Carbon-to-phosphorus bond breaking

The importance of transition-metal mediated decomposition of ligands has been reviewed by Garrou [30] with an emphasis on oxidative addition as the mechanism. 

Oxidative addition. The oxidative addition of a phosphine to a low valent transition metal can be most easily understood by comparing the Ph2P-fiagment with a Cl- or Br- substituent at the phenyl ring electronically they are very akin, cf Hammett parameters and the like. The phosphido anion formed during this reaction will usually lead to bridged structures, which are extremely stable. 

Cluster or bimetallic reactions have also been proposed in addition to monometallic oxidative addition reactions. The reactions do not basically change. Reactions involving breaking of C-H bonds have been proposed. For palladium catalysed decomposition of triarylphosphines this is not the case [32], Likewise, Rh, Co, and Ru hydroformylation catalysts give aryl derivatives not involving C-H activation [33], Several rhodium complexes catalyse the exchange of aryl substituents at triarylphosphines [34]  

These authors propose as the mechanism for this reaction a reversible oxidative addition of the aryl-phosphido fragments to a low valent rhodium species. A facile aryl exchange has been described for complexes Pd(PPh3)2(C6H4CH3)I. The authors [35] suggest a pathway involving oxidative additions and reductive eliminations. The mechanism outlined below, however, can also explain the results of these two studies. 

Related to this is the mechanism for the catalytic oxidation of phosphines by water. This reaction plays a role in catalysis in water using water-soluble phosphine ligands. 

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As a second step in the reaction, following co-ordination, most authors propose an oxidative addition of the C-S fragments to the transition metal, similar to the reaction found for carbon-to-phosphorus bond breaking. Since the C-S bond is rather weak it is easy to break and indeed several model reactions can be found in the literature. 

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