Investigation and comparison of the mechanistic steps in the [(Cp*MCl2)2] (Cp* = C5Me5; M = Rh, Ir)-catalyzed oxidative annulation of isoquinolones with alkynes

Abstract

The mechanism of the [(Cp*MCl(2))(2)] (M = Rh, Ir)-catalyzed oxidative annulation reaction of isoquinolones with alkynes was investigated in detail. In the first acetate-assisted C-H-activation process (cyclometalated step) and the subsequent mono-alkyne insertion into the M-C bonds of the cyclometalated compounds, both Rh and Ir complexes participated well. However, the desired final products, dibenzo[a,g]quinolizin-8-one derivatives, were only formed in high yield when the Rh species participated in the final oxidative coupling of the C-N bond. Moreover, a Rh(I) sandwich intermediate was isolated during this transformation. The iridium complexes were found to be inactive in the oxidative coupling processes. All of the relevant intermediates were fully characterized and determined by single-crystal X-ray diffraction analysis. Based on this mechanistic study, a Rh(III)→Rh(I)→Rh(III) catalytic cycle was proposed for this reaction.