Iron(0)‐Mediated Stereoselective (3+2)‐Cycloaddition of Thiochalcones via a Diradical Intermediate

Reactions of α,β‐unsaturated aromatic thioketones 1 (thiochalcones) with Fe 3 (CO) 12 leading to η 4 ‐1‐thia‐1,3‐diene iron tricarbonyl complexes  2 , [FeFe] hydrogenase mimics 3 , and the thiopyrane adduct 4 are described. Obtained products have been characterized by X‐ray crystallography and by computational methods. Completely regio‐ and diastereoselective formation of the five‐membered ring system in products  3 , containing four stereogenic centers, can be explained by an unprecedented, stepwise (3+2)‐cycloaddition of two thiochalcone molecules mediated by Fe 3 (CO) 12 . Quantum chemical calculations aimed at elucidation of the reaction mechanism, suggest that the formal (3+2)‐cycloaddition proceeds via sequential intramolecular radical transfer events upon homolytic cleavage of one carbon‐sulfur bond leading to a diradical intermediate.