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

The aromatic osmacyclopropenefuran bicycles [OsTp{κ3 -C1 ,C2 ,O-(C1 H2 C2 CHC(OEt)O)}(Pi Pr3 )]BF4 (Tp=hydridotris(1-pyrazolyl)borate) and [OsH{κ3 -C1 ,C2 ,O-(C1 H2 C2 CHC(OEt)O)}(CO)(Pi Pr3 )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 =CCHCO2 Et)(OCMe2 )(Pi Pr3 )]BF4 and [OsH(η2 -CH2 =CCHCO2 Et)(CO)(OH2 )(Pi Pr3 )2 ]BF4 , and their aromaticity analyzed by DFT calculations. The bicycle containing the [OsH(CO)(Pi Pr3 )2 ]+ metal fragment is a key intermediate in the [OsH(CO)(OH2 )2 (Pi Pr3 )2 ]BF4 -catalyzed regioselective anti-Markovnikov hydration of ethyl buta-2,3-dienoate to ethyl 4-hydroxycrotonate.