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2.1 Electronic Structure of Boron Reagents

DOI: 10.1055/sos-SD-230-00002

Carbó, J. J.; Maseras, F.Science of Synthesis: Advances in Organoboron Chemistry towards Organic Synthesis, (201915.

The element boron has three valence electrons that typically form three linkages to other atoms, such as main-group elements of the p-block, alkali and alkaline-earth metals, or transition metals. These trivalent boron compounds have a trigonal planar geometry with an empty p-orbital perpendicular to the molecular plane, the overall structure being isoelectronic to a carbocation. The electron deficiency at boron can be compensated for by π-donation from the substituents into the empty p-orbital of boron (Scheme 1). The pπ–pπ lone-pair donation from atoms such as oxygen has a significant stabilizing effect, while in transition-metal–boryl species, the dπ–pπ back donation is assumed to be weak. Therefore, both organoboron compounds and boryl–transition metal species have been traditionally regarded as electrophilic reagents via the Lewis acidity of the boron center. However, trivalent boron compounds can change from having electrophilic character to exhibiting nucleophilic behavior, depending on the substituents attached to the boron atom.

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