Metal-substituted Molecular Sieves as Catalysts for Allylic and Benzylic Oxidations

4 Metal-substituted Molecular Sieves as Catalysts for Allylic and Benzylic Oxidations 

Soluble chromium compounds are known to catalyze the allylic oxidation of olefins [22,23] and benzylic oxidations of alkyl aromatics [22,24] using tert-butyl-hydroperoxide as the primary oxidant. Chromium-substituted aluminophosphates, e. g. CrAPO-5, were shown to catalyze the allylic oxidation of a variety of terpene substrates with TBHP to give the corresponding enones [25,26]. For example, a-pinene afforded verbenone with 77% selectivity (Eq. 6) and 13% of the corresponding alcohol. 

Careful investigation [25,26] in which the catalyst was removed by filtration at the reaction temperature and the filtrate was left to react further revealed, however, that the observed catalysis was by soluble chromium(VI) leached from the framework. In separate experiments it was shown that a few ppm of soluble chromium VI) could account for the observed catalysis. It was further shown that leaching was a result of reaction of framework chromium atoms with the alkyl hydroperoxide. 

Similarly, the CrAPO-5- and chromium silicalite-1 (CrS-l)-catalyzed oxidation of aromatic side-chains with TBHP or O2 as the primary oxidant [27-31] almost certainly arises as a result of soluble chromium(VI) leached from the catalyst. The same probably applies to benzylic oxidations with TBHP catalyzed by chromium-pillared montmorillonite [32]. More recently, a chromium Schiff s base complex tethered to the mesoporous silica, MCM-41, was claimed [33] to be an active and stable catalyst for the autoxidation of alkylaromatic side-chains. It would seem unlikely, however, that Schiff s base ligands can survive autoxidation conditions. Indeed, on the basis of our experience with chromium-substituted molecular sieves we consider it unlikely that a heterogeneous chromium catalyst can be developed that is both active and stable to leaching under normal oxidizing conditions with O2 or RO2H in the liquid phase. Similarly, vanadium-substituted molecular sieves are also unstable towards leaching under oxidizing conditions in the liquid phase [6,34]. 

Jacobs and coworkers [35] described the use of CoAPO-5 and CoAPO-11 as heterogeneous catalysts for the autoxidation of cyclohexane. The cobalt did not seem to be leached at low conversions. At higher conversions secondary products, such as adipic acid, leach the cobalt by forming chelate complexes. It would be interesting to investigate the activity and stability towards teaching of these and other cobalt-substituted molecular sieves [36,37] in benzylic oxidations with O.  

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