Mechanism for phenanthridines synthesis by nitrogenation of 2-acetylbiphenyls in acidic solution: a DFT study.

The mechanism of phenanthridines synthesis from the nitrogenation of 2-acetylbiphenyls (1) by TMSN3 in TFA has been studied by DFT calculations. Results at the B3LYP/6-311G(d) level showed that: 1) reaction of TMSN3/HN3 with the protonated form of 1 (1H+), which generates the key intermediate C x+ by removal of TMSOH/H2O, is the rate determining step, and TMSN3 as the nitrogen source is certainly preferred over HN3. 2) from C x+, the two pathways leading to 2 x H+ and 3 x H+ are both thermodynamically and kinetically feasible and competitive to each other. 3) The high barriers of the reverse reactions suggest that the ratio of the final products 2 x :3 x is determined by the branching ratio of reaction rates of C x+ to intermediates D x+ in pass_I and E x+ in pass_II. Graphical Abstract DFT results indicate that the replacement of -OH by -N3 which generates C x+ controls the consumption rate of 1 x H+, and the ratio of C x+ transforms to D x+ and C x+ transforms to E x+ (k:k') determines the final ratio of products 2x:3x.