Metal‐Mediated Carbon–Hydrogen Bond Activation

Abstract

Along with the carbon-carbon bond, the carbon-hydrogen bond forms the foundation of organic chemistry. The combination of the typical strength of CÀH bonds (CÀH bond dissociation energies (BDEs) are often in the range of 95-110 kcal/mol) and their nonpolar nature often renders CÀH bonds chemically inert. These features make it difficult to selectively transform CÀH bonds into new functionalities, which is a substantial impediment to the synthesis of complex organic molecules as well as the production of materials on the commodity scale. In the past several decades, the emergence of metal-mediated activation of CÀH bonds has resulted in the expectation that homogeneous catalysts for the selective and efficient transformation of CÀH bonds can be developed.Since the demonstration that CÀH groups of a ligand on a transition metal can bond with the metal in an intramolecular fashion (i.e., intramolecular agostic interactions; Chart 11.1), 1,2 much effort has been directed toward understanding transition metal/CÀH bonding and subsequent CÀH bond cleavage by the metal center. Although both fundamental and applied challenges remain, a substantial understanding of how metals coordinate and break CÀH bonds has been developed. It is the aim of this chapter to provide an overview of the mechanisms by which metals activate and cleave CÀH bonds. This chapter is not intended to present a comprehensive review of all contributions to this field, but rather to provide a representative sampling of seminal studies and a perspective on the history and scope of the studies that have led to the state of the art in metal-mediated CÀH activation.It is generally accepted that metal-mediated rupture of CÀH bonds is typically preceded by direct coordination of the CÀH unit to the metal center. Similar to the coordination of dihydrogen to metal centers, which was first reported in 1984 by Kubas, 3,4 the metal/CÀH bonding interaction involves the donation of electrons from the CÀH s-bonding molecular orbital to an empty metal orbital. Complexes in which the ligand-to-metal electron donation comes from an electron pair that originates from a s-bonding molecular orbital have been termed s-complexes (Chart 11.2). 5 In some regards, the coordination of a CÀH unit is similar to the coordination of classic Lewis bases (e.g., amines and phosphines) in that the primary bonding interaction often involves the donation of two electrons from the ligand to an empty metal orbital; however, for CÀH coordination, the origin of the donated electrons (s-bonding molecular orbital versus a lone pair or, in some cases, a p molecular orbital) provides an important distinction from typical Lewis bases. Due to the donation of low-energy electrons from a s-bonding molecular orbital, CÀH units coordinate weakly to the metal center relative to the coordination of amines, phosphines, olefins, and so on. In fact, the unique nature of metal/CÀH bonding prompted designation of the term "agostic" to describe "situations in which a hydrogen atom is covalently bonded simultan...

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