# Synthetic Approaches to 1,3,4-Oxadiazole-Containing Boronic Derivatives
## 3 Synthesis of Boronic Derivatives from Reagents Containing 1,3,4-Oxadiazole Moiety
### Indirect Connection of Boron with 1,3,4-Oxadiazole Ring via Heteracyclic Linker
#### 3.3.2 Boronic Acid Derivatives of 1,3,4-Oxadiazol-2-yl-indoles
The second approach is based on the formation of a 1,3,4-oxadiazol ring from a carboxythiophene boronic ester and an appropriate hydrazide (method B). This reaction is carried out in the presence of polystyrene-supported triphenylphosphine (PPh\({}_{3}\)) and trichloroacetonitrile in dry acetonitrile under microwave irradiation at 130 \({}^{\circ}\)C for 2 h. The resulting thienyl boronic ester (with the boron atom at the 2- or 3-position relative to the sulfur atom) is subsequently converted to the corresponding boronic acids: 5-(1,3,4-oxadiazol-2-yl)thiophen-3-ylboronic acid (**84**), 5-(1,3,4-oxadiazol-2-yl)thiophen-2-ylboronic acid (**85**), 3-(1,3,4-oxadiazol-2-yl)thiophen-2-ylboronic acid (**86**) using sodium periodate and ammonium acetate in an acetone/water mixture. The boronic acid can be isolated using various methods, including precipitation with 1\(N\) HCl, extraction, recrystallization, trituration, or flash column chromatography. This approach has also been successfully applied to the derivatives containing furan and ethenylbenzene linkers such as 4-(1,3,4-oxadiazol-2-yl)furan-2-ylboronic acid and 2-(2-(5-methyl-1,3,4-oxadiazol-2-yl)vinyl)phenylboronic acid (Figure 11) [15]. 3.4 Other Boronic Acid Derivatives of 1,3,4-Oxadiazole Substituted with Dibenzothiophene, Dibenzofuran, Naphthalene, and Biphenyl  
Dibenzo[b,d]furan and dibenzothiophene have also been used as heterocyclic linkers between 1,3,4-oxadiazol and boronic esters [21; 31; 32]. These compounds were synthesized via Miyaura bordism of the corresponding chloro derivatives using bis(pinacolato)diboron (B\({}_{2}\)(pin)\({}_{2}\)), potassium acetate (AcOK) as the base, palladium(II) acetate (Pd(OAc)\({}_{2}\)) as the catalyst, and tricyclohexylphosphine (P(Cy)\({}_{3}\)) as the ligand. The reactions were conducted in refluxing 1,4-dioxane, yielding the desired products containing dibenzofuran and dibenzothiophene linkers in 85% and 71% yields (Figure 12). The same approach was also applied to naphthalene and biphenyl linkers [21]. Boronic esters of biphenyl derivatives of 1,3,4-oxadiazol: 4\({}^{\prime}\)-(5-phenyl-1,3,4-oxadiazol-2-yl)biphenyl-4-ylboronic acid pinacol ester and 3\({}^{\prime}\)-(5-phenyl-1,3,4-oxadiazol-2-yl)biphenyl-2-ylboronic acid pinacol ester (Figure 12) were synthesized in 90% [32] and 59% [31] yields, respectively. The reactions were conducted in 1,4-dioxane in the presence of bis(diphenylphosphino)ferrocene)palladium(II) dichloride (Pd(dppf)Cl\({}_{2}\)) and potassium acetate (AcOK). Figure 11: Boronic acid derivatives of 1,3,4-oxadiazole containing thiophene, furan, and ethenylbenzene linkers: 5,5\({}^{\prime}\)-(1,3,4-oxadiazol-2,5-diyl)bis(thiophene-5,2-diyl)diboronic acid dipinacol ester (**83**), 5-(1,3,4-oxadiazol-2-yl)thiophen-3-ylboronic acid (**84**), 5-(1,3,4-oxadiazol-2-yl)thiophen-2-ylboronic acid (**85**), 3-(1,3,4-oxadiazol-2-yl)thiophen-2-ylboronic acid (**86**), 4-(1,3,4-oxadiazol-2-yl)furan-2-ylboronic acid (**87**), 2-(2-(5-methyl-1,3,4-oxadiazol-2-yl)vinyl)phenylboronic acid (**88**).