Hydroamination of Dihapto‐Coordinated Benzene and Diene Complexes of Tungsten: Fundamental Studies and the Synthesis of γ‐Lycorane

Reactions are described for complexes of the form WTp(NO)(PMe3)(η2‐arene) and various amines, where the arene is benzene or benzene with an electron‐withdrawing substituent (CF3, SO2Ph, SO2Me). The arene complex is first protonated to form an η2‐arenium species, which then selectively adds the amine. The resulting η2‐5‐amino‐1,3‐cyclohexadiene complexes can then be subjected to the same sequence with a second nucleophile to form 3‐aminocyclohexene complexes, where up to three stereocenters originate from the arene carbons. Alternatively, 1,3‐cyclohexadiene complexes containing an ester group at the 5 position (also prepared from an arene) can be treated with acid followed by an amine to form trisubstituted 3‐aminocyclohexenes. When the amine is primary, ring closure can occur to form a cis‐fused bicyclic γ‐lactam. Highly functionalized cyclohexenes can be liberated from the tungsten through oxidative decomplexation. The potential utility of this methodology is demonstrated in the synthesis of the alkaloid γ‐lycorane. An enantioenriched synthesis of a lactam precursor to γ‐lycorane is also described. This compound is prepared from an enantioenriched version of the tungsten benzene complex. Regio‐ and stereochemical assignments for the reported compounds are supported by detailed 2D‐NMR analysis and 13 molecular structure determinations (SC‐XRD).