Cyclic Bis(trifluoromethyl)borates [cyclo-O(CH2)nB(CF3)2]− (n=3, 4, 5): structure of Cs[cyclo-O(CH2)4B(CF3)2]

Hydroboration of H2CCH(CH2)(n−2)(CF3)2B·NMe3 (n=3 (1), 4 (2), 5 (3)) with 9-BBN followed by alkaline oxidation with H2O2 yields HO(CH2)n(CF3)2B·NMe3 (n=3 (4), 4 (5), 5 (6)). Upon heating HO(CH2)n(CF3)2B·NMe3 rearranges under elimination of HNMe3+ to form the cyclic borates [cyclo-O(CH2)nB(CF3)2]− (n=3 (7), 4 (8), 5 (9)). In case of n=4 and 5 side products [cyclo-O(CH2)nC(CF3)BF3]− (10) and (11) could be identified and the Cs salts of 7, 8, 9 and 11 have been isolated. The constitution of the novel boron compounds has been deduced from multinuclear NMR, Raman and mass spectra. The structure of Cs[cyclo-O(CH2)4B(CF3)2] has been determined by X-ray diffraction.