The Lemieux–Johnson or Malaprade–Lemieux–Johnson oxidation is a chemical reaction in which an olefin undergoes oxidative cleavage to form two aldehyde or ketone units. The reaction is named after its inventors, Raymond Urgel Lemieux and William Summer Johnson, who published it in 1956.
The reaction proceeds in a two step manner, beginning with dihydroxylation of the alkene by osmium tetroxide, followed by a Malaprade reaction to cleave the diol using periodate. Excess periodate is used to regenerate the osmium tetroxide, allowing it to be used in catalytic amounts. The Lemieux–Johnson reaction ceases at the aldehyde stage of oxidation and therefore produces the same results as ozonolysis.
The classical Lemieux–Johnson oxidation often generates many side products, resulting in low reaction yields; however the addition of non-nucleophilic bases, such as 2,6-lutidine, can improve on this.
OsO may be replaced with a number of other Osmium compounds. Periodate may also be replaced with other oxidising agents, such as oxone.