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Denmark, S.; Chang, W.-T. T., Science of Synthesis: Cross Coupling and Heck-Type Reactions, (2013) 1, 384.
The generally accepted mechanism for palladium-catalyzed cross-coupling reactions involves (1) oxidative addition of an organohalide, (2) transmetalation of the organyl group from the organometallic donor, and (3) reductive elimination to yield the coupling product. Amongst these steps, the mode of transmetalation is distinctive to the organometallic reagent employed. In the field of silicon-based cross coupling, it is generally believed that the generation of a pentacoordinate siliconate from the activation of a tetracoordinate silane is a prerequisite for transmetalation (Scheme 1). This is a reasonable assumption because neutral organosilanes do not undergo cross coupling in the absence of activators, usually fluoride or Brønsted bases. Moreover, silicon can expand its coordination number and distribute the additional electron density to the ligands.[12–15] Finally, an isolable hypervalent silicon reagent, tetrabutylammonium difluorotriphenylsiliconate (TBAT) has demonstrated the ability to transfer a phenyl group without an additional activator (Scheme 2).[16,17]
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References
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