# Synthetic Approaches to 1,3,4-Oxadiazole-Containing Boronic Derivatives
## 1 Introduction  
Boron, the fifth element in the periodic table, is a small atom and a semimetal from the \(p\)-block, containing three valence electrons. This leads to remarkable potential in its chemistry, especially in the field of organoboron compounds [1]. Elemental boron is used as a dopant in semiconductors, while its derivatives are widely exploited in the production of glass, ceramics and glass fibers, wood preservatives and insecticides, as well as detergents and bleaching agents [1, 2]. It also plays a fundamental role in organic synthesis. Boronic acids (Figure 1) are organic derivatives of boric acid, B(OH)\({}_{3}\), in which one hydroxyl group is replaced by an alkyl or aryl group (represented as R in the general formula R-B(OH)\({}_{2}\)). Boronic esters, such as the well-known pinacol boronic ester, are formed by the reaction of boronic acids with alcohols. In addition to traditional boronic acids and esters, new classes of organoboron compounds--including trifluoroborate salts and MIDA (methyliminodiacetic acid) boronic ester (Figure 1)--have been developed, offering improved stability and reactivity in various synthetic applications [3]. Boronic acids are a versatile class of reagents due to their unique properties: they act as mild Lewis acids, exhibit good stability, and are relatively easy to handle. Furthermore, their low toxicity and biodegradability into boric acid make them environmentally friendly and \"green\" compounds [4]. The acids in question and their esters are known to form from both saturated and unsaturated organic compounds [1]. However, arylboronic acids represent the most prominent class in organic chemistry. Organoboron compounds are widely used in the synthesis of pharmaceuticals, materials, and other valuable molecules. They serve as powerful building blocks in modern organic chemistry, especially in carbon-carbon cross-coupling reactions such as Suzuki-Miyaura, Petasis, Chan-Lam, and Liebeskind-Srogl couplings. Furthermore, organoboron compounds play crucial roles in catalysis, materials science, biology, and imaging [4; 5]. Over the past decades, oxadiazoles--classified as five-membered aromatic heterocyclic systems--have attracted considerable scientific interest [6]. Compounds of this type belong to the diazole family and contain two nitrogen atoms and one oxygen atom within their structure. Four isomers of oxadiazoles are known, differing in the relative positions of heteroatoms (Figure 2). Unlike the unstable 1,2,3-oxadiazoles (Figure 2), sydnones (1,2,3-oxadiazol-3-ium-5-olate) are recognized as their stable derivatives [7]. The other three isomers--1,2,4-oxadiazole, 1,2,5-oxadiazole, and 1,3,4-oxadiazole--are actively being investigated by both medicinal and materials chemists, who are keenly interested in achieving specific properties through optimized substitution patterns. However, 1,3,4-oxadiazole and 1,2,4-oxadiazole are favored by researchers and have been extensively studied for their significant chemical and biological properties (Figure 3). Derivatives of both 1,2,4- and 1,3,4-oxadiazoles exhibit activity against bacteria, fungi, parasites, and viruses. These compounds are also being explored in research related to anti-inflammatory, anticancer, antidiabetic, antioxidant, and neurological applications [6; 8]. The properties of compounds containing 1,2,4- and 1,3,4-oxadiazole strongly depend on the substitution patterns, but some general trends can be recognized. For example, in medicinal chemistry applications, 1,3,4-oxadiazoles demonstrate enhanced properties compared to 1,2,4-oxadiazoles, including an order of magnitude lower lipophilicity (LogD), improved metabolic stability and aqueous solubility, and decreased hERG inhibition [9]. In materials science, 1,3,4-oxadiazoles find primary applications in polymers, particularly in heat-resistant materials, insulators/semiconductors, macromolecular scintillators, and organic light-emitting devices (OLEDs).