Microwave Synthesis of Derivatives of [B10H10]2– and [B12H12]2– Anions with Crown Ethers

A novel method has been developed for the synthesis of mono-substituted derivatives of the closo-decaborate and closo-dodecaborate anions bearing macrocyclic oxonium-type substituents. This method involves the reaction between the [B10H11]– and [B12H12]2– anions with crown ether molecules (12-crown-4, 15-crown-5, and 18-crown-6) under microwave-assisted synthesis conditions (2.45 GHz, 700 W). These processes proceed via the EINS mechanism. H+ (as part of [B10H11]–) was used as the electrophilic inducer for derivatizing the [B10H10]2– anion, while BF3·Et2O was employed for derivatizing the [B12H12]2– anion. The developed approach utilizing microwave synthesis enables the preparation of substituted closo-borates without the use of a solvent, which is highly relevant for such processes. Furthermore, it significantly reduces reaction times (by a factor of 3–8) compared to previously known methods, while maintaining high yields (81–92%). The proposed method exhibits regioselectivity (in the case of the [B10H10]2– anion, the derivative with the substituent in the equatorial position is formed). The obtained derivatives of [B10H10]2– and [B12H12]2– anions were characterized by elemental analysis, IR, 11B, 13C, 1H NMR spectroscopies, and ESI mass spectrometry. The synthesized closo-borates continue the series of derivatives of the [B10H10]2– and [B12H12]2– anions with oxonium-type substituents, expanding the possibilities of the employed functionalization method.