Palladium-Catalyzed Cyclocarbonylation Approach to Thiadiazafluorenones. A Correction

The palladium-catalyzed carbonylation of 2-(propynylthio)benzimidazoles bearing a terminal triple bond leads to 2-methyl-1-thia-4a,9-diazafluoren-4-ones, instead of the previously reported 3-methyl isomers. This has been unequivocally established by XRD analysis of a representative product, that is, 2-methyl-1-thia-4a,9-diazafluoren-4-one. A correction is therefore provided here in order to rectify the previous erroneous assignment of the position of the methyl group in the final products. Moreover, the process has been generalized to the use of substrates bearing an internal triple bond, which led to 3-alkyl-2-methyl-1-thia-4a,9-diazafluoren-4-ones, whose structure was confirmed by XRD analysis of two representative derivatives. An unexpected structural rearrangement must take place to account for product formation with both kind of substrates. This probably corresponds to a palladium-promoted propargyl-allene rearrangement, with elimination a thiolate and an allenylpalladium intermediate, which is then followed by nucleophilic attack of the thiolate to the central carbon of allene moiety and cyclocarbonylation.