Palladium catalysis with sulfurated substrates under aerobic conditions: A direct oxidative carbonylation approach to thiophene-3-carboxylic esters

The first example of a palladium-catalyzed oxidative carbonylative S-cyclization process carried out under aerobic conditions is reported. The method is based on the use of the PdI2/KI catalytic system and an acetylenic substrate bearing a sulfurated group in suitable position for attacking the coordinated triple bond. To avoid the possible oxidation of a free thiol group, a methylthio group was used as sulfur nucleophile, since the methyl group on sulfur could be removed under the reaction conditions by reaction with the iodide anion to give methyl iodide. In particular, we have employed as substrates 1-(methylthio)-3-yn-2-ols, which, upon S-cyclization, demethylation, and alkoxycarbonylation, led to thiophene-3-carboxylic esters in one step, in alcohols as external nucleophile and solvents, and under 40 atm of a 4:1 mixture of CO-air (air being the source of oxygen, used as external oxidant). The Pd(0) species deriving from the alkoxycarbonylation process was reoxidized back to catalytically active PdI2 by the action of oxygen in combination with the hydrogen iodide ensuing from the alkoxycarbonylation step and from water attack to methyl iodide. This new synthetic transformation could be successfully applied to a variety of differently substituted 1-(methylthio)-3-yn-2-ols, including substrates bearing geminal dialkyl substituents.