[6 + 3] Cycloaddition of Fischer aminocarbene complexes: An efficient annulation of fulvenes to indene derivatives

Alkenyl(amino)carbene chromium(0) complexes 1 undergo regioselective [6 + 3] cycloaddition to pentafulvenes 2, affording substituted dihydroindenes 5a–d and indene 6. In the case of 6,6-dimethylpentafulvene 2a dihydroindene chromium complexes 4a–c were also isolated. Isopropylfulvene 2b similarly produces 5d or, under oxidative reaction conditions, the indene 6. The reaction involves: (i) 1,2-nucleophilic addition of fulvene to the carbene carbon, (ii) [1,2]-Cr(CO)5 migration, (iii) either metal–indene coordination or reductive metal elimination/aromatization.