Carbon dioxide effect on palladium-catalyzed sequential reactions with carbon monoxide, acetylenic compounds and water

The effect of CO 2 on the reaction of a terminal alkyne with CO and H 2 O in the presence of PdI 2 –KI was studied. With simple 1-alkynes such as alkyl- or arylacetylenes in the absence of added CO 2 a catalytic process takes place with formation of substituted furan-2(5 H )-ones 1 , deriving from a reductive carbonylation process. Oxidation of CO to CO 2 accounts for the stoichiometry of the reaction. The latter may occur through decarboxylation of a Pd–CO 2 H species with formation of a Pd–H species, which is in turn responsible for the reduction process. It has been found that working in the presence of added CO 2 the latter can also work as hydrogen acceptor, probably via its insertion into the Pd–H bond. In this way a catalytic synthesis of maleic anhydrides 2 , corresponding to an oxidative carbonylation process, can be obtained even in the absence of external oxidants. In contrast with simple 1-alkynes, the use of 2-methylbut-3-yn-2-ol in the absence of added CO 2 results in additive carbonylation with formation of teraconic anhydride 5 , formally deriving from the combination of oxidative dicarbonylation and reduction. No CO 2 effect is observed in this case, which rules out the intervention of Pd–H species in the catalytic cycle leading to 5 . As expected, however, carbon dioxide effect is felt by secondary products resulting from reductive carbonylation, their formation being curtailed by CO 2 addition to the reaction mixture.