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

DA·2BH 2 CN ( 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·2BH 2 CNEt)(BF 4 ) 2 ( 2 ), and DA·2BH 2 COOH ( 3 ) complexes were prepared from Me 2 S solution of (BH 2 CN) 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·BH 2 COOH ( 5a, b ) and DA·BH 2 COOMe ( 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. p K a values of 12a, b (p K 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 (p K a ≈ 4.2), was prepared as hexafluorophosphate and tetraphenylborate salt. The latter was transformed with aqueous KOH into the corresponding zwitterion ( 21a ).