Termination of the Metal-promoted or catalysed Reactions and a Catalytic Cycle

7 Termination of the Metal-promoted or -catalysed Reactions and a Catalytic Cycle 

The Grignard reaction proceeds via oxidative addition and insertion. The reaction product 93 is isolated after hydrolysis of the insertion product 92 with dilute aqueous 

giving MgCl2, and it is practically impossible to reduce the generated Mg(II) to Mg(0) in situ, and hence the Grignard reaction is stoichiometric. In other words, Mg(0) is oxidized to Mg(II) by the Grignard reaction. However, reactions involving transition metal complexes proceed with a catalytic amount of the metal compounds in many cases whenever they are attacked by nucleophiles. 

The catalytic reaction that can be carried out with a small amount of expensive metal complex is the most useful feature of synthetic reactions involving transition metal complexes. In catalytic reactions, the active catalytic species must be regenerated in the last step of the reaction. Reductive elimination and elimination of -hydrogen are two key reactions that can regenerate the catalytic species, making the whole reaction catalytic. Not all transition metal complexes undergo the catalytic reactions. 

As a typical example of the catalytic cycle, the Pd-catalysed reaction of benzoyl chloride (99) with MeSnBu3 to afford acetophenone (100) is explained by the 

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