Deprotonation of the doubly arylene-bridged diborane(6) derivative 1H with (Me Si) CLi or (Me Si) NK gives the B-B σ-bonded species M[1H] in essentially quantitative yields (THF, room temperature; M=Li, K, arylene=4,4'-di-tert-butyl-2,2'-biphenylylene). With nBuLi as the base, the yield of Li[1H] drops to 20 % and the 1,1-bis(9-borafluorenyl)butane Li[2H] is formed as a side product (30 %). In addition to the 1,1-butanediyl fragment, the two boron atoms of Li[2H] are linked by a μ-H bridge. In the closely related molecule Li[3H], the corresponding μ-H atom can be abstracted with (Me Si) CLi to afford the B-B-bonded conjugated base Li [3] (THF, 150 °C; 15 %). Li[1H] and Li[2H] were characterized by NMR spectroscopy and X-ray crystallography.

Targeted C(sp3)–H activation or nucleophilic substitution reactions have been achieved through the interaction of a diborane dianion with haloalkanes.

Double reduction of the THF adduct of 9H‐9‐borafluorene (1⋅THF) with excess alkali metal affords the dianion salts M2[1] in essentially quantitative yields (M=Li–K). Even though the added charge is stabilized through π delocalization, [1]2− acts as a formal boron nucleophile toward organoboron (1⋅THF) and tetrel halide electrophiles (MeCl, Et3SiCl, Me3SnCl) to form B−B/C/Si/Sn bonds. The substrate dependence of open‐shell versus closed‐shell pathways has been investigated.

Subvalent boron compounds contain boron atoms with oxidation numbers lower than +III. Over the last decades, the development of isolable derivatives has relied heavily on the use of specially designed ligands capable of stabilizing the electron‐rich boron centers electronically or through steric protection. Herein, we are exclusively reviewing anionic organo(hydro)boranes largely devoid of stabilizing ligands or heteroatom substituents. The restriction to these subvalent species is intended to minimize the risk of ligand artifacts being included when carving out the characteristic properties of the respective boron centers, such as nucleophilic or carbenoid behavior. The scope of this review encompasses triorganoborane radical monoanions ([·BR3]–) along with closed‐shell dianions ([:BR3]2–), boryl anions ([:BR2]–), as well as B–B single‐bonded diborane(6) dianions ([R3B–BR3]2–) and diborane(5) monoanions ([R2B–BR3]–), and finally B=B double‐bonded diborane(4) dianions ([R2B=BR2]2–). We are showing how these species are related to each other and comment on their bonding situations from an experimentalist's perspective.

Two subvalent, redox‐active diborane(4) anions, [3]4− and [3]2−, carrying exceptionally high negative charge densities are reported: Reduction of 9‐methoxy‐9‐borafluorene with Li granules without stirring leads to the crystallization of the B(sp3)−B(sp2) diborane(5) anion salt Li[5]. [5]− contains a 2,2′‐biphenyldiyl‐bridged B−B core, a chelating 2,2′‐biphenyldiyl moiety, and a MeO substituent. Reduction of Li[5] with Na metal gives the Na+ salt of the tetraanion [3]4− in which two doubly reduced 9‐borafluorenyl fragments are linked via a B−B single bond. Comproportionation of Li[5] and Na4[3] quantitatively furnishes the diborane(4) dianion salt Na2[3], the doubly boron‐doped congener of 9,9′‐bis(fluorenylidene). Under acid catalysis, Na2[3] undergoes a formal Stone–Wales rearrangement to yield a dibenzo[g,p]chrysene derivative with B=B core. Na2[3] shows boron‐centered nucleophilicity toward n‐butyl chloride. Na4[3] produces bright blue chemiluminescence when exposed to air.

Deprotonation of the doubly arylene‐bridged diborane(6) derivative 1H2 with (Me3Si)3CLi or (Me3Si)2NK gives the B−B σ‐bonded species M[1H] in essentially quantitative yields (THF, room temperature; M=Li, K, arylene=4,4′‐di‐tert‐butyl‐2,2′‐biphenylylene). With nBuLi as the base, the yield of Li[1H] drops to 20 % and the 1,1‐bis(9‐borafluorenyl)butane Li[2H] is formed as a side product (30 %). In addition to the 1,1‐butanediyl fragment, the two boron atoms of Li[2H] are linked by a μ‐H bridge. In the closely related molecule Li[3H], the corresponding μ‐H atom can be abstracted with (Me3Si)3CLi to afford the B−B‐bonded conjugated base Li2[3] (THF, 150 °C; 15 %). Li[1H] and Li[2H] were characterized by NMR spectroscopy and X‐ray crystallography.

Two challenging but rewarding topics in chemical synthesis are C–F-bond activation and the development of B-centered nucleophiles. We have now tackled both subjects simultaneously by forming aryl–B bonds via SNAr-type...

Two subvalent, redox‐active diborane(4) anions, [3]4− and [3]2−, carrying exceptionally high negative charge densities are reported: Reduction of 9‐methoxy‐9‐borafluorene with Li granules without stirring leads to the crystallization of the B(sp3)−B(sp2) diborane(5) anion salt Li[5]. [5]− contains a 2,2′‐biphenyldiyl‐bridged B−B core, a chelating 2,2′‐biphenyldiyl moiety, and a MeO substituent. Reduction of Li[5] with Na metal gives the Na+ salt of the tetraanion [3]4− in which two doubly reduced 9‐borafluorenyl fragments are linked via a B−B single bond. Comproportionation of Li[5] and Na4[3] quantitatively furnishes the diborane(4) dianion salt Na2[3], the doubly boron‐doped congener of 9,9′‐bis(fluorenylidene). Under acid catalysis, Na2[3] undergoes a formal Stone–Wales rearrangement to yield a dibenzo[g,p]chrysene derivative with B=B core. Na2[3] shows boron‐centered nucleophilicity toward n‐butyl chloride. Na4[3] produces bright blue chemiluminescence when exposed to air.