Solvents aqueous palladium catalysis

The combination of water and an organic nitrile as the solvent system (aqueous bipha-sic system, ABS) permits one to separate the catalyst in the water solution, coordinated to a water-soluble phosphine, TPPTS, the trisodium salt of trisulfonated triphenylphos-phine.f" The groups of Sinou and Genet have studied this strategyy. " ° " ° Allylic carbonates are quite stable to the potentially hydrolytic conditions since the reactions occur in neutral medium and only traces of base are generated in the catalytic cycle. Organic-aqueous phase palladium catalysis has been reviewed extensively. 

In this way, the consecutive reactions to give higher amines in the aqueous phase are avoided in contrast, these reactions are unavoidable in the homogeneous one-phase catalysis. The catalyst system consists of palladium acetate/tppts dissolved in water the second phase is an organic solvent such as toluene. 

To illustrate the inner-sphere characteristics of the CH activation chemistry, an analogy can be made between CH activation by coordination of an alkane CH bond to a metal center and the known catalysis resulting from coordination of olefins via the CC double bond (note that the nature of the orbitals involved in bonding are quite different). It is well known that coordination of olefins to electrophilic metal centers can activate the olefin to nucleophilic attack and conversion to organometallic, M-C, intermediates. The M-C intermediates thus formed can then be more readily converted to functionalized products than the uncoordinated olefin. An important example of this in oxidation catalysis is the Wacker oxidation of ethylene to acetaldehyde. In this reaction, catalyzed by Pd(II) as shown in Fig. 7.14, ethylene is activated by coordination to the inner-sphere of an electrophilic Pd(II) center. This leads to attack by water and facile formation of an organometallic, palladium alkyl intermediate that is subsequently oxidized to acetaldehyde. The reduced catalyst is reoxidized by Cu(II) to complete the catalytic cycle. The Wacker reaction is very rapid and selective and it is possible to carry out the reaction is aqueous solvents. This is largely possible because of the favorable thermodynamics for coordination of olefins to transition metals that can be competitive with coordination to the water solvent. The reaction is very selective presumably because the bonds of the product (po-... 

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