Combined oxidative and reductive carbonylation of terminal alkynes with palladium iodide-thiourea catalysts

Oxidative carbonylation of alkynes can be carried out catalytically in the absence of added oxidants if it is coupled with a reductive carbonylation process at the expense of the same alkyne involved in the oxidative process. Maleic esters (from oxidative carbonylation) and unsaturated lactones (from reductive carbonylation) are the main products formed under the catalytic action of palladium iodide complexes with thiourea (tu). A complex, formally corresponding to the ionic formula [PdI(tu),]I, allows the reaction of alkylacetylenes at room temperature and atmospheric pressure. With activated alkynes such as phenylacetylene, or with alkynes containing coordinating groups, other palladium complexes with two or four molecules of thiourea are also active, although to a lesser extent. Identification of the organic by-products gives a hint of the mechanism by which coupling of oxidative and reductive carbonylation occurs.