018) and C–C (Sonogashira) (Orha et al. 2019) coupling reactions previously. In the present work, we demonstrated that its utilization can be extended for Hiyama coupling, as well, which makes it an attractive biomass-based, environmentally friendly alternative for common, fossil-based solvents, and opens the possibility for its application in wider range of chemical transformations.