Hard Catalysis with Transition Metal Compounds

1 Hard Catalysis with Transition Metal Compounds 

An example of hard catalysis is the oxidation of aldehydes with Co or Mn salts (Eq. 2-84). 

Initially, acetaldehyde is oxidized to peracetic acid via hard acetyl radical intermediates (Eq. 2-85). The peracetic acid then oxidizes acetaldehyde to acetic acid. 

Oxidation catalysts often have a large proportion of ionic bonding, mostly with simple a bonding of hard ligands (H2O, ROH, RNH2, OH, COO ) to the metal ion. An example is the selective epoxidation of olefins with organic hydroperoxides (Eq. 2-86). The key step of this process is the nondissociative coordination of the hydroperoxide molecule by a hard-hard interaction of the type  

The metal center lowers the electron density on the peroxide oxygen atom, activating it towards nucleophilic attack of the olefin. Typical catalysts are Mo, W, and Ti compounds. 

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Scheme 2-4 Hard and soft catalysis with transition metal compounds...

Of the many substrates which have been oxidized in the presence of transition metal complexes, one of the most extensively studied group of compounds has been olefinic hydrocarbons. The obvious incentive for the pursuit of this research is the identification of catalysts which could convert the abundant olefinic hydrocarbons into more valuable oxygen-containing derivatives under mild conditions in high selectivity. Since hydrocarbon-soluble complexes of the transition metals have been successfully applied to the catalytic addition of other small molecules to olefinic substrates, attempts have been made to activate and catalytically transfer dioxygen to olefins in a similar manner. It has been difficult, however, to exclude other pathways by which oxygen can interact with olefins and to achieve selective reactions. In cases where selective oxidations are achieved it is often hard to decide whether products arise via coordination catalysis or autoxidation pathways. 

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