Oppenauer oxidation catalytic process

A major part of the work described in this section has been carried out with the aim of applying these silsesquioxane complexes of Ti, Zr and Hf in catalytic processes such as ethylene polymerization, olefin epoxidation and Oppenauer oxidation. These catalytic aspects have been highlighted in several recent review articles. 

Indium trichloride promotes catalytically the addition of alkynylstannanes to aldehydes (Table 25).42 Metallic indium also mediates the same Barbier-type coupling between alkynyl halides and aldehydes or ketones to give secondary or tertiary propargyl alcohols (Table 26). Secondary alcohols can be oxidized in situ according an Oppenauer process.395 Thus, alkynyl ketones have been prepared from aldehydes via an indium-mediated alkynylation reaction followed by an indium-mediated Oppenauer oxidation. They are also obtained via an indium-mediated alkynylation of the relevant acyl chlorides (Table 27).396... 

Aluminum alkoxides, particularly those formed from secondary alcohols, have been of interest to synthetic chemists since the mid-1920s due to their catalytic activity. Examples of these trialkoxides include aluminum isopropoxide (AIP) and aluminum sec-butoxide (ASB). They are easily prepared at lab or plant scale and provide highly selective reductions and oxidations under mild conditions. These reductions are termed Meerwein-Ponndorf-Verley (MPV) reactions after the chemists (1-3) who first investigated their utility. Because a MPV reaction are accuratelybe described as an equilibrium process, the reverse reaction (oxidation) can also be exploited. These associated reactions are termed Oppenauer oxidations (4). Meerwein-Ponndorf-Verley reductions and Oppenauer oxidations as well as other reaction types and applications will be discussed, but first some background is provided concerning structure, preparation, and characterization of aluminum isopropoxide and related compounds. 

An example of a flow chemistry process is the Oppenauer oxidation of secondary benzylic alcohols using partially hydrated zirconia and various carbonyl compounds as oxidants (Scheme 53). The authors applied this procedure to electron-rich and electron-deficient substrates, with improvement in temperature (as low as 40 °C) and an easy reaction workup. The reuse of the catalyst was performed several times, without loss in catalytic efficiency. 

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