CuO/SiO2 as a simple, effective and recoverable catalyst for alkylation of indole derivatives with diazo compounds.

The purely inorganic copper oxide on silica catalyzes the reaction of methyl phenyldiazoacetate with N-methyl indole under mild reaction conditions, giving the alkylation (formally a C-H insertion) in position 3, and the catalyst can be recovered and reused at least in 5 consecutive runs with only minor loss in activity. The scope of the reaction includes various diazo compounds and indole or pyrrole derivatives leading to alkylation or cyclopropanation depending on the heterocycle structure. An alternative mechanism, without reduction of Cu(II) to Cu(I), is proposed on the basis of the obtained results.