Since the right-hand disubstituted aryl ring in 2a could be incorporated from either aryl group of 1a we sought to confirm that it originates from the bromobenzene moiety (rather than the N-phenyl ring). As expected, use of deuterated benzamide 1a-d5 gave phenanthridinone 2a-d5 as a single product in 61% yield after 2 h at 130 °C. We further probed the regioselectivity of the reaction by employing methyl substituted benzamide 1b, which afforded phenanthridinone 2b as a single regioisomeric product in 39% yield, the structure of which was confirmed by single crystal X-ray diffraction analysis and NMR spectroscopic analysis. This molecular substitution pattern confirms 8e the mechanism proceeds through a biaryl intermediate to D (Scheme 2), under our catalytic reaction conditions. Scheme 3. A: Synthesis of N-phenyl phenanthridinones 2a, 2a-d5 and 2b. B: Reaction of 1a at 80 °C in DMF mediated by Pd(OAc)2/dppe (1:1, 5 mol%), showing molar fraction of 1a (consumption) over time (GC analysis).