Construction of Hexahydrophenanthrenes By Rhodium(I)-Catalyzed Cycloisomerization of Benzylallene-Substituted Internal Alkynes through C−H Activation

The treatment of benzylallene‐substituted internal alkynes with [RhCl(CO)2]2 effects a novel cycloisomerization by C(sp2)−H bond activation to produce hexahydrophenanthrene derivatives. The reaction likely proceeds through consecutive formation of a rhodabicyclo[4.3.0] intermediate, σ‐bond metathesis between the C(sp2)−H bond on the benzene ring and the C(sp2)−RhIII bond, and isomerization between three σ‐, π‐, and σ‐allylrhodium(III) species, which was proposed based on experiments with deuterated substrates. Hexahydrophenanthrene derivatives were obtained from internal alkynes with a pendant benzylallene moiety through cycloisomerization by C(sp2)−H bond activation. The reaction likely proceeds by formation of a rhodabicyclo[4.3.0] intermediate, σ‐bond metathesis between a C(sp2)−H bond on the benzene ring and the C(sp2)−RhIII bond, and isomerization between three σ‐, π‐, and σ‐allylrhodium(III) species.