Building a Lewis Base with Boron

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Science  29 Jul 2011:
Vol. 333, Issue 6042, pp. 530-531
DOI: 10.1126/science.1209588

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In introductory chemistry courses, acids are defined as substances that increase the concentration of H+ (or H3O+) in solutions (the Arrhenius concept) or act as proton donors (the Brønsted-Lowry concept). The more general Lewis concept defines an acid as a substance that can accept an electron pair and a base as a substance that can donate an electron pair. Compounds with atom centers that are inherently electron-deficient, such as boron or aluminum, readily accept electron pairs. Thus, simple compounds of these elements, such as borane (BH3) and aluminum chloride (AlCl3), are typically strong Lewis acids. If a monovalent boron center is to bear an electron pair that could be donated as a Lewis base, it would exhaust its own supply of valence electrons and end up with empty orbitals. For example, borylene compounds (such as compound A in the figure) (1) would be bases, but they are so reactive that they have only been observed as transient intermediates (2, 3). On page 610 of this issue, Kinjo et al. (4) used carbenes to stabilize a borylene (see compound 1 in the figure). This unusual type of base (and analogs yet to be prepared) may open up new avenues in synthesis and catalysis.

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