Dihydrogen activation, mechanism

In order to appreciate the mechanism of rhodium asymmetric hydrogenation, it may be useful first to examine the simpler achiral case of alkene hydrogenation by Wilkinson s catalyst. In early studies [13] assumptions were made about the dihydrogen activation step which proved to be incorrect because of the direct... 

The reactivity of the bis-dppm-bridged complexes described above, although diverse, suggests a common mechanism of dihydrogen activation initiated by a single-metal addition. After this common activation step, the feasibility of subsequent reactions such as hydride migrations, further dihydrogen additions, or reductive eliminations is determined by the specific features of each compound, which also define the nature of the final reaction product. 

Hydrogenation of unsaturated substrates. Hydrogenation of alkenes and alkynes is one of the most widely useful reactions involving dihydrogen activation. The mechanism of Wilkinson s complex offers a good example of the process (Fig. 2.3) [100]. The cycle shown is just one of several that operate in the real system, depending on the exact conditions. It serves to show a key point, that the activation step is the hrst step in the cycle. As in the Vaska case, the ligands fold back so that the cis-dihydride is formed. An early NMR experiment (Fig. 

Computations showed that indeed in that case the oxidative addition of H2 to the re-coordinated complex, for example, 14, is the fastest mechanism of dihydrogen activation among all conceivable processes (hydrogenations of solvate complex, of nonchelating catalyst-substrate complexes and of sf-coordinated catalyst-substrate complexes are slower). Nevertheless, after formation of the dihydride intermediate 58, the double bond dissociation via the TS3 is much faster than migratory insertion via the TS2 (Scheme 1.17). As a result, the nonchelating dihydride intermediate 60 is formed, and (R)-57a is formed after stereoselective double bond coordination in a Rh(III) octahedral nonchelating complex vide infra). ° ... 

For unsaturated lactones containing an endocyclic double bond also the two previously described mechanisms are presumably involved and the regio-selectivity of the cyclocarbonylation is governed by the presence of bulky substituents on the substrate. Inoue and his group have observed that the catalyst precursor needs to be the cationic complex [Pd(PhCN)2(dppb)]+ and not a neutral Pd(0) or Pd(II) complex [ 148,149]. It is suggested that the mechanism involves a cationic palladium-hydride that coordinates to the triple bond then a hydride transfer occurs through a czs-addition. Alper et al. have shown that addition of dihydrogen to the palladium(O) precursor Pd2(dba)3/dppb affords an active system, in our opinion a palladium-hydride species, that coordinates the alkyne [150]. 

In the reactions above we have not explicitly touched upon the reactions of dihydrogen and transition metal complexes. Here the reactions that involve the activation of dihydrogen will be summarised, because they are very common in homogeneous catalysis and because a comparison of the various mechanisms involved may be useful. Three reactions are usually distinguished for hydrogen ... 

---