Activation of Styrenes toward Diels−Alder Cycloadditions by Osmium(II):  Synthesis of Stereodefined Decalin Ring Systems

The complex [Os(NH3)5(η2-anisole)]2+ reacts with acetals or Michael acceptors to form 4-methoxystyrene complexes that are exclusively coordinated at the arene (η2). These styrene complexes are active dienes and readily participate in Diels−Alder reactions with electron-deficient olefins to form tetrahydronaphthalene complexes. The cycloadducts are formed typically as single diastereomers and are valuable as precursors to functionalized tetralins and decalins.