Boron Attacks

In a bold move for boron chemistry, scientists in Japan have synthesized, isolated, and characterized a boryllithium compound for the first time (Science 2006, 314, 113). The novel boryl anion behaves efficiently as a base and as a nucleophile, opening "important synthetic pathways to boron-containing compounds," according to the report.

Even though boron sits immediately to the left of carbon on the periodic table, the atom does not share its neighbor's reactive versatility. Boron compounds are almost always electrophilic, preferring to accept electrons rather than donate them in a nucleophilic attack.

"Although there have been examples in which metal boryl species may be considered nucleophilic, this is a clear-cut case of nucleophilic boron," says Todd B. Marder, a boron expert at Durham University in England. A handful of researchers have reported anionic organoboron alkali metal salts as reactive intermediates, but none were able to isolate or conclusively characterize the anions.

Theoretical predictions led University of Tokyo chemists Kyoko Nozaki, Makoto Yamashita, and Yasutomo Segawa to believe that a diamino-substituted boryllithium would be stable. To generate the species, the researchers reductively cleaved the B-Br bond in bulky 2-bromo diazaborole. They were able to verify that they had indeed made a boryllithium species by X-ray crystallography and solution characterization.

This type of boron anion shares the same number of valence electrons as its popular carbon cousin, the N-heterocyclic carbene. The Tokyo team demonstrated boryllithium's nucleophilic prowess in reactions with n-butyl chloride and benzaldehyde.

"This is a very exciting time for boron chemistry," writes Marder in a commentary that accompanies the report. "The work opens the door to new pathways in boron chemistry that will have a substantial impact on both organic and inorganic synthesis."