Aromatic Osmacyclopropenefuran Bicycles and Their Relevance for the Metal-Mediated Hydration of Functionalized Allenes

The aromatic osmacyclopropenefuran bicycles [OsTp{κ3‐C1,C2,O‐(C1H2C2CHC(OEt)O)}(PiPr3)]BF4 (Tp=hydridotris(1‐pyrazolyl)borate) and [OsH{κ3‐C1,C2,O‐(C1H2C2CHC(OEt)O)}(CO)(PiPr3)2]BF4, with the metal fragment in a common vertex between the fused three‐ and five‐membered rings, have been prepared via the π‐allene intermediates [OsTp(η2‐CH2=CCHCO2Et)(OCMe2)(PiPr3)]BF4 and [OsH(η2‐CH2=CCHCO2Et)(CO)(OH2)(PiPr3)2]BF4, and their aromaticity analyzed by DFT calculations. The bicycle containing the [OsH(CO)(PiPr3)2]+ metal fragment is a key intermediate in the [OsH(CO)(OH2)2(PiPr3)2]BF4‐catalyzed regioselective anti‐Markovnikov hydration of ethyl buta‐2,3‐dienoate to ethyl 4‐hydroxycrotonate. Der ungesättigte dreigliedrige Ring von Osmacyclopropenfuran‐Bicyclen weist signifikante σ‐Aromatizität auf. Solche Verbindungen spielen eine Schlüsselrolle in der Hydratation von funktionalisierten Allenen, der Reaktionsverlauf hängt dabei von der LnOs‐Einheit ab (siehe Schema).