A new method for the synthesis of oxonium derivatives of the closo decaborate anion in high yields was described. Reactions of the anion B 10 H 10 2-with cyclic ethers (tetrahydrofuran (THF) and tetrahydropyran) in the presence of dry HCl are stepwise processes. Depending on the reaction temperature and time, either mono or disubstituted products were obtained. Their structures were determined using 11 B, 13 C, and 1 H NMR and IR spectroscopy, ESI mass spectrometry, and elemental analysis for boron.Key words: boranes, closo decaborate anion, oxonium derivatives of the closo decaborate anion, electrophile induced nucleophilic substitution.Oxonium derivatives of closo decaborates are of con siderable interest for the preparation of novel com pounds and materials for various areas of medicine and engineering. 1,2Oxonium closo decaborates can be synthesized in dif ferent ways, e.g., by reactions of the anion B 10 H 11 -with cyclic ethers, 3 by reactions of the closo decaborate anion with appropriate nucleophiles in the presence of CF 3 COOH, 3 and by using Lewis acids as initiators. 4,5 However, these methods involve expensive reagents and complicated procedures for isolation of the target products.In the present work, we developed a new preparative method for the synthesis of oxonium derivatives by reac tions of cyclic ethers with salts of the decahydro closo decaborate anion (1) in the presence of dry HCl. This method is distinguished from the previous ones by a sim ple procedure and high yields (up to 95%) and allows the synthesis of disubstituted derivatives.
Results and DiscussionWe studied reactions of salts of the closo decaborate anion (1) with cyclic ethers in the presence of dry HCl as an initiator. Depending on the reaction conditions (tem perature and time), mono (2) and disubstituted deriva tives (3) were obtained (Scheme 1).The highest yields of monosubstituted products 2 (up to 95%) were achieved at 60 °C and a reaction time of 1 h. The course of the reactions was monitored by 11 B NMR spectroscopy. The 11 B NMR spectrum of product 2b (see Scheme 1) shows a singlet at δ 5.1 for the substituted B atom (the singlet is not split into a doublet without broad band proton decoupling) and two singlets at δ -1.1 and -7.8 with equal integral intensities for two nonequivalent apical B atoms, which indicates the presence of one sub stituent in an equatorial position of the boron cluster. The spectrum also contains signals for the other B atoms at δ -23.2, -25.1, and -31.4 with an integral intensity ratio of Scheme 1 n = 1 (a), 2 (b) Nu is tetrahydrofuran (a), tetrahydropyran (b) Conditions: i. HCl, 1 h, 50-88 °C; ii. HCl, 3 h, reflux.
