Mechanistic studies on catalytic alkane oxidation by Murahashi's O2/copper(ii)/aldehyde system

Abstract

Mechanistic studies on catalytic alkane hydroxylation by Murahashi's O₂/copper(II)/aldehyde system have been conducted to show that the autoxidation of an aldehyde (RCHO) by an O₂ generating acyl radical intermediate (RC(O)˙) is involved as an initiation step of the catalytic cycle. The generated RC(O)˙ is trapped by O₂ to give an acylperoxyl radical intermediate RC(O)OO˙, which may react with another RCHO to generate an adduct intermediate RC(O)OOC(R)(H)O˙. The following O–O bond homolytic cleavage of this intermediate will give acyloxyl intermediate RC(O)O˙ and RCOOH, in which the former acts as a reactive species for hydrogen atom abstraction (HAA) from alkane substrates (R¹R²CH₂; R¹ and R² are alkyl groups or hydrogen atoms), giving R¹R²CH˙. The generated R¹R²CH˙ reacts with O₂ to generate alkylperoxyl radical intermediate R¹R²CHOO˙, which then undergoes the Russell reaction to give R¹R²CHOH (alcohol) and R¹R²CO (ketone) in a 1 : 1 ratio as the oxidation products. The acyloxyl intermediate RC(O)O˙ also reacts with RCHO to give carboxylic acid RC(O)OH and RC(O)˙, constructing the catalytic cycle. The role of copper(II) ions in the above catalytic process is also investigated using a series of copper(II) complexes. Furthermore, Murahashi's system was adopted in the catalytic oxidation of methane.