Amine-bis(N-ethylcarbamoyl)boranes [A·BH(CONHEt)2, 3; A = trimethylamine (Me3N, a), quinuclidine (Q, b), pyridine (py, f), 4-picoline (pic, g)] have been prepared after deprotonation of [amine-bis(C-hydroxy-N-ethylimidate)hydroboron(2+)] cations (2), which were formed by the hydrolysis of [amine-bis(ethylnitrilium)-hydroboron(2+)]tetrafluoroborates (1). Numerous representatives of 3 [A = diethylamine (Et2NH, c), piperidine (pip, d), pyrrolidine (pyrr, e), 4-aminopyridine (4-NH2-py, h), 4-(dimethylamino)pyridine (DMAP, i), imidazole (Him, j), 1-methylimidazole (Mim, k)] have been prepared by base exchange reactions from 3a. 3a-e are extremely stable in aqueous media, either acidic or alkaline, probably because of the considerable steric hindrance of possible reaction centers. However, they were transformed into amine-dicarboxyboranes [A·BH(COOH)2, 4a-e] in acidic medium under vigorous conditions (100-130 °C). This transformation was accompanied by significant decomposition, probably owing to the protonation on the N atom, resulting in the rupture of the B-N bond. As an exception, 4b, where N atom in a rigid bicycle is not prone to attacks, could be isolated in very good yield. On the other hand, amine-bis(N-ethylcarbamoyl)boranes containing amines with sp2-hybridized N atoms (3f-k) undergo complete decomposition under similar conditions probably because of the increased hydridic character of the hydrogen adjacent to boron. Base exchange reactions starting from 4b resulted in the ammonium salts of 4c-e, h, i of composition [A·BH(COOH)(COO-)][AH+], which in turn could be transformed into the diacids 4, except 4h, by protonation. As salt formation indicates, the 4 compounds are stronger acids as univalent acids than the corresponding A·BH2(COOH) complexes. 4a-e, i were readily esterified into amine-bis(methoxycarbonyl)-boranes (5a-e, i) in methanol, employing a catalytic amount of HBr. 5a-e, i are stable in alkaline medium but are readily hydrolyzed in acidic medium. Hydrolysis of [amine-bis(C-methoxy-N-ethylimidate)hydroboron(2+)] cations did not give the corresponding bisesters 5 in alkaline, neutral, or acidic medium.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry