Mechanistic insight into conjugated N–N bond cleavage by Rh(III)-catalyzed redox-neutral C–H activation of pyrazolones

Density functional theory (DFT) calculations have been performed to investigate the detailed mechanism of Rh(iii)-catalyzed redox-neutral C-H activation of pyrazolones with PhCCPh. It is found that (1) the methylene C-H activation is prior to the phenyl C-H activation, (2) the N-N bond cleavage is realized via Rh(iii) Rh(i) Rh(iii) rather than via Rh(iii) -> Rh(v) -> Rh(iii). The zwitterionic Rh(i) complex is identified to be a key intermediate in promoting the N-N bond cleavage. (3) Different from the Rh(iii)-catalyzed hydrazine-directed C-H activation for indole synthesis, the rate-determining step of the reaction studied in this work is the Rh(iii) Rh(i) Rhliii) process resulting in the N-N bond cleavage rather than the alkyne insertion step. The present theoretical study provides new insight into the mechanism of the conjugated N-N bond cleavage.