Synthesis of amine-dicyanohydroboranes, [amine-bis(ethylnitrilium)hydroboron(2+)] tetrafluoroborates, and their derivatives as precursors of amine-dicarboxyboranes

Numerous amine-dicyanohydroboranes [A.BH(CN)(2), 1, A = quinuclidine (Q, c), trimethylamine (Me3N, d), 4-picoline (Pic, e), 4-(dimethylamino)pyridine (DMAP, f), N,N,N',N'-tetramethylethylenediamine (TMEDA, j), 1.4-diazabicyclo[2.2.2]octane (DABCO, k)] have been prepared by base exchange reactions from 4-cyanopyridine-dicyanohydroborane (4-CN-py.BH(CN)(2), 1a). In analogous experiments with secondary amines [piperidine (g), diethylamine (h), and morpholine (i)] 1a underwent aminodecyanation also, probably via SNAr mechanism, which demonstrates the strong electron-withdrawing effect of the >N.BHX2 moiety toward the substituents on the nitrogen. Amine-dicyanohydroboranes have been transformed into [amine-bis(C-hydroxy-N-ethylimidate)hydroboron(2+)] (3) [amine-N-ethylcarbamoyl(C-hydroxy-N-ethylimido)hydroboron(1+)] (4), [amine-bis(C-methoxy-N-ethylimidate)hydroboron(2+)] (5), [amine-bis(amidinium)hydroboron(2+)] (6), and [amine-bis(triethylamidinium)hydroboron(2+)] (7) cations, precursors of amine-dicarboxyboranes and their derivatives. These transformations were carried out in two steps. First, the otherwise nonreactive cyano groups were activated by ethylation employing Et3OBF4, yielding [amine-bis(ethylnitrilium)hydroboron(2+)] tetrafluoroborates (2), then 3-7 were obtained by nucleophilic addition to 2. The pK(a) values corresponding to the protonation of the N-ethylamide group were found to be extremely high (3.1-3.3), which demonstrates the strong electron-donating effect of >N.BHX2 moiety toward the substituents on the boron.