Insight into the Mechanism of Quinoline Formation by the Chromium(0) Fischer Carbene Catalytic Transmetallation to Palladium and Rhodium: Application to the Synthesis of the Alkaloids of Ruta chalepensis

The Pd- and Rh-mediated cyclization of group 6 (2-aminoethenyl)carbene complexes incorporating aryl iodides or boronates in their structures is reported. The process led to the formation of quinoline rings and has been applied to the synthesis of quinoline alkaloids 12 and 13 isolated from the roots of the perennial grass Ruta chalepensis. The computational (DFT) study of the cyclization processes revealed unusual features. Thus, for the Pd-mediated cyclization of (2-aminoethenyl)carbene complexes incorporating aryl iodides in their structures, the first step in the pathway to the quinolines is the Cr-to-Pd transmetallation reaction, leading to palladacarbenes instead of the expected oxidative addition to the aryl iodide. For Rh-I-mediated cyclizations in (2-aminoethenyl) carbene complexes incorporating aryl boronates, the sequence of events involves an initial Cr-to-Rh transmetallation followed by oxidative addition to the aryl-boron bond. The evolution of these intermediates to the final quinolines parallels the Pd-catalyzed pathway.