Synthesis, Characterization, and Bromine Substitution of Diamine Complexes of Carboxyborane and Methoxycarbonylborane. Diazabora Rings Containing B-Carboxyl and B-Carboxylato Groups

DA·2BH₂CN (1) [DA = N,N,N‘,N‘-tetramethylethanediamine (TMEDA, a), N,N,N‘,N‘-tetramethylpropanediamine (TMPDA, b), and N,N,N‘,N‘-tetramethylbutanediamine (TMBDA, c)], (DA·2BH₂CNEt)(BF₄)₂ (2), and DA·2BH₂COOH (3) complexes were prepared from Me₂S solution of (BH₂CN)_n (→ 1 → 2 → 3) in fast procedures, differently from those usually applied, in good yields. Dimethyl esters (4) were prepared from 3 with methanol in very fast reactions catalyzed by HBr. 3 and 4 were transformed into DA·BH₂COOH (5a,b) and DA·BH₂COOMe (6) with DA in THF. Bromination of 3 in aqueous HBr yielded diamine-bis(monobromocarboxyboranes) (7), in vigorous conditions diamine-bis(dibromocarboxyboranes) (9) were produced. N-Bromosuccinimide in methanol transformed 3a,b directly into diamine-bis(bromomethoxycarbonylboranes) (8a,b). BrH·DA·BH(Br)COOH (10a,b) could be prepared from 5a,b with bromine in strongly acidic medium. N-Deprotonated 10a quickly transformed into a five-membered cyclic cation [TMEDA·BHCOOH]⁺ (cation in 12a) in both water and methanol. Thus, this ion was formed by dissolution of 10a in water, bromination of 5a or destroying one of the borane moieties in 7a by a base. N-Deprotonated 10b in water decomposed with a pH-dependent rate, but in methanol, in the presence of base, it transformed into a six-membered cyclic cation [TMPDA·BHCOOH]⁺ (cation in 12b). 8a,b could also be transformed into cyclic cations by destroying one of their borane moieties in methanol, (8b in absence of acids only). All cations were prepared as bromide, hexafluorophosphate and tetraphenylborate salts. pK_a values of 12a,b (pK_a = 5.5−5.6) are 3 units lower than those of amine-carboxyboranes. Zwitterions (18a,b) formed by their deprotonation were prepared. Cyclic cation [TMEDA·B(Br)COOH]⁺ (cation in 19a) formed on destroying one dibromocarboxyborane of 9a. This cation, quite a strong acid (pK_a ≈ 4.2), was prepared as hexafluorophosphate and tetraphenylborate salt. The latter was transformed with aqueous KOH into the corresponding zwitterion (21a).