Formal dihydrogenation

Interestingly, when benzophenone imine itself is employed as substrate, higher reaction efficiency is observed in the absence of ancillary ligand (Scheme 31). Under identical reaction conditions, a 1.3 1 ratio of the mono and bis ortho arylated benzophenone imine is obtained (256% overall yield with respect to NaBPh4). Under these reaction conditions, the excess of benzophenone imine participates in the reoxidation of the catalyst as a formal dihydrogen acceptor, resulting in the concomitant formation of diphenylmethaneamine. 

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). 

The reaction of a metal-dimer with H2 can also be regarded as an oxidative addition reaction. For instance, a dimer of a d7 metal complex reacts with dihydrogen to give two d6 species. In this process dihydrogen also gives formally two hydride anions. A well-known example in the present context is the conversion of dicobaltoctacarbonyl into hydridocobalttetracarbonyl ... 

Palladium(O) forms a complex with quinone that is now electron rich and can be protonated to give hydroquinone and palladium(II). The latter can start a new cycle via a carbomethoxy species after reaction with methanol and CO (c.f. reaction (6), Figure 12.4). Thus we have formally switched from a hydride initiator to a carbomethoxy initiator species. Addition of quinone to a nonactive or moderately active palladium system is a diagnostic tool that tells us whether zerovalent palladium is involved as an inactive state. Likewise, one might add dihydrogen to a system to see whether palladium(II) salts need to be converted to a hydride to reactivate our dormant catalyst. 

Fig. 5. Insertion of dihydrogen. Complex 12 is formed from the formal insertion of H2 into the Fe-S bond. The energy of complex 12 is relative to the energy of 1 + H2 and given in kcal mol-1.

The dihydrogen bond X-H H-M may be formally regarded as a normal hydrogen bond... 

Dihydrogen is said to be heterolytically cleaved when it is dissociated into a proton and a metal bonded hydride. It has been the topic of much study and discussion, and the evidence for its occurrence has been growing steadily [54,55]. In the ideal case the heterolytic splitting is catalyzed by the metal ion and a base which assists in the abstraction of the proton. In this reaction there is no formal change in the oxidation state of the metal. The mechanism has been proposed for Ru(II) complexes which can react with dihydrogen according to ... 

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