Boryl anions

Segawa Y, Yamashita M, Nozaki K (2007) Boryl anion attacks transition-metal chlorides to form boryl complexes Syntheses, spectroscopic, and structural studies on group 11 borylmetal complexes. Angew Chem Int Ed 46 6710... 

Braunschweig H (2007) Boryl anions lithiumboryl—a synthon for a nucleophilic boryl anion. Angew Chem Int Ed 46 1946... 

Unlike carbanions and silyl anions, boryl anions (BR2"), in the form of boryllithiums, have been unknown until recently. In 2006, Japanese researchers reported the synthesis of a boryllithium via the reduction of a cyclic (diamino)bromoborane with lithium/naphthalene in THF, that is, Li+Np , where Np is the anion radical of naphthalene (Segawa, Y. Yamashita, M. Nozaki, K. Science 2006, 314, 113-115) ... 

Suggest a rationale for the apparent inaccessibility and instability of boryl anions (BR2 ). Why, for instance, can you not obtain a boryl anion by deprotonating a dialkylborane with n-butyllithium ... 

Yamashita, M. Nozaki, K. Recent Developments of Boryl Anions Boron Analogues of Carban-ion, Bull. Chem. Soc. Jpn. 2008, 81, 1377-1392. A history of these elusive anions. 

Free ftom a transition metal, tiie boryl group is highly basic. The first fuUy diaracter-ized alkali metal boryl anion was reported in 2006 by Yamashita, Nozaki and co-workers. Previous studies on the generation of boryl anions have not been reproduced. Theoretical studies imply that much of the charge on a free boryl anion would actually reside on the substituents, and that the boron would bear positive charge. ... 

Very recently, diamino-substituted boryl, -B(R NCH=CHNR ) (R = 2,6- Pr2CgH3), complexes of Li", Mg ", and Zn have been synthesized and structurally characterized [78-82]. As expected, the boryl anion displays its nucleophilicity and undergoes... 

As a side note, it is worth mentioning that NHC-stabilized r-boryl anions (30 and 31) were recently reported [84-86]. The potassium salt of 30 has been synthesized and structurally characterized [84]. Computational and reactivity studies indicate that there is a n-nucleophilic boron atom and 30 cleanly reacts with the electrophile Mel [84]. 31 was suggested as a reactive intermediate, which easily reacts with a diverse range of electrophiles to give different types of NHC boranes [86]. 

The search for boron-centered nucleophiles has been a fundamental challenge owing to the six-valence electron, hypovalent nature of boron which renders boron-based molecules electrophilic. A major breakthrough in this area came in 2006 when Nozaki and Yamashita isolated and structurally characterized a boryllithium reagent that was isoelectronic to an NHC and reacted as a boron-centered nucleophile. Since, there have been additional reports of boron-centered nucleophiles where the nucleophilic boron atom was a ligand to a transition metal [i.e. borylene). In the past five years, however, carbenes have emerged as excellent ligands to stabilize sp nucleophilic boryl anions. 

---