Hydrogen addition, stepwise 11/11 transfer

Figure 7.9 The mechanism for the hydrogenation of an alkene as catalyzed by finely divided platinum metal (a) hydrogen adsorption (b) adsorption of the alkene (c) and (d), stepwise transfer of both hydrogen atoms to the same face of the alkene (syn addition).

Scheme 13 Stepwise transfer of H2 from an aminoborane to an iridium center by oxidative addition and B-to-M a-hydrogen migration. Counterion omitted for clarity. ...

In the present chapter, a classification of the hydrogenation reaction mechanisms according to the necessity (or not) of the coordination of the substrate to the catalyst is presented. These mechanisms are mainly classified between inner-sphere and outer-sphere mechanisms. In turns, the inner-sphere mechanisms can be divided in insertion and Meerweein-Ponndorf-Verley (MPV) mechanisms. Most of the hydrogenation reactions are classified within the insertion mechanism. The outer-sphere mechanisms are divided in bifunctional and ionic mechanisms. Their common characteristic is that the hydrogenation takes place by the addition of H+ and H- counterparts. The main difference is that for the former the transfer takes place simultaneously, whereas for the latter the hydrogen transfer is stepwise. 

Figure 14 Mechanisms for the migration of C1 (OH) of propane-1,2-dioi in DDH. Stepwise and concerted mechanisms both require acid cataiysis for the migration of OH from C2 to Cl of propane-1,2-dioi. in the stepwise mechanism, the eiimination of OH from C1 is cataiyzed by proton transfer to form a discrete water moiecuie and oxycation radicai intermediate. Addition of the water moiecuie to Cl with proton transfer compietes the rearrangement, in the concerted mechanism, water is not a discrete intermediate but the migrating oxygen remains partiaiiy bonded to C1 and C2 as weii as hydrogen bonded in a transition state. His143, Asp155, and potassium ion are in contact with the migrating OH group.

Addition of hydrogen to Wilkinson s catalyst promotes oxidative addition of hydrogen. Dissociation of a bulky phosphine ligand and co-ordination of the alkene is followed by stepwise stereospecific cis transfer of the two hydrogen atoms from the metal to the alkene by way of an intermediate with a carbon-metal bond (7.26). Diffusion of the saturated substrate away from the transfer site allows the released complex to combine with dissolved hydrogen and repeat the catalytic reduction cycle. 

The cluster cation can be isolated from the reaction mixture as the BF4 salt. The authors suggest that the catalytic hydrogenation involves a stepwise mechanism, as depicted in Figure 15.8.2 they believe that the hydrogenation of the aromatic substrate occurs within the hydrophobic pocket of the catalyst, via cyclohexadiene and cyclohexene intermediates. This implies that after the dihydride transfer, the partially reduced substrate leaves the hydrophobic pocket. The catalyst is finally regenerated by H2 oxidative addition. 

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