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

The aromatic osmacyclopropenefuran bicycles [OsTp{κ 3 ‐C 1 ,C 2 ,O‐(C 1 H 2 C 2 CHC(OEt)O)}(P i Pr 3 )]BF 4 (Tp=hydridotris(1‐pyrazolyl)borate) and [OsH{κ 3 ‐C 1 ,C 2 ,O‐(C 1 H 2 C 2 CHC(OEt)O)}(CO)(P i Pr 3 ) 2 ]BF 4 , 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(η 2 ‐CH 2 =CCHCO 2 Et)(OCMe 2 )(P i Pr 3 )]BF 4 and [OsH(η 2 ‐CH 2 =CCHCO 2 Et)(CO)(OH 2 )(P i Pr 3 ) 2 ]BF 4 , and their aromaticity analyzed by DFT calculations. The bicycle containing the [OsH(CO)(P i Pr 3 ) 2 ] + metal fragment is a key intermediate in the [OsH(CO)(OH 2 ) 2 (P i Pr 3 ) 2 ]BF 4 ‐catalyzed regioselective anti‐Markovnikov hydration of ethyl buta‐2,3‐dienoate to ethyl 4‐hydroxycrotonate.