The first example of an additive cyclocarbonylation process leading to 1-thia-4a,9-diazafluoren-4-ones is reported. This process is based on the reaction of readily available 2-(propynylthio)benzimidazoles with carbon monoxide carried out in EtOH at 100 °C under a 5/2 mixture of CO-CO2 at 70 atm in the presence of the PdI2/KI catalytic system. Experimental evidence suggests a mechanistic pathway involving N-palladation of the substrate followed by CO insertion, triple bond insertion, protonolysis, and isomerization.