Anionic Organoboranes: Delicate Flowers Worth Caring for

Abstract

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.

B–B vs. B–H Bond Activation in a (μ‐Hydrido)diborane(4) Anion upon Cycloaddition with CO2, Isocyanates, or Carbodiimides

“…[31] The 1 Ha nd 13 [22] which points toward as ignificantly altered electronic situation within the B(m-H)B core.Alow-field resonance at d( 13 C) = 195.5 ppm is consistent with the presence of aB-O-C(O)-B fragment. [29,32] In line with that, two 11 BNMR resonances are found in the region of tetracoordinated boron nuclei (d( 11 B) = À1.6, À5.1 ppm). [33] X-ray crystallography on the centrosymmetric dimer {[K(pmdta)][2H]} 2 •THF finally confirmed the molecular structure of the cycloaddition product (Figure 2 3) ,r espectively.W en ote that the BÀO bond of K[2H],which is inert toward excess CO 2 over hours, is shorter by 0.065 than the B À Ob ond of ac omparable cycloadduct Na 2 [A](Scheme 2b), which readily inserts asecond molecule of CO 2 .…”

“…Thea ddition of iPrNCNiPr (1 equiv) to K[1H] in [D 8 ]THF resulted in the selective formation of K [3](Scheme 2a). The 1 HNMR spectrum shows only one set of tBu(C 6 H 3 ) and iPr signals with an overall integral ratio suggesting an equimolar reaction between the two starting materials.A resonance assignable to aB-bonded Hatom is not detectable, even upon 11 Bd ecoupling.I nstead, we observe as inglet resonance at d( 1 H) = 8.07 ppm (1 H), showing across-peak to as ignal at d( 13 C) = 158.8 ppm in the 1 H- 13 CH SQC experiment. Theassociated CÀHmoiety is obviously not part of an…”

Synthetic Approaches to Triarylboranes from 1885 to 2020

“…Furthermore, their synthesis should be possible from all knownb oront rihalides. Recently,t he synthesis of triarylboranes from potassium aryltrifluoroborates [27][28][29][30][31] and boronic esters [32] was reported.W ed on ot discuss dibenzoboroles [33] or boron-containing polyaromatic hydrocarbons (B-PAHs) [34][35][36] in our review as both topics have been reviewed recently.…”

Stabilization of the Elusive 9‐Carbene‐9‐Borafluorene Monoanion

“… [3] Despite the high reactivity of these types of compounds, they can be used in situ to generate boron species that cannot be prepared using higher oxidation state starting materials. For example, anionic organoboranes have been shown to activate the C=O, H−H, C sp −H, O−O, and C−F bonds of catalytically relevant small molecules [1b, 2b, 4] . Braunschweig reported an N‐heterocyclic carbene‐stabilized borole anion which reacts with organotetrel halides via nucleophilic substitution or single‐electron transfer [5] .…”

“…However, with the appropriate ligand system, electrondeficient boron compounds can undergo chemical reduction; and the additional electrons transform these species into powerful nucleophiles. Indeed, since the seminal reports of boryllithium by Yamashita and Nozaki, [1] boron-centered anions have become valuable synthons in organometallic chemistry, and have afforded the isolation of a wide range of unique boron-element chemical bonds [2] (element = s-, d-, pblock element) (Figure 1 a). Very recently, Aldridge reported a boron complex that approaches the "naked" boryl anion (Figure 1 a).…”