Dihydrogen bond activation

Why are transition metals so well suited for catalysis A complete treatment of this critical question lies well beyond the scope of this book, but we can focus on selected aspects of bond activation and reactivity for dihydrogen and alkene bonds as important special cases. Before discussing specific examples that involve formal metal acidity or hypovalency, it is convenient to sketch a more general localized donor-acceptor overview of catalytic interactions in transition-metal complexes involving dihydrogen49 (this section) and alkenes (Section 4.7.4). 

Early metal complexes often avoid the 18e configuration - Me3TaCl2 is ostensibly lOe, for example. This can provide coordinative unsaturation at the metal Schrock carbene complexes with <18e commonly have agostic C H bonds see Alkane Carbon-Hydrogen Bond Activation, Dihydrogen Complexes Related Sigma Complexes). When... 

C-H and C-C bond activations by ruthenium complexes have formed the focus of this chapter, and consequently other important reactions to cleave chemical bonds such as dihydrogen, C-S and M-R have not been described. Today, ruthenium is regarded as a powerful tool for cleaving a variety of both activated and unactivated chemical bonds under homogeneous conditions. Important factors that provide these activities include 1) coordinative unsaturation of the ruthenium center 2) a close proximity of the bond to the ruthenium metal and 3) kinetic preference and thermodynamic stability of the products. It is likely that the combined use of ruthenium complexes and modern strategies in organic synthesis and catalysis will provide many opportunities for the creation of new reaction processes in the futtue. 

One of the most interesting recent topics in ruthenium chemistry is undoubted ) C—H bond activation in which the generation of coordinatively unsaturated specie> may play an important role. These species are usually produced by thermal or photo-mediated reductive elimination of dihydrogen, alkanes, alkenes or arenes. Recently, dehydrochlorination from RuHCI(CO) (P BuTMe) is reported to give a 7C-allyl complex via C—H activation of propylene (eq (42)) [144]. 

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