Cp*CoIII Catalyzed Site-Selective CH Activation of Unsymmetrical O-Acyl Oximes: Synthesis of Multisubstituted Isoquinolines from Terminal and Internal Alkynes

The synthesis of isoquinolines by site-selective CH activation of O-acyl oximes with a Cp*Co-III catalyst is described. In the presence of this catalyst, the CH activation of various unsymmetrically substituted O-acyl oximes selectively occurred at the sterically less hindered site, and reactions with terminal as well as internal alkynes afforded the corresponding products in up to 98% yield. Whereas the reactions catalyzed by the Cp*Co-III system proceeded with high site selectivity (15:1 to 20:1), use of the corresponding Cp*Rh-III catalysts led to low selectivities and/or yields when unsymmetrical O-acyl oximes and terminal alkynes were used. Deuterium labeling studies indicate a clear difference in the site selectivity of the CH activation step under Cp*Co-III and Cp*Rh-III catalysis.