Cobalt-Catalyzed Cross-Benzannulation of Conjugated Enynes and Diynes

The
[4 + 2] cross-benzannulation of conjugated enynes and diynes
under cobalt-catalysis led to 1,2,3-trisubstituted benzene derivatives
in good yields. The reaction proceeds smoothly in absolute regiospecific
control when symmetrical diynes are applied. Moreover, the use of
unsymmetrical diynes was investigated, resulting in the formation
of the unprecedented regioisomers as major products, which is in contrast
to the results obtained in palladium-catalyzed benzannulation reactions.
Also, 4-bromophenyl-substituted starting materials could be applied
successfully in the cobalt-catalyzed process, which can be problematic
in the palladium-catalyzed counterpart.