## 2 Results
HBF4) as a ligand it gave 35% and 23% yield, respectively (Table 2, entries 1-3). With bis(triphenylphosphine)palladium(II) chloride (PdCl2(PPh3)2) similarly low yield of 36% was achieved (Table 2, entry 4). Further we screened palladium(II) acetate (Pd(OAc)2) in combination with three different ligands, \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Entry** & **Solvent** & **Temperature (\({}^{\circ}\)C)** & **Yield (\%) 12** \\ \hline
1 & THF & 60 & 38 \\
2 & DMF & 100 & 31 \\
3 & 1,4-dioxane & 100 & 71 \\
4 & isopropanol & 80 & 52 \\ \hline \hline \end{tabular} \({}^{1}\)Reaction conditions: **1** (149 mg, 1.0 mmol), _solvent_ (7 mL), K2 CO3 (415 mg, 3.0 mmol), **2** (122 mg, 1.0 mmol), Pd(PPh3)4 (58 mg, 0.05 mmol, 5 mol%), heating (60, 80 or 100 \({}^{\circ}\)C), 24 h. \({}^{2}\) Isolated yield of **3** after extraction and column chromatography.  
\end{table} 
Table 1: The isolated yield of product **3** after Suzuki coupling of 2,4-dichloropyrimidine (**1**) with phenylboronic acid (**2**) in four selected solvents. namely tricyclohexylphosphine (PCy3), triphenylphosphine (PPh3), and triphenylphosphine bound of divinylbenzene (PPh3 on DVB), with yields not significantly improved (Table 2, entries 5-7). According to the mechanism proposed, Pd(II) pre-catalysts have to be reduced to Pd(0) prior to their involvement in oxidative addition. It has been reported that trimeric Pd(OAc)\({}_{2}\) is more susceptible to reduction in its monomeric form, whose formation has been shown to be proportional to the dipole moment of the solvent used [34]. In our screening, 1,4-dioxane with low dipole moment of only 0.45 D was used, which might be the reason for lower yields of Suzuki coupling with Pd(OAc)\({}_{2}\). Solid-supported catalysts represent an attractive approach in the field of green organic chemistry, since they can often be recycled without loss of activity, in addition to the products and solution waste remaining free from metal contamination [35]. Despite several successful applications in Suzuki-Miyaura coupling [36; 37; 38], no product was formed when palladium on multiwall carbon nanotubes was used as a catalyst. Additionally, the screening was performed on a set of classic and solid-supported catalysts Pd(OAc)\({}_{2}\) with PPh3 and Pd(OAc)\({}_{2}\) with PPh3 on DVB, respectively, with the latter exhibiting inferior catalytic ability and yield in our model reaction. Only one catalyst ([1,1'-Bis(diphenylphosphino)ferrocene]dichloropalladium(II), in complex with dichloromethane (Pd(dppf)Cl\({}_{2}\cdot\) CH\({}_{2}\)Cl\({}_{2}\)) led to a higher yield of 70% (Table 2, entry 8), which could be due to its ability (i.e., wide bite-angle P-Pd-P) of driving very effective reductive elimination [39]. Since there was no improvement compared to Pd(PPh3)\({}_{4}\) (Table 2, entry 9), all further reactions were performed with Pd(PPh3)\({}_{4}\). One parameter was screened additionally after selection of the appropriate catalyst, i.e., temperature (see Supporting Information, Table S2). Lowering the temperature significantly reduced the yield, thus all further reactions were performed at 100 \({}^{\circ}\)C.  
After the first optimization process, we decided to further upgrade the reaction procedure with the aim to improve the yields and shorten the reaction time by performing it in a microwave reactor. We started with the solvent scan, where H\({}_{2}\)O due to favorable dielectric constant for microwave heating was added in different proportions to THF or 1,4-dioxane. With this experiment, where total solvent volume was 6 mL, we tried to find the optimal solvent ratio (_v/v_) to obtain the highest yield of 3 and as little as possible of any side products (Table 3). Generally, the yield of 3 was improved, when H\({}_{2}\)O was present in the reaction mixture, with the peak at solvent ratio 1:1 or 2:1 in the favor of non-polar solvent for THF and 1,4-dioxane, respectively (Table 3, entries 1 and 8).