“Umpolung” von Bor im [B(CN)₄]⁻ Anion durch Reduktion zum Dianion [B(CN)₃]²⁻

Abstract

Ein homoleptisches Cyanoborat mit nukleophilem Boratom: Das beispiellose Dianion in den Salzen M2[B(CN)3] (M=Li, Na, K), das nukleophiles Bor in der Oxidationsstufe +I enthält, wurde durch reduktive B‐C‐Bindungsspaltung im [B(CN)4]−‐Anion erhalten (siehe Schema) und durch 11B‐ und 13C‐NMR‐ und Schwingungsspektroskopie sowie durch Strukturanalyse charakterisiert.

Deprotonation of a Borohydride: Synthesis of a Carbene‐Stabilized Boryl Anion

“…[13] Subsequent treatment with dibenzo-18-crown-6 allowed for the isolation of the monomeric boryl anion 4, with the potassium cation trapped within the sphere of the crown ether (Figure 2, lower). As observed by Bernhardt et al, in the case of dianion E, [7] the potassium cation is bound to a nitrile nitrogen (2.7884(15) ). The B-C nitrile bond lengths in 4 (B1-C2: 1.553 (2), B1-C3: 1.552 (2) ) are slightly shorter than in 3 (B1-C2: 1.605 (6), B1-C3: 1.595 (6) ), but longer than in E (1.513 ), which indicates a weaker delocalization of the negative charge on the nitrile groups.…”

“…[6] Also noteworthy is the synthesis by Bernhardt et al of the dianion [B(CN) 3 ] 2À (E), generated by reduction of K[B(CN) 4 ] with potassium. [7] Lastly, in 2011 our group isolated the neutral tricoordinate nucleophilic boron F, which is isoelectronic with amines and phosphines. [8] Unfortunately, the steric environment around the boron center of F hampered its reactivity from any electrophile larger than a proton.…”

Isolation of a Bis(oxazol‐2‐ylidene)–Phenylborylene Adduct and its Reactivity as a Boron‐Centered Nucleophile

“…Interestingly, Bernhardt et al. showed that the reaction of K[B(CN) 4 ] with potassium yields the dianionic boron species K 2 [B(CN) 3 ] ( IV ), where the anionic charge is delocalized onto the cyanide groups 7. Recently, Bertrand and co‐workers reported the first deprotonation reaction of an adduct between a neutral borohydride and a cyclic (alkyl)(amino)carbene (CAAC),8, 9 which provided the corresponding CAAC‐stabilized boryl anion V 10.…”

Isolation of a Bis(oxazol‐2‐ylidene)–Phenylborylene Adduct and its Reactivity as a Boron‐Centered Nucleophile

“…[5c] Interestingly, Bernhardt et al showed that the reaction of K[B(CN) 4 ] with potassium yields the dianionic boron species K 2 [B(CN) 3 ] (IV), where the anionic charge is delocalized onto the cyanide groups. [7] Recently, Bertrand and co-workers reported the first deprotonation reaction of an adduct between a neutral borohydride and a cyclic (alkyl)(amino)carbene (CAAC), [8,9] which provided the corresponding CAAC-stabilized boryl anion V. [10] Remarkably, by using two CAACs, Bertrand et al also synthesized the neutral tricoordinate nucleophilic boron species VI, in which the strong electrophilicity of the CAACs as p-acceptors is critical for delocalizing and stabilizing the lone pair of the HBD fragment of VI. [11,12] Although VI is reluctant to react with most electrophiles because of steric congestion around the boron center, it readily reacted with Brønsted acids to yield the corresponding boronium ions, indicating its potential as an electron-donating ligand.…”