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 Cx+ by removal of TMSOH/H2 O, is the rate determining step, and TMSN3 as the nitrogen source is certainly preferred over HN3 . 2) from Cx+ , the two pathways leading to 2x H+ and 3x 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 2x :3x is determined by the branching ratio of reaction rates of Cx+ to intermediates Dx+ in pass_I and Ex+ in pass_II. Graphical Abstract DFT results indicate that the replacement of -OH by -N3 which generates Cx+ controls the consumption rate of 1x H+ , and the ratio of Cx+ transforms to Dx+ and Cx+ transforms to Ex+ (k:k') determines the final ratio of products 2x:3x.