## 2 Results
\begin{tabular}{c c c c} \hline \hline
**Entry** & **Non-Polar** & **Polar Solvent** & **Ratio Non-Polar vs. Polar** & **Yield (\%) 1,2** \\  & **Solvent (\(\mathit{v/v}\))** & **Yield (\%) 1,2** \\ \hline
1 & THF & H\({}_{2}\)O & 1:1 & 53 \\
2 & THF & H\({}_{2}\)O & 2:1 & 23 \\
3 & THF & H\({}_{2}\)O & 3:1 & 20 \\
4 & THF & H\({}_{2}\)O & 5:1 & 13 \\
5 & THF & H\({}_{2}\)O & 9:1 & 14 \\
6 & THF & / & / & 21 \\ \hline
7 & 1,4-dioxane & H\({}_{2}\)O & 1:1 & 45 \\
8 & 1,4-dioxane & H\({}_{2}\)O & 2:1 & 80 \\
9 & 1,4-dioxane & H\({}_{2}\)O & 3:1 & 59 \\
10 & 1,4-dioxane & H\({}_{2}\)O & 5:1 & 58 \\
11 & 1,4-dioxane & H\({}_{2}\)O & 9:1 & 24 \\
12 & 1,4-dioxane & / & / & 21 \\ \hline \hline \end{tabular}   
A scale-up of the model reaction on proposed reaction conditions has further been performed. The reaction was performed on 4.0 mmol scale of starting reagents, with obtained yield of **3** being 53%, and 30% of **1** still remaining unreacted in the reaction mixture. Therefore, we prolonged the microwave reaction time for 5 min to 20 min and improved the yield of **3** to 74%, with only 19% of **1** still being present in the reaction mixture. Taken together, we can conclude that our microwave-assisted procedure is also suitable for production of aryl pyrimidines on larger scales. With the optimal conditions being determined, the scope of several aryl and heteroaryl boronic acids was investigated (Table 6). On the series of methoxyphenyl boronic acids, different substituent positions on the aromatic ring were evaluated (products **26**, **27**, and **28**); however, no significant effect on the yield and selectivity was found between _ortho_, _meta_ or _para_ position of a methoxy group. Therefore, we preferentially focused on _meta_-substituents in the following reactions. Overall, the main product was consistently the 4-substituted 2-chloropyrimidine, with little or usually none of 2-substituted 4-chloropyrimidine being formed during the reaction. Several different substituents were screened for and it was noticed that yields of phenylboronic acids with electron-withdrawing groups on _meta_ position (**30** to **33**) were rather high and comparable to the model reaction (**3**). Other boronic acids with carboxylic and electron-donating groups (i.e., amino group) led to lower yields. As expected, the naphthyl boronic acid (product **37**) displayed similar yield to our model reaction with phenylboronic acid **2 (3)**. Some heteroaryl boronic acids were also screened, with the reaction being selective and successful for both furanylboronic acids (**38** and **39**) with comparable yield. However, the coupling of 2,4-dichloropyrimidine with thiophen-2-boronic acid failed due to the sulphur poisoning of the palladium catalyst [40]. Furthermore, in case of pyridinylboronic acids, the reaction was also unsuccessful. The microwave-assisted procedure for formation of 4-phenylpyrimidines was further screened on the series of differently substituted 2,4-dialogenopyrimidines (Table 7). Despite chloroarenes being generally inert to oxidative addition, some electron-deficient heteroaryl chlorides (i.e., 2,4-dichloropyrimidines) are enough reactive for successful cross-coupling reactions [30]. Interestingly, the coupling of **2** with 2,4-dibromopyridine (**19**) did not offer improved yield (product **40**) and the formation of a side product 2-phenyl-4-bromopyrimidine was neither increased. The substitutions on the position 6 of 2,4-dichloropyrimidine (**19-25**) with electron-withdrawing and electron-donating groups did not significantly affect the yield of the reaction (e.g., **43** and **45** with electron donor substituent versus **42** with electron withdrawing group), with the exception of **41** and **46**, which led to lower yields of 27% and 54%, respectively. Thus, we wanted to improve the yield of **41**. The addition of 3 mol% of Pd(PPh3)4 significantly improved the formation of **41**, leading to 45% yield.