Mild oxidation of alkanes and toluene by tert-butylhydroperoxide catalyzed by an homogeneous and immobilized Mn(salen) complex Original Research Article

Mild oxidations with t-BuOOH of linear (n-pentane, n-hexane, n-heptane and n-octane) and cyclic (1,2-dimethylcyclohexane) alkanes to the corresponding secondary alcohols and ketones, as well as of toluene to benzaldehyde, catalyzed by homogeneous and immobilized Mn(salen) (on a polydimethylsiloxane (PDMS) based membrane, Mn(salen)-PM) systems [Mn(salen) = {(R,R)-N,N-bis(3,5-di-tert-butylsalicylidenato)-1,2-cyclohexanediamine(2-)}chloro manganese(III)], have been studied. These oxidations are markedly promoted by the addition of various acids, namely nitric, pyrazine-2-carboxylic, acetic, benzoic and oxalic acids. Regio- and stereoselectivity parameters are dependent on the immobilization of the catalyst on the support and on the type of acid promoter, and correlate with the sorption values of alkane substrates on the PDMS membrane. The oxidation was shown to proceed with participation of t-BuO and alkylperoxyl (ROO) radicals. The toluene oxidation systems feature a high selectivity towards the formation of benzaldehyde (mainly in the case of the supported catalyst) and remarkably high catalyst turnover numbers (up to 1.7 × 103).