Peroxide Disproportionation Catalase Reactivity

Catalases are structural relatives of the peroxidases in that they contain a heme tetramer at the active site [241]. Their biological function is to control the cellular concentration of hydrogen peroxide by the following disproportionation reaction  

Benzylic and allylic positions are hydroxylated by CPO in halide-dependent catalytic transformations. Toluene and p-xylene are oxidized to the respective aldehydes and carboxylic acids [247, 248]. Ethylbenzene and other substrates with longer alkyl chains form the respective benzylic/allylic alcohols with high enantio-selectivity. Straight-chain aliphatic and cyclic (Z)-alkenes are hydroxylated, favoring small unsubstituted substrates in which the double bond is not more than two carbon atoms from the terminus. Steric control is observed for benzylic hydroxylations. 

Ethylbenzene is transformed into (R)-p hen ethyl alcohol with an ee of 97% whereas propylbenzene yields the S-configured alcohol in 88% ee. The enantioselectivity remains high even with longer-chain substrates, for which the catalytic performance is rather inefficient [249]. 

Systematic investigations into this reaction have been undertaken and showed that for straight-chain aralkyl hydroperoxides and their cyclic analogues the (R)-alcohol forms, whereas the stereoselection is the opposite for branched hydroperoxides. The reaction could be applied to functionalized hydroperoxides such as a- and (3-hydroperoxy esters or hydroperoxy alcohols with good to excellent diastereo- and enantioselectivities. Up to 99% ees were obtained for small, sterically non-hindered substrates, whereas for tertiary hydroperoxides and for substrates with substituents at the co-position neither good ees nor useful turnover numbers have been reported. HRP reacts very sluggishly also with sterically demanding silyl-substituted allyl hydroperoxides. 

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