Syntheses of the first amine-dicarboxyboranes and their bis(methylester) and Bis(N-ethylamide) derivatives

Amine-bis(N-ethylcarbamoyl)boranes [A.BH(CONHEt)(2), 3; A = trimethylamine (Me3N, a), quinuclidine (Q, b), pyridine (py, I), 4-picoline (pic, g)] have been prepared after deprotonation of [amine-bis(C-hydroxy-N-ethylimidate)h ydroboron(2+)] cations (2), which were formed by the hydrolysis of [amine-b is(ethylnitrilium)hydroboron(2+)]tetrafluoroborates (1). Numerous represent atives 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 b ase exchange reactions from 3a. 3a-e are extremely stable in aqueous media, either acidic or alkaline. probably because of the considerable steric hin drance of possible reaction centers. However, they were transformed into am ine-dicarboxyboranes [A.BH(COOH)(2), 4a-e] in acidic medium under vigorous conditions (100-130 degreesC). This transformation was accompanied by signi ficant decomposition, probably owing to the protonation on the N atom, resu lting 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 yie ld. On the other hand, amine-bis(N-ethylcarbamoyl)boranes containing amines with sp(2)-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 re sulted in the ammonium salts of 4c-e, h. i of composition [A.BH(COOH)(COO-) ][AH(+)]. which in Nm could be transformed into the diacids 4, except 4h, b y protonation. As salt formation indicates, the 4 compounds are stronger ac ids 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 alk aline medium but are readily hydrolyzed in acidic medium. Hydrolysis of [am ine-bis(C-methoxy-N-ethylimidate)hydroboron(2+)] cations did not give the c orresponding bisesters 5 in alkaline, neutral, or acidic medium.