Rhodium(III)-Catalyzed Heteroannulation Route to Isoquinolines

Significance

Reported is the rhodium(III)-catalyzed synthesis of isoquinolines 3 from the reaction of O-acetyloximes 1 with alkynes 2 using a [Cp*RhCl₂]₂/NaOAc catalytic system. The reaction proceeds in good yields with diphenylacetylene using a wide range of R^² = EDGs and EWGs. In case of meta R^² substituents, a mixture of regio­isomeric isoquinoline 3 products was obtained. Unsymmetrical and dialkyl-substituted alkynes also afforded the isoquinolines, albeit in modest yields. Based on deuteriation studies, a mechanism involving ortho C-H activation of O-acetyl­oximes 1 via the oxime sp^² nitrogen was proposed. Regioselective insertion of the alkyne into the arylrhodium intermediate leads to the formation of a seven-membered rhodacycle, which finally undergoes C-N reductive elimination. Analogously, isoxazol-5-ones 4 also undergo direct ortho-rhodation affording isoquinolines 5 with different R⁷ groups.